What is Tribology and Why Should You Care?

By Patrick de Vos, Corporate Technical Education Manager, Seco Tools AB

Tribology is a relatively new area of metalcutting load analysis. It studies how surfaces in contact with each other, such as the cut chip and tool, interact at certain temperatures and pressures. Its main focus is e.g. learning what causes the negative phenomenon of built-up edge and what can be done to minimize the problem.

Tribological research has determined that the cutting process does not simply involve a single shearing event and subsequent disconnection of chip and tool. In fact, secondary and tertiary connections and disconnections also occur. The chip shears away, adheres to the rake face and then shears away again before finally sliding off the tool. The main wear mechanism is repeated shearing, not friction.

Built-up edge occurs when thin layers of the workpiece material adhere to and build-up on the tool rake face. If a significant amount of material accumulates on the tool, it can change the profile of the cutting edge. The built-up material can also break off and damage the edge or be deposited on to the workpiece. Regardless, edge buildup makes the cutting process unpredictable and results in poor surface finishes and a need to change tools frequently.

We know that the prime factors that promote edge buildup are high ductility, high adhesion tendencies, abrasiveness and the high pressure and temperatures that are generated when machining tough alloys that have poor thermal conductivity. The possibility of built-up edge formation is much greater in newer workpiece materials such as low carbon steels, aluminum, and the family of aerospace and energy industry materials encompassing titaniums, nickel-based alloys and heat-resistant metals.

Tooling engineers are applying the findings of tribological research in the development of machining processes and tools that will meet the higher demands from these new materials. On the process side, we know that minimizing adherence and the chances for forming built-up edge involves reducing the contact time between the chip and the rake face.

The most straightforward solution is to increase the cutting speed and apply a sharper tool. Faster cutting speeds reduce the time the tool and workpiece material are in contact with each other. The resulting higher process temperatures can also reduce the strength of any edge buildup or eliminate it entirely. The sharper tool has a higher primary rake angle that forces the chip to move more quickly. Other tool geometry choices, such as use of positive rake tools, can help direct cut material away from the workpiece.

We have also used tribology research findings to understand the role tool coatings play in minimizing edge buildup. For example, the newest generation of Seco’s CVD aluminium-oxide Duratomic® coating is based on tribological principles. Development engineers manipulated coating components in response to expanded knowledge of the interactions between the chips and the cutting tool.

Another example of Seco coatings aimed at controlling built-up edge is the new silver PVD uni-coating developed for MS2050 milling inserts. The coating has high heat resistance capabilities and also practically eliminates the occurrence of built-up edge when cutting sticky materials such as titanium. With the absence of built-up edge, the inserts last about 50 percent longer and run at much higher cutting parameters as compared with existing tools.

Tool engineers are also using tribology to research ways to turn edge buildup from a liability into a positive contributor to machining productivity. For example, in some cases, a thin layer of workpiece material on the surface of the cutting tool actually slows the progress of wear. The key is to find a perfect thickness of this tool protection layer that does not affect tool geometry and also does not separate from the tool surface.

While tribology may not be a topic you think about everyday, it is offering an important new perspective for developers of cutting tools and machining processes. It’s giving us another tool to use as we respond to and solve increasingly tougher machining challenges in innovative ways.

If you have questions about tribology, please contact me.

About the Author
Based in The Netherlands, Patrick is the corporate technical education manager for Seco Tools AB with global responsibilities for the technical education activities that help train Seco employees and customers worldwide. He led the creation of the Seco Technical Education Program (STEP) and since its launch more than 200,000 people  worldwide have participated in the program. He has been with the Seco organization for more than 30 years, and during that time he has trained more than 70,000 people in   over 57 countries. He is also the author of the books “Metal Cutting, theories in practice”, “Tool Deterioration, Best Practices” and “Applied Metal Cutting Physics, Best Practices”.

