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New to centerless grinding, need some help please.

Joined
Nov 3, 2017
Hi everyone,

I recently got a new job and I have been working on a Cincinnati centerless grinder. It's the 330-10z-xg.

I have been training with an individual who is not familiar with this machine but he has worked with an older manual centerlessachine for years.

We are having a few issues and would love some advice if anyone has some for us.

A little background:. We manufacture punch and die mainly using the centerless to remove about 0.010" after the material returns from heat-treatment. We primarily use M4 material and the diameters range from .25-1.25". With exceptions of larger parts of rare occasions.

We are using a Norton wheel it's the
55a80-lvs3 it's 10" wide and 24" diameter. Our machine runs at 1049 rpm. Our parts generally have tolerances of +.0002-+.0005 but often times it's is tighter than this.

First issue. The machine grinding wheel slows down when grinding larger diameters. The longer the part the worse it is. Just doesn't seem to be able to handle the load. Maybe we are feeding to fast or our wheel is too hard I. Not sure. Any thoughts on this? I can provide all feedrates if needed.

Second issue I am having is with the parts. Finish is good but I get a higher mid compare to the ends. No idea how to fix this. Example. We are grinding a punch length is 5" with a diameter of .500". Tolerance on size is +.0002 +.0005. I can get the ends of the part within half a tenth apart but the middle will be larger sometime as much as .0002"

What would cause this? The larger parts show this more. Shorter parts are easier to maintain size but still issue is there.

Thanks!!
 
What speed is your regulating wheel at? The barrel shape sounds as though your guides might be a little close. I assume that your regulating wheel is dressed properly. This is a through feed grind, right? How much are you taking off in a pass?
 
Hi,

This is one of the many things I am unsure about.

1. Regulating wheel speed should run at what? or rather what rpm is ideal for the part to be rotating at? like 400rpm or like 800rpm? We run a 14" diameter regulating wheel that for a 1/2" diameter I would probably run at about 20rpm. If I am doing the math right that would equal to about 560rpm for the work piece.
2. this is not through feed, there is a head on the punches. we use the blade as a stop.
3. I apologize but I am not familiar what the guides your referring too. If you mean the V shape guides for the work piece we don't use that. We have a blade with and angle on it.
4. the cycle time is roughly 28-32 seconds to remove approx .008"-.010". then we put that part in to remove that last .001-.002" to finish the part. For roughing we use a feed rate of about .025-.035 in/min and finishing is about .015-.020

we have the angle of inclination on the machine set to approx 1/4 degree and the angle on the truing unit for the regulating wheel at about 1/4 degree to match. there is no step over adjustment on the diamond on this unit.

please comment, thanks.
 
So you are doing infeed or plunge grinding like the bottom one in the picture. http://www.acegrinding.com/images/art4.jpg
I would put the reg. wheel and the reg wheel dresser as close to zero angle as possible. You will need just enough angle on the reg wheel for the part to infeed up to the stop. If you don't have enough angle the part will work out or away from the stop.
 
Ahhh, got it. You would not have guides on end feed grinding. If your set over and inclination match then you are good there. The most likely cause in my guestimation is the dresser. It should follow a cam that, in your case should be perfectly straight. It is subject to wear and I would bet that is the problem. I don't know of any way to check it other than remove it and check on a surface plate. If you don't have the capacity to regrind it there are several companies that specialize in centerless service and repair that can probably do an exchange.
 
So you are doing infeed or plunge grinding like the bottom one in the picture. http://www.acegrinding.com/images/art4.jpg
I would put the reg. wheel and the reg wheel dresser as close to zero angle as possible. You will need just enough angle on the reg wheel for the part to infeed up to the stop. If you don't have enough angle the part will work out or away from the stop.

The bottom picture, I guess that's called plunge. Yeah, we did adjust the andle just enough to indeed to the stop which it does.

Ahhh, got it. You would not have guides on end feed grinding. If your set over and inclination match then you are good there. The most likely cause in my guestimation is the dresser. It should follow a cam that, in your case should be perfectly straight. It is subject to wear and I would bet that is the problem. I don't know of any way to check it other than remove it and check on a surface plate. If you don't have the capacity to regrind it there are several companies that specialize in centerless service and repair that can probably do an exchange.

I will look into the dresser on the grinding wheel and see what's up there.

Any thoughts on why the rooms are dropping during the cycle. Getting some burn marks on the part which causes us to redress the wheel. But it's my opinion that we are dressing the wheel more often then we should be. Maybe I am just misinformed.
 
So you are doing infeed or plunge grinding like the bottom one in the picture. http://www.acegrinding.com/images/art4.jpg
I would put the reg. wheel and the reg wheel dresser as close to zero angle as possible. You will need just enough angle on the reg wheel for the part to infeed up to the stop. If you don't have enough angle the part will work out or away from the stop.

