Steel is one of the most machined materials in the world. It is strong, abundant, and has a wide variety of different types. Some steels can be annealed or hardened, even to over 70 HrC in some cases. There are a wide variety of different steels that can range from low carbon 1018 to higher carbon content tool steels, such as D2. The combination of different hardness and grades of materials creates a wide variety of different applications to machine. Steels can vary widely in recommended feeds and speeds on the same exact application depending on the material. Manufactures will normally offer a variety of indexable inserts for turning applications, available with different shapes, angles, grind tolerances, sizes, chip breaker and grades.Use the information below or contact one of Travers Tool’s experienced technical advisors for product selection assistance at email@example.com or via phone at 800.234.9985
CHOOSING THE BEST INSERT WILL DEPEND
ON A VARIETY OF FACTORS SUCH AS:
• What type of steel are you machining?
• What is the hardness of your workpiece?
• Is there interruption or surface-scale in your workpiece?
• What type of finish does your application require?
• What type of speeds and feeds is your application capable of running?
WHAT TYPE OF STEEL ARE YOU MACHINING?
Steels have a wide variety of different grades and a wide range of hardness. They are normally separated into 4 categories:
- Low Carbon Steels: Usually normally lower in carbon content and very free machining. Can be machined very quickly and tool changes due to failure can be very infrequent (e.g. 12L14, 1018, etc.). These steels are very machinable and require a sharper edge and good chip control. In CNC operations, it is a good idea to look for low carbon steel specific chip breakers and harder/more wear resistant grades when machining. If the application allows for coolant, which if possible, is preferred, coolant thru tool holders help to remove the chips. In the right application, speeds that exceed 1,000 SFM can be achieved when machining low carbon steels.
- Carbon Steels: Normally higher carbon content and more difficult to machine (e.g. 4140, 4340, 8620, etc.). These applications are more demanding then low carbon steels. The use of a honed edge and reduced speeds and feeds versus low carbon steels are typically recommended. Just like low carbon steels, a multi-layered CVD coating is recommended for more wear resistant applications. A manufacture may offer many chip breakers, but normally when they state steel ‘roughing’, ‘medium’ or ‘finishing’ chip breakers, this is what they’re targeting. It’s always recommended that you compare the numbers on the depth of cut, feed rate and how much interruption there is (if any) as the previously listed terms are relative and one shop’s roughing is another shop’s medium machining parameters. A typical turning application is normally in the 400-600 SFM range.
- Tool Steels: Very high carbon content steels (01, A2, D2, etc.). These are normally carbon and alloy steels that are abrasive resistant, deformation resistant, and hold a very good cutting edge under high temperatures. These are some of the most difficult steels to machine. Speeds and feeds are the most conservative with these tools and tool changes are very frequent. Typically, the same chip breakers that are used on carbon steels can be used on Tool Steels, thought the feed rate and depth of cut may need to be reduced. A typical turning application is normally in the 300-400 SFM range.
- Hardened Steels: These are technically not a different steel grade, but rather steel grades that have been hardened to above 50 HrC. These applications are so specific that this has a different ISO material class: “ISO H”. These can be machined with carbide inserts, but potentially with a tremendous amount of frustration. These materials are normally machined with ceramics and CBN inserts, which are harder and can perform better under the high heat that these applications produce. It’s recommended that you generally take smaller depths of cut and lighter feed rates for the best productivity. Modern ceramic and CBN inserts are reinforced and may even come coated to provide even greater tool life and productivity. Speeds will vary significantly depending on how you approach these applications, but due to the high heat these inserts may produce, coolant is not recommended on many applications.
WHAT IS THE HARDNESS OF YOUR WORKPIECE?
One of the first rules of cutting tools is.... your tool must be harder than the object that you are cutting. The harder the steel you are machining the harder the tool you should use. Since steels are machined both in its annealed state and hardened. The hardness of the steel will determine what tools you can use and will dictate your feeds and speeds. For example, if you attempted to machine a hardened piece of 4140 (42CrMo4), 50+ HrC with carbide the machinist should consider a harder grade of carbide. At this point a C5 (P25-P35) or even a C6 (P20-P30) substrate would probably wear or produce plastic deformation. A C7 (P05-P15) substrate with a wear/heat resistant coating would be suitable.
Though most of the steel machined is in its annealed state, a coated C6 or C5 would be your first choice for roughing applications and a coated C6 or C7 would be your first choice for finishing applications.
IS THERE INTERRUPTION OR SURFACE-SCALE IN YOUR WORKPIECE?
Many times, carbon steels are forged and getting through the scale is an important cut. Scale will vary in hardness and is usually an inconsistent surface. The best way to handle this is to get under the scale. Utilize a large radius and a roughing chip breaker to help fortify the edge. In a CNC operation, it maybe even favorable to have a rough turning tool simply to break through the scale.
