Cylindrical Grinder problems

[Dani j]

The dwell of change of direction either side is 3 to 5 seconds.
The wheel is a multi purpose one that is used for the variety of material I do. So probably not the best.
The wheel is probably smaller then full diameter as I know it probably due changing soon.
Sorry I'm not used to some of the words you use Michiganbuck.
What is take stock in feeds?
And is spark out the last few finishing passes it does before it rapids to safety zone.
Thanks for your help
Dani

 

[Richard King]

I see a lot of guesses. How about doing some tests and be a detective and figure this out. I have never heard of the brand of grinder you have, can you post some photo's, the complete machine and the part in the machine, the head-stock and tail-stocks. Is it hand feed or hydraulic, what sort of lube system does it have, pressurized or roll up wheels. Have you recently taken the table off? Did you do something just before this issue started? What grit wheel are you using, are you using coolant? How does the headstock get oil? Put new V-Belts on the wheel head? Do you have a precision test bar to check between centers and indicate to see if the bed ways are the issue? If you don't, they sell some really good Indian made test bars on eBay. You can also buy test bars that fit into the head and tail stock tapers to see if the head and tail stocks are aligned. Lets start to eliminate some issues to find the problem.

 

[Dani j]

To be honest with you Richard I was taught by someone who taught himself who didn't have alot of engineering background. I do however but when it comes to Grinders that where my knowledge is a little shy. From what I can see it's a
Robbie o micron 2000 Pt6
It's only got a 4 jaw attached to headstock
The tailstock is air pressured
The oil system is all interconnected. Haven't touched table or anything the problem happened like overnight sort of thing.
I have discovered it can be on any job I put in the chuck.
I can't always put jobs between centres due to nature of what we do. And when I do I still get same problem.
Haven't put new v belts in.
Haven't got a test bar.
The coolant I do think is iffy is a brown orange colour.
I don't know what grit wheel is.
I will have to sort pics out of later.
Thanks Dani

 

[eKretz]

A dial indicator run on a part you've ground to full cleanup and sparked out will tell you plenty without a test bar as long as you can accurately measure diameter. I'd start there.

Without doing some testing, Richard is right; all we can do is guess. The fact that the problem is now repetitive on many different parts tends to rule out coolant and such as a likely cause. Is this machine manually controlled or automatic? What is the depth of cut at finish? And spark out refers to traversing the wheel along the part and letting it grind all remaining tool pressure out until the wheel "sparks out" - in other words, until the wheel is no longer grinding any significant amount of material away. Is this occuring?

Also, what is the air pressure on the tailstock when it's engaged? Is there hopefully a pressure gauge? I'll say again: You should not have a whole lot of pressure on the tailstock on a thin long part.

 

[gbent]

After you chuck your part, put an indicator on the tailstock end and release the tailstock. Does the tailstock end want to move? What you are describing absolutely will occur with a part chucked rigid in a headstock when the headstock is turned into the wheelhead, even slightly.

 

[michiganbuck]

After you chuck your part, put an indicator on the tailstock end and release the tailstock. Does the tailstock end want to move? What you are describing absolutely will occur with a part chucked rigid in a headstock when the headstock is turned into the wheelhead, even slightly.

Agree, it is the tail's purpose to pull to center and stabilize the out end of the part. The extent of that force should be noted to consider if that action is great enough to bend or distort a part. Some parts can be loosened and re-tightened at the head end to better align a part, or set on the tail and then tightened the head end, and as Eric mentioned some can be held in a wire or like to let the head end find center with lowered stress.

Again, it may not be the head-induced stress causing the problem, there are such a number of factors. We are just shooting out guesses with not knowing enough about the machine, wheel, part, and process.

Eric mentioned spark-out.. What happens if the near-finished part is allowed to completely cool and then a come-to-the-part for a .0002 stock removal, how and where does that slight grind occur?

 

[Richard King]

You have a large machine. Have you checked the bed and then the bed travel with a precision level? A machine that long and if the machine has changed that could grind the part smaller in the center. It sounds as if the coolant is full of rust if it is brown. Normal coolants are white or pink or green. Also have you dressed the wheel...with a sharp diamond? https://grinding-machines-robbi.com/semi-automatic-grinding-machine-robbi-omicron-pt/

 

[Richard King]

How old is your machine? Is it still under warrantee? Is there a factory technician you can call? Do you have the original maintenance and operators manual? If not order one.
file:///C:/Users/richa/Downloads/Omicron_T6_Rid_00_En.pdf

 

[Dani j]

Machine is quite old so I would say out of warrenty. It's been there longer then I have.
I have put a dial test indicator on the face of the chuck and that's still running true to the head of the grinder.
Yes when you put the tailstock centre in the job does move. The amount of movement varies from job to job.
Unfortunately there isn't anyone I can call hence why I have tried this way and you lovely people have been amazing
Wheel sparks out after 15 passes
Wheel delay and each side is 3 seconds
Per cut per pass is 4 microns roughing and 3 microns finishing cut
The machine is mainly manual but I put the cuts on in automatic in the max of 3 thou so -75 .
The airpressure on tailstock is on the lowest setting I can get it as to me I don't know if it has a pressure that is a set that you can't go below.
I've got the operators manual I've read through it but it didn't really help me much.

 

[michiganbuck]

Reaching for straws and likely not the culprit is the travel direction of the dress.
As strange as it may seem some grinders have a prefeed dress direction and can grind adversely with dressing from or to a certain dress direction.

