The "secret" to successful hard turning is high surface speed. Applications typically involve tripling or quadrupling the surface speed used for carbide inserts when CBN inserts are substituted. CBN (also called PCBN, polycrystalline cubic boron nitride) is a man-made material second only to diamond in hardness.
At these speeds, heat goes out with the chip and not into the tool or the workpiece. Cutting dry--without coolant--is one benefit. Improved surface finish and reduced machining time are other benefits.
The greatest advantage to hard turning, however, is that it eliminates a grinding operation to finish the part. Therein lies the payoff that makes hard turning such an attractive alternative to many machine shops.
So why isn't everybody doing it? Obviously, hard turning is not an option in every case, but not every application for hard turning is being taken advantage of. One reason is that some potential users are under the impression that they'll have to make a big investment in new tool-holders and/or new fixtures, not to mention the higher cost of those CBN inserts. The other reason is that they are afraid their existing machines and setups aren't up to the higher speeds and resulting higher cutting forces that are involved in hard turning.
Bob McCarthy, the CBN/PCD Product Manager at Fansteel VR/Wesson, a supplier of cutting tool inserts and toolholders, says that both of these concerns need to be re-examined. In a great number of applications, his experience shows, existing toolholders are more than adequate for hard turning. For example, if a shop is tooled up for 80-degree diamond inserts in carbide, he encourages them to run CBN-tipped inserts of the same shape in the same toolholders for hard turning--at much higher cutting speed (surface feet per minute or sfm), of course.
Likewise, Mr. McCarthy believes that many shops are too quick to rule out hard turning because they do not have the latest turning equipment. What is important, he says, is the condition of the machine, not necessarily its age. For hard turning, a lathe must be rigid, have sufficient spindle rpm, and adequate horsepower. A well-maintained CNC (computer numerical control) lathe is a likely candidate for hard turning.
The point is, every successful application of hard turning has to be engineered on a case-by-case basis.
"It's simply not practical to say that for a given operation and workpiece material, this or that style and grade of insert is best for hard turning," says Mr. McCarthy. "So many other factors have to be considered. But understanding those factors will help many shops recognize opportunities for hard turning that they might otherwise have overlooked.
"The biggest hurdle isn't always working out the technical issues," he observes, "it's often the psychological factor. Some shops hesitate to give hard turning a try because they are unaccustomed to the surface speeds it calls for. Today we know enough about hard turning to determine when and how to do it safely--and effectively," he points out.
Numerous case histories back him up and show what real-world results hard turning is capable of providing.
What Materials Are Being Hard Turned?
Hard turning is a process in which hardened steels (above 45 Rc) are finish turned. In other words, a lathe or turning center provides the last operation bringing the workpiece to final shape and surface condition. Hard turned parts do not need to be finish ground.
Typical materials that are hard-turned are 5120 steel (62 Rc), 1050 steel (62 Rc), 9310 (60 Rc) and 4320 steel (60-62 Rc).
Many plants that manufacture hardened steel bearings, gears and axle shafts use this process. According to Mr. McCarthy, they can consistently hold dimensional tolerances to ±0.0004 inch or better over long production runs and achieve excellent surface finishes.
Plus, he points out that grinding machines may be two or three times the purchase price of a lathe. In numerous companies where hard turning has replaced conventional grinding, these plants are averaging one-third the machine tool investment. Further, cycle times and setups are generally much shorter on lathes, he notes.
"How the hard-turning process works is simple," says Mr. McCarthy. "If you are finish-turning a shaft of 60 Rc hardness at 400 sfm with an 0.008-inch depth of cut, the material being removed will have an Rc hardness approximately in the 30s range. That's because all of the heat generated at this cutting speed is concentrated where the chip is being formed, causing it to anneal ahead of the cutting action, while the parent material remains unaffected. In fact, this annealing effect is even more pronounced if the speed is increased, so the Rc of the chips drops even further. Hence, the faster the speed, the more parts per tool. We suggest starting at the low end of the recommended sfm and work up to the optimum speed."
Table I (below) gives some representative parameters proven for hard turning. As these figures show, the essence of hard turning is high sfm and low depth of cut.
Speeds and Feeds
|SFM||IPM||DOC (Inch Depth of Cut)|
|Carbon And Alloy
Steels (50-60 Rc)
|Die Steels (55-65 Rc)||150-350||0.002-0.008||0.003-0.008|
Are My Machine And Setup Rigid Enough?
The first question a shop needs to address regarding hard turning focuses on the machine tool. The issue, summarizes Mr. McCarthy, is rigidity. Hard turning requires a rigid machine with a rigid toolholder system. Otherwise, the cutting tools will wear quickly and be prone to chipping. Surface finishes will be poor and dimensional accuracy will be questionable. "Vibration is a CBN tool's worst enemy," he says.
CNC lathes are recommended for hard turning, but manual lathes can be effectively utilized as well. A manual lathe must be in excellent condition and have very little play in the cross-slide toolpost and tailstock. Mr. McCarthy notes that shop managers should be careful not to assume that older equipment will be inadequate or that newer equipment will automatically measure up.
In an occasional instance, a shop that has been under pressure to get a good return on the investment represented by a high-ticket turning center may find that heavy use has produced more wear than might be expected, whereas a well-maintained manual lathe subjected to less demanding duty may actually turn out to be a candidate for hard turning. The lesson here, Mr. McCarthy stresses, is that the best piece of equipment for taking advantage of the hard turning process is an open mind.