Along with scale, interruption can also determine what type of insert you use. A large honed edge is normally used in these applications. A strong roughing chip breaker with a tough C5 coated substrate should get the job done. Chip breaking is less of a concern if there is enough interruption, as the interruption will naturally break the chip. In order to combat heavy scale a larger nose radius can help reinforce the cutting edge as well. However, producing too much of a honed edge can produce an excessive amount of heat, so if you are unable to take the appropriate depth of cut, you can utilize finer chip breakers with a tough grade.
WHAT TYPE OF FINISH DOES YOUR APPLICATION REQUIRE?
The surface finish of an application will determine shape, angles, chip breaker, grades and the radius of an insert. When a coarse finish is acceptable a roughing or medium chip breaker should be utilized, and the focus of the insert should be tool life during the roughing application. If your application requires a fine finish on your lathe, utilize a positive insert with an acute cutting edge and a ground tolerance, ie.. VCGT. Choose a small radius, 0.008” – 0.016”, a fine finishing chip breaker and hard wear resistant grade. Cermets (a hybrid substrate) should be considered for these applications, as theses substrates are harder than tungsten carbide substrates. Though some finesse is necessary to properly run these types of inserts, low depth of cut, low feed rate and high speeds will improve the workpiece finish. A wiper cutting edge can also by used to help produce a finer cutting surface as well.
80° diamond shaped inserts are the most common in the industry. CNMG style inserts for negative tool holders and CCMT style inserts for positive tool holders. Positive inserts are normally used in applications where power, tool pressure and surface finish pose an issue. You’ll find the most variety in CNMG & CCMT style inserts, both 80° diamonds. These inserts come in a variety of radii, tolerances, grades and chip breakers for heavy roughing applications to fine finishing applications. Although this insert shape does not handle all applications, it is a great place to start because of the variety. When in doubt start with a medium chip breaker and a coated C5 (P25-P35) class grade. This will give you the best balance of surface finish, versatility and edge strength. If the 80° diamond is too weak, consider going to a stronger shaper such as a square or round insert. If the surface finish is unacceptable a sharper 55° or 35° diamond are good choices.
WHAT TYPE OF SPEEDS AND FEEDS IS YOUR APPLICATION CAPABLE OF RUNNING?
Along with the type of steel, another determining factor will depend on application, such as the depth of cut, feed rate, interruption, rigidity in the setup, and workpiece size. How well the part is held will help reduce vibration under high speeds. Harder substrates (C7) with fine finishing chip breaker tend to not perform as well under these conditions, and to compensate, a tougher substrate or coarser chip breaker may need to be utilized.
Many times, the application can dictate the speed and chip breaker that should be used on an application. For example, a finish- ing pass on a clean 1.0” cylinder of 1018 steel on a very rigid setup can be pushed to the limits of the tool, potentially reaching close to 1000 SFM; vs. a giant 12.0’ workpiece rough forged 4140 on a VTL, which must be run slower due to the shear size
at 2 RPM or 75 SFM. In the first example a standard 3/8” or 1/2” IC insert can be used to successfully machine the part, both efficiently and effectively. Though in the later, typically these applications utilize 1.0” or 1.25” inserts, that only offer a limited selection. The largest variety of insert grades and chip breakers are in the 3/8” & 1/2” size range.
SUBSTRATES, COATINGS, & POST TREATMENT SUMMARY
In summary steel turning applications can be very productive when approached the right way. The geometry you choose can help produce better edge strength or provide better surfaces. Identifying your parameters such as, what type of steel you’re machining, the hardness of your workpiece, interruption levels, what type of finish your workpiece requires and what type of feeds and speeds you can run your part at will help refine the tool selection process and dictate how productive you can turn your next piece of steel.
COMMON ISSUES AND INSERT FAILURE
One of the most common problems when machining aluminum is built-up edge (BUE). Built-up edge is when the metal that is being cut will accumulate on the rake face of the insert. This accumulation dulls the cutting edge, degrading the tool life and leaves a poor surface finish. There are several means of preventing BUE, which include using sharp geometries, using an aluminum wear-resistant specific grade, utilizing a polished surface or coating, utilizing ample coolant and applying the correct speeds. Though coolant tooling is recommended if not available ample flood coolant.
Chip evacuation is another common problem when machining aluminum. This issue is more commonly seen with solid round tools but can be seen when machining aluminum in high volumes, at high speeds. Milling tools with too many teeth can ‘pack’ chips and this can lead to tool failure. Wide insert pockets with plenty of clearance for chips to evacuate is recommended. Utilizing too many teeth is not recommended, while coarser teeth selection with higher feed rates.
Aluminum is a relatively sturdy material that is very common and highly machinable. Utilizing indexable tooling is a great way to remove material from aluminum, because it is cost-effective and there are plenty of tooling options. Choose a sharp chipbreaker, use a ground tolerance when possible, access whether you need a polished surface or a coating, consider PCD tipped inserts on high volume production runs, and use coolant when possible.
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