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Reducing Tool Runout

Tool runout is a given in any machine shop, and can never be 100% avoided. Thus, it is important to establish an acceptable level of runout for any project, and stay within that range to optimize productivity and prolong tool life. Smaller runout levels are always better, but choice of machine and tool holder, stick-out, tool reach, and many other factors all have an influence on the amount of runout in every setup.

Defining Tool Runout

Tool runout is the measurement of how far a cutting tool, holder, or spindle rotates off of its true axis. This can be seen in low quality end mills where the cutting diameter is true to size when measured while stationary, but measures above tolerance while rotating.

The first step to minimizing runout is understanding what individual factors cause runout in every machine setup. Runout is seen in the accuracy of every cutting tool, collet, tool holder, and spindle. Every added connection between a machine and the workpiece it is cutting will introduce a higher level of runout. Each increase can add to the total runout further and further. Steps should be taken with every piece of tooling and equipment to minimize runout for best performance, increased tool life, and quality finished products.

Measuring Runout

Determining the runout of your system is the first step towards finding how to combat it. Runout is measured using an indicator that measures the variation of a tool’s diameter as it rotates. This is done with either a dial/probe indicator or a laser measuring device. While most dial indicators are both portable and easy to use, they are not as accurate as the available laser indicators, and can also make a runout measurement worse by pushing on a tool. This is mostly a concern for miniature and micro-tooling, where lasers should be strictly used due to the tool’s fragile nature.  Most end mill manufacturers recommend using a laser runout indicator in place of a dial indicator wherever possible.

Z-Mike Laser

Z-Mike laser measurement devices are common instruments used to measure levels of tool runout.

Runout should be measured at the point where a tool will be cutting, typically at the end of the tools, or along a portion of the length of cut. A dial indicator may not be plausible in these instances due to the inconsistent shape of a tool’s flutes. Laser measuring devices offer another advantage due to this fact.

High Quality Tools

The amount of runout in each component of a system, as-manufactured, often has a significant impact on the total runout of a given setup. Cutting tools all have a restriction on maximum runout allowed when manufactured, and some can have allowances of .0002” or less. This is often the value that should be strived for in a complete system as well. For miniature tooling down to .001” diameter, this measurement will have to be held to an even smaller value. As the ratio of tool runout to tool diameter becomes larger, threats of tool failure increase. As stated earlier, starting with a tool that has minimal runout is pivotal in keeping the total runout of a system to a minimum. This is runout that cannot be avoided.

Precision Tool Holders

The next step to minimizing runout is ensuring that you are using a high quality, precision tool holder. These often come in the form of shrink-fit, or press-fit tool holders offering accurate and precise tool rotation.  Uniform pressure around the entire circumference of a shank is essential for reducing runout. Set screw based holders should be avoided, as they push the tool off-center with their uneven holding pressure.  Collet-based tool holders also often introduce an extra amount of runout due to their additional components. Each added connection in a tool holding system allows more methods of runout to appear. Shrink-fit and press-fit tool holders are inherently better at minimizing runout due to their fewer components.

tool runout

Included in your tool holding considerations should be machine tool cleanliness. Often, chips can become lodged in the spindle, and cause an obstruction between two high-precision surfaces in the system. Ensuring that your tool holder and spindle are clean and free of chips and debris is paramount when setting up for every job.

Shank Modifications

Apart from equipment itself, many other factors can contribute to an increasing amount of tool runout. These can include how long a tool is, how rigid a machine setup is, and how far a tool is hanging out of its holder.  Shank modifications, along with their methods of tool holding can have a large impact. Often thought of as an older, obsolete technology, Weldon flats are found guilty of adding large amounts of runout in many shops. While many shops still use Weldon flats to ensure a secure grip on their tools, having a set screw pushing a tool to one side can push it off center, yielding very high levels of runout. Haimer Safe Lock™ is another option increasing in popularity that is a much higher performance holding technology. The Safe-Lock™ system is designed with the same tolerances as shrink fit and other high precision tool holders. It is able to minimize runout, while firmly holding a tool in place with no chance of pull-out.

haimer safe lock

The Haimer Safe-Lock™ system is one option to greatly reduce tool runout.

