Understanding Threads & Thread Mills

Thread milling can present a machinist many challenges. While thread mills are capable of producing threads with relative ease, there are a lot of considerations that machinists must make prior to beginning the job in order to gain consistent results. To conceptualize these features and choose the right tool, machinists must first understand basic thread milling applications.

 

What is a thread?

The primary function of a thread is to form a coupling between two different mechanisms. Think of the cap on your water bottle. The cap couples with the top of the bottle in order to create a water tight seal. This coupling can transmit motion and help to obtain mechanical advantages.  Below are some important terms to know in order to understand threads.

Root – That surface of the thread which joins the flanks of adjacent thread forms and is immediately adjacent to the cylinder or cone from which the thread projects.

Flank – The flank of a thread is either surface connecting the crest with the root. The flank surface intersection with an axial plane is theoretically a straight line.

Crest – This is that surface of a thread which joins the flanks of the thread and is farthest from the cylinder or cone from which the thread projects.

Pitch – The pitch of a thread having uniform spacing is the distance measured parallelwith its axis between corresponding points on adjacent thread forms in the same axial plane and on the same side of the axis. Pitch is equal to the lead divided by the number of thread starts.

Major Diameter – On a straight thread the major diameter is that of the major cylinder.On a taper thread the major diameter at a given position on the thread axis is that of the major cone at that position.

Minor Diameter – On a straight thread the minor diameter is that of the minor cylinder. On a taper thread the minor diameter at a given position on the thread axis is that of the minor cone at that position.

Helix Angle – On a straight thread, the helix angle is the angle made by the helix of the thread and its relation to the thread axis. On a taper thread, the helix angle at a given axial position is the angle made by the conical spiral of the thread with the axis of the thread. The helix angle is the complement of the lead angle.

Depth of Thread Engagement – The depth (or height) of thread engagement between two coaxially assembled mating threads is the radial distance by which their thread forms overlap each other.

External Thread – A thread on a cylindrical or conical external surface.

Internal Thread – A thread on a cylindrical or conical internal surface.

Class of Thread – The class of a thread is an alphanumerical designation to indicate the standard grade of tolerance and allowance specified for a thread.

Source: Machinery’s Handbook 29th Edition

Types of Threads & Their Common Applications:

ISO Metric, American UN: This thread type is used for general purposes, including for screws. Features a 60° thread form.

British Standard, Whitworth: This thread form includes a 55° thread form and is often used when a water tight seal is needed.

NPT: Meaning National Pipe Tapered, this thread, like the Whitworth Thread Form, is also internal. See the above video for an example of an NPT thread.

UNJ, MJ: This type of thread is often used in the Aerospace industry and features a radius at the root of the thread.

ACME, Trapezoidal: ACME threads are screw thread profiles that feature a trapezoidal outline, and are most commonly used for power screws.

Buttress Threads: Designed for applications that involve particularly high stresses along the thread axis in one direction. The thread angle on these threads is 45° with a perpendicular flat on the front or “load resisting face.”         

Thread Designations

Threads must hold certain tolerances, known as thread designations, in order to join together properly. International standards have been developed for threads. Below are examples of Metric, UN, and Acme Thread Designations. It is important to note that not all designations will be uniform, as some tolerances will include diameter tolerances while others will include class of fit.

Metric Thread Designations              

M12 x 1.75 – 4h – LH

In this scenario, “M” designates a Metric Thread Designation, 12 refers to the Nominal Diameter, 1.75 is the pitch, 4h is the “Class of Fit,” and “LH” means “Left-Hand.”

UN Thread Designations

¾ 10 UNC 2A LH

For this UN Thread Designation, ¾ refers to the thread’s major diameter, where 10 references the number of threads per inch. UNC stands for the thread series; and 2A means the class of thread. The “A” is used to designate external threads, while “B” is for internal threads. For these style threads, there are 6 other classes of fit; 1B, 2B, and 3B for internal threads; and 1A, 2A, and 3A for external threads.

ACME Thread Designations

A 1 025 20-X

For this ACME Thread Designation, A refers to “Acme,” while 1 is the number of thread starts. The basic major diameter is called out by 025 (Meaning 1/4”) while 20 is the callout for number of threads per inch. X is a placeholder for a number designating the purpose of the thread. A number 1 means it’s for a screw, while 2 means it’s for a nut, and 3 refers to a flange.

How are threads measured?

Threads are measured using go and no-go gauges. These gauges are inspection tools used to ensure the that the thread is the right size and has the correct pitch. The go gauge ensures the pitch diameter falls below the maximum requirement, while the no-go gauge verifies that the pitch diameter is above the minimum requirement. These gauges must be used carefully to ensure that the threads are not damaged.

Thread Milling Considerations

Thread milling is the interpolation of a thread mill around or inside a workpiece to create a desired thread form on a workpiece. Multiple radial passes during milling offer good chip control. Remember, though, that thread milling needs to be performed on machines capable of moving on the X, Y, and Z axis simultaneously.

5 Tips for Successful Thread Milling Operations:

1.  Opt for a Quality Tooling Manufacturer

There is no substitute for adequate tooling. To avoid tool failure and machining mishaps, opt for a quality manufacturer for High Performance Drills for your starter holes, as well as for your thread milling solutions. Harvey Tool fully stocks several types of threadmills, including Single Form, Tri-Form, and Multi-Form Thread Milling Cutters. In addition, the 60° Double Angle Shank Cutter can be used for thread milling.

thread milling

Image Courtesy of  @Avantmfg

2. Select a Proper Cutter Diameter

Choose only a cutter diameter as large as you need. A smaller cutter diameter will help achieve higher quality threads.