Rarely would this have a .002 effect, usually a few millionths or variance in surface finish.

 

[eKretz]

Machine is quite old so I would say out of warrenty. It's been there longer then I have.
I have put a dial test indicator on the face of the chuck and that's still running true to the head of the grinder.
Yes when you put the tailstock centre in the job does move. The amount of movement varies from job to job.
Unfortunately there isn't anyone I can call hence why I have tried this way and you lovely people have been amazing
Wheel sparks out after 15 passes
Wheel delay and each side is 3 seconds
Per cut per pass is 4 microns roughing and 3 microns finishing cut
The machine is mainly manual but I put the cuts on in automatic in the max of 3 thou so -75 .
The airpressure on tailstock is on the lowest setting I can get it as to me I don't know if it has a pressure that is a set that you can't go below.
I've got the operators manual I've read through it but it didn't really help me much.

The work shouldn't move when you put the center in - that was Glen's point when he mentioned it, and the same "bind" I was talking about earlier also. If the end of the work is moving when you put the center in, you are creating a bow in the part. That's almost certainly where your taper is coming from. You'll need to check your grinding wheelhead alignment as well as runout of the part at the tailstock end while it's held only in the chuck. Both can cause trouble like you're describing having.

 

[michiganbuck]

list of job process aspects ;
Has this part run correctly/better in the recent past?
Wheel grit and grade
Part material and hardness
Part turning RPM
Part big end indicator run-out amount? Is that the same part after part or varies?
Running an indicator on the finished part shows the same error as a micrometer check(1/2 of the mic check.
Is the undersize a long tapering or a quick drop of size? about one wheel width?
Is the part middle dwell the same as at the end of the part?
Is the wheel going off the part at the small end?
Is the dressing diamond short shanked, 3/8" or bigger dia, and turned to a fresh facet?
Can the middle dwell be shortened to 1 second/less, or step stopped a few times?
Are the in-feeds done in middle, at the out end, at random, or both?
Part centers honed with a point-mounted center hone? does a two-finger push on the head or tail center show a change?
Does a one-hand push-pull (elbow swing not shoulder push) on the wheel, and a right-left push show an indicator change? more one way? (that's about 20 to 35 LB force.)
If the tail is live, how much run-out when loaded?
Is the wheel tight?
Is there any machine shifting/wobble at the end of travel?
Is the machine level and solid setting.?
Has the change direction of long feed direction gotten harsher?
If oiled ways, are they all wet with oil?
What are the part tolerances (size/run out) on the small diameter, big diameter?
Is the big end chuck held in a 3jaw, 4 or other /is it indicared in radialiy.
Is it possible to run an entire part skim in one opration?

General stuff
(all grinder wheels should have a sharpie mark so quick to know/see if it turned on the mount)
Wheel diameter can get too small so SFPM is low..
Belts can get loose enough to change grinding.
*Bumping to a change like the part corner/step radius with some wheel load at a change diameter can cause some chatter to make undersize, so one more possible cause.

 

[Richard King]

I would start from the beginning and check the machine level. Use a precision level to compare the travel of the table. With the table centered, set a .000"/12" per division level on top of the table, Shim under one end if needed to get the level vial bubble sitting in the middle, then move the table to the left in a slow feed rate and watch the bubble. Make up a sketch and write down every 12" of travel. Then repeat it from middle to right end. Or go from one end to the other. The bed in the machine I found and linked too has a long bed and the bubble should not move. If the bed is off .001" on the bubble or it reads higher on the wheel head side in the middle, that's your problem. Also order an air pressure gage and plumb it into the system. You can't guess. The relief valve on the pressure regulator could be stuck. We can lead you to water, but you have to take the drink.

All machines need to be re-leveled after a while as floors settle. Where did you say you were located again?

 

[michiganbuck]

Dani j,
I hope my post #32 is not overwhelming but those are the many things you have to consider, likely even more. A part error of .002 is a mile in grinding. A good chance that running the whole part between centers, or leaving a stub (to hold) on the big end to be cut off might solve your problem.

This is so the part can be entirely ground with having no stress

Do let us know what solves the problem.
Buck

 

[Dani j]

Cheers for all your responses I'm just trialing a few things at the moment. Stil with little success I appreciate all your help

 

[Dani j]

I think I might have sorted the problem.
I found out after the tailstock centre decided overnight that it wouldn't line up with any job anymore. So time to start having a fiddle. We discovered that the head was 39 thou out to the part I had in there that was about 24 inches maybe from chuck to tailstock. So actually I realized after wow how did I get on without too many problems for so long.
But also discover one of the head nuts are snapped off so the heads been held by 1 bolt which probably isn't ideal but I'm going to talk to my work and see what we can do to fix that.

But blimey I've never ground anything this straight.
See it's knowing the little things about the machine and know I'm knowing this it is helping you guys have been amazing to thankyou so much

 

[michiganbuck]

Wow, we should have stated that ...to line up the head and tail.

Shame on us. Good that you found the problems.

Still, it was a fun thread that stuck to the subject, with everybody trying to share.

 

[eKretz]

Hey now, I did state that - post #31. Well, kinda'... Just looked back. Meant to say workhead, not wheelhead. Oopsie. The best laid plans of mice and men...

 

[Richard King]

I recall stating go back to basics and level the machine. Pull out the manual and start reading it.