Runout will never be completely eliminated from a machining system. However, steps can (and should) be taken to keep it to a minimum using every method possible. Keeping a tool running true will extend tool life, increase performance, and ultimately save your shop time and money. Runout is a common concern in the metalworking industry, but it is often overlooked when it could be main issue causing part rejections and unacceptable results. Every piece of a machine tool plays a part in the resultant runout, and none should be overlooked.

Most Common Methods of Tool Entry

Tool entry is pivotal to machining success, as it’s one of the most punishing operations for a cutter. Entering a part in a way that’s not ideal for the tool or operation could lead to a damaged part or exhausted shop resources. Below, we’ll explore the most common part entry methods, as well as tips for how to perform them successfully.


Pre-Drilled Hole

Pre-drilling a hole to full pocket depth (and 5-10% larger than the end mill diameter) is the safest practice of dropping your end mill into a pocket. This method ensures the least amount of end work abuse and premature tool wear.

tool entry predrill

 


Helical Interpolation

Helical Interpolation is a very common and safe practice of tool entry with ferrous materials. Employing corner radius end mills during this operation will decrease tool wear and lessen corner breakdown. With this method, use a programmed helix diameter of greater than 110-120% of the cutter diameter.

helical interpolation

 


Ramping-In

This type of operation can be very successful, but institutes many different torsional forces the cutter must withstand. A strong core is key for this method, as is room for proper chip evacuation. Using tools with a corner radius, which strengthen its cutting portion, will help.

ramping

Suggested Starting Ramp Angles:

Hard/Ferrous Materials: 1°-3°

Soft/Non-Ferrous Materials: 3°-10°

For more information on this popular tool entry method, see Ramping to Success.


Arcing

This method of tool entry is similar to ramping in both method and benefit. However, while ramping enters the part from the top, arcing does so from the side. The end mill follows a curved tool path, or arc, when milling, this gradually increasing the load on the tool as it enters the part. Additionally, the load put on the tool decreases as it exits the part, helping to avoid shock loading and tool breakage.


Straight Plunge

This is a common, yet often problematic method of entering a part. A straight plunge into a part can easily lead to tool breakage. If opting for this machining method, however, certain criteria must be met for best chances of machining success. The tool must be center cutting, as end milling incorporates a flat entry point making chip evacuation extremely difficult. Drill bits are intended for straight plunging, however, and should be used for this type of operation.

tool entry

 


Straight Tool Entry

Straight entry into the part takes a toll on the cutter, as does a straight plunge. Until the cutter is fully engaged, the feed rate upon entry is recommended to be reduced by at least 50% during this operation.

tool entry

 


Roll-In Tool Entry

Rolling into the cut ensures a cutter to work its way to full engagement and naturally acquire proper chip thickness. The feed rate in this scenario should be reduced by 50%.

tool entry

 

Magnuson Superchargers – Featured Customer

Magnuson Superchargers is a manufacturer of aftermarket and OEM (Original Equipment Manufacturer) supercharger systems for the automotive industry, located in Ventura, California. Started by industry legend Jerry Magnuson, Magnuson Superchargers has quickly grown into one of the most respected brands in the automotive industry. Magnuson creates products for various brands, including GM, Mopar, Ford, AUDI, Mercedes-Benz, Lexus, Toyota, and Jeep. Magnuson Superchargers are most commonly found in “hot rods,” everyday vehicles, off-road vehicles, and vehicles purpose built for competitive racing, as they are used to significantly yet reliably increase horsepower.

The Magnuson Superchargers team of technicians combine modern and time-tested prototyping and fabrication techniques to construct each component to exact specifications and the highest quality. Magnuson has a complete machine shop in house for fabrication of new prototype system components. This allows them to operate efficiently with short runs and high volume production.

magnuson superchargers

Hubert Gromek, Magnuson Superchargers’ Machine Shop Manager, is a 15-year veteran of the industry. We spoke with Hubert about his experiences building a career in the manufacturing industry, his advice for young machinists, and the way he and his team use both Harvey Tool and Helical Solutions tools in their machine shop every day.


Tell us a little bit about yourself.

I started with Magnuson Superchargers 15-plus years ago as a young kid who didn’t know anything about machining at all. Being a major car guy and drag racer, working for a company that makes superchargers was a perfect fit for me.  I started by deburring and washing parts and worked my way up to operating our Fadal Vertical Mills.