3. Ensure You’re Comfortable with Your Tool Path

Your chosen tool path will determine left hand or right hand threads.

Right-hand internal thread milling is where cutters move counterclockwise in an upwards direction to ensure that climb milling is achieved.

Left-hand internal thread milling a left-hand thread follows in the opposite direction, from top to bottom, also in a counterclockwise path to ensure that climb milling is achieved.

4. Assess Number of Radial Passes Needed

In difficult applications, using more passes may be necessary to achieve desired quality. Separating the thread milling operation into several radial passes achieves a finer quality of thread and improves security against tool breakage in difficult materials. In addition, thread milling with several radial passes also improves thread tolerance due to reduced tool deflection. This gives greater security in long overhangs and unstable conditions.

5. Review Chip Evacuation Strategy

Are you taking the necessary steps to avoid chip recutting due to inefficient chip evacuation? If not, your thread may fall out of tolerance. Opt for a strategy that includes coolant, lubricant, and tool retractions.

In Summary

Just looking at a threading tool can be confusing – it is sometimes hard to conceptualize how these tools are able to get the job done. But with proper understanding of call, methods, and best practices, machinists can feel confident when beginning their operation.

Get to Know Machining Advisor Pro

Machining Advisor Pro (MAP) is a tool to quickly, seamlessly, and accurately deliver recommended running parameters to machinists using Helical Solutions end mills. This download-free and mobile-friendly application takes into account a user’s machine, tool path, set-up, and material to offer tailored, specific speeds and feed parameters to the tools they are using.

How to Begin with Machining Advisor Pro

This section will provide a detailed breakdown of Machining Advisor Pro, moving along step-by-step throughout the entire process of determining your tailored running parameters.

Register Quickly on Desktop or Mobile

To begin with Machining Advisor Pro, start by accessing its web page on the Harvey Performance Company website, or use the mobile version by downloading the application from the App Store or Google Play.

Whether you are using Machining Advisor Pro from the web or your mobile device, machinists must first create an account. The registration process will only need to be done once before you will be able to log into Machining Advisor Pro on both the mobile and web applications immediately.

machining advisor pro

Simply Activate Your Account

The final step in the registration process is to activate your account. To do this, simply click the activation link in the email that was sent to the email address used when registering. If you do not see the email in your inbox, we recommend checking your spam folders or company email filters. From here, you’re able to begin using MAP.

Using MAP

A user’s experience will be different depending on whether they’re using the web or mobile application. For instance, after logging in, users on the web application will view a single page that contains the Tool, Material, Operation, Machine, Parameter, and Recommendation sections.

 

 

On the mobile application, however, the “Input Specs” section is immediately visible. This is a summary of the Tool, Material, Operation, and Machine sections that allow a user to review and access any section. Return to this screen at any point by clicking on the gear icon in the bottom left of the screen.

machining advisor pro

Identify Your Helical Tool

To get started generating your running parameters, specify the Helical Solutions tool that you are using. This can be done by entering the tool number into the “Tool #” input field (highlighted in red below). As you type the tool number, MAP will filter through Helical’s 4,800-plus tools to begin identifying the specific tool you are looking for.

 

Once the tool is selected, the “Tool Details” section will populate the information that is specific to the chosen tool. This information will include the type of tool chosen, its unit of measure, profile, and other key dimensional attributes.

 

Select the Material You’re Working In

Once your tool information is imported, the material you’re working in will need to be specified. To access this screen on the mobile application, either swipe your screen to the left or click on the “Material” tab seen at the bottom of the screen. You will move from screen to screen across each step in the mobile application by using the same method.

In this section, there are more than 300 specific material grades and conditions available to users. The first dropdown menu will allow you to specify the material you are working in. Then, you can choose the subgroup of that material that is most applicable to your application. In some cases, you will also need to choose a material condition. For example, you can select from “T4” or “T6” condition for 6061 Aluminum.

 

Machining Advisor Pro provides optimized feeds and speeds that are specific to your application, so it is important that the condition of your material is selected.

Pick an Operation

The next section of MAP allows the user to define their specific operation. In this section, you will define the tool path strategy that will be used in this application. This can be done by either selecting the tool path from the dropdown menu or clicking on “Tool Path Info” for a visual breakdown and more information on each available toolpath.

 

Tailor Parameters to Your Machine’s Capabilities

The final section on mobile, and the fourth web section, is the machine section. This is where a user can define the attributes of the machine that you are using. This will include the Max RPM, Max IPM, Spindle, Holder, and work holding security. Running Parameters will adjust based on your responses.

 

Access Machining Advisor Pro Parameters

Once the Tool, Material, Operation, and Machine sections are populated there will be enough information to generate the initial parameters, speed, and feed. To access these on the mobile app, either swipe left when on the machine tab or tap on the “Output” tab on the bottom menu.

 

Please note that these are only initial values. Machining Advisor Pro gives you the ability to alter the stick out, axial depth of cut, and radial depth of cut to match the specific application. These changes can either be made by entering the exact numeric value, the % of cutter diameter, or by altering the slider bars. You are now able to lock RDOC or ADOC while adjusting the other depth of cut, allowing for more customization when developing parameters.

The parameters section also offers a visual representation of the portion of the tool that will be engaged with the materials as well as the Tool Engagement Angle.

MAP’s Recommendations

At this point, you can now review the recommended feeds and speeds that Machining Advisor Pro suggests based on the information you have input. These optimized running parameters can then be further refined by altering the speed and feed percentages.

