Causes & Effects of Built-Up Edge (BUE) in Turning Applications

In turning operations, the tool is stationary while the workpiece is rotating in a clamped chuck or a collet holder. Many operations are performed in a lathe, such as facing, drilling, grooving, threading, and cut-off applications. it is imperative to use the proper tool geometry and cutting parameters for the material type that is being machined. If these parameters are not applied correctly in your turning operations, built-up edge (BUE), or many other failure modes, may occur. These failure modes adversely affect the performance of the cutting tool and may lead to an overall scrapped part.

When inspecting a cutting tool under a microscope or eye loupe, there are several different types of turning tool failure modes that can be apparent. Some of the most common modes are:

  • Normal Flank Wear: The only acceptable form of tool wear, caused by the normal aging of a used cutting tool and found on the cutting edges.
    • This abrasive wear, caused by hard constituents in the workpiece material, is the only preferred method of tool wear, as it’s predictable and will continue to provide stable tool life, allowing for further optimization and increased productivity.
  • Cratering: Deformations found on the cutting face of a tool.
    • This tool mode is a chemical and heat failure, localized on the rake face area of the turning tool, or insert. This failure results from the chemical reaction between the workpiece material and the cutting tool and is amplified by cutting speed. Excessive Crater Wear weakens a turning tool’s cutting edge and may lead to cutting edge failure.
  • Chipping: Breaking of the turning tool along its cutting face, resulting in an inaccurate, rough cutting edge.
    • This is a mechanical failure, common in interrupted cutting or non-rigid machining setups. Many culprits can be to blame for chipping, including machine mishaps and tool holder security.
  • Thermal Mechanical Failure (Thermal Cracking): The cracking of a cutting tool due to significant swings in machining temperature.
    • When turning, heat management is key. Too little or too much heat can create issues, as can significant, fast swings in temperature (repeated heating and cooling on the cutting edge). Thermal Mechanical Failure usually shows in the form of evenly spaced cracks, perpendicular to the cutting edge of the turning tool.
  • Built-Up Edge (BUE): When chips adhere to the cutting tool due to high heat, pressure, and friction.

Effects of Built-Up Edge in Turning Application

A built-up edge is perhaps the easiest mode of tool wear to identify, as it may be visible without the need for a microscope or an eye loupe. The term built-up edge means that the material that you’re machining is being pressure welded to the cutting tool. When inspecting your tool, evidence of a BUE problem is material on the rake face or flank face of the cutting tool.

Image Source: Carbide inserts Wear Failure modes. | machining4.eu, 2020

This condition can create a lot of problems with your machining operations, such as poor tool life, subpar surface finish, size variations, and many other quality issues. The reason for these issues is that the centerline distance and the tool geometry of the cutting edge are being altered by the material that’s been welded to the rake or flank face of the tool. As the BUE condition worsens, you may experience other types of failures or even catastrophic failure.                     

Causes of Built-Up Edge in Turning Applications

Improper Tooling Choice

Built-Up Edge is oftentimes caused by using a turning tool that does not have the correct geometry for the material being machined. Most notably, when machining a gummy material such as aluminum or titanium, your best bet is to use tooling with extremely sharp cutting edges, free cutting geometry, and a polished flank and rake face. This will not only help you to cut the material swiftly but also to keep it from sticking to the cutting tool.

Using Aged Tooling

Even when using a turning tool with correct geometry, you may still experience BUE. As the tool starts to wear and its edge starts to degrade, the material will start building up on the surface of the tool. For this reason, it is very important to inspect the cutting edge of a tool after you have machined a few parts, and then randomly throughout the set tool life. This will help you identify the root cause of any of the failure modes by identifying them early on.

Insufficient Heat Generation

Built-up edge can be caused from running a tool at incorrect cutting parameters. Usually, when BUE is an issue, it’s due to the speed or feed rates being too low. Heat generation is key during any machining application – while too much heat can impact a part material, too little can cause the tool to be less effective at efficiently removing chips.

4 Simple Ways to Mitigate BUE in Turning Applications

  1. When selecting a tool, opt for free cutting, up sharp geometries with highly polished surfaces. Selecting a tool with chipbreaker geometry will also help to divide chips, which will help to remove it from the part and the cutting surface.
  2. Be confident in your application approach and your running parameters. It’s always important to double-check that your running parameters are appropriate for your turning application.
  3. Make sure the coolant is focused on the cutting edge and increase the coolant concentration amount.
  4. Opt for a coated Insert, as coatings are specifically engineered for a given set of part materials, and are designed to prevent common machining woes.

Save Time With Quick Change Tooling

Making a manual tool change on any CNC machine is never a timely or rewarding process. Typically, a tool change in a standard holder can take up to 5 minutes. Add that up a few times, and suddenly you have added significant minutes to your production time.

