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Cylindrical Grinder problems

Dani j

Plastic
Joined
Aug 18, 2022
Hello this is my first post on here. Wondering if you fabulous people could help me out. I work on a Robbie cylindrical grinder and recently I have started to get 2 thou run out in the centre of jobs. The ends are dimensionally parallel. It's just this 2 thou smaller section in the middle.
I have thought it could be push off pressure but it's only stainless so normally it's not that much push off maybe half thou to 1 thou.
So my question is to would you know where this random 2 thou might be coming from?
Thanks in advance
Dani
 
Two elements that can cause a wobble are part center quality and machine center quality. The first test might be to Hone lap your part centers and run a test part with dead machine centers, also inspect machine centers to be true round.
A live tail center should run-out less than .0002 when loaded.
These are not a solution but first check items.
I ran an older Od grinder that changing the wheel height caused a concave or convex part size.

Heat growth can be a factor as the part center often gains the most heat and so when warm takes more stock. The remedy for that can be a more open wheel. Test: If you let the part cool for 1/2 hour and then come back to the part for a .0002 take to find only a spark at the part ends then heat may be your problem.

* You should add your location.
Bargain center lap $10 (I use and prefer Norton center laps)

Norton: $16.
* https://www.mscdirect.com/product/details/43069749
 
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I have started to get 2 thou run out in the centre of jobs. The ends are dimensionally parallel. It's just this 2 thou smaller section in the middle.
Please clarify. Is the part hourglass shaped with a smaller diameter in the middle, or is it bowed with eccentric runout in the middle?
 
Yeah, I'm not sure what we're talking about here - runout or hourglass-shaped part. We need significantly more information to be of any help. Part diameter, length, etc. It won't be tool pressure causing the problem if it's smaller in the middle. If that was an issue it would be bigger in the middle.

Next: are you running dead centers preferably? If it's an hourglass shape you're getting, that can come about from excess pressure on the centers - either initially or as the part heats up. Most cylindrical grinders have spring-loaded centers for this reason. It could of course also be way wear, but if the parts had recently been grinding straight and now out of the blue you are getting hourglass, I'd be taking a close look at the pressure on the centers.

And Buck, the OP is in the UK.
 
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I am not holding between centres all the time but still get this issue.
I have recently ground jobs with no issues. It's just recently. I have been having it on 1.5inch dia shafts and .5 inch dia shafts that are about 24 inch long from chuck to tail stock roughly.
It grinds concentric it's just the centre is 2thou small where the ends measure the same generally always bigger.

I appreciate all your advice and help so much I got taught all I know about grinding from a guy who taught him self so any new information is much appreciated
 
You should be grinding between centers if possible, chucks can easily put the work in a bind and cause the same issue as centers that are too tight, bowing the workpiece - especially at that length: diameter ratio.
 
QT Op: (I have started to get 2 thou run out in the center of jobs.)
Does that imply that in the past it did grind straight?
Have you changed the wheel, material, or process?
I think that the wheel might need changing as when you touch on a job, my last 3 for example have been relatively similar in length but it only touches on in centre first not on ends so I have to fiddle around with it to get it to cut whole job like usual but alot more but still leaves 2thou smaller size in middle. But process is same.

As of today I have checked chuck and cleaned and greased all jaws and put a new center in and still getting same result .
 
I'm leaning heavy with michiganbuck;
heat distortion in the middle section, and possibly a loaded abrasive wheel adding even more heat potential
 
I'll take a different direction. How is the waylube on the table? If its dry, you need to pull the table to clean it and fix the waylube.
 
.5 and even 1 1/2 x 24" long are pretty flimsy parts being that long to the diameter.

eKretz's point about chuck stressing the part is very valid. Dressing the wheel to reduce heat and pushing away and deflection is important. Just push the parked part with your thumb under an indicator will show how easy it can deflect. Between centers would be best, but still, you may need to rig up a steady to stop push-away and downward deflection. The part should be turning fairly slow and you might use as course a wheel that will give a sufficient surface finish. Coolant can be an aid.​


I have used (round nose) spring calipers set at the high size limit to measure the size/diameter with the part running to select grind the high places, and as last resort used crocs paper to finish
 
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.5 and even 1 1/2 x 24" long are pretty flimsy parts being that long to the diameter.

eKretz's point about chuck stressing the part is very valid. Dressing the wheel to reduce heat and pushing away and deflection is important. Just push the parked part with your thumb under an indicator will show how easy it can deflect. Between centers would be best, but still, you may need to rig up a steady to stop push-away and downward deflection. The part should be turning fairly slow and you might use as course a wheel that will give a sufficient surface finish. Coolant can be an aid.​


I have used (round nose) spring calipers set at the high size limit to measure the size/diameter with the part running to select grind the high places, and as last resort used crocs paper to finish
I have been dressing wheel quite a lot and don't seem to make much difference. Most of the jobs I do have a big head part that sits in the chuck but there's a shaft that's comes out of that that is the 1inch to 1 to 1 1/2
The smaller ones tend to be general size all way along .
I've never had a problem in past it just seems to be recently. Coolant doesn't look great so I might see if I can get that changed next week.

Waylube??;

If that's the lube on the bed ways that's of a pumps system that I do 1 time a day.

Your all very helpful by the way
 
Might you fixture the part between the chuck and the tail, and then release the tail's holding while that area is under an indicator? that will tell if the chuck end is putting a strain on the part
If the big end is machined in another operation it may not be running with the same axial center line as the entire part, If the chuck is putting a strain on the part, again the axial centerline of the big end and the turning may be different.
You may have to tell the entire operation.
 
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Simply pumping the waylube once/day isn't enough. On a regular basis you need to feel the ways (both side of the vee and the flat) to ensure the oil is getting to the ways. They shouldn't be dripping wet, but a wipe of your finger should leave a slight film of CLEAN oil on your finger.

If you have to hold the head end of the part in a chuck, make sure you are grabbing the part as short as possible. You should be able to hold the part well enough with 1/8" of contact. You can use a spider to space the part off the back of the chuck.
 
We are just tossing out bandages not knowing the whole story.

Another bandage: is the wheel full diameter so running at proper SFM?
Is it the best wheel for the part/job?.
Are the part and machine centers good?
Does the long travel check near zero with indicator to a test bar held between centers?
Is the big end indicating close to zero?
Is the part-turning RPM reasonable so part weight and balance do not fudge the job.?
Have the wheel spindle and work head been checked lately?
How much are the take-stock in-feeds?
Are the take-stock feeds done at the out-end, at the center, or both?
Can the entire part be run between centers, or off a sacrifice stub at the big end?
 
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I guess step 1 if I were fighting this issue would be to drop an indicator on that part after it's ground to cleanup (making sure the indicator is registering exactly where the wheel contacts the part - which should automatically be dead center on a cylindrical grinder, they don't usually adjust for wheelhead height) and see if it reads zero all the way down the machine travel. If not, you probably have some sort of deflection going on. If so, you likely have a worn machine.
 
Ops; In my post#18 add, how much dwell do you have at the change of direction and at the end of the part?.. Agree that .002 would be a crazy much for dwell considering spark-out but might as well consider that also.
 








 
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