 

Machining Advisor Pro recommendations can be saved by clicking on the PDF button that is found in the recommendation section on both the web and mobile platforms. This will automatically generate a PDF of the recommendations, allowing you to print, email, or share with others.

Machining Advisor Pro Summarized

The final section, exclusive to the mobile application, is the “Summary” section. To access this section, first tap on the checkmark icon in the bottom menu. This will open a section that is similar to the “Input Specs” section, which will give you a summary of the total parameter outputs. If anything needs to change, you can easily jump to each output item by tapping on the section you need to adjust.

 

 

This is also where you would go to reset the application to clear all of the inputs and start a new setup. On the web version, this button is found in the upper right-hand corner and looks like a “refresh” icon on a web browser.

Contact Us

For the mobile application, we have implemented an in-app messaging service. This was done to give the user a tool to easily communicate any question they have about the application from within the app. It allows the user to not only send messages, but to also include screenshots of what they are seeing! This can be accessed by clicking on the “Contact Us” option in the same hamburger menu that the Logout and Help & Tips are found.

Click this link to sign up today!

How To Maximize High Balance End Mills

High speed machining is becoming increasingly widespread in machine shops all over the world due to the proven benefits of greater efficiency and productivity through increased spindle speeds and metal removal rates.  However, at such high spindle speeds, otherwise negligible errors and imperfections can cause negative effects such as reduced tool life, poor surface finish, and wear on the machine itself. Many of these negative effects stem from an increase in total centrifugal forces leading to vibration, commonly referred to in the industry as chatter. A key contributor to vibrations and one of the more controllable factors, is tool unbalance.

Why Balance is Critical to Machining

Unbalance is the extent to which the tool’s center of mass diverges from its axis of rotation.  Small levels of unbalance may be indistinguishable at lower RPMs, but as centrifugal force increases, small variations in the tool’s center of mass can cause substantial detrimental effects on its performance. High Balance End Mills are often used to help solve the problem of vibrations at the increased spindle speeds. Balancing is used to make compensation for the intrinsic unsymmetrical distribution of mass, which is typically completed by removing mass of a calculated amount and orientation.

Image Source: Haimer; Fundamentals of Balancing

Helical Solutions offers High Balance End Mills in both 2 and 3 flute options (see Figure 2), square and corner radius, along with coolant-through on the 3 fluted tools. These end mills are balanced at the industry standard of G2.5 at 33,000 RPM: G stands for the potential damage due to unbalance, which can be expressed as “Balancing Quality Grade” or G and 2.5 is the vibration velocity in MM per second. These tools are designed specifically to increase performance in highly balanced machining centers that are capable of elevated RPMs and feed rates. With high balance tooling, improved surface finishes are also achieved due to reduced vibrations during the machining process. Additionally, these end mills have been designed around current high-end tool holding, and come in a variety of neck lengths at specific overall lengths. These dimensional combinations result in maximum rigidity and reduced excess stick out, allowing for optimal performance and the ability to push the tools to the limit.

High Balanced Tooling Cost Benefits

Machinists who choose to use High Balance End Mills will see certain benefits at the spindle, but also in their wallets. Cost benefits of opting to run this type of tool include:

Utilizing Tap Testers

What Tap Testers Do

Vibrations are your applications worst enemy, especially at elevated RPMs and feed rates. Using resources such as a Tap Tester can help decrease vibrations and allow you to get the most out of your High Balance End Mills by generating cutting performance predictions and chatter limits.

How Tap Testing Works

High balance

Image Source: Manufacturing Automation Laboratories Inc.

Tap Testing generates cutting performance predictions and chatter limits. In a tap test, the machine-tool structure is “excited,” or tested, by being hit with an impulse hammer. In milling, the machine-tool structure is usually flexible in all three directions: X, Y, and Z, but in milling applications where High Balance Tooling is used, the flexibility is commonly only considered in two planes – the X and Y directions. By hitting the X and Y directions with the impulse hammer, the impact will excite the structure over a certain frequency range that is dependent on the hammer’s size, the type of tool being used, and the structure itself. The frequencies generated from the initial hit will produce enough information that both the impact force measurement and the displacement/accelerometer measurement are available. Combining these two measurements will result in the Frequency Response Function, which is a plot of the dynamic stiffness of the structure in frequencies.

After the information from the Tap Test is gathered, it will then process the information into useful cutting parameters for all spindles speeds such as cut depths, speed rates, and feed rates. In knowing the optimum running parameters, vibrations can be minimized and the tool can be utilized to its full potential.

High Balanced Tooling Summarized

Keeping vibrations at bay during the machining process is extremely important to machining success. Because one cause of vibration is tool unbalance, utilizing a balanced tool will result in a smoother job, a cleaner final product, and a longer life of both the tool and spindle. Machinists who choose to use High Balance Tooling can utilize a Tap Tester, or a method for generating the perfect running parameters for your tool and machine setup to ensure that machining vibration is as minimal as possible.

Shank Tolerances, Collet Fits, & h6 Benefits

A cutting tool’s shank is one of the more vital parts of a tool, as it’s critical to the collet-tool connection. There are several types of shanks, each with their own tolerances and suitable tool holder methods. One of the most popular and effective tool holding styles is a shrink fit tool holder, which works with h6 shanks, but what does this mean and what are the benefits of it? How is this type of shank different from a shank with standard shank tolerances? To answer these questions, we must first explore the principals of tolerances.