From there I started to get the concept of what it actually takes to machine things and started learning how to do all the setups; I even started making my own fixtures here and there. After a couple of years of being the setup guy for our shop, I started looking into the programming aspect of the job and that really grabbed my interest right away. It’s one thing to run and set up machines with other people’s programs and instruction, but it’s a whole new world when you have to do the entire job from scratch on your own.

magnuson superchargers

After a couple of years of being the Lead Setup Programmer here at our shop, I was given the opportunity to be the Machine Shop Manager. I was very honored that the owner of such a big and great company thought I had what it takes to run the whole shop. Let me tell you, when you are responsible for everything that goes on in a machine shop, it really opens your eyes to how much every little thing matters. The one thing I learned very quickly is how important it is to have the right team in your shop to support you and reach the goals that are set. It doesn’t matter how great a manager or programmer you are, if you don’t have the right team of machinists in your shop, you are setting yourself up for failure. After many years of trying, I think I have finally found that team that I’ve been looking for.

What made you get into machining?

It was when I first saw a raw piece of material (billet aluminum) become a billet bracket for a hot rod my boss was working on. I thought that was the coolest thing ever. You start with nothing and the finished product was a work of art to me. I knew right then that I wanted to do that someday.

What is your greatest challenge as a machinist?

This is a two-part answer. First, it is finding the right core team that you can trust and not have to worry about what they are doing. My current team is comprised of experienced and disciplined machinists and they know what needs to get done. I don’t have to watch over them, I just try to guide them and teach them everything that I have learned over the years.

The second part has always been fixture design. I am always learning how to make better, more user-friendly fixtures to help speed up production but still maintain very high part quality.

magnuson superchargers

What is your favorite part of this profession?

I really love the fact that I learn something new every day. It doesn’t matter how much you think you know, there is always a job that will test your ability as a machinist.

What made you decide to use Harvey Tool and Helical products?

Actually I have a great local tool supplier that I deal with all the time. His name is Mike Baldino over at PM Industrial, and he is the one who first introduced me to both of these products. We make tiny Dovetail O-Ring grooves in a lot of our parts and I couldn’t find a tool that would do the job like I wanted it to. Mike recommended the Harvey Tool .135″ Dovetail Cutter and I haven’t used anything else since. As for the Helical End Mills, since 98% of our jobs are in aluminum, Mike also recommended I try these new (at the time) Zplus coated Helical End Mills. Just like the Harvey Tool Dovetail Cutter, I haven’t used anything else since I found out how amazing these cutters worked for us.

magnuson superchargers

Why is high quality tool performance important to your team at Magnuson Superchargers?

We work with a lot of castings here at Magnuson Superchargers, and even though they are aluminum, they can be very abrasive. Because of this, tool life and part finishes are very important to us. The Helical End Mills hold up very well to cast and billet materials and the Harvey Tool Dovetail Cutters are the only thing that works for us.

Tell us about your favorite projects that Harvey Tool or Helical Solutions tools helped you create.

We make most of our casting tooling in-house, which includes master patterns and core boxes, usually in 6061 Billet Aluminum. The Helical Zplus coated End Mills are amazing for doing these jobs. Using the dynamic toolpaths and utilizing the entire flute length is great. As for the Harvey Tool Dovetail Cutters, I haven’t used anything that works better than these. Every project has become easier with the use of both Harvey Tool and Helical Solutions tools.

magnuson superchargers

A 2016 Chevrolet Camaro loaded with the TVS2300 supercharger at the track.

One of our most exciting projects is our new TVS2300 supercharger that we built for our 2016 Chevrolet Camaro. We took a completely stock engine and transmission, and with just our supercharger and a couple of modifications it was able to run a 9 second 1/4 mile drag race. This was very impressive and has made a huge impact in the automotive industry. We are very excited about this kit and the potential it has in the market.

We have also been working on the biggest supercharger that our company has ever made, the new TVS2650. We are very proud of the all the R&D work that has gone into this kit and we are seeing some incredible horsepower numbers from these units. We displayed this at last year’s Specialty Equipment Market Association (SEMA) show in Las Vegas. We are still in the prototype stages of this project but will have production units coming very soon.

magnuson superchargers

A prototype of the new TVS2650 supercharger, the largest ever built by Magnuson.

Would you recommend that young people take the #PlungeIntoMachining and start a career as a machinist?