Don’t Let Mechanical Loads Weigh Down Your Milling Process

By Patrick de Vos, Corporate Technical Education Manager, Seco Tools AB

When planning a milling application, there are several factors you must consider for optimum results. First and foremost, you want to have the right cutting tool for the job. But before diving too deep into a tooling supplier catalog, it’s important to understand the variables that impact cutting tool performance, with mechanical loads being one of them.

A mechanical load, not to be confused with cutting force, can be thought of in terms of pressure (force per unit of surface area). This pressure has significant influence on tool life and failure. Consider this: a high cutting force spread over a large tool area produces a relatively insignificant load, while a low cutting force concentrated in a small section of the tool can create a problematic load.

Milling exposes multiple cutting edges to continuously changing loads that go from small to large and back again. And, no matter what milling cutter type you use, its cutting edges will repeatedly enter and exit the workpiece material. Loads on the milling teeth will go from zero before entry to peak values in the cut and back to zero at exit.

Therefore, you want to moderate these intermittent loads so you can achieve the best possible tool life, reliability and productivity in your application. Elements such as cutter positioning, entry and exit strategies and chip thickness are key to controlling mechanical loads and ensuring your success.

Cutter Positioning
When approaching a workpiece, you must consider what milling direction will best meet your goals. In conventional “up” milling, the cutter rotates against the direction of the workpiece feed, while climb “down” milling moves in the same direction as the feed.

Whether you go “up” or “down,” you’ll want to position the cutter to one side or the other of the workpiece centerline. Central positioning mixes the forces of conventional and climb milling, which can result in unstable machining and vibration.

Entry and Exit Strategies
The way the cutter and its cutting edges enter the workpiece largely determine mechanical loads in milling. More times than not climb milling will offer the best point of entry over conventional milling, but there are pros and cons to both.

• Climb milling pros: Full-thickness entry into the workpiece allows for proper heat transfer into the chips, protecting both the part and the tool. Chips flow behind the cutter, minimizing recuts and yielding better part surface finishes.
• Climb milling cons: Full-thickness entry into the workpiece can subject the tool to heavy mechanical loads (which is not a problem for most cutting tool materials). Face milling via the climb method creates a downward force that can cause backlash on older manual machines.
• Conventional milling pros: Gradual entry into the workpiece protects brittle, super hard cutting tools from damage when machining rough-surfaced or thin-walled materials. It also handles heavy cuts on less stable machines.
• Conventional milling cons: Shallow-thickness entry into the workpiece creates excessive friction and heat that can have detrimental effects on a tool. Chips drop in front of the cutter, increasing recuts and lowering part surface finish quality.

Furthermore, how your cutter exits the workpiece is just as important as how it enters. If your cutter’s exit is too sudden or uneven, the cutting edges will chip or break. When handled properly, however, you stand to benefit from up to 10 times more tool life. The exit angle, defined as the angle between the milling cutter radius line and the exit point of the cutting edge, should be the primary focus of your exit strategy. Keep in mind your exit angle can be negative (above the cutter radius line) or positive (below the radius line).

Chip Thickness

Chip thickness is the thickness of the non-deformed chip at the right angles of the cutting edge, and it’s influenced by the radial engagement, edge preparation of the insert and feed per tooth.

When chips are too thick, they tend to generate heavy loads that can chip or break a tool’s cutting edges. When chips are too thin, cutting takes place on a smaller portion of the cutting edge, creating friction and increased heat that results in rapid tool wear.

Cutting tool manufacturers typically have the average chip thickness data for their milling products, so be sure to ask your supplier for this important information. When the average chip thickness data for your cutting tool is applied and maintained, you benefit from maximum tool life and productivity.

Milling cutters have significantly evolved over the years, allowing us to achieve levels of productivity and profitability never before possible. However, many fail to take full advantage of this technical progress. Don’t be one of them. By taking the time to understand the variables that influence cutting tool performance and planning out a proper milling strategy, you’ll have it made.