The Principals of Tolerances

Defining Industry Standard Tolerances

There are two categories of shank tolerances that machinists and engineers operating a CNC machine should be familiar with: hole basis and shank (or shaft) basis. The hole basis system is where the minimum hole size is the starting point of the tolerance. If the hole tolerance starts with a capital “H,” then the hole has a positive tolerance with no negative tolerance. The shank basis system is where the maximum shank size is the starting point. This system is relatively the same idea as the hole basis system but instead, if the tolerance starts with a lowercase “h,” the shank has a negative tolerance and no positive tolerance.

Letter Designations

The limits of tolerance for a shank or hole are designated by the appropriate letter indicating the deviation. For instance, the letter “k” has the opposite minimum and maximum designations as “h”. Tolerances beginning with “k” are exclusively positive, while tolerances beginning with “h” are exclusively negative. The number following the given letter denotes the International Tolerance (IT) grade. For example, a tolerance with the number 6 will have a smaller tolerance range than the number 7, but larger than the number 5. This range is based on the size of the shank. A hole that has a 0.030” diameter will have an h6 tolerance of (+0.0000,-0.0002), while a 1.00” hole with have an h6 tolerance band of (+0.0000,-0.0005).

It is important to note that most sources list IT tolerances in millimeters, while the graph below has been translated to inches. Operations that require more precise manufacturing, such as reaming, will have lower IT grades. Operations that do not require manufacturing to be as precise will have higher IT grades.

shank tolerances

Preferred Collet Fits

Different types of combinations of hole basis and shank basis tolerances lead to different types of collet fits. The following table offers insight into a few different types of preferred fits and the shank tolerances that are required for each.

collet fits

Image: Machinery’s Handbook 29th Edition.

Shrink Fit Tool Holders

The shrink fit holder is one of the more popular styles of tool holders because of its ability to be more customizable, as evident in the chart above. In this method, a collet is heated to expand, then cooled to contract around the shank of a tool. At room temperature, a cutting tool should not be able to be inserted into a shrink fit holder – only when the holder has undergone thermal expansion due to the introduction of a significant amount of heat should the tool fit. As the holder cools, the tool is held tighter and tighter in place. Typically, a holder is heated through a ring of coils by an induction heater. It is important to heat the holder uniformly, paying mind to not overheat it. Doing so could cause the shank that is being held to expand within the holder and remain stuck.

 

Benefits of Shrink Fit Tool Holders

  1. Gripping power. The shank is held flush and uniform against the holder, resulting in a tighter connection.
  2. Low runout. A more secure connection will result in extended tool life, and a higher quality surface finish.
  3. Better balance for high RPM. With a tighter tool-to-holder connection, the opportunity exists for more aggressive running parameters.

Shank Tolerances Summarized

Understanding shank tolerances is an intricate part of the machining process as it impacts which tool holder is appropriate for your job. A secure holder connection is vital to the performance of the tool in your application. With an h6 shrink fit holder, the result is a secure connection with stronger gripping power. However, only certain shanks are able to be used with this type of holder. From the letter designation assigned to a shank, to whether that letter is upper or lowercase, each detail is vital to ensuring a proper fit between your tools shank and its corresponding shrink fit holder.

Best Practices of Tolerance Stacking

Tolerance stacking, also known as tolerance stack-up, refers to the combination of various part dimension tolerances. After a tolerance is identified on the dimension of a part, it is important to test whether that tolerance would work with the tool’s tolerances: either the upper end or lower end. A part or assembly can be subject to inaccuracies when its tolerances are stacked up incorrectly.

The Importance of Tolerances

Tolerances directly influence the cost and performance of a product. Tighter tolerances make a machined part more difficult to manufacture and therefore often more expensive. With this in mind, it is important to find a balance between manufacturability of the part, its functionality, and its cost.

Tips for Successful Tolerance Stacking

Avoid Using Tolerances that are Unnecessarily Small

As stated above, tighter tolerances lead to a higher manufacturing cost as the part is more difficult to make. This higher cost is often due to the increased amount of scrapped parts that can occur when dimensions are found to be out of tolerance. The cost of high quality tool holders and tooling with tighter tolerances can also be an added expense.

Additionally, unnecessarily small tolerances will lead to longer manufacturing times, as more work goes in to ensure that the part meets strict criteria during machining, and after machining in the inspection process.

Be Careful Not to Over Dimension a Part

When an upper and lower tolerance is labeled on every feature of a part, over-dimensioning can become a problem. For example, a corner radius end mill with a right and left corner radii might have a tolerance of +/- .001”, and the flat between them has a .002” tolerance. In this case, the tolerance window for the cutter diameter would be +/- .004”, but is oftentimes miscalculated during part dimensioning. Further, placing a tolerance on this callout would cause it to be over dimensioned, and thus the reference dimension “REF” must be left to take the tolerance’s place.

stacking tolerances

Figure 1: Shape of slot created by a corner radius end mill

Utilize Statistical Tolerance Analysis:

Statistical analysis looks at the likelihood that all three tolerances would be below or above the dimensioned slot width, based on a standard deviation. This probability is represented by a normal probability density function, which can be seen in figure 2 below. By combining all the probabilities of the different parts and dimensions in a design, we can determine the probability that a part will have a problem, or fail altogether, based on the dimensions and tolerance of the parts. Generally this method of analysis is only used for assemblies with four or more tolerances.

stacking tolerances

                                                               Figure 2: Tolerance Stacking: Normal distribution

Before starting a statistical tolerance analysis, you must calculate or choose a tolerance distribution factor. The standard distribution is 3 . This means that most of the data (or in this case tolerances) will be within 3 standard deviations of the mean. The standard deviations of all the tolerances must be divided by this tolerance distribution factor to normalize them from a distribution of 3  to a distribution of 1 . Once this has been done, the root sum squared can be taken to find the standard deviation of the assembly.