As CNC machine tool and cutting tool technology has advanced, there are more multi-functional tools available to help you avoid tool changes. However, sometimes it just isn’t feasible, and multiple tool changes are needed. Luckily, Micro 100 has developed a revolutionary new method to speed up tool changes significantly.

What is the Micro-Quik™ Tooling System?

Developed in Micro 100’s world-class grinding facility in Meridian, Idaho, the Micro 100 Micro-Quik™ tooling system is held to the same standards and tight tolerances as all of the Micro 100 carbide tooling.

The quick change tooling system allows for highly repeatable tool changes that save countless hours without sacrificing performance. This system combines a unique tool holder with a unique tool design to deliver highly repeatable and accurate results.

Each quick change tool holder features a locating/locking set screw to secure the tool and a locating pin which helps align the tool for repeatability. Removing a tool is as simple as loosening the set screw and inserting its replacement.

During tool changes, the precision ground bevel on the rear of the tool aligns with a locating pin inside the tool holder. The distance from this locational point to the tip of the tool is highly controlled under tight tolerances, meaning that the Micro-Quik™ tooling system ensures a very high degree of tool length and centerline repeatability. The “L4” dimension on all of our quick change tools, as seen in the image above, remains consistent across the entire product line. Check out the video below for a demonstration of the Micro 100 Micro-Quik™ system in action!

Quick Change Tooling Benefits

The most obvious benefit to using Micro 100’s Micro-Quik™ system is the time savings that come with easier tool changes. By using the quick change holders in combination with quick change tooling, it is easy to reduce tool changes from 5 minutes to under 30 seconds, resulting in a 90% decrease in time spent swapping out tools. This is a significant benefit to the system, but there are benefits once the tool is in the machine as well.

As mentioned above, the distance from the locational point on each tool shank to the tip of the tool is highly controlled, meaning that regardless of which type of tool you insert into the holder, your stick out will remain the same. This allows you to have confidence in the tooling and does not require additional touch offs, which is another major time saver.

By removing additional touch-offs and tool changes from your workflow, you also reduce the chances for human or machine error. Improper touch-offs or tool change errors can cause costly machine crashes and result in serious repairs and downtime. With the Micro 100 Micro-Quik™ system, initial setups become much easier, allowing you to hit the cycle start button with total confidence for each run.

By making a few simple changes to your tool holding configurations and adopting the Micro-Quik™ system, your shop can save thousands in time saved, with less machine downtime and increased part production. To learn more about the Micro 100 Micro-Quik™ cutting tools and tool holders, please visit (URL here to quick change page).

How Boring Bar Geometries Impact Cutting Operations

Boring is a turning operation that allows a machinist to make a pre-existing hole bigger through multiple iterations of internal boring. It has a number of advantages over traditional drilling methods:

  • The ability to cost-effectively produce a hole outside standard drill sizes
  • The creation of more precise holes, and therefore tighter tolerances
  • A greater finish quality
  • The opportunity to create multiple dimensions within the bore itself

Solid carbide boring bars, such as those offered by Micro 100,  have a few standard dimensions that give the tool basic functionality in removing material from an internal bore. These include:

Minimum Bore Diameter (D1): The minimum diameter of a hole for the cutting end of the tool to completely fit inside without making contact at opposing sides

Maximum Bore Depth (L2): Maximum depth that the tool can reach inside a hole without contact from the shank portion

Shank Diameter (D2): Diameter of the portion of the tool in contact with the tool holder

Overall Length (L1): Total length of the tool

Centerline Offset (F): The distance between a tool’s tip and the shank’s centerline axis

Tool Selection

In order to minimize tool deflection and therefore risk of tool failure, it is important to choose a tool with a max bore depth that is only slightly larger than the length it is intended to cut. It is also beneficial to maximize the boring bar and shank diameter as this will increase the rigidity of the tool. This must be balanced with leaving enough room for chips to evacuate. This balance ultimately comes down to the material being bored. A harder material with a lower feed rate and depths of cut may not need as much space for chips to evacuate, but may require a larger and more rigid tool. Conversely, a softer material with more aggressive running parameters will need more room for chip evacuation, but may not require as rigid of a tool.

Geometries

In addition, they have a number of different geometric features in order to adequately handle the three types of forces acting upon the tool during this machining process. During a standard boring operation, the greatest of these forces is tangential, followed by feed (sometimes called axial), and finally radial. Tangential force acts perpendicular to the rake surface and pushes the tool away from the centerline. Feed force does not cause deflection, but pushes back on the tool and acts parallel to the centerline. Radial force pushes the tool towards the center of the bore.

