Ask the Grinding Doc

Dear Doc: What’s more important when specing out a grinding machine: motor power or stiffness?

The Doc replies: Stiffness. I almost never see a grinding machine maxing out its motor. In fact, most motors, even in rough grinding, are running at maybe 30% of their maximum horsepower.

But low stiffness? That causes all sorts of problems: deflection in grinding and in dressing, increased chatter risk, the wheel not biting into the part during finish grinding, etc. If I’m suffering in grinding — burn, chatter, surface finish, long cycle times, deflection, and just about any problem — I seldom go to the back of the machine and look at the spindle-motor plate. Instead, I measure the stiffness. What do I find? That I’m working on an unstiff machine. I then have to lower my expectations: lower material removal rates, longer cycle times, more headaches.

That’s one reason why I like machines from the 1960s and 1970s: they’re often super stiff. Sure, they weigh several tons. But all that weight usually means higher stiffness.

If somebody said to me: “We’re designing a new machine. We can give you double the motor power but half the stiffness. Or we can give you half the motor power but double the stiffness.” Not knowing anything else, I would take the double stiffness every time.

Dear Doc: I have chatter marks on my workpiece. Is there a quick way to determine if it’s coming from an outof-balance or eccentric wheel?

The Doc replies: Yes. Easy. First, take 4″ (100 mm) of length in the ground workpiece in the direction of grind (for surface grinding or cylindrical grinding). Then count the number of chatter marks (or chatters). Divide the number of chatters by 4″ (or 100 mm). That’ll give you the number of chatters per inch (or millimeter). Now take the workpiece surface velocity and multiply by the number of chatters per inch (or millimeter). That’ll give the chatter frequency in chatters per minute. (Yes, chatter is usually given in chatters per second, or Hz, but here we’re doing it in chatters per minute.)

If that result is close to the wheel rpm — maybe within ±10% — then those chatter marks might be caused by an out-of-balance or eccentric wheel. If the answer isn’t close, then your chatter is coming from somewhere else.

An example: Joe the Grinder counts 57 chatters in 100 mm. His chatter spacing is 0.57 chatters per millimeter (57 ÷ 100). His workpiece is running at 3,000 mm/minute. His chatter frequency is 1,710 chatters/minute (0.57 × 3,000). His wheel is running at 1,800 rpm. Joe decides 1,710 chatters/minute is close to 1,800 rpm, and concludes he may have an out-of-balance or eccentric wheel.