Metal cutting is definitely a complex process, so any time you have questions or require applications advice, please don’t hesitate to contact our technical support team. Also, be on the lookout for future posts on thermal and tribological loads in milling.

Read the published Seco technical article “Controlling Mechanical Loads In Milling.”

About the Author
Based in The Netherlands, Patrick is the corporate technical education manager for Seco Tools AB with global responsibilities for the technical education activities that help train Seco employees and customers worldwide. He led the creation of the Seco Technical Education Program (STEP) and since its launch more than 200,000 people  worldwide have participated in the program. He has been with the Seco organization for more than 30 years, and during that time he has trained more than 70,000 people in   over 57 countries. He is also the author of the books “Metal Cutting, theories in practice”, “Tool Deterioration, Best Practices” and “Applied Metal Cutting Physics, Best Practices”.

How to Best Deal With (Part) Rejection

By Todd Miller, Manager of Product Marketing

No one likes rejection, especially in the world of parts production. Manufacturers running critical applications know there’s nothing worse than having to scrap expensive, time-consuming components because of burrs or unacceptable edge conditions. Sound familiar? Then it might be time for you to tackle these unsightly manufacturing blemishes differently.

Mechanized Edge Profiling, or MEP, could be your answer, especially if you’re deburring components using hand grinders or other manual processes. Because, even when performed by skilled craftsmen, manual techniques are slow and lack the required process consistency from part to part.

MEP, however, is a controlled strategy involving precision engineered tools, guided by a machine tool’s CAM program, to remove burrs and sharp edges quickly and consistently. It’s also a documentable method that can increase your repeatability and reduce your setup and part handling expenses.

Before implementing MEP, it’s important to make sure you have the right tools for the job. When machining non-rotating components, such as aircraft engine casings, it’s best to use solid-carbide chamfering end mills and edge-break tooling. However, when profiling edges on contouring components, ball nose and lollipop-style tools are your best bet.

Custom MEP tooling is necessary when cutting critical rotating parts that require perfect surfaces, such as aircraft engine fans. These special tools have specific radii, chamfers and angles that are essential to machining flawless, lab-certified edge profiles. The most sophisticated deburring tools have edge designs that produce a chamfer with a radiused edge preceded by lead-in and lead-out angles to prevent formation of secondary burrs.

When putting MEP into action, you should use it as a portion of the actual part feature machining operation for maximum accuracy, consistency and productivity. Deburring should occur after all machining operations are completed. The CAM program directs the MEP tools to deburr all the holes and break sharp edges in sequence. Some MEP tools can be used to deburr a variety of holes, and some profiling tools can be applied on three or four different locations or features, such as the bottom of a hole as well as the bottom of a scallop contour.

To ensure that the edge profiling takes place in the correct location and with the proper amount, your part’s hole or feature must be defined or measured before the MEP operation begins. When part tolerances are very tight, the location of the part surface should be well defined and in-process measurement may be unnecessary. However, if the location is in question, measurement will be necessary after initial machining to determine the location of the edge or feature to be profiled.

In addition, the tool itself must be measured and located to ensure that it will profile the part correctly. Because the tool radii are so small — and for practical purposes, unmeasurable — the tool length is specified in the CAM program. You can confirm the tool length away from the machine with a presetter or on the machine via a laser or touch probe. Feed rates are calculated relative to the measured dimensions of the part features and the tool.

Overall, you should consider the deburring or chamfering process as a finishing pass, with your primary focus on quality. Productivity is always important, but especially in the case of expensive components, pushing the tool to maximize output can have negative, and costly, repercussions. Consistency, reliability and elimination of scrap parts are paramount because no one likes the feeling of rejection.

If you have questions about MEP, please don’t hesitate to contact me or our technical support team. You might also want to check out the published Seco technical article “How MEP Takes the Edge Off Parts Manufacturing.”