As a general rule, if the lathe generates vibration, chatter, and poor surface finishes with carbide tools, it would not be a good candidate for hard turning.
Regarding toolholders, Mr. McCarthy has this advice: Standard toolholders work fine to clamp the insert for single-point cutting. The toolholder clamping components, however, must be in good condition. Carefully examine the top clamps, cam lock pins, and screws. If they are worn, replace them.
Ditto for the chuck or fixture that will hold the workpiece for hard turning. Be sure it is in good condition. Check jaws for wear. A solid grip on the workpiece is essential because "vibration is a CBN tool's worst enemy," as Mr. McCarthy reiterates.
The point is, in most cases, where toolholders and chucks are in good repair and are properly used, they can also be used for hard turning.
What About Insert Tooling?
Hard turning does call for a change in thinking about the shape and style of inserts that are applicable but not a radical shift in tooling concepts. That common sense is a principle guiding factor becomes clear in some general guidelines about hard-turning Mr. McCarthy offers.
For example, in most applications, hard-turning inserts should be negative geometries. Negative rake angles give good support to the cutting edge, where it is needed most because higher speeds and relatively light depths of cut concentrate forces there. (However, in certain boring operations, positive geometries are best because they are available in different inscribed-circle sizes, such as 0.125, 0.156 and 0.250 inch.)
Likewise, to protect the cutting edge from chipping, a T-land or K-land is a must on hard-turning inserts. An edge hone also helps the edge resist chipping. Standard K-lands appropriate for hard turning are from 10 to 25 degrees by 0.004 to 0.010 inch wide, depending on the material and depth of cut.
Commonly used inserts for hard turning are CNGA, DNGA, VNGA, CNMP and TNG styles. Positive inserts for hard turning are CCMT, CPGM, and DCMT.
Mr. McCarthy points out another oddity about hard turning with CBN that sometimes defies expectations. Generally, the grades used for hard turning inserts are those having roughly 50 percent CBN content, depending on the manufacturer. Grades with a higher content of CBN, on the other hand, are used for conventional turning of softer materials such as powdered metal, gray irons, and certain super alloys.
Mr. McCarthy stresses that shops should not be afraid to experiment once they get a feel for hard turning. Similarly, they should not expect to find definitive answers, from outside sources, about what hard turning parameters are best for their application. Nobody's got an infallible cookbook to refer to that takes in all of the variables that come into play in every shop. Shops should expect to do some tweaking, he says.
CBN Is Too Expensive!
No, says Mr. McCarthy. Compared to the carbide inserts, CBN inserts are considerably more expensive (from four to five times more costly per insert), but one CBN tool produces many, many parts. But that isn't the point at all, he says. The cost of tooling is trivial when one looks at savings from eliminating a finish grinding operation.
For example, he cites one plant that adopted hard turning to finish the bore ID on a pinion gear of 5120 steel (62 Rc). In this case, 11 grinders were replaced with five lathes. Many shops find that reduced coolant costs from dry cutting more than offset the higher price of CBN inserts, Mr. McCarthy says.
Can I Really Cut That Fast?
The one aspect of hard turning that may take some getting used to is the high surface speed involved. Nonetheless, these high speeds are essential. "If you fall below what is recommended, the insert will wear quickly and fail," warns Mr. McCarthy. "Hard turning has certain running parameters that must be followed to achieve success," he says, pointing again to Table I, "but the results are worthwhile."
Mr. McCarthy recalls one shop owner who hesitated initially when confronted with the speeds hard turning calls for. The shop was cutting 45 Rc steel with carbide inserts on a three-year old CNC lathe. To produce just one part, the shop was using two carbide inserts, indexing each one time (four cutting points altogether.)
The workpieces were 2 inches in diameter, 1 inch long. The part was faced, then a shoulder was turned, followed by turning of the OD. Each cut removed 0.02 inch, with the carbide inserts. Cycle time totaled 3.5 minutes. The parts were subsequently finish ground to complete.
To improve the process and reduce high insert usage, the shop owner went to a CBN-tipped, Fansteel VR/Wesson DNGA 432 insert. This insert had a 0.006-inch by 20-degree K-land. A cutting speed of 600 sfm, at 0.02 inch depth of cut, was recommended, with no coolant. "The owner was a little nervous because the speeds were much higher than he was accustomed to," Mr. McCarthy recalls, "but once he let the machine and tool do its work, the results were reassuring."
The machine was able to make 40 parts with one CBN insert, holding a 0.0002-inch tolerance on all 40 parts. Cycle time was reduced to 1.5 minutes, allowing what used to take 20 hours to be completed in one shift. Surface finish was superior, too, making the finish grinding operation unnecessary. Cutting dry also eliminated costs associated with handling and disposing of coolant.
Turning It On
Why hard turn with CBN inserts? Mr. McCarthy sums up the case in its favor: Hard turning:
- promotes faster cycle times,
- cuts cost of machine tool investment,
- improves accuracy,
- imparts excellent surface finishes,
- permits higher metal removal rates (two to four times),
- allows multiple operations to be performed in a single chucking,
- makes wet or dry cutting an option (dry cutting avoids coolant expense and disposal issues).
"Hard turning with CBN inserts can streamline the manufacturing process, increase productivity, provide consistent quality and lower rejection rates," he says.
Question is, why aren't you hard turning everything you can?