I personally would recommend a career in machining to anyone who has an interest in how things are made. I believe it is a great career choice. There are always going to be parts that have to get made somehow, so there is no shortage of open jobs available in the industry. I have a 4 year old son and as soon as he is old enough, I will teach him everything I know about this profession. If he chooses not to go that route, that is completely okay, but at least he will know what it takes to make something from scratch.

If you could give one piece of advice to a new machinist, what would it be?

Learn the basics. Start with a manual mill or lathe and get some experience with how it feels to cut something. Lots of people start on a CNC as an operator and call themselves “machinists.” It took me 5 years before my boss officially called me a machinist! Trust me, it feels really good when your boss hands you a print or CAD model and says “make this,” and you come back with a perfect part that you were able to make yourself.

magnuson superchargers

The Magnuson Superchargers machine shop team. From left: John, Jesus, Jun, Miguel, Jesse, Kenton, and Armando.

Would you like to be considered for a future “Featured Customer” blog? Click here to submit your information.

Photos courtesy of Magnuson Superchargers

3 Ways to Help Solve the Machinist Shortage

The manufacturing industry is on the rise, but there is a shortage in the workforce that is limiting the abilities of machine shops to find great talent and fulfill their needs. As manufacturing continues to move back to the US, the shortage will only grow larger. With nearly 70% of the machinist workforce over the age of 45, there has to be an injection of youth in the industry over the next 20 years to keep American manufacturing alive and well. Currently employed machinists are the best source to encourage today’s youth to join the profession, so the community will be an integral part of solving this machinist shortage.

1. Reach Out and Get Involved

The best thing machinists can do to make an immediate impact is to begin reaching out to their local communities, sharing their craft with families and students in the area. If we want to solve the machinist shortage, we have to get students excited about the industry. One great way to get students interested is to hold an open house at your machine shop and open your doors to local schools for visits. Since machining is a very visual craft, students will appreciate seeing finished projects in-person and watching the machines at work. Shops could also open their doors to vocational schools and have a “Career Night,” where students who are interested in the trades can come with their families and learn more about what it is like to be a machinist. It is important to get the families of interested youth involved, as colleges will do the same at their open houses, and it gives the family a better sense of where they may be sending their son or daughter after graduation.

machinist shortage

As great as it is to get students and families inside the machine shop, it is equally important for machinists to branch out and attend career days at local schools, as the trades are often underrepresented at these events. Bringing in a few recent projects and videos or photos of more advanced machining processes will be sure to open a few eyes, and might inspire a student who had never considered machining to do some research on the profession.

2. Join Communities on Social Media

According to a report from the Pew Internet Research Center, 92% of high school students use social media daily – a staggering number that must be taken into consideration when it comes to inspiring the younger generations. One easy way machinists can share their work is by using social media apps like Instagram, Facebook, Twitter, and YouTube. Instagram in particular has a great community of machinists, who are constantly sharing videos, tips and tricks, photos of their finished work, and talking to each other about best practices. Many machinist-related Instagram accounts have thousands of followers, and every machine shop should be jumping on this trend not only for their own marketing efforts, but also to get in front of the younger audience present in that space.

machinist shortage

Machinists love sharing their work with the community on social media, like this example from Reboot Engineering (@rebooteng) on Instagram.

If Instagram is not an option, there are several Facebook groups with tens of thousands of machinists talking about the trade, and quite a few influential machinists on YouTube who have substantial followings and are working to raise awareness about their trade. The machinist community on Twitter is smaller than the others, but it is growing and could be a valuable resource going forward.

3. Share Your Knowledge

New machinists will be more likely to embrace the profession and stick around if they are welcomed with open arms and in-depth, hands-on training from the senior machinists in a shop. This will decrease turnover, and keep younger machinists connected to the trade from the start. A machine shop full of veteran machinists can be an intimidating environment for a new hire, so this is a vital step in solving the machinist shortage.

It is also a great idea to share knowledge and stories with younger relatives. Nieces and nephews, younger cousins, grandchildren, and sons and daughters may find inspiration to follow in the footsteps of someone they look up to, but they’ll never know unless those experiences are shared with them.

If you already know someone who is considering a career as a machinist, share our “How to Become a Machinist” blog post with them, which is a great resource for all machine shops looking to hire young talent. This article could be handed out at open houses, career days, or school visits, and is part of Harvey Performance Company’s ongoing effort to improve the manufacturing industry and help solve the machinist shortage.

