Hard Turning | Lathe Technique

Hardly turning it all - hard turning process

Tom Beard

Hard turning may sound like something best left to the specialists. But with a decent lathe and sound methods, many more shops con use this efficient process to bypass that final grinding operation.

Hard turning can't be called a new process, but it is still largely unknown outside the auto industry where it has been well accepted for years. Thus, it is a process ripe for broader application in a host of industries who seek a viable alternative to post-heat-treat grinding. Hard turning offers all the flexibility of conventional CNC turning - quick setup and tool change as well as the capability to generate complex or contoured surfaces. Though hard turning will not easily yield precision grinding tolerances, a good lathe can consistently hold five tenths either side of nominal and deliver a surface finish in the 10 to 15 microinch rms range.

No one suggests that hard turning is a replacement for grinding on all round parts. But turning is proving the more viable process for an increasingly large number of applications, for a number of different reasons. Contrary to what you might expect, hard turning is not necessarily faster than grinding, though that is often the case. But hard turning can be executed on a conventional CNC lathe, which is generally more accessible technology to the average shop as well as being less expensive than the grinder that would be required for comparable work. It also generates less heat than grinding does, which proponents claim makes the hard turning process considerably less inclined to thermally damage the workpiece surface. It doesn't even require coolant.

But hard turning does present its own set of technical challenges. Perhaps most important of these is the proper selection of tooling. Carbide won't last very long up against steel hardened to Rockwell C 50 or above. That forces a switch either to CBN inserts. or to the more commonly applied ceramic materials. For that reason, we spoke to Kyocera Engineered Ceramics, Inc. (Mountain Home, North Carolina) - an experienced developer of tooling for hard turning applications - to get some practical advice on how to go about mastering the process. And for a user's perspective, we toured the shop of L.T.C. Roll & Engineering (Mt. Clemens, Michigan) who has been hard turning their parts for over twenty years.

A Pressure Situation

By Kyocera's definition, hard turning is the machining of materials hardened to the range of 52 to 65 Rc. With such a hard workpiece material, the most obvious process requirement is to find an insert formulated to resist rapid edge deterioration, and that is indeed a prime consideration. But there is an even more significant aspect of hard turning to bear in mind because it quite literally impacts every physical component of the process. Cutting forces are generally 30 to 80 percent greater in hard turning than in conventional "soft" machining processes. It is, in a sense, the proverbial case of the irresistible force meeting the immovable object, which will exploit to the fullest extent any lack of rigidity in the machine tool, toolholder or chuck.

So good advice is to keep the cut light, and the setup tight. Specially designed machine tools are not required, but the lathe should be very rigid from top to bottom, and spindle runout and feed drive play should be tightly controlled. High-precision, balanced chucks are recommended as well.

Likewise, lightly designed tooling systems will have too much give to produce acceptable results. Kyocera recommends selecting toolholders with a 1 1/4 inch square shank or larger, and to keep tool overhang to a minimum. Use standard tools wherever possible, but should special tools be required, be sure they are designed to take advantage of the best insert applicable to the specific job requirements.

As with any turning, the ideal insert geometry is highly dependent on the part to be machined, but generally speaking, always select the strongest insert available for the job. Round or square inserts are recommended for roughing; so save the diamond inserts for a light finishing touch. And in all cases, use negative rake geometry.

While tool material selection is always a tradeoff between toughness and wear resistance, it is most acute here. For hard turning to be economically viable, you can't have the operator constantly stopping the process to change tools, so wear resistance comes to the fore as the leading requirement. Carbide simply will not deliver acceptable service life for the majority of applications. Thus, there are basically two options for ferrous workpieces: CBN or ceramics. The tougher CBN makes sense in high-impact work, such as when cutting through heavy scale or in interrupted cuts. But, asserts Kyocera, for general continuous cutting of hard materials, hot-pressed ceramic will achieve longer tool life, higher part quality and better surface finish.

Still, insert toughness is an issue with ceramics, so it must be kept in mind in managing other process variables. For one, selection of the proper edge preparation is very important to minimize chipping and to deliver a suitable surface finish. A typical edge prep might be a straight chamfer (specified by width and angle), a honed radius,or a combination of both. The ideal edge prep may vary for different materials, hardness levels, and even machine tools. A tooling manufacturer can usually offer some rule-of thumb guidance on edge-prep selection, but optimization will likely be a matter of trial and error on your own shop floor.

Establishing the best combination of cutting parameters will take some experimentation as well, but the general rule is that they all must be adjusted downward as hardness of the part increases. As for speed, it is usually best to cut as fast as possible with ceramics, but the rpm must come down substantially as parts get harder. Kyocera's recommended speed range (see chart) goes as high as 1300 sfpm for 36 Rc steel, and down as low as 100 sfpm for steel hardened to 64 Rc. Similarly, recommended maximum feed rates run from 0.014 to 0.004 inch. over the same 36 to 64 Rc hardness range, and depth of cut runs from 0.200 to 0.040 inch. Coolant is not required, or even recommended, but some practitioners do contend that it helps extend tool life.