Think of it like a cup of coffee being made with 3 different sized beans. In order to make a delicious cup of joe, you must first grind down all of the beans to the same size so they can be added to the coffee filter. In this case, the beans are the standard deviations, the grinder is the tolerance distribution factor, and the coffee filter is the root sum squared equation. This is necessary because some tolerances may have different distribution factors based on the tightness of the tolerance range.

The statistical analysis method is used if there is a requirement that the slot must be .500” wide with a +/- .003” tolerance, but there is no need for the radii (.125”) and the flat (.250”) to be exact as long as they fit within the slot. In this example, we have 3 bilateral tolerances with their standard deviations already available. Since they are bilateral, the standard deviation from the mean would simply be whatever the + or – tolerance value is. For the outside radii, this would be .001” and for the middle flat region this would be .002”.

For this example, let’s find the standard deviation (σ) of each section using equation 1. In this equation represents the standard deviation.

standard deviation

The standard assumption is that a part tolerance represents a +/- 3  normal distribution. Therefore, the distribution factor will be 3. Using equation 1 on the left section of figure 1, we find that its corrected standard deviation equates to:

tolerance stacking

This is then repeated for the middle and right sections:

standard deviation

After arriving at these standard deviations, we input the results into equation 2 to find the standard deviation of the tolerance zone. Equation 2 is known as the root sum squared equation.

root sum

At this point, it means that 68% of the slots will be within a +/- .0008” tolerance. Multiplying this tolerance by 2 will result in a 95% confidence window, where multiplying it by 3 will result in a 99% confidence window.

68% of the slots will be within +/- .0008”

95% of the slots will be within +/- .0016”

99% of the slots will be within +/- .0024”

These confidence windows are standard for a normal distributed set of data points. A standard normal distribution can be seen in Figure 2 above.

Statistical tolerance analysis should only be used for assemblies with greater than 4 toleranced parts. A lot of factors were unaccounted for in this simple analysis. This example was for 3 bilateral dimensions whose tolerances were representative of their standard deviations from their means. In standard statistical tolerance analysis, other variables come into play such as angles, runout, and parallelism, which require correction factors.

Use Worst Case Analysis:

Worst case analysis is the practice of adding up all the tolerances of a part to find the total part tolerance. When performing this type of analysis, each tolerance is set to its largest or smallest limit in its respective range. This total tolerance can then be compared to the performance limits of the part to make sure the assembly is designed properly. This is typically used for only 1 dimension (Only 1 plane, therefore no angles involved) and for assemblies with a small number of parts.

Worst case analysis can also be used when choosing the appropriate cutting tool for your job, as the tool’s tolerance can be added to the parts tolerance for a worst case scenario. Once this scenario is identified, the machinist or engineer can make the appropriate adjustments to keep the part within the dimensions specified on the print. It should be noted that the worst case scenario rarely ever occurs in actual production. While these analyses can be expensive for manufacturing, it provides peace of mind to machinists by guaranteeing that all assemblies will function properly. Often this method requires tight tolerances because the total stack up at maximum conditions is the primary feature used in design. Tighter tolerances intensify manufacturing costs due to the increased amount of scraping, production time for inspection, and cost of tooling used on these parts.

Example of worst case scenario in context to Figure 1:

Find the lower specification limit.

For the left corner radius

.125” – .001” = .124”

For the flat section

.250” – .002” = .248”

For the right corner radius

.125” – .001” = .124”

Add all of these together to the lower specification limit:

.124” + .248” + .124” = .496”

Find the upper specification limit:

For the left corner radius

.125” + .001” = .126”

For the flat section

.250” + .002” = .252”

For the right corner radius

.125” + .001” = .126”

Add all of these together to the lower specification limit:

.126” + .252” + .126” = .504”

Subtract the two and divide this answer by two to get the worst case tolerance:

(Upper Limit – Lower Limit)/2 = .004”

Therefore the worst case scenario of this slot is .500” +/- .004”.

Drill / End Mills: Drill Style vs. Mill Style

Drill / End Mills are one of the most versatile tools in a machinist’s arsenal. These tools can perform a number of different operations, freeing space on your carousel and improving cycle times by limiting the need for tool changes. These operations include:

  1. Drilling
  2. V-Grooving
  3. Milling
  4. Spot Drilling
  5. Chamfering

The ability of the Drill / End Mill to cut along the angled tip as well as the outer diameter gives it the range of operations seen above and makes it an excellent multi-functional tool.

drill mill operations

Drill Style vs. Mill Style

The main difference between Drill / End Mill styles is the point geometry.  They are defined by how the flutes are designed on the end of the tool, using geometry typically seen on either an end mill or a drill.  While mill style tools follow the features of an end mill or chamfer mill, the drill style geometry uses an S-gash at the tip.  This lends strength to the tip of the tool, while giving it the ability to efficiently and accurately penetrate material axially.  While both styles are capable of OD milling, mill style tools will be better for chamfering operations, while drill style will excel in drilling.  The additional option of the Harvey Tool spiral tipped Drill / End Mill is an unprecedented design in the industry.  This tool combines end geometry taken from our helical flute chamfer cutters with a variable helix on the OD for enhanced performance. Versatility without sacrificing finish and optimal performance is the result.

