February 2010 / Volume 62 / Issue 2|
Cutting cycle time
By Dr. Jeffrey Badger
Dear Doc: I surface grind 20 "×12 "×0.5 " steel plates using a 14 "-dia., 1 "-wide WA46IV5 wheel. I must remove 0.025 " of stock on both sides, doing it in 0.0004 " passes and dressing 0.0004 " before every pass. It’s taking 12 to 15 hours to grind this plate and cycle time is killing me. What can I do?
The Doc Replies: There are two keys to reducing cycle time: mixing up your dressing and being smart about your cross-feed. I’d do the following:
First, dress the wheel aggressively by taking a 0.0012 " DOC and racing the diamond across the wheel as fast as it will go. Then, take a fast finish dress with no infeed to clean the grits you’ve missed. This will give you a super-sharp wheel that will generate less heat and reduce grinding forces by as much as 80 percent, depending on the initial dressing.
Second, grind with a 0.001 " DOC in each pass using the original table speed. That’s two and a half times the material in each pass! But don’t worry about burn and the motor stalling because you’ve opened the wheel, grinding forces and heat will be much lower and your machine should be able to handle it.
Finally, here’s the kicker: make sure the grinding overlap ratio is less than 1.0:1.0. That means for a 1 "-wide wheel, cross-feed will be around 0.95 ". That’s imperative to reduce cycle time. When rough grinding, you don’t want to “regrind” material to improve the surface finish because you don’t care about surface finish. Therefore, use nearly 100 percent of that wheel for roughing—not 10 percent for roughing and 90 percent for regrinding, which is all too common.
Now, the surface finish is going to be a disaster, but don’t worry. Continue to grind until you’ve removed 0.0238 " of material, leaving 0.0012 ".
Once you have 0.0012 " of material left to grind, redress the wheel, but with more timid parameters. If the original parameters were imparting the desired surface finish, go with those. If not, go with a smaller dressing depth and slower diamond traverse. But remember: You must remove the “residual topography” from the aggressive dress, so you’ll need to dress around one-grit diameter to remove that. For a 46-mesh wheel, that’s around 0.013 ".
Finish grind with your original parameters, with the smaller DOC and a smaller grinding cross-feed. If you went for a more timid dress to improve surface finish, you may have to decrease the DOC. Grind that last 0.0012 " of material with a few spark-out passes.
Next, flip the plate and repeat the procedure for the other side. Again, be sure to remove the residual topography from the previous fine dress.
This approach can dramatically reduce cycle times and improve surface finish. Of course, depending on surface finish requirements, machine stiffness, spindle power and wheel wear, you may need to tweak the parameters I’ve provided. But the concept remains the same: Rough grind with a sharp wheel and large cross-feed, and finish grind with a dull wheel and small cross-feed.
I’ve helped companies switch to this approach and have seen cycle time drop from 15 hours to 2 hours. It’s not a difficult concept, so why aren’t more using it? In some cases, it’s because they don’t understand the concept of rough dress, rough grind, finish dress, finish grind. In other cases, it’s something else.
A few years ago I visited a company in England grinding shaper cutters. The manager was complaining about 15-hour cycle times. I introduced the rough/finish approach. A year later, I visited again and noticed they still had a 15-hour cycle time. I asked why. The answer? “Yeah, it all made sense and we tried it and it worked great, but the short cycle time was messing up Simon’s lunch and tea breaks.” CTEAbout the Author: Dr. Jeffrey Badger is an independent grinding consultant. His Web site is www.TheGrindingDoc.com.
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