You can also share our new infographic, which outlines the current state of the industry, and provides a visual representation of how you can help solve this shortage as a current machinist. Use the hashtag #PlungeIntoMachining and share to your Facebook, Instagram, Twitter, and LinkedIn pages to help us start a movement!

Solving the Machinist Shortage

Corner Engagement: How to Machine Corners

Corner Engagement

During the milling process, and especially during corner engagement, tools undergo significant variations in cutting forces. One common and difficult situation is when a cutting tool experiences an “inside corner” condition. This is where the tool’s engagement angle significantly increases, potentially resulting in poor performance.

Machining this difficult area with the wrong approach may result in:

  • Chatter – visible in “poor” corner finish
  • Deflection – detected by unwanted “measured” wall taper
  • Strange cutting sound – tool squawking or chirping in the corners
  • Tool breakage/failure or chipping

Least Effective Approach (Figure 1)

Generating an inside part radius that matches the radius of the tool at a 90° direction range is not a desirable approach to machining a corner. In this approach, the tool experiences extra material to cut (dark gray), an increased engagement angle, and a direction change. As a result, issues including chatter, tool deflection/ breakage, and poor surface finish may occur.

Feed rate may need to be lessened depending on the “tool radius-to-part radius ratio.”

corner engagement

More Effective Approach (Figure 2)

Generating an inside part radius that matches the radius of the tool with a sweeping direction change is a more desirable approach. The smaller radial depths of cut (RDOC) in this example help to manage the angle of engagement, but at the final pass, the tool will still experience a very high engagement angle.  Common results of this approach will be chatter, tool deflection/breakage and poor surface finish.

Feed rate may need to be reduced by 30-50% depending on the “tool radius-to-part radius ratio.”

corner engagement

Most Effective Approach (Figure 3)

Generating an inside part radius with a smaller tool and a sweeping action creates a much more desirable machining approach. The manageable RDOC and smaller tool diameter allow for management of the tool engagement angle, higher feed rates and better surface finishes. As the cutter reaches full radial depth, its engagement angle will increase, but the feed reduction should be much less than in the previous approaches.

Feed rate may need to be heightened depending on the “tool-to-part ratio.” Utilize tools that are smaller than the corner you are machining.

corner engagement

Ramping to Success

Poor tool life and premature tool failure are concerns in every machining application. Something as simple as tool path selection – and how a tool first enters a part – can make all the difference. Tool entry has a great deal of influence on its overall success, as it’s one of the most punishing operations for a cutter. Ramping into a part, via a circular or linear toolpath, is one of the most popular and oftentimes the most successful methods (Figure 1). Below, learn what ramping is, its benefits, and in which situations it can be used.

ramping

What is Ramping?

Ramping refers to simultaneous radial and axial motion of a cutting tool, making an angular tool path. Oftentimes, this method is used to approach a part when there is a need to create closed forms such as pockets, cavities, engravings, and holes. In doing so, the need to plunge with an end mill or drill to create a starting point is eliminated. Ramping is particularly important in micromachining where even the slightest imbalance in cutting forces can cause tool failure.

There are two types of ramping toolpaths: Linear and Circular (Figure 2 ).

ramping

Linear Ramping involves moving a cutting tool along two axes (the z-axis and one of the x, y axes). This method has significant more radial engagement with complementary increased cutting forces distributed across only two axes.

Circular Ramping (Helical Interpolation) has a spiral motion of the cutting tool that engages all three axes (x, y, and z axes). This method typically has less radial engagement on the cutting tool, with the cutting forces distributed across the three different axes. This is the recommended method, as it ensures the longest tool life.

Suggested Starting Ramp Angles:

Soft/Non-Ferrous Materials: 3° – 10°

Hard/Ferrous Materials 1° – 3°

Benefits of Ramping

When a tool enters the part via a Ramping method, it gradually increases in depth, preventing any shock loading on end mills. This reduces costs resulting from unnecessary tool breakage. Ramping produces smaller chips when compared to plunging, which makes chip evacuation faster and easier. As a result, cycle time can be decreased by running the end mill at faster parameters. Ramping also creates an extra space in the tool changer that would otherwise be occupied by a drill purposed with machining a starter hole.

Arcing

Similar to ramping in both method and benefit, arcing is another technique of approaching a workpiece (See Figure 3).