drill mills

Left to Right: 2 Flute Drill Style End, 2 Flute Mill Style End, 4 Flute Mill Style End

Drill Mills: Tool Offering

Harvey Tool currently offers Drill / End Mills in a variety of styles that can perform in different combinations of machining applications:

Mill Style – 2 Flute

This tool is designed for chamfering, milling, drilling non-ferrous materials, and light duty spotting. Drilling and spotting operations are recommended only for tools with an included angle greater than 60°. This is a general rule for all drill mills with a 60° point. Harvey Tool stocks five different angles of 2 flute mill-style Drill / End Mills, which include 60°, 82°, 90°, 100° and 120°. They are offered with an AlTiN coating on all sizes as well as a TiB2 coating for cutting aluminum with a 60° and 90° angle.

drill mill

Mill Style – 4 Flute

4 flute mill-style Drill / End Mills have two flutes that come to center and two flutes that are cut back. This Drill / End Mill is designed for the same operations as the 2 flute style, but has a larger core in addition the higher flute count. The larger core gives the tool more strength and allows it to machine a harder range of materials. The additional flutes create more points of contact when machining, leading to better surface finish. AlTiN coating is offered on all 5 available angles (60°, 82°, 90°, 100°, and 120°) of this tool for great performance in a wide array of ferrous materials.

drill mill

Drill Style – 2 Flute

This tool is specifically designed for the combination of milling, drilling, spotting and light duty chamfering applications in ferrous and non-ferrous materials. This line is offered with a 90°, 120°, and 140° included angle as well as AlTiN coating.

drill mills drill style

Helical Tip – 4 Flute

The Helically Tipped Drill / End Mill offers superior performance in chamfering, milling and light duty spotting operations. The spiral tip design allows for exceptional chip evacuation and surface finish. This combined with an OD variable helix design to reduce chatter and harmonics makes this a valuable tool in any machine shop. It is offered in 60°, 90°, and 120° included angles and comes standard with the latest generation AlTiN Nano coating that offers superior hardness and heat resistance.

 

Tips for Maintaining Tight Tolerances

In manufacturing large production runs, one of the biggest difficulties machinists experience is holding tooling to necessary tolerances in holes, walls, and threads. Typically, this is an iterative process that can be tedious and stressful, especially for inexperienced machinists. While each job presents a unique set of challenges, there are rules of thumb that can be followed to ensure that your part is living up to its accuracy demands.

What is a Tolerance?

A tolerance is an allowable amount of variation in a part or cutting tool that a dimension can fall within. When creating a part print, tolerances of tooling can’t be overlooked, as tooling tolerances can result in part variations. Part tolerances have to be the same, if not larger, than tool tolerances to ensure part accuracy.

Cutting tool tolerances are oftentimes applied to a tool’s most critical dimensions, such as Cutter Diameter, Length of Cut, Shank Diameter, and Overall Length. When selecting a cutting tool for a job, it’s critical to choose a brand that adheres to strict tolerance standards and reliable batch-to-batch consistency. Manufacturers like Harvey Tool and Helical Solutions prominently display tolerances for many critical tool dimensions and thoroughly inspect each tool to ensure that it meets the tolerances specified. Below is the table header for Harvey Tool’s line of Miniature End Mills – Square – Stub & Standard.

tolerances

Tolerances help to create repeatability and specificity, especially in an industry in which even a thousandth of an inch can make or break a final product. This is especially true for miniature tooling, where Harvey Tool is experienced in the designing and manufacturing of tooling as small as .001” in diameter.

How are Tolerances Used?

When viewing a tolerance, there’s an upper and lower dimension, meaning the range in which the dimension of the tool can stray – both above and below what its size is said to be. In the below example, a .030″ cutter diameter tool’s size range would be anywhere between .0295″ and .0305.”

tooling tolerance

Maintaining Tolerances in Holemaking Operations

Holes oftentimes mandate the tightest dimensional tolerances, as they generally are meant to align perfectly with a mating part. To maintain tolerances, start first by testing the runout of both your machine and your tool. This simple, yet often overlooked step can save machinists a great amount of time and frustration.

Spotting Drills

Spotting Drills allow for drills to have a very precise starting point, minimizing walking or straying from a desired path. This can be especially beneficial when machining irregular surfaces, where accessing a hole’s perfect location can be more difficult.

spotting drills

Reamers

Reaming is great for any very tight tolerance mandate, because many Miniature Reamers have much tighter tolerances than a drill. Harvey Tool’s Miniature Reamers, for example, have tolerances of +.0000″/-.0002. for uncoated options and +.0002″/-.0000″ for AlTiN coated tools. Reamers cut on their chamfered edge, removing a minimal amount of material within a hole with the ultimate goal of bringing it to size. Because the cutting edge of a reamer is so small, the tool has a larger core diameter and is thus a more rigid tool.

miniature reamers

Maintaining Tight Tolerances While Machining Walls

Be Wary of Deflection

Maintaining tolerances when machining walls is made difficult by deflection, or the curvature a tool experiences when a force is applied to it. Where an angle is appearing on a wall due to deflection, opt for a reached tool to allow for less deflection along the tool’s neck. Further, take more axial depths of cut and machine in steps with finishing passes to exert less pressure on the tool. For surface finish tolerances, a long fluted tool may be required to minimize evidence of a tool path left on a part. For more information on ways to minimize deflection, read Tool Deflection & Its Remedies.tool deflection

Corner Radius End Mills

Corner radius End Mills, because they do not feature a sharp edge, will wear slower than a square end mill would. By utilizing corner radius tooling, fracturing on the tool edge will be minimized, resulting in an even pressure distribution on each of the cutting edges. Because the sharper edge on a square tool is less durable and more prone to cracking because of the stress concentration on that point, a corner radius tool would be much more rigid and thus less susceptible to causing a tolerance variation. For this reason, it’s recommended to use a roughing tool with a corner radius profile and a finisher with a square profile for an edge tolerance. When designing a part and keeping manufacturing in mind, if there is a potential for a wall with a radius as opposed to a wall with a square edge, a wall with a radius allows for easier machineability and fewer tool changes.

