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Centerless Grinding Adjusting Tips, General and Correcting Hourglass

ClappedOutBport

Cast Iron
Joined
Mar 30, 2016
All,

I need a few tips.

My work is not a machine shop. I am not a professional machinist, I am an engineer, but I do regular machine work at home. Grinding is not part of my skill set.

Anyways, we produce one product we grind in house. The material is a refractory metal with highly inconsistent hardness, so it's not even worth discussing. (Also proprietary) Our parts started coming out hourglass shaped and I need to fix them.

The part is approximately the 3/32 size diameter. We are removing roughly .010 to make a round finished part. There were some obvious problems I found. For starters the regulating wheel dresser was locked up due to varnished oil (mineral oil coolant). We stripped, cleaned and reassembled it and I dressed the regulating wheel, it was very clearly out after 3 years of minimal-marginal use. My questions are:


  1. The regulating wheel dresser can swivel on it's mount both in an approximately horizontal and vertical offset. I understand the horizontal angle should match the swivel angle of the regulating wheel spindle. Does the vertical angle matter much? I leveled it out as best I could, but I only had a painted surface to level to. Or is there another way?
  2. The diamond can be offset. I understand this to be half of the work diameter. Above or below?
  3. The top slide has an angle adjustment. Do I set this to the arctangent of the diameter delta divided by the wheel length? I.e., .095 degrees? Or leave it at zero?
  4. How close does one get the blade to the work wheel? What is the general procedure to setting the top slide vs bottom slide? What is the general infeed procedure for setting up to a new diameter?

Info:

  1. 6" width wheels I believe. Unsure on diameters. Guessing 12 and 8". No data on grit or material.
  2. I believe the machine is an Supertec STS-C 1206 Supertec
  3. Very narrow blade, probably 24 or 30 degrees. The blade holder has been cleaned and readjusted, but I don't know if correctly. Nobody here has formal training.
  4. Regulating wheel angle: currently 2 degrees.
  5. Dresser angle: 0 degrees
  6. Diamond offset: None
  7. Top slide angle: 0 degrees
  8. Mineral oil coolant.
  9. Parts are ~3/32 x 3
  10. Desired stock removal is ~10-15 thou, one pass to finish.
  11. Parts may have a small tit running down the side, up to .005, but this is generally removed beforehand.
  12. Tolerance is .001 on the diameter.

So in the immediate I know I need to:


  1. Adjust the dresser angle to match the spindle angle for the regulating wheel.
  2. Adjust the diamond offset to the part height.
  3. Inspect the blade holder to determine the part height.
  4. Inspect the blade guides for straightness.

I know the wheels, coolant, etc may not be perfect, but we made good parts before. I need to get it back to there and hopefully better. Any advice would be appreciated. Thanks, MB.
 

brucecu

Aluminum
Joined
Aug 3, 2014
Location
New Jersey USA
centerless grinder tips

PM me.
I have a document that may help you. I use it as an introduction for machinists unfamiliar with centerless grinding. It's an overview but takes some of the mystery out of the process eg helps tame the beast.
 

michiganbuck

Diamond
Joined
Jun 28, 2012
Location
Mt Clemens, Michigan 48035
You might pick up a Dedtru to run parts on the surface grinder for a backup.

Not as good or as fast as a dedicated centerless but can easily make your part to .001 and better.
Unison Dedtru centerless grinding attachment for surface grinder, #1 | eBay

But I am not a centerless or Debtru hand so that is about all have on this.

That would be a good sideline for a good retired centerless hand..to go out making service calls.

With a grinder
UNISON DEDTRU CENTERLESS GRINDER MODEL 288, 208 3PH. | eBay

Used Dedtru for sale. Harig equipment & more | Machinio
 

ClappedOutBport

Cast Iron
Joined
Mar 30, 2016
Ok. So I set the wheel dresser angle for the regulator, offset the diamond, and dressed the wheel. I fiddled with the guides (crap adjuster) and did get it cutting back in tolerance and straight. Very good surface finish, indistinguishable from pre-ground rod. I think we still need some adjustments, but it's close for now. Looks like we'll need a finish pass for best quality.