While ramping enters the part from the top, arcing enters from the side. The end mill follows a curved tool path (or arc) when milling, thus gradually increasing the load on the tool as the tool enters the part, as well as gradually decreasing the load as the tool exits the part. In this way, shock loading and possible tool breakage are avoided.

For more information on ramping, arcing, and other tool entry methods, please see Helical Solutions’ “Types of Tool Entry.” 

Climb Milling vs. Conventional Milling

There are two distinct ways to cut materials when milling: Conventional Milling (Up) and Climb Milling (Down). The difference between these two techniques is the relationship of the rotation of the cutter to the direction of feed. In Conventional Milling, the cutter rotates against the direction of the feed. During Climb Milling, the cutter rotates with the feed.

Conventional Milling is the traditional approach when cutting because the backlash, or the play between the lead screw and the nut in the machine table, is eliminated (Figure 1). Recently, however, Climb Milling has been recognized as the preferred way to approach a workpiece since most machines today compensate for backlash or have a backlash eliminator.

 


Key Conventional and Climb Milling Properties:

Conventional Milling (Figure 2)

  • Chip width starts from zero and increases which causes more heat to diffuse into the workpiece and produces work hardening
  • Tool rubs more at the beginning of the cut causing faster tool wear and decreases tool life
  • Chips are carried upward by the tooth and fall in front of cutter creating a marred finish and re-cutting of chips
  • Upwards forces created in horizontal milling* tend to lift the workpiece, more intricate and expansive work holdings are needed to lessen the lift created*

climb milling

 

Climb Milling (Figure 3)

  • Chip width starts from maximum and decreases so heat generated will more likely transfer to the chip
  • Creates cleaner shear plane which causes the tool to rub less and increases tool life
  • Chips are removed behind the cutter which reduces the chance of recutting
  • Downwards forces in horizontal milling is created that helps hold the workpiece down, less complex work holdings are need when coupled with these forces
  • Horizontal milling is when the center line of the tool is parallel to the work piece

climb milling


When to Choose Conventional or Climb Milling

Climb Milling is generally the best way to machine parts today since it reduces the load from the cutting edge, leaves a better surface finish, and improves tool life. During Conventional Milling, the cutter tends to dig into the workpiece and may cause the part to be cut out of tolerance.

However, though Climb Milling is the preferred way to machine parts, there are times when Conventional Milling is the necessary milling style. One such example is if your machine does not counteract backlash. In this case, Conventional Milling should be implemented. In addition, this style should also be utilized on casting, forgings or when the part is case hardened (since the cut begins under the surface of the material).

 

 

Dodging Dovetail Headaches: 7 Common Dovetail Mistakes

Cutting With Dovetails

While they are specialty tools, dovetail style cutters have a broad range of applications. Dovetails are typically used to cut O-ring grooves in fluid and pressure devices, industrial slides and detailed undercutting work. Dovetail cutters have a trapezoidal shape—like the shape of a dove’s tail. General purpose dovetails are used to undercut or deburr features in a workpiece. O-ring dovetail cutters are held to specific standards to cut a groove that is wider at the bottom than the top. This trapezoidal groove shape is designed to hold the O-ring and keep it from being displaced.

Avoiding Tool Failure

The dovetail cutter’s design makes it fragile, finicky, and highly susceptible to failure. In calculating job specifications, machinists frequently treat dovetail cutters as larger than they really are because of their design, leading to unnecessary tool breakage. They mistake the tool’s larger end diameter as the critical dimension when in fact the smaller neck diameter is more important in making machining calculations.

As the tools are downsized for micro-applications, their unique shape requires special considerations. When machinists understand the true size of the tool, however, they can minimize breakage and optimize cycle time.

Miniature Matters – Micro Dovetailing

As the trend towards miniaturization continues, more dovetailing applications arise along with the need for applying the proper technique when dovetailing microscale parts and features. However, there are several common misunderstandings about the proper use of dovetails, which can lead to increased tool breakage and less-than-optimal cycle times.

There are seven common mistakes made when dovetailing and several strategies for avoiding them:

1. Not Taking Advantage of Drop Holes

Many O-ring applications allow for a drop hole to insert the cutter into the groove. Take advantage of a drop hole if the part design allows it, as it will permit usage of the largest, most rigid tool possible, minimizing the chance of breakage (Figure 1).

dovetail cutters
Figure 1. These pictured tools are designed to mill a groove for a Parker Hannifin O-ring groove No. AS568A-102 (left). These O-rings have cross sections of 0.103″. There is a large variation in the tools’ neck diameters. The tool at right, with a neck diameter of 0.024″, is for applications without a drop hole, while the other tool, with a neck diameter of 0.088″, is for drop-hole applications. The drop-hole allowance allows application of the more rigid tool.