Maintaining Tight Tolerances While Threading

Making threads to tolerance is all about chip evacuation. Evacuating chips is an issue commonly overlooked; If chips within a hole have not been removed before a threading operation, there could be interference in the tool tip that leads to vibration and chatter within a thread. This would decrease the continuity of the thread while also altering the points of contact. Discontinuity of a thread could be the difference between passing and failing a part, and because threading is typically the last application when machining to decrease damaging the threads, it also increases the likelihood of chips remaining within the hole from other applications.

Tolerances Summarized

If you continue to experience troubles maintaining tight tolerances despite this blog post, consult the Harvey Tool or Helical Solutions tech team, as the problem may exist outside of your machine. Temperature and humidity can vary how gummy a material is, and can lead to workpiece expansion and contraction. Additionally, the foundation of buildings can expand and contract due to outside temperature, which can result in upped runout and irregular vibration in a spindle.

Tool Deflection & Its Remedies

Every machinist must be aware of tool deflection, as too much deflection can lead to catastrophic failure in the tool or workpiece. Deflection is the displacement of an object under a load causing curvature and/or fracture.

For Example: When looking at a diving board at rest without the pressure of a person’s weight upon it, the board is straight. But as the diver progresses down further to the end of the board, it bends further. Deflection in tooling can be thought of in a similar way.

Deflection Can Result In:

  • Shortened tool life and/or tool breakage
  • Subpar surface finish
  • Part dimensional inaccuracies

Tool Deflection Remedies

Minimize Overhang

Overhang refers to the distance a tool is sticking out of the tool holder. Simply, as overhang increases, the tool’s likelihood of deflection increases. The larger distance a tool hangs out of the holder, the less shank there is to grip, and depending on the shank length, this could lead to harmonics in the tool that can cause fracture. Simply put, For optimal working conditions, minimize overhang by chucking the tool as much as possible.

extended reach tool

Image Source: @NuevaPrecision

Long Flute vs. Long Reach

Another way to minimize deflection is having a full grasp on the differences between a long flute and a long reach tool. The reason for such a difference in rigidity between the two is the core diameter of the tool. The more material, the more rigid the tool; the shorter the length of flute, the more rigid the tool and the longer the tool life. While each tooling option has its benefits and necessary uses, using the right option for an operation is important.

The below charts illustrate the relationship between force on the tip and length of flute showing how much the tool will deflect if only the tip is engaged while cutting. One of the key ways to get the longest life out of your tool is by increasing rigidity by selecting the smallest reach and length of cut on the largest diameter tool.

tool deflection

 

tool deflection

 

When to Opt for a Long Reach Tool

Reached tools are typically used to remove material where there is a gap that the shank would not fit in, but a noncutting extension of the cutter diameter would. This length of reach behind the cutting edge is also slightly reduced from the cutter diameter to prevent heeling (rubbing of noncutting surface against the part). Reached tools are one of the best tools to add to a tool crib because of their versatility and tool life.

 

When to Opt for a Long Flute Tool

Long Flute tools have longer lengths of cut and are typically used for either maintaining a seamless wall on the side of a part, or within a slot for finishing applications. The core diameter is the same size throughout the cutting length, leading to more potential for deflection within a part. This possibly can lead to a tapered edge if too little of the cutting edge is engaged with a high feed rate. When cutting in deep slots, these tools are very effective. When using HEM, they are also very beneficial due to their chip evacuation capabilities that reached tools do not have.

 

Deflection & Tool Core Strength

Diameter is an important factor when calculating deflection. Machinists oftentimes use the cutter diameter in the calculation of long flute tools, when in actuality the core diameter (shown below) is the necessary dimension. This is because the fluted portion of a tool has an absence of material in the flute valleys. For a reached tool, the core diameter would be used in the calculation until its reached portion, at which point it transitions to the neck diameter. When changing these values, it can lower deflection to a point where it is not noticeable for the reached tool but could affect critical dimensions in a long flute tool.

Deflection Summarized

Tool deflection can cause damage to your tool and scrap your part if not properly accounted for prior to beginning a job. Be sure to minimize the distance from the tool holder to the tip of the tool to keep deflection to a minimum. For more information on ways to reduce tool deflection in your machining, view Diving into Depth of Cut.

Selecting the Right Harvey Tool Miniature Drill

Among Harvey Tool’s expansive holemaking solutions product offering are several different types of miniature tooling options and their complements. Options range from Miniature Spotting Drills to Miniature High Performance Drills – Deep Hole – Coolant Through. But which tools are appropriate for the hole you aim to leave in your part? Which tool might your current carousel be missing, leaving efficiency and performance behind? Understanding how to properly fill your tool repertoire for your desired holemaking result is the first step toward achieving success.

