Grinding: A Dynamic Machining Process

Reducing per-part cost means examining capital, consumable, and labor costs

August 1, 2010

Machine uptime and utilization in a grinding process influences final cost-per-part.

Grinding is a dynamic process. In a constantly changing environment, the part is ground, the wheel breaks down from use, and thermal issues cause changes in the machine.

All of these factors affect the success of CNC grinding, an application that is deemed successful only if the desired results are consistent from the first to the last part of a batch. By controlling capital costs, consumable costs, and labor costs, the final cost of the part can be lowered, helping companies to remain competitive in today’s global marketplace.

“The most important factor in reducing costs in grinding today lies in the utilization of capital,” said Machine Tool Systems President John Manley. “There are no barriers, other than available working capital, to someone in a lower-labor-cost country buying an identical machine to that of a Canadian shop.”

What this means is that in order to differentiate themselves, Canadians must make better use of their equipment.

“The amount of uptime that you can have on a machine is the key here,” said Manley. “If you are running a machine only a single shift, 40 hours per week, your capital costs on the part you are producing are higher than someone who is running the machine more shifts per week.”

This uptime influences the cost per piece depending on the volumes being processed on the machine.

“If a shop wants to process only 50 parts a day and can do it in one shift, the uptime is irrelevant. However, if instead they are competing globally and they are going to run it 24/7, the uptime is absolutely essential,” said Manley.

As the volume of parts per print drops, machine uptime is reduced, and multiple setups can mean that the machine is actually removing material fewer hours per day.

“You may be setting up as many hours per day as you are grinding,” said Manley.

Suddenly uptime and utilization can be at 50 percent.

“On the more sophisticated machines you can get quick-change clamping systems for the part, quick-change systems for the wheels and for the dressing systems, and programming that can ‘talk’ to all of the peripheral devices and reconfigure the grinder for the next part quickly,” said Manley.

This makes it realistic and cost-effective to perform multiple setups for different parts in a single day no matter if you are performing cylindrical grinding, tool and cutter grinding, surface grinding, or even gear grinding.

This is what is changing the landscape in grinding operations today. Shops can now justify spending more on a grinding machine if the machine can be set up quickly and easily between jobs to handle smaller runs. Reducing this nongrinding time is important.

One specific way that a manufacturer can reduce nonproductive time in grinding is by using the same abrasive for different parts with similar material characteristics.

For example, when flat honing a range of different aluminum alloy parts – parts with varying mechanical properties - the grinding wheel now can be sped up or slowed down thanks to the variable-speed drives of today’s grinding machines, instead of changing out the wheels.

“This makes the wheel act harder or softer depending on the speed it is rotating, and is a feature seen across today’s better grinders,” said Manley.

Understanding Consumables

Understanding the true cost of consumables for grinding operations is important for reducing per-part costs.

If a shop still uses one traditional aluminum-oxide grinding wheel for every application it is missing out, said Manley. Superabrasive grinding wheels and more sophisticated techniques often can produce more parts per hour with lower consumable costs.

What’s changed in abrasives is the development of superabrasives, which have cubic boron nitride (CBN) or diamond in the bond.

“These bonds allow wheels to maintain profiles much longer,” said Manley.

Proper wheel dressing is also paramount.

“In the old days it was common to have a single-point diamond dresser. Today we are actively using rotary-disk dressers,” said Manley.

According to Manley, a single rotary-disk dresser can handle most any conventional or free spline form without the need to change dressers.

“It’s only as good as the quality of the disk,” said Manley. “The key to using a rotary-disk dresser is to have minimal runout (TIR) in that disk. We can reliably change from one form to another without having to perform multiple offsets.”

Whether you use a single-point dresser or a rotary-disk dresser, maintaining the form is critical.

“One of the biggest challenges that I see in grinding is maintaining the form on the wheel,” said Manley. “Even a square profile will go convex or concave, or have corner breakdown, and that is detrimental to the process.”

Dressing time per part has been reduced significantly thanks to the use of superabrasives.

“This can have a huge impact on your nonproductive time,” said Manley. “What it also does is allow you to be very, very predictable in your process.”

While a superabrasive wheel costs more than a conventional wheel, the cost per part is far less.

“If you are changing a wheel and calibrating the machine every couple of months with a superabrasive wheel compared to every couple of days with a conventional wheel, you can see how much savings there will be,” said Manley.

However, he added, suberabrasives are not the solution for everyone, but they are certainly worth  a look, in most cases.

Labor Content

Like many other processes, the reduction of labor also lowers cost per part.

For example, if a single worker can run multiple machines, by complementing him with autoloaders, labor cost will come down.

“You can also have one programmer work centrally to produce the program for the creation of the first part off the grinder, and have operators on the shop floor running the machines, acting as processes maintenance workers in order to reduce the labor cost,” explained Manley. “The shop floor personnel monitor the process and make adjustments when necessary, but are not responsible for the prototyping of the first piece.”

For more information, visit www.machinetoolsystems.com.