Grinding Holds Its Ground

Despite the growing popularity of hard turning, there are still plenty of reasons to grind parts instead.

Whether or not shops choose to follow the trend of hard turning rather than grinding may depend on their machining requirements. Hard turning may be favored over grinding by shops that seek shorter setup times, higher metal-removal rates, and faster machining cycles. However, this greater speed may be of little value to shops that require superior consistency and quality of tolerances and finishes. For precision and versatility, grinding remains the best means of material removal.

A single grinding wheel can take off large amounts of material and still produce the desired finish. Unlike a turning insert, which must be replaced when it dulls or wears out, a dull grinding wheel can be dressed in-process to expose an aggressive cutting surface.

Using a grinding wheel, the operator can easily create a new cutting surface without stopping for tool replacement. And since many different forms and angles can be dressed into one wheel over time, a new wheel isn’t always required to produce a new part. With this flexibility, adjustments can be made to achieve the required tolerances and finishes on different parts.

Grinding offers many other significant advantages over hard turning, such as the production of fewer burrs. Sharp corners with radii of 0.001" to 0.002" can be achieved by grinding, but not by hard turning. Interrupted cuts that require close tolerances typically can be made only by grinding; the repeated impact the insert suffers as it crosses the interruption frequently shatters or cracks tools in hard turning. And because hard turning doesn’t have as much forgiveness as grinding, there is a greater chance of scrapped parts due to broken inserts and heat resulting from the aggressive rigidity of the tooling.

Fine Finishes

Even when machining to tight tolerances is not required, grinding is preferred over hard turning for the production of flat, smooth finishes. Unlike a grinding wheel, a turning insert cannot kiss the surface of the part and then gradually work down into the material. The sharp cutting edge of the insert mars the surface finish by creating helical forms and grooves in the part.

Although tests have produced hard-turning finishes of 8µin. to 9µin. rms, microfinishes in real-world applications are higher due to inconsistencies in material composition and operator performance. At best, hard-turning finishes are more likely to be 15µin. or 16µin. rms; grinding typically produces finishes of 2µin. or 3µin. rms. Thus, if fine, consistent finishes are required, hard turning has limited application.

Part Production

The benefits of grinding’s consistency and versatility extend to long production runs. Many CNC grinding machines can be used for both OD and ID grinding and allow setup of automatic feeds, automatic dresses, and adjustments to compensate for wheel wear. Even as it wears, a grinding wheel retains its hardness and cutting action. As a result, it can make cuts in a long run of parts with consistent quality. The quality of cuts a turning tool makes over a long run may deteriorate, because the tool may not be able to hold a sharp edge.

Although conventional grinding is better than hard turning for the mass production of a single part, it may be challenged in batch production of one or two parts. For the one- or two-piece runs, equipment setup is much easier and faster for hard turning than for grinding. For several short runs on the same machine, however, grinding offers more versatility, as long as the operator is just modifying the dress on the wheel.

Furthermore, the simplicity of hard turning is beneficial for short runs only if the part can be brought to a finished state in one operation. Rarely is this possible. The operator typically must turn the part and then grind it to finish it. This two-station setup is much less efficient than rough cutting and finishing the part using grinding alone in a one-station setup.

Cost Comparison

Despite the drawbacks of hard turning, some shops consider using this method because they believe that grinding is more expensive. However, the differences between turning and grinding make a true “apples-to-apples” cost comparison difficult. For instance, although grinders typically cost more than lathes, they provide faster output, which may reduce production costs in the long run.

Any conclusion a user might draw after examining the projected costs of hard turning and grinding must be questioned, because the comparison is being made between known and unknown figures. The costs of a grinding procedure can be readily and accurately determined. The price of the grinding wheel will depend on its size and will vary between five and several hundred dollars. Wheel life can be calculated based on factors such as material-removal rates, wheel wear, and dressing. These costs can be verified and documented, based on shop experience and the wear records of abrasives manufacturers. Hard turning, being a relatively new process, does not have this track record, and any costs that may be assigned to the operation are more speculative.

Comparing the tool costs of hard turning and grinding also can be misleading. The tools used in hard turning, which are generally polycrystalline cubic boron nitride or aluminum-oxide/titanium-carbide ceramic, are extremely expensive compared to standard inserts. Even in cases where a turning tool costs less than a wheel, the value may be lost, since the inserts aren’t reused and have to be changed often. Even when using very small inserts, hard-turning tools wear down rapidly, especially those made of the lower-priced ceramic material. Tool replacement may occur as often as every 150 to 250 parts, depending on the tool composition, the material being turned, and the shape of the part. The rapid tool wear also forces the operator to adjust the lathe frequently. When comparing the costs of tools, the downtime and labor costs for insert replacement and machine resetting must be factored into hard turning’s cost.

Success Stories

Shops that have switched to hard turning for some applications have found it is a good idea to retain their grinding capabilities for certain operations. Camcraft Corp., Hanover Park, IL, a manufacturer of precision components, sometimes uses hard turning instead of rough grinding or finish grinding. However, grinding is almost invariably the choice to finish the OD of the cages used in hydraulic applications, according to Mark Bossert, manufacturing engineer at Camcraft. The tool pressure that would have to be exerted on these thin-walled stainless-steel parts to cut them using turning tools would distort the parts sufficiently to affect both the tolerance and the consistency of the process. In fact, Camcraft has found that part deflection from the pressure of the tool can be as much as 0.0002".

Other factors make grinding the best option for the secondary machining of powder-metal parts at Coldwater Products Inc., Coldwater, MI. After a powder-metal part has been pressed, machining often is done to close tolerances, according to William Wolf, the company’s vice president. If Coldwater had to make fixtures to hold the part in a lathe, there would be little room for any part deviation in the blank. For example, Coldwater received a part with an OD of 0.630". All of the tooling and fixtures were made for this particular size. The next month, the same part came in with an OD of 0.665", because the die had worn or the furnace atmosphere was not the same for both runs. With that 0.035" discrepancy, Coldwater would likely have problems with fixtures not accepting parts. Since Coldwater had no control over tolerances on the incoming parts, costs for tooling and fixtures custom-designed for the blanks each time a new batch came in would have made hard turning too expensive. Therefore, centerless grinding was the more cost-effective option.

Wolf says that after experimenting to determine which grade and style of wheel works best for a particular application, an operator is often able to run for an entire shift on a centerless grinder without having to make any major adjustments or dress the wheel. The company’s centerless grinders run all day long, and even when two individuals operate each grinder, costs still come out far ahead. While a variance of 0.005" to 0.010" on the OD of part blanks would normally break an insert, the grinding wheel still does its job, because of its taper and gradual feed into the part.

Optimistic Outlook

As tool materials and machines improve and become less expensive, hard turning will become more competitive with grinding, particularly in applications that require less precision. Most likely, grinding and hard turning will exist side by side or work in tandem.

Before switching from grinding to hard turning, however, a shop should plan on investing a lot of time in training and development. A shop’s grinding experience is not directly applicable to hard turning, and the time to develop proficiency in hard turning may not justify the projected cost savings. But the most important consideration remains the customer’s demands for precision. If only the highest level of accuracy will do, grinding remains the best way to do it.

About the Author
Kimberly B. Pigeon is vice president of marketing for CGW Camel Grinding Wheel, Niles, IL.