Pre-Drilling Considerations

Miniature Spotting Drills

Depending on the depth of your desired machined hole and its tolerance mandates, as well as the surface of the machine you will be drilling, opting first for a Miniature Spotting Drill might be beneficial. This tool pinpoints the exact location of a hole to prevent common deep-hole drilling mishaps such as walking, or straying from a desired path. It can also help to promote accuracy in instances where there is an uneven part surface for first contact. Some machinists even use Spotting Drills to leave a chamfer on the top of a pre-drilled hole. For extremely irregular surfaces, however, such as the side of a cylinder or an inclined plane, a Flat Bottom Drill or Flat Bottom Counterbore may be needed to lessen these irregularities prior to the drilling process.

spotting drill

Tech Tip: When spotting a hole, the spot angle should be equal to or wider than the angle of your chosen miniature drill. Simply, the miniature drill tip should contact the part before its flute face does.

spotting drill correct angle

Selecting the Right Miniature Drill

Harvey Tool stocks several different types of miniature drills, but which option is right for you, and how does each drill differ in geometry?

Miniature Drills

Harvey Tool Miniature Drills are popular for machinists seeking flexibility and versatility with their holemaking operation. Because this line of tooling is offered uncoated in sizes as small as .002” in diameter, machinists no longer need to compromise on precision to reach very micro sizes. Also, this line of tooling is designed for use in several different materials where specificity is not required.

miniature drill

Miniature High Performance Drills – Deep Hole – Coolant Through

For situations in which chip evacuation may be difficult due to the drill depth, Harvey Tool’s Deep Hole – Coolant Through Miniature Drills might be your best option. The coolant delivery from the drill tip will help to flush chips from within a hole, and prevent heeling on the hole’s sides, even at depths up to 20 multiples of the drill diameter.

miniature drill coolant through

Miniature High Performance Drills – Flat Bottom

Choose Miniature High Performance Flat Bottom Drills when drilling on inclined and rounded surfaces, or when aiming to leave a flat bottom on your hole. Also, when drilling intersecting holes, half holes, shoulders, or thin plates, its flat bottom tool geometry helps to promote accuracy and a clean finish.

flat bottom drill

Miniature High Performance Drills – Aluminum Alloys

The line of High Performance Drills for Aluminum Alloys feature TiB2 coating, which has an extremely low affinity to Aluminum and thus will fend off built-up edge. Its special 3 flute design allows for maximum chip flow, hole accuracy, finish, and elevated speeds and feeds parameters in this easy-to-machine material.

drill for aluminum

 

Miniature High Performance Drills – Hardened Steels

Miniature High Performance Drills – Hardened Steels features a specialized flute shape for improved chip evacuation and maximum rigidity. Additionally, each drill is coated in AlTiN Nano coating for hardness, and heat resistance in materials 48 Rc to 68 Rc.

drill for hardened steel

Miniature High Performance Drills – Prehardened Steels

As temperatures rise during machining, the AlTiN coating featured on Harvey Tool’s Miniature High Performance Drills – Prehardened Steels creates an aluminum oxide layer which helps to reduce thermal conductivity of the tool and helps to promote heat transfer to the chip, as well as improve lubricity and heat resistance in ferrous materials.

drill for prehardened steel

Post-Drilling Considerations

Miniature Reamers

For many operations, drilling the actual hole is only the beginning of the job. Some parts may require an ultra-tight tolerance, for which a Miniature Reamer (tolerances of +.0000″/-.0002″ for uncoated and +.0002″/-.0000″ for AlTiN Coated) can be used to bring a hole to size. miniature reamer

Tech Tip: In order to maintain appropriate stock removal amounts based on the reamer size, a hole should be pre-drilled at a diameter that is 90-94 percent of the finished reamed hole diameter.

Flat Bottom Counterbores

Other operations may require a hole with a flat bottom to allow for a superior connection with another part. Flat Bottom Counterbores leave a flat profile and straighten misaligned holes. For more information on why to use a Flat Bottom Counterbore, read 10 Reasons to Use Flat Bottom Tools.

flat bottom counterbores

Key Next Steps

Now that you’re familiar with miniature drills and complementary holemaking tooling, you must now learn key ways to go about the job. Understanding the importance of pecking cycles, and using the correct approach, is vital for both the life of your tool and the end result on your part. Read this post’s complement “Choosing the Right Pecking Cycle Approach,” for more information on the approach that’s best for your application.

Choosing The Right Pecking Cycle Approach

Utilizing a proper pecking cycle strategy when drilling is important to both the life of your tool and its performance in your part. Recommended pecking cycles vary depending on the drill being used, the material you’re machining, and your desired final product.

What are Pecking Cycles?

Rather than drill to full drill depth in one single plunge, pecking cycles involve several passes – a little at a time. Pecking aids the chip evacuation process, helps support tool accuracy while minimizing walking, prevents chip packing and breakage, and results in a better all around final part.

Recommended Pecking Cycles / Steps

Miniature Drills

miniature drill pecking cycles

High Performance Drills – Flat Bottom

pecking cycles

High Performance Drills – Aluminum & Aluminum Alloys

aluminum pecking cycles

Note: For hole depths 12x or greater, a pilot hole of up to 1.5X Diameter is recommended.

High Performance Drills – Hardened Steels

hardened steels pecking cycles
High Performance Drills – Prehardened Steels

prehardened steels pecking cycles

Key Pecking Cycle Takeaways

From the above tables, it’s easy to identify how recommended pecking cycles change based on the properties of the material being machined. Unsurprisingly, the harder the material is, the shorter the recommended pecking depths are. As always, miniature drills with a diameter of less than .010″ are extremely fragile and require special precautions to avoid immediate failure. For help with your specific job, contact the Harvey Tool Technical Team at 800-645-5609 or [email protected]