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Ecampbell

Plastic
Joined
Jun 1, 2013
Location
SouthBend IN
Hello all gotta tricky one needs some info
I run an s33 studer grinder and am having trouble holding size it is 4 different Dia. and 2 shoulders that I am grinding in one program. everytime I think I have it dialed in 1or2 of the lands will go undersize.. the part is only 1.5 in. long and I am running between centers The wheel I am using is a 150 grit 36A.
Any thoughts any ideas are appriciated.
 
I do not do much od grinding....I have noticed in alot of lathes that as your diameters get bigger or smaller the finished diameters after cutting will not all be the same amount from nominal, the larger the differences in diameter....the bigger the differences in finished diameter. I have always assumed this is due to slight imperfections in the machines......maybe the slant angle or something may be ever so slightly off....or maybe tool centerline is varying due to z axis not being perfectly aligned. I would think similar thoughts about your grinder.....but probably closer to 0 than the lathes would be. I have always programmed around such annomalies in the lathe. Can you program around the problem? Or does it constantly change? Is it a dress cycle issue?
 
Should be between dead centers, or with the head side center ground into spindle and opposite dead.

For super close work I would often back away to let part cool and then come back take last .0001 or .0002

How are you driving? the driver should be balanced type. How tight are you making the driver could that be a difference? do you locate the driver the same way?

Are the part centers polished or good? Are the machine centers Good?

Does the tail end have a locking control, or any free play if no?

What do you mean by lands?

That is a close size to just come to make size.

Let us know more about the part, set up and machine.

Buck
 
My first thought would be coolant ,if the supply is erratic or insufficient this could be the problem.

Is it safe to assume that the machine is in good order,is the problem just with this part?
 
Totally off the wall, and this is coming from someone who does not do grinding for money, but one time I saw a guy get good parts off a grinder by changing the way lube. It took him a few days to figure out that's what he needed to do.
 
Ecampbell said:
Hello all gotta tricky one needs some info
I run an s33 studer grinder and am having trouble holding size it is 4 different Dia. and 2 shoulders that I am grinding in one program. everytime I think I have it dialed in 1or2 of the lands will go undersize.. the part is only 1.5 in. long and I am running between centers The wheel I am using is a 150 grit 36A.
Any thoughts any ideas are appriciated.
Click to expand...

Are you programing in inch or metric? If you are programing in inch than do it in metric. 2 1/2 times more accurate.
 
Why such a fine grit? This will cut very slowly and will easily over heat your part, even with great coolant. This fine of a grit is usually only required it you want to hold 0.002 corner radii.
 
Please expand on why the grinder would be 2.5 times more accurate in metric. My old imperial dog reads on the dial .0002 on D and a fine feed is .00005 on D.

My initial read isBS, but open to hear facts.
 
Resolution in metric is exactly as good as resolution in imperial.

The machine uses some form of motion control to move bits.
The motion control does not use metric or imperial, it uses units.
The leadscrew does not know what scale you have taped to it - and changing the scale wont improve the leadscrew.

It also wont change the servo driver, servo motor, stepper motor, or the old coot turning the handles.

If using a cnc machine, the units are either perfect or something like 31 digits long, and about 100.000.000 more accurate than the scaling of metric to imperial.
 
It would be interesting to hear how you grind.
Means:
Do you first rough and then finish-grind? How much material do you have to remove? Do you dress between grinding fits? Do you use coolant?

If you have to remove 1 mm from each dimension and grind dimension after dimension without dressing (or correcting) in between, the wheel-wear will ruin your dimensions.
Just my experience with a manual cylindrical grinder.

Nick
 
Resolving the "Accuracy" confusion

juergenwt said:
Are you programing in inch or metric? If you are programing in inch than do it in metric. 2 1/2 times more accurate.
Click to expand...

Not really.

Toms Wheels said:
Please expand on why the grinder would be 2.5 times more accurate in metric. My old imperial dog reads on the dial .0002 on D and a fine feed is .00005 on D.