Defining the Geometric Features of Boring Bars:

Nose Radius: the roundness of a tool’s cutting point

Side Clearance (Radial Clearance): The angle measuring the tilt of the nose relative to the axis parallel to the centerline of the tool

End Clearance (Axial Clearance): The angle measuring the tilt of the end face relative to the axis running perpendicular to the centerline of the tool

Side Rake Angle: The angle measuring the sideways tilt of the side face of the tool

Back Rake Angle: The angle measuring the degree to which the back face is tilted in relation to the centerline of the workpiece

Side Relief Angle: The angle measuring how far the bottom face is tilted away from the workpiece

End Relief Angle: The angle measuring the tilt of the end face relative to the line running perpendicular to the center axis of the tool

Effects of Geometric Features on Cutting Operations:

Nose Radius: A large nose radius makes more contact with the workpiece, extending the life of the tool and the cutting edge as well as leaving a better finish. However, too large of a radius will lead to chatter as the tool is more exposed to tangential and radial cutting forces.

Another way this feature affects the cutting action is in determining how much of the cutting edge is struck by tangential force. The magnitude of this effect is largely dependent on the feed and depth of cut. Different combinations of depth of cuts and nose angles will result in either shorter or longer lengths of the cutting edge being exposed to the tangential force. The overall effect being the degree of edge wear. If only a small portion of the cutting edge is exposed to a large force it would be worn down faster than if a longer portion of the edge is succumb to the same force. This phenomenon also occurs with the increase and decrease of the end cutting edge angle.

End Cutting Edge Angle: The main purpose of the end cutting angle is for clearance when cutting in the positive Z direction (moving into the hole). This clearance allows the nose radius to be the main point of contact between the tool and the workpiece. Increasing the end cutting edge angle in the positive direction decreases the strength of the tip, but also decreases feed force. This is another situation where balance of tip strength and cutting force reduction must be found. It is also important to note that the angle may need to be changed depending on the type of boring one is performing.

Side Rake Angle: The nose angle is one geometric dimension that determines how much of the cutting edge is hit by tangential force but the side rake angle determines how much that force is redistributed into radial force. A positive rake angle means a lower tangential cutting force as allows for a greater amount of shearing action. However, this angle cannot be too great as it compromises cutting edge integrity by leaving less material for the nose angle and side relief angle.

Back Rake Angle: Sometimes called the top rake angle, the back rake angle for solid carbide boring bars is ground to help control the flow of chips cut on the end portion of the tool. This feature cannot have too sharp of a positive angle as it decreases the tools strength.

Side and End Relief Angles: Like the end cutting edge angle, the main purpose of the side and end relief angles are to provide clearance so that the tools non-cutting portion doesn’t rub against the workpiece. If the angles are too small then there is a risk of abrasion between the tool and the workpiece. This friction leads to increased tool wear, vibration and poor surface finish. The angle measurements will generally be between 0° and 20°.

Boring Bar Geometries Summarized

Boring bars have a few overall dimensions that allow for the boring of a hole without running the tool holder into the workpiece, or breaking the tool instantly upon contact. Solid carbide boring bars have a variety of angles that are combined differently to distribute the 3 types of cutting forces in order to take full advantage of the tool. Maximizing tool performance requires the combination of choosing the right tool along with the appropriate feed rate, depth of cut and RPM. These factors are dependent on the size of the hole, amount of material that needs to be removed, and mechanical properties of the workpiece.

Confidently Select Your Next Thread Mill

Do you know the key differences between a Single Form Thread Mill and a Multi-Form Thread Mill? Do you know which tooling option is best for your job? This blog post examines how several factors, including the tool’s form and max depth of thread, are important to ultimately making the appropriate Harvey Tool thread mill decision.

Thread Mill Product Offering

Single Form Thread Mill

The single form thread mill is the most versatile threading solution Harvey Tool offers. These tools are ground to a sharp point and are capable of milling 60° thread styles, such as UN, metric, and NPT threads. With over 14 UN and 10 Metric sized tools, Harvey Tool’s single form selections allow machinists the opportunity to machine many different types of threads.

Thread Mill

Harvey Performance Company, LLC.

Single Form Thread Mills for Hardened Steels

Similar to the standard single form thread mills, Harvey Tool’s thread mills for hardened steels offer machinists a quality option when dealing with hardened steels from 46-68 Rc. The following unique geometries helps this tool machine tough alloys:

  1. Ground Flat – Instead of a sharp point these tools have a ground flat to help ensure long tool life.
  2. Eccentric Relief – Gives the cutting edges extra strength for the high feeds at relatively low RPMs required for harder materials.
  3. AlTiN Nano Coating – Allows for superior heat resistance.

thread mill

Harvey Performance Company, LLC.

