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Turning to hard-part turning.

The term "hard-part turning", or simply "hard turning" as it is commonly called, refers to the process of cutting hardened steel to final form and finish, thus avoiding the customary grinding operations that are common in industry today. Generally, it involves cutting hardened steel (54 to 63 HRc) to accuracies of |+ or -~0.0003" to |+ or -~0.0005" and finishes better than 20 rms on a lathe or turning center, using a polycrystalline cubic boron nitride (PCBN) tipped insert or a ceramic insert.

Beyond choosing an appropriate insert, there need not be anything special about the lathe or turning center. If the lathe or turning center is rigid enough and tight enough to provide the required accuracies and finishes in soft steel, it can produce the same in hardened steel.

"We find that with both PCBN and ceramics, accuracies of |+ or -~0.0004" are generally attainable. CBN will tend to hold the tolerance much longer than the ceramic, as it wears slower. Surface finish achievable with CBN is better than 13 microinch rms; with ceramic it's about 25 microinch," according to John Israelsson, a tooling specialist for Sandvik Coromant.

Larry Brenner, zone manager for Mitsubishi Fabmet agrees. "Typically, in production applications where you are shooting for a 10 to 15 rms surface finish, you can expect to obtain |+ or -~0.0005". By slowing the feedrate, we've seen customers achieve |+ or -~0.0004" at 12 rms. As far as ceramics, a lot of shops are still using them for steels as hard as the low 50s, but anything much above 50 HRc, you're much better off with a PCBN."

Turning vs grinding

When compared to grinding, hard turning offers many favorable advantages:

* Capital investment for a lathe is substantially less than a grinder.

* Material removal rate is up to three times faster than with grinding.

* Overall part accuracy is better with hard turning.

* Environmental concerns with grinding are non-existent with hard turning.

The initial cost of a lathe is one-third to one-half the cost of a grinder for the same production volume. Not only does the lathe cost less to purchase, it takes up less floor space and requires less expense for supporting systems. Grinders require a flume system, coolant, and filters plus an environmentally safe way of disposing grinding swarf, an expense that does not arise with turning.

"Although grinding methods are becoming more aggressive, machining is still up to three times faster in most applications," according to Mark Deming, manager, technical marketing, GE Superabrasives. "Overall part accuracy is greater with machining because a turning machine can perform multiple operations during one chucking as opposed to multiple setups on a grinding machine to do the same number of operations." For example, multiple operations--OD, ID, and grooving--can be performed in one setup. Double-end machines can be utilized to reduce load/unload times, for up to three spindles, to as little as 2 to 3 seconds. Unloading/loading on one end is performed as parts are being machined on the other end.

A dramatic example of productivity gains that can be achieved with hard turning can be found with a manufacturer of lead screws. Previously, the company cut threads by machining soft bar stock, hardening the threaded steel, and then finish grinding the piece. The operation required nearly 170 hours.

To increase productivity, the manufacturer converted to a hard-steel whirling operation using BZN*-8100 Compacts, polycrystalline cubic boron nitride inserts from GE Superabrasives. The inserts, composed of CBN particles and ceramic grains sintered together and integrally bonded to a tungsten carbide substrate, provide high material removal rates and excellent surface finish.

In the new whirling process, the steel bar stock was hardened before whirling to produce the finished lead screw. Hard whirling replaced both the soft machining and finish grinding steps. Elimination of these operations, plus the sharply increased material removal rates versus grinding, dramatically shortened the thread whirling process time to only 105 minutes.

Form and finish

It is often assumed, incorrectly, that grinding ensures a higher quality level than turning. However, grinding generates surface heat which can lead to surface burns and checks. "Not so with hard turning," says Jim Galimberti, Kennametal cutting-tool product specialist. "In hard turning, most of the heat is carried away with the chips. The only reason one would use a coolant or cutting fluid in hard turning is to keep the part cool enough for in-process gaging."

When subsequent superfinishing or microfinishing is desired, hard turning is the better choice, says Norm Lewis, Kasper Machine Co sales manager. "A key benefit to hard turning compared to grinding is its ability to produce true roundness. Grinding tends to create lobing on workpieces while generating the desired surface finish."

Another example of how hard turning can be used to improve part quality and reduce cost can be found at Ford Motor Co. For a forged front-wheel drive sun gear, manufacturing engineers decided that to meet production consistency, accuracy, and cost goals, they needed to take a hard look at the entire part-manufacturing process.

Typically, the sun gear's two concentric diameters had been turned, then heat treated, ground to desired roundness, and polished to the correct surface finish. This involved three different machine tools, part handling between the machines, and three part fixturings or chuckings.