About the Author
Todd oversees the product marketing team at Seco Tools LLC. and works with the company’s sales department to further enhance the customer experience. He and his team also support product introductions while working globally on new product testing to ensure customers gain access to the industry’s most advanced tooling as quickly as possible. In his spare time, Todd likes to go bowling as well as cheer on the University of Michigan football team.

Take the Load Off Your Turning Tools

By Chad Miller, Product Manager – Turning and Advanced Materials

Do you find yourself frequently frustrated by premature tool deterioration or failure during turning operations? Understanding the four basic types of machining loads and how each affects your cutting tools can help you achieve slow, predictable tool deterioration as well as increase productivity and profitability.

The four load types are: mechanical, thermal, chemical, and tribological, and each one contributes to the wear and failure of your cutting tool. You’ll find these loads do not act independently, but rather interact and influence the sum of their effects on tool deterioration.

• Mechanical loads in turning are steady; however, any interrupted cuts during the process produces impact loads that can chip or break the tool. 
• Thermal loads occur when workpiece deformation generates heat up to 1,650 degrees Fahrenheit, causing tool deformation and blunting. 
• Chemical loads occur when heat and pressure combine between the cutting and workpiece materials, producing tool wear in the form of diffusion or cratering.
• Tribological loads happen when there is friction between the tool and chip, creating abrasion and erosion-type wear.

You can, however, mitigate the negative impact these loads have on your tools using two strategies.

Strategy 1:

Manipulating your cutting parameters can influence the level of load impact on your tool. Keep in mind, however, depth of cut, feed and speed all have differing effects on machining loads, so you should consider the following:

• Doubling the depth of cut doubles the cutting force but also doubles the length of the cutting edge in cut. This results in the load remaining the same per unit of cutting edge length.
• Increasing feed rate increases cutting forces, but to a lesser, non-linear degree. Because the greater feed increases chip thickness, not the length of the tool in the cut, loads are seriously increased on the cutting edge.
• When increasing cutting speed, forces generally remain the same, but power requirements rise.
• Cutting forces rise at lower cutting speeds and decrease at higher cutting speeds. Watch closely for a built-up edge, which may indicate inappropriate cutting speed.
• Too high of cutting speeds can reduce the reliability of a process through uncontrolled chip formation, extreme tool wear and vibration that can make a tool chip or fracture.
• Higher feeds and depths of cut combined with low or moderate cutting speeds offer the best potential for operational security and reliability. Higher cutting speeds, if depth of cut and feed are sufficiently low to limit cutting forces, can provide greater productivity.

Strategy 2:

The basic size and shape of a tool determines its strength and capabilities. Often times, tool geometries are engineered with a specific application or workpiece material in mind. So, it's critical to have the best tool for the job right from the start. Here are a few things to keep in mind when selecting tools:

• A large, strong insert enables use of highly productive feeds and depths of cut because cutting forces acting on a large insert will result in lighter loads than the same forces would create on a smaller insert.
• A round insert shape is the strongest, and a 90-degree corner of a square insert is stronger than a 35-degree corner of a diamond insert. However, a round insert cannot cut the same variety of part profiles as a 35-degree tool. There is a tradeoff of strength for flexibility of application.
• Generally, the best tool for cutting steel, where toughness is required, has a honed edge. The best tool for cutting stainless steel, which tends to be gummy, has a sharp edge.
• Very sharp cutting edges may not necessarily provide the best surface finish. The best results often are obtained after an edge has run for a period of time.

Tool deterioration during machining is inevitable, but as you can see there are ways to slow the process and better predict when a tool will fail. Metal cutting is definitely a complex process, so any time you have questions or require applications advice, please don’t hesitate to contact me or our technical support team.

Read the published Seco technical article “Mechanical Loads and Cutting Geometries in Turning Operations.”

About the Author
Chad manages Seco's turning and advanced materials product lines, including all CBN and PCD products. When he's not helping customers implement advanced metal cutting solutions, you can find him training for and running 5K, 10K and 1/2 marathon races and triathlons.