2. Misunderstanding a Dovetail’s True Neck Diameter.

The dovetail’s profile includes a small neck diameter behind a larger end-cutting diameter. In addition, the flute runs through the neck, further reducing the tool’s core diameter. (In the example shown in Figure 2, this factor produces a core diameter of just 0.014″.) The net result is that an otherwise larger tool becomes more of a microtool. The torque generated by the larger diameter is, in effect, multiplied as it moves to the narrower neck diameter. You must remember that excess stress may be placed on the tool, leading to breakage. Furthermore, as the included angle of a dovetail increases, the neck diameter and core diameter are further reduced. O-ring dovetail cutters have an included angle of 48°. Another common included angle for general purpose dovetails is 90°. Figure 3 illustrates how two 0.100″-dia. dovetail tools have different neck diameters of 0.070″ vs. 0.034″ and different included angles of 48° vs. 90°.

dovetail cutters
Figure 2. The dovetail tool pictured is the nondrop-hole example from Figure 1. The cross section illustrates the relationship between the end diameter of the tool (0.083″) and the significantly smaller core diameter (0.014″). Understanding this relationship and the effect of torque on a small core diameter is critical to developing appropriate dovetailing operating parameters.
dovetail cutters
Figure 3: These dovetail tools have the same end diameter but different neck diameters (0.070″ vs. 0.034″) and different included angles (48° vs. 90°).

3. Calculating Speeds and Feeds from the Wrong Diameter.

Machinists frequently use the wrong tool diameter to calculate feed rates for dovetail cutters, increasing breakage. In micromachining applications where the margin for error is significantly reduced, calculating the feed on the wrong diameter can cause instantaneous tool failure. Due to the angular slope of a dovetail cutter’s profile, the tool has a variable diameter. While the larger end diameter is used for speed calculations, the smaller neck diameter should be used for feed calculations. This yields a smaller chip load per tooth. For example, a 0.083″-dia. tool cutting aluminum might have a chip load of approximately 0.00065 IPT, while a 0.024″-dia. mill cutting the same material might have a 0.0002-ipt chip load. This means the smaller tool has a chip load three times smaller than the larger tool, which requires a significantly different feed calculation.

4. Errors in Considering Depth of Cut.

In micromachining applications, machinists must choose a depth of cut (DOC) that does not exceed the limits of the fragile tool. Typically, a square end mill roughs a slot and the dovetail cutter then removes the remaining triangular-shaped portion. As the dovetail is stepped over with each subsequent radial cut, the cutter’s engagement increases with each pass. A standard end mill allows for multiple passes by varying the axial DOC. However, a dovetail cutter has a fixed axial DOC, which allows changes to be made only to the radial DOC. Therefore, the size of each successive step-over must decrease to maintain a more consistent tool load and avoid tool breakage (Figure 4).

dovetail cutters
Figure 4: In microdovetailing operations, increased contact requires diminishing stepover to maintain constant tool load.

5. Failing to Climb Mill.

Although conventional milling has the benefit of gradually loading the tool, in low-chip load applications (as dictated by a dovetail cutter’s small neck diameter) the tool has a tendency to rub or push the workpiece as it enters the cut, creating chatter, deflection and premature cutting edge failure. The dovetail has a long cutting surface and tooth pressure becomes increasingly critical with each pass. Due to the low chip loads encountered in micromachining, this approach is even more critical to avoid rubbing. Although climb milling loads the tool faster than conventional milling, it allows the tool to cut more freely, providing less deflection, finer finish and longer cutting-edge life. As a result, climb milling is recommended when dovetailing.

6. Improper Chip Flushing.

Because dovetail cuts are typically made in a semi-enclosed profile, it is critical to flush chips from the cavity. In micro-dovetailing applications, chip packing and recutting due to poorly evacuated chips from a semi-enclosed profile will dull the cutter and lead to premature tool failure. In addition to cooling and lubricating, a high-pressure coolant effectively evacuates chips. However, excessive coolant pressure placed directly on the tool can cause tool vibration and deflection and even break a microtool before it touches the workpiece. Take care to provide adequate pressure to remove chips without putting undue pressure on the tool itself. Specific coolant pressure settings will depend upon the size of the groove, the tool size and the workpiece material. Also, a coolant nozzle on either side of the cutter cleans out the groove ahead of and behind the cutter. An air blast or vacuum hose could also effectively remove chips.