Because the grinding process uses a constantly wearing abrasive, it frequently produced inconsistent surface finishes and could not reliably produce desired roundness. Irregular wear of the stone and structural inconsistencies are inherent to the material. Wheels had to be dressed and regularly changed, further affecting the ability of the abrasive to consistently generate specified part roundness and surface finish, and resulting in machine downtime.

To improve the quality of the sun gear, Ford engineers at the Batavia, OH, transmission plant decided to heat treat the part, hard turn it with a Kasper Machine Co turning machine, and then apply a patented IMPCO microfinishing process to achieve final size to |+ or -~5 microns, less than 3 microns roundness, and surface finish to 0.3 Ra or better in a 20 second cycle. The result was a shorter total production cycle and more consistently accurate parts at lower manufacturing costs.

IMPCO GBQ (generating bearing quality) microfinishing with in-process size control was essential to achieving Ford's quality goals for the sun gear without adding cost to manufacturing. GBQ uses abrasive film backed by a nonresilient "shoe." The noncompressible film is circulated around cylindrical workpieces to achieve roundness, size, and surface finish to within just a few microns of nominal part print tolerance in high-volume production.

"This process is very widely accepted for processing crankshafts, camshafts, and power transmission shafts because of the accuracy and quality it generates, but it is our first application of the process to eliminate grinding," according to IMPCO vice president Norm Judge.

Hard turning requirements

Selecting inserts for hard turning is no more difficult than selecting carbide inserts for conventional turning. Generally, insert manufacturers offer two or more grades of PCBN, which are selected on the basis of whether the insert will be used for continuous or interrupted turning.

According to Mr Brenner, two grades of PCBN inserts that Mitsubishi Fabmet commonly recommend for hard-turning applications are formulated to either provide optimum wear resistance in continuous-turning applications, or optimum toughness to resist chipping in interrupted-turning applications. Both are formulated to provide the best possible finish given the type of workpiece being turned.

Similar is the case with Sumitomo PCBN inserts, which offer three hard-turning grades, each composed of different percentages of CBN and, consequently, carrying different Vickers hardness ratings. Sumitomo's BN200 carries a Vickers hardness of 3000 to 3100 and provides good tool life in continuous-turning applications. BN250 carries a 3100 to 3300 Vickers hardness and offers a good combination of wear resistance and toughness in continuous- or interrupted-turning applications. BN300 has a Vickers hardness rating of 3300 to 3500 and offers the superior toughness needed in heavy interrupted-turning applications. According to Tod Callaby, Sumitomo engineering, all three grades are offered both as full inserts and as the increasingly popular petite or "one-use" type.

Teledyne Firth Sterling's Greg Mooreland confirms that ceramics are still very popular in steel mills for steels rated up to the low 50s HRc; however, for material hardnesses from 55 HRc to 65 HRc, PCBNs such as its Grade FE5 inserts are used almost exclusively.

The important point to note here is that choosing an insert for hard turning need not be any more difficult than selecting an insert for any other turning application. All of the major cutting tool manufacturers provide data sheets containing recommended feeds and speeds for the various grades of hard-turning inserts that they offer.

The hard-turning mystery

Despite all of the advantages associated with hard turning, not many machine shops outside the automotive industry are doing much of it. While conducting interviews for this article, participants were asked to give their "best guess" as to the percentage of shops performing hard turning. Their consensus was that only about 10% of the shops capable of performing hard turning are actually doing any.

So why aren't more machine shops using hard turning? According to Mr Brenner, "education seems to be the biggest obstacle. Either people are unaware of the results that can be obtained with hard turning, or the perceived cost of the materials has kept people away." Originally, when CBNs were first introduced, cost of a CBN insert could be ten times that of a comparable carbide insert. Today, with petite inserts like those from Fabmet, or Compacts from GE Superabrasives, the cost of a CBN tipped insert is only slightly more expensive than a common carbide insert.

Mr Deming agrees that education is the key. GE Superabrasives, for one, has made a big investment in educating junior college, vocational, and technical school instructors in the benefits of superabrasives and hard turning. As part of GE Superabrasives' Partnership for Manufacturing Productivity program, the company provides extensive teacher-training courses, teaching manuals, video tapes, slides, etc, all aimed at upgrading machining methods in the United States through the use of superabrasives.

Mr Galimberti agrees that education is part of the problem, but the simple answer is that most shops have a hefty investment in grinders and aren't about to abandon them for lathes, even if it can be clearly demonstrated that hard turning will produce a superior and less costly part.

For more information on hard turning from companies mentioned in this article, circle the appropriate numbers found below on your reader service card:
GE Superabrasives 290
Kasper Machine Co 292
Kennametal 293
Mitsubishi Fabmet 294
Sandvik Coromant 295
Sumitomo 296
Teledyne Firth Sterling 297
COPYRIGHT 1993 Nelson Publishing
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Author:Stovicek, Donald R.
Publication:Tooling & Production
Date:Jan 1, 1993
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