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In Action

At least, that's what a supplier says. But what does someone who makes his living making parts say about hard turning? Partners Ned Cavallaro and Andy Ligda have been applying the process for the last twenty years in their business, L.T.C. Roll & Engineering. The company manufactures roll form tooling, and does roll form production, primarily for the automotive and building products industries. Their tooling typically produces such parts as formed molding strips or structural members for auto door frames.

They also do a lot of retrimming, where worn rolls are returned to the factory, and recut to the original form specification. Rolls can be "resharpened" in this manner as many as ten times over their life. Whether the rolls are new or refurbished, the final cutting is done with hard turning. Here's how it plays out in the plant.

The business places a premium on flexibility. A complete set of roll form tooling may cover anywhere from four to sixty stations, each of which consists of a top and bottom roll. A small, simple roll may be a single piece, but a more complex form may have as many as eight pieces which are bolted together to make the complete roll. And every single piece is different. Exacting precision is a requirement on the multipiece rolls, as the fit of one piece to another is critical. And in all cases, accurate machining is essential. L.T.C. Roll & Engineering does nothing but quantity-of-one machining, and botching the final hard turning process creates a very expensive piece of scrap.

Traditionally, form rolls were soft turned hardened and then ground. But some years ago, L.T.C. Roll & Engineering began hard turning, the rolls on manual machines using carbide tooling. The process wasn't as accurate as one might hope, forced frequent tool changes, and required highly skilled operators, but Mr. Cavallaro still felt it was more efficient than grinding for their application. The move to CNC came about eight years ago, and along with it came a significant boost in quality and a dramatic reduction in labor.

The process begins with a blank, a sawed-off piece of round stock. Most of the steel is D-2, and they occasionally use some oil-hardened grades as well. A center hole is drilled, and the blank is pressed on to an arbor so the piece can be turned between centers. This workholding arrangement is important initially because it allows the lathe to get at the OD and both sides of the soft workpiece in a single setup. Later, it will provide the firm support necessary to stand up to the heavy cutting forces of hard turning. The art is soft turned on a Mazak Quick

Turn 20 CNC lathe. Next it is removed from the arbor, ID stamped, and then sent to heat treat where it will be through hardened to 60 to 62 Rc.

The hard part is remounted on an arbor and turned between centers on a Mazak Quick Turn 28. On roughing cuts, they will generally take a depth of cut between 10 and 20 thousandths of an inch, says operator Tony Harlukowicz. Cuts have gone as deep as 75 thousandths, but with extremely reduced speeds and feeds. For a 0.010-inch cut, they will generally set speed in the neighborhood of 450 surface feet per minute, and feed at a rate of 0.008 inches per revolution. A finish pass may be as light as two thousandths, with speed of 450 surface feet per minute, and feed of 0.008 ipr.

The results? According to Mr. Cavallaro, jobs frequently must beheld to [+ or -] 0.0005, and they have no problem meeting the tolerance. He believes that the surface finish is generally in the 16 rms range. though they do not formally track that attribute since the rolls will be polished anyway. Mr. Harlukowicz adds, however, that surface finish is extremely consistent, unless there has been a problem with heat treat. The only other significant detriment to surface finish is flank wear on the insert, but that also provides a sensitive process indicator as to when tool changes are warranted. Changes are not necessary as often as one might think. On average, he estimates they get roughly 25 minutes in the cut per edge.

As mentioned earlier, conventional wisdom says run the speeds up as high as possible with ceramic tooling, and forego the coolant. However, Mr. Cavallaro has found that slower speeds and a flood coolant both contribute to longer tool life, so they almost always cut conservative and wet. Moreover, they often will tend both lathes with a single operator, so a somewhat longer cycle time is a highly acceptable tradeoff.

All final profiles are generated through hard turning, though the center-bore is still ground. And finally, they are polished to generate a near mirror finish. The tools are then tried out on one of the shop's own roll forming machines to ensure they produce an acceptable part. Occasionally, some modifications are required, which means some rolls will go back to the lathe for a recut.

An interesting sidelight to the shop's roll-making operation is how they go about part programming - it's all done on the shop floor. Both Mazak lathes have CNCs designed for shopfloor programming, meaning they have CAMlike software that assists the operator in the preparation of a part program. Moreover, they have "background programming" capability, which means one part can be programmed while another is being machined, without compromising the CNC's performance in either function. Once a part is set up, and running, they will go ahead and program the next component part. The program is graphically simulated on the CNC, which helps the operators spot any errors before the tool hits real metal.

Also, rather than generating a new part program for hard turning, they simply import the program that was generated for the soft turning operation via floppy disk. This way, they simply pick up the tool path for the final soft cut, offset it to the proper depth for the hard cut, and then edit the feeds and speeds. The technique makes for very efficient overall programming, and essentially provides a "try out" for the hard turning routine in the form of the soft turned part.

Bottom line-what hard turning does for L.T.C. Roll & Engineering is enhance their competitiveness in the markets they serve throughout the Midwest. They are not a price shop. Rather, they stake their reputation on quality and service, and a quick and consistent manufacturing process goes a long way to enhance their reputation.