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MEW-169 Tool-post Grinder Article

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Steve Garnett29/10/2010 23:31:42
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Posted by MICHAEL WILLIAMS on 29/10/2010 14:31:37: 
 
The direction of rotation of the workpiece relative to the wheel does not affect the grinding process at all if there is no backlash present . Since there is always some backlash this has to be taken into account in practical grinding.

So that puts you firmly in what I called the 'non-climb' grinding school. It's the one that makes intuitive sense to me, but just so that I'm sure of what you are saying, can you confirm that you are suggesting that the grinding wheel energy (even with minimal contact) is sufficient to take up the backlash in the drive to the work? If that is indeed the case, then it would be a very convincing reason for having the surface travel directions opposing each other.
 
I'm not trying to provoke any arguments here, just trying to get a good grip on this so that next time I try it, I'll have some sort of idea what might really be happening. There appear to be two somewhat different approaches to what might actually make a difference...
blowlamp30/10/2010 01:00:25
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Steve.
If you're grinding in the conventional way, that is, by the non-climb method, then the backlash is automatically removed as soon as the work head is powered up and turning the job.
Think of how all the backlash becomes irrelevant from the screwcutting train on a lathe when threading.
 
Using a climb grinding technique requires the workhead drive motor to actually function as a brake, because the workpiece will tend to be driven by the wheel and isn't ideal.
 
Martin. 
chris stephens30/10/2010 12:26:46
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Hi Michael,
As an example of how little force is involved, I have been known to true up rotating centre cones, which rotate with no more drive than the oil film in the bearings! If that is not negligible, it must be very close to it.
chriStephens 
Steve Garnett30/10/2010 14:53:39
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Yes I do understand what backlash is - that's not the issue. I just wanted to make sure that I'd understood what Michael was saying correctly. I think that Martin's point about the workhead motor then acting as a brake sums it all up rather nicely - the less the force applied by the grinding head, the greater the chance of 'chatter' in the work drive, which translates into changes in the rate of surface progress of grinding - obviously to be avoided.
 
It's been a useful discussion though, because it now means that if I contrive a means of rotating work on the tool cutter (which I was thinking of), then I'll do it with a stepper motor and a toothed drive, I think. More control over the rate of progress, and potentially zero backlash, whatever I do with it.
Steve Garnett31/10/2010 11:33:09
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Posted by MICHAEL WILLIAMS on 31/10/2010 10:48:01:
Hi Steve ,
 
The drag load on the work (and wheel) in cylindrical grinding varies in a highly non linear way with depth of cut . The worst case is in internal grinding where the drag force varies as something like depth of cut to the power three so careful management and light cuts allow you to do delicate work with almost no drag whereas a quite modest increase of depth of cut generates a much higher drag and an ill judged or accidental over deep depth of cut leads causes a massive level of drag and wheel snatch inevitably follows with all its destructive and potentially dangerous consequences .
 

I had begun to wonder about the degree of drag and how it might vary, so I did a bit more research into that myself. The obvious additional factor seems to be that you have to add the rate of progress to this (work rotation speed), as well as the cut depth, but interestingly, I also found a reference to potential problems to do with grinding spindle flex, especially with small wheels. It went on to say in this particular context that there is also a corollary issue with letting the grinding wheel completely lose contact with the work at any point (if it is going to return to it, of course), and suggests that steps should be taken to prevent this from happening.  Anyway, it's certainly not too difficult to see why the drag load vs. everything else relationship is complex!

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