It's plunge grinding. I can check on the dressing unit for the grinding wheel. It's on rails. If the rails check out to be straight then what else should I check?

As far as the regulating wheel slowing down, any thoughts on that?

Thanks
 
...
Any thoughts on why the rooms are dropping during the cycle. Getting some burn marks on the part which causes us to redress the wheel. But it's my opinion that we are dressing the wheel more often then we should be. Maybe I am just misinformed.

Are you sure that the diamond(s) are in good shape? If not, you wouln't end up with a well-dressed wheel, therefore the issues typical of a wheel begging to be dressed.

Paolo
 
What speed is your regulating wheel running at? What speeds are available? The 80 grit wheel will be susceptable to burning, especially on a heavy cut. How is your coolant flow? A part that long and that much to remove will need a LOT of coolant and at the right place. Any chance of pics or a video of a part or two running?
 
The diamond is good, we checked.

Regulating wheel speed is adjustable based on the size parts we running. Aiming to achieve around 400rpm for the workpiece. Generally we are 15-30rpm at the regulating wheel.

Thoughts?
 
First off M4 probably sucks to grind if it is anything like M2. Like tdmidget said use lots of coolant.

Next if your dressing cams are good then how are you dressing the reg wheel? Are you using Coolant on the reg wheel to dress? Are you dressing at the fastest speed the reg wheel will spin? How much are you taking a pass? .001" or so. How much are you taking per pass on the grinding wheel? .001" or so? Also use some way oil on the cams to make sure it slides good and there is no sticking.

Here is what I do. I run the highest rpm I can when I dress the reg wheel. I use coolant to dress with. I take about .0005" per pass. I dress the grinding wheel about .0002-.0003". When taking light dressing cuts you have to increase the traverse speed. Give this a try.

Now for the technical part. You should dress at and set the reg wheel inclination to 0. What you do when you dress the wheel at lets say 1/4 degree is you create a hyperboloid curve on the reg wheel. Take a look. Hyperboloid Structures in GSA | Oasys Software Blog
So if you are not perfect on the correct centerline you start to go up the curve which gives you the barrel shape you are talking about.
There are calculations for this but it is way easier to set things to zero. Let us know what you come up with.
 
If his setover and inclination match exactly then it should not produce the barrel shape. But are they exactly the same? At zero I don't see the wheel pulling the part to a stop. I wonder too about coolant placement. If his nozzle does not distribute coolant equally along the length of the part then the ends may be warming up and thus ground more than the center. When cool this will produce a barrel shape. I sympathize with him. I was thrown into centerless by a guy who knew little but had manged to make decent parts. He convinced management that he was the be all and end all of centerless. He could not answer a simple question. I got help here on PM . Payback time. Centerless seems like B S to a noob because there is so much to check and adjust.
 
If everything is set to zero the part will not pull in but it will not move out either. Coolant placement plays a big role in sizing just like you said.
 
Never ground with a CNC. One place I worked wanted to grind the balls for self aligning rod end type bearings. Some required .0001 sphericity and single digit finishes. The only builders who offered were Kellenberger and Studer. They bought a Studer OD grinder, right at 200 large. The dresser was 19 more on top of that. It would actually do it with ease. I lived in fear of being the first one to fuck that up. I never did. You don't get many chances to run something that nice.
 
Did you find any solution to the problem? Please follow up so we all can learn. I don't know about the rest of you but I like to help people find the solution to the problem because I learn a lot also.
 
Sorry for the delay but I literally found the issue earlier today.

So just a recap. I was getting barrel shape occuring with my parts. This means I have low ends to my part with approx .0002" high spots in the middle.

I finally found out that the angle set of my dresser for the regulating wheel was too much. I believe the scale used is incorrect and I don't think I was actually at the 1/4 degree but rather probably a fair bit more.

I test my suspicions but first further increasing the angle up to 1 degree and ran some parts. Sure enough the barrel effect of my parts got worse. I then reduced this angle to the point where my parts came out straight.

I will run it this way for a while and see how things go but I think this is the fix. The regulating wheel was dressed with too much of a concave shape leaving not enough pressure on the workpiece during the plunge.
 
Glad you found it. Like I said earlier you need to get the dresser closer to zero but I thought you would need to set the Reg Wheel angle more towards zero also. My Monza and Estarta grinder don't have a separate dresser setting like Cincinnati's. That is why I like the Monza and Estarta better. HAHA
 
Thanks for getting back to us , so many don't.
Donovan, if the regulating wheel is at zero it won't pull the part against the stop.
 








 
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