7. Giving the Job Away.

As discussed in item number 3, lower chip loads result in significantly lower material-removal rates, which ultimately increase cycle time. In the previous example, the chip load was three times smaller, which would increase cycle time by the same amount. Cycle time must be factored into your quote to ensure a profitable margin on the job. In addition to the important micro-dovetailing considerations discussed here, don’t forget to apply the basics critical to all tools. These include keeping runout low, using tools with application-specific coatings and ensuring setups are rigid. All of these considerations become more important in micro-applications because as tools get smaller, they become increasingly fragile, decreasing the margin of error. Understanding a dovetail cutter’s profile and calculating job specifications accordingly is critical to a successful operation. Doing so will help you reach your ultimate goal: bidding the job properly and optimizing cycle time without unnecessary breakage.

This article was written by Peter P. Jenkins of Harvey Tool Company, and it originally appeared in MicroManufacturing Magazine.

How To Avoid Common Part Finish Problems

Finishing cuts are used to complete a part, achieving its final dimensions within tolerance and its required surface finish. Most often an aesthetic demand and frequently a print specification, surface finish can lead to a scrapped part if requirements are not met. Meeting finish requirements in-machine has become a major point of improvement in manufacturing, as avoiding hand-finishing can significantly reduce costs and cycle times.

Circular Interpolation: Machining Circular Tool Paths

When machining, proper speeds and feeds are very important to avoid breakage and maximize performance. Traditional end milling formulas use Surface Footage (SFM) and Chip Load (IPT) to calculate Speed (RPM) and Feed (IPM) rates. These formulas dictate the correct machining parameters for use in a linear path in which the end mill’s centerline is travelling in a straight line. Since not all parts are made of flat surfaces, end mills will invariably need to move in a non-linear path. In the case of machining circular tool paths, the path of the end mill’s centerline is circular. Not surprisingly, this is referred to as Circular Interpolation.

Cutting Circular Tool Paths

All rotating end mills have their own angular velocity at the outside diameter. But when the tool path is circular, there is an additional component that is introduced, resulting in a compound angular velocity. Basically, this means the velocity of the outside diameter is travelling at a substantially different velocity than originally expected. The cause of the compound angular velocity is seen in the disparity between the tool path lengths.

Internal Circular Tool Paths

Figure A shows the cross section of a cutting tool on a linear path, with the teeth having angular velocity due to tool rotation, and the center of the tool having a linear feed. Note that the tool path length will always be equal to the length of the machined edge. Figure B shows the same cutting tool on an internal circular path, as done when machining a hole. In this case, the angular velocity of the teeth is changed as a result of an additional component from the circular path of the tool’s center. The diameter of the tool path is smaller than that of the major diameter being cut. Or, in other words, the tool path length is shorter than the machined edge length, increasing the angular velocity of the teeth. To prevent overfeeding and the possibility of tool breakage, the increased angular velocity of the teeth must be made the same as in the linear case in Figure A. The formula below can be used to properly lower the feed rate for internal machining:

Internal Adjusted Feed = (Major Diameter-Cutter Diameter) / (Major Diameter) × Linear Feed

 

External Circular Tool Paths

Figure C shows the same cutting tool on an external circular path, as done when machining a post. In this case, the diameter of the tool path is larger than the major diameter being cut. This means that the tool path length is longer than the machined edge length, resulting in a decreased angular velocity. To prevent premature dulling and poor tool life due to over-speeding, use the formula below to properly raise the feed rate for external machining. In this way, the decreased angular velocity of the teeth is made the same as in the linear case in Figure A.

External Adjusted Feed = (Major Diameter+Cutter Diameter) / (Major Diameter) × Linear Feed

Optimize Your Performance

By adjusting the feed in the manner provided, internal applications can avoid tool breakage and costly down time. Further, external applications can enjoy optimized performance and shorter cycle times. It should also be noted that this approach can be applied to parts with radiused corners, elliptical features and when helical interpolation is required.