A key difference between the standard Single Form Thread Mill and the Single Form Thread Mills for Hardened Steels is that the thread mills for hardened steels are actually only capable of milling 83% of the actual thread depth. At first, this may seem detrimental to your operation. However, according to the Machinery’s Handbook 29th Edition, “Tests have shown that any increase in the percentage of full thread over 60% does not significantly increase the strength of the thread. Often, a 55% to 60% thread is satisfactory, although 75% threads are commonly used to provide an extra margin of safety.” With the ability to preserve tool life and effectively perform thread components, Harvey Tool’s single form thread mills for hardened steels are a natural choice when tackling a hardened material.

Tri-Form Thread Mills

Tri-Form thread mills are designed for difficult-to-machine materials. The tri-form design reduces tool pressure and deflection, which results in more accurate threading. Its left-hand cut, left-hand spiral design allows it to climb mill from the top of the thread to the bottom.

thread mill

Harvey Performance Company, LLC.

Multi-Form Thread Mills

Our multi-form thread mills are offered in styles such as UN, NPT, and Metric. Multi-Form Thread Mills are optimized to produce a full thread in single helical interpolation. Additionally, they allow a machinist to quickly turn around production-style jobs.

thread mill

Harvey Performance Company, LLC.

Coolant-Through Multi Form Thread Mills

Coolant-Through Multi Form Thread Mills are the perfect tool for when a job calls for thread milling in a blind hole. The coolant through ability of the tool produces superior chip evacuation. These tools also improve coolant flow to the workpiece – delivering it directly from the tip of the tool – for decreased friction and high cutting speeds.

thread mill

Harvey Performance Company, LLC.

Long Flute Thread Mills

These tools are great when a job calls for a deep thread, due to their long flute. Long Flute Thread Mills also have a large cutter diameter and core, which provides the tool with improved tool strength and stability.

thread mill

Harvey Performance Company, LLC.

N.P.T. Multi-Form Thread Mills

While it may seem obvious, N.P.T. Multi-Form Thread Mills are perfect for milling NPT threads. NPT threads are great for when a part requires a full seal, different from traditional threads that hold pieces together without the water-tight seal.

thread mill

Harvey Performance Company, LLC.

Multi-Start Thread Reference Guide

A multi-start thread consists of two or more intertwined threads running parallel to one another. Intertwining threads allow the lead distance of a thread to be increased without changing its pitch. A double start thread will have a lead distance double that of a single start thread of the same pitch, a triple start thread will have a lead distance three times longer than a single start thread of the same pitch, and so on.

By maintaining a constant pitch, the depth of the thread, measured from crest to root, will also remain constant. This allows multi-start threads to maintain a shallow thread depth relative to their longer lead distance. Another design advantage of a multi-start thread is that more contact surface is engaged in a single thread rotation. A common example is a cap on a plastic water bottle. The cap will screw on in one quick turn but because a multi start thread was used there are multiple threads fully engaged to securely hold the cap in place.

multi-start thread

Figure 1 displays a triple start thread with each thread represented in a different shade. The left side of the image represents a triple start thread with just one of the three threads completed. This unfinished view shows how each individual thread is milled at a specific lead distance before the part is indexed and the remaining threads are milled. The right side of the image displays the completed triple start thread with the front view showing how the start of each thread is evenly spaced. The starting points of a double start thread begin 180° apart and the starting points of a triple start thread begin 120° apart.

multi-start thread

Figure 2 displays the triangle that can be formed using the relationship between the lead distance and the circumference of a thread. It is this relationship that determines the lead angle of a thread. The lead angle is the helix angle of the thread based on the lead distance. A single start thread has a lead distance equal to its pitch and in turn has a relatively small lead angle. Multi-start threads have a longer lead distance and therefore a larger lead angle. The graphic depicted on the right is a view of the lead triangle if it were to be unwound to better visualize this lead angle. The dashed lines represent the lead angle of a single start thread and double start thread of the same pitch and circumference for comparison. The colors represent each of the three intertwined threads of the triple start thread depicted in Figure 1.

Lead Angle Formula

multi-start thread

The charts below display the information for all common UN/Metric threads as well as the lead and lead angle for double and triple start versions of each thread. The lead angle represented in the chart is a function of a thread’s lead and major diameter as seen in the equation above. It is important to be aware of this lead angle when manufacturing a multi-start thread. The cutting tool used to mill the thread must have a relief angle greater than the lead angle of the thread for clearance purposes. All Harvey Tool Single Form Thread Milling Cutters can mill a single, double, and triple start thread without interference.

Machining a Multi-Start Thread

  1. Use the table or equation to determine the pitch, lead, and lead angle of the multi-start thread.
  2. Use a single form thread mill to helically interpolate the first thread at the correct lead. *The thread mill used must have a relief angle greater than that of the multi-start thread’s lead angle in order to machine the thread.
  3. Index to the next starting location and mill the remaining parallel thread/threads.

Click here for the full chart – starting on Page 2.

multi-start thread