One thing I noticed is that most all of the cutting happens in the first 1/2" of the wheel. From what I've seen, that is normal?
 

tdmidget

Diamond
Joined
Aug 13, 2005
Location
Tucson AZ
That is pretty common. It is really in the dresser profile. As long as the finish is good and no burning it's not a problem. If that occurs then do it in 2 passes. Now that you are getting good parts make any further adjustments small ones and one at a time so you can undo it if goes the wrong way.
 

ClappedOutBport

Cast Iron
Joined
Mar 30, 2016
That is pretty common. It is really in the dresser profile. As long as the finish is good and no burning it's not a problem. If that occurs then do it in 2 passes. Now that you are getting good parts make any further adjustments small ones and one at a time so you can undo it if goes the wrong way.

Cheers. Thanks for the advice. I think 2 passes is a great idea, that way they will all be consistent for the finish pass, as they do not start consistent in shape.

I dressed an ~6 thou lead in on the regulating wheel. I guess better the regulating wheel than the cutting wheel? But it seemed to help cut the violent start and get them to feed in consistently.

Project manager was gung ho on replaceing the machine with a new CNC one the moment it went down. I tried to convince him that that would be a lot more down time than just fixing what we've got. Seems like he understands better now. My question is, we have two diameters we need to make that are probably different enough to change the blade out and fiddle with all the guides. Where we're at in production, we need to flow these parts steadily, we can't let the rods build up before grinding them, so we need to switch between the two sizes commonly. I am thinking a second machine, even though they will both sit idle for a lot of the time just to avoid the constant changeout and redadjustment of the guides, and the scrap hazard that comes with that. Then again, with two blade holders, it wouldn't be too bad.

Thoughts on those that have faced that dilemma?
 

tdmidget

Diamond
Joined
Aug 13, 2005
Location
Tucson AZ
What are the 2 diameters? Odds are that if not drastically different you might only have to change the workrest height.
 

ClappedOutBport

Cast Iron
Joined
Mar 30, 2016
What are the 2 diameters? Odds are that if not drastically different you might only have to change the workrest height.


.080 and .145. Doesn't seem significant. Parts are still coming out good, quality is happy and so are the machinists. I think I will average out the dresser height someday when I have downtime but it doesn't seem to matter a lot.

I know the grab is still a bit rough and the leading edge of the wheel wears a good bit. I read this in the manual:

9-3 Rotary Base
The rotary base is attached above the sliding bed and serves to adjust the contact
Surface amount between grinding wheel and workpiece. While performing the thrufeed
grinding operation, the rotary base may be rotated in the clockwise or counter clockwise
direction for 5° while the contact surface is larger, the grinding roughness is better.

Which seems to agree with my original theory. I guess I should just try it.
 

bigbert74

Plastic
Joined
Aug 5, 2016
Hourglass

What type of grinder do you have? I would definitely try the rotating/swivel adjustment a little at a time and try to reference your starting point so you can get back there if needed
 

ClappedOutBport

Cast Iron
Joined
Mar 30, 2016
What type of grinder do you have? I would definitely try the rotating/swivel adjustment a little at a time and try to reference your starting point so you can get back there if needed

Supertec STS-C 1206

I did try it. I finally measured it, and it was at 7 to 8 thou which is now approximately our stock removal. I adjusted it to more to try to reduce pressure as we were having some breakage with the fragile material we were running through it. I ended up finding 7-8 thou was idea and the parts are running really smoothly now. It could probably be a bit more optimized, but with the hour of work it gets every day it's fine. Breakage is now really low, the grinding action is very smooth and even across the entire wheel.
 

ClappedOutBport

Cast Iron
Joined
Mar 30, 2016
I ran the machine for about 5 hours today on specialty steel tubes, about 1/2". I have learned a lot in that time. For one, this machine works tremendously better on this size range. The guides are very easy to adjust, the 15hp motor makes more sense, the servo speed is a godsend, etc.

As for the wheel angle or "funnel" as some call it, I find it should be just over one's stock removal. So if your stock removal is 5 thou per pass, your funnel should be maybe 5.5 or 5.25. Then it grabs right past the start and you can even get a feel for wheel wear by how far the rod goes in before it starts.

Just had a major spindle service on the regulating wheel. Machine is running good. I still need to balance the main wheel as nobody knew to do that when it was mounted. But I'm holding good tolerances and no more than a tenth taper.

I guess that's my advise to anyone new. Start with a medium size, and get some "seat" time.
 








 
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