How to purposely burn workpieces

Dear Doc: I implemented acid etching to evaluate burn when grinding hardened-steel parts. I want to purposely burn some parts to calibrate the test. What’s the best method?

The Doc Replies: The following procedures practically guarantee you’ll metallurgically damage the steel during grinding.

1. Dress the wheel dull, with a depth of dress in single-point dressing of 0.0002" (5µm) and a very slow diamond traverse speed of, say, 1 ipm.

2. Increase the wheel speed by, say, 20 percent—but within safe limits.

3. Double the DOC.

4. Decrease the workpiece speed by 80 percent.

5. Reduce coolant flow.

6. Be careful! If you do all of these at once you’ll be grinding very hot with large grinding forces. You should implement them one at a time and check the workpiece after each one.

Also, be careful with the coolant. When cylindrical or surface grinding, turning down the coolant probably won’t cause anything crazy to happen. But when creep-feed grinding that might not be the case. In creep-feed grinding, the coolant removes a significant portion of the heat, and reducing the amount can cause the workpiece to crack or the wheel to explode. Therefore, skip the final step when creep-feed grinding.

In addition, slowing the workpiece speed is key to inducing burn, so don’t be timid. In surface grinding, the equation for how many seconds a point on the workpiece spends in the hot zone equals the arc length in inches divided by the feed rate in ips. The equation for arc length in inches equals the square root of the DOC in inches times the square root of the wheel diameter in inches.

In addition to increasing the time in the hot zone and, therefore, the surface temperature, slowing the feed rate means the grits don’t penetrate deeply—i.e., a small chip load or grit penetration depth. This causes the grits do more rubbing and less cutting, which is good for getting burn.

Dear Doc: I cylindrical grind the IDs of bearing races and have a tough time holding tolerances, but I do just fine on the ODs. Why?

The Doc Replies: Holding tolerances when ID grinding can be more difficult than when OD grinding, and it has to do with normal forces and wheel deflection. When ID grinding, the contact length is much longer than when OD grinding. This means more grits in the action and a smaller penetration depth for each grit, resulting in more rubbing and a larger normal force.

The longer arc length increases impingement and hydrodynamic forces from the coolant. This produces a larger normal force. If that wasn’t bad enough, that little ID wheel sticks way out on the end of the spindle, meaning it’s not very stiff. So the normal force—big or small—causes a larger wheel deflection than when OD grinding.

In a perfect world, you’d just spark-out until that wheel deflection gradually disappears and you achieve final workpiece size. But it’s not that easy. If coolant impingement and hydrodynamic forces are large enough, the wheel may never get there and just ride over the workpiece indefinitely.

Therefore, the grits must dig in and reduce the normal force. You can do that by:

1. Dressing the wheel sharper.

2. Increasing the workpiece rpm.

3. Decreasing the wheel rpm.

4. Reducing coolant flow to a trickle during spark-out, which most machines won’t allow you to do automatically, only manually.

5. Sparking out longer.

In any case, it’s an uphill battle when trying to hold size when ID grinding. It’s just the nature of the beast. CTE

About the Author: Dr. Jeffrey Badger is an independent grinding expert. His Web site is www.TheGrindingDoc.com. He’ll be giving his “High Intensity Grinding Course” Feb. 2-4 near Chicago, hosted by Greenlee Diamond Tool.