My initial read isBS, but open to hear facts.
Click to expand...

Tom, your initial read is on target (accurate).

juergenwt said:
Resolution metric .001 = .00004 inch
Resolution inch .ooo1

Here is more info: CNC tip: Which mode is better, inch or metric?
Click to expand...

Now a different word is used "resolution" and is theoretically true.


If you go to the link posted by Juergenwt here is what is actually said:

"you probably don't know about the accuracy advantage of the metric mode. This advantage has to do with the least input increment of the input mode. The least input increment in the inch mode for the vast majority of CNC machines is 0.0001 in. In the metric mode, the least input increment for these machines is 0.001 mm. 0.001 mm is less than half of 0.0001 in (0.001 mm is equivalent to 0.000039 in), meaning your CNC machine will have a much finer resolution when the metric mode is selected.
To get an understanding of this implication, consider a common indexer. A five degree indexer has 72 positions (360 divided by 5). A one degree indexer has 360 positions. Though the one degree indexer is no more accurate than the five degree indexer, you can program it with a finer resolution. You can, of course, index 34 degrees with a one degree indexer and cannot with a five degree indexer. One way to compare this to the inch/metric mode selection is to say that working exclusively in the inch mode when the metric mode is available is like having a one degree indexer but only programming it in five degree increments!
Said another way, a ten inch long linear axis has 100,000 programmable positions in the inch mode. In the metric mode, the same ten inch long axis has over 254,000 programmable positions!"

So those who questioned the idea of increased ACCURACY are correct. There is the potential for greater resolution if your lead screw really is as precise and accurate (gotta be careful with the true meaning of these words) as your theoretical resolution. In the real world YMMV.

Denis
 
Last edited:
juergenwt said:
For anyone interested here is some more info on this subject: Imperial Versus Metric : Modern Machine Shop
Click to expand...

Juergen

Quite unfortunate that the above MMS article does not describe the specific control and drive system, it's encoder resolution, accuracy and repeatability of the control loop.

You see, My Wire EDM is programmable to .00001 increments, but it's loop is only capable of .00002 resolution with a repeatability of .000035.
The ability to program such equipment in metric to the 4th decimal would not make a single bit of difference.
Similarly, the Hardinge Super Precision lathes with the linear encoders can be programmed to the 5th decimal, but they can only resolve to 20 mil. Again, no metric can help there either.

I believe my lathe's Fanuc Oi can resolve to 40 mil with a repeatability of 60mil or so. In that case your 2.5x advantage is immediately reduced to less than 50%.

So, the MMS article and your claim that metric programming is inherently more accurate is a meaningless and .... inaccurate statement without qualifying the control and the drive system's capability.
 
Ok- I'm no grinder wizzard, but my thought was why such a coarse grit wheel- 150 grit. I do a fair amount of under 1 inch dia grinding on an old Norton grinder and use mostly a 60 grit 32A wheel for both rough and finish grinding of carburized 8620., but I'm only going +/-.0002. Red James seems to think the 150 grit is too fine.

I'd like to know more- please educate me.

Dan

On edit- boy I just went back and read what I typed- sure screwed that up- not sure what I was thinking! The wheel I use is a 46 grit, so I'm going to shut up and go away now.
 
Last edited:
I don't care what the size requirement is, you pick the wheel grit and structure (close, medium or wide open) more by the surface finish requirements than size tolerance. A 150 grit on an od grinder is awfully fine, many of them run 46's or 60's with open structure for coolant delivery, 16 rms is pretty easy to achieve. Grinding for tenths everything matters, time especially on sparkouts. I'd be looking at a courser more open wheel and about double your sparkout passes before finish.
 
We had to have our Kellenberger table and wheelhead slide reflaked to minimize stick slip.Only then would it hold a tenth.Holding a tenth requires that you closely monitor part temperature. We actually heat our coolant slightly with an aquarium heater so when you blow off your part it will be closer to room temp.It takes a little experimentation.
 
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