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Size is jumping during cylindrical grind

kgrinde

Plastic
Joined
Jun 7, 2023
Good Morning All,

I was hoping that I could get a few more eyes to look at this problem,

I am grinding a new part. It is a boss that extends from a shoulder and is 0.170" in length and 1.53" in diameter. This is solid carbide, and I am grinding the diameter down to 1.50" +/- 0.0003" (So 0.030" stock). I can attach images for better clarification if requested.

I am using a CNC cylindrical grinder, a single 16" resin bond diamond wheel to get my finish size. My Workholding is -- faceplate, with a live center holding the boss (There is a center hole).

I have verified flatness and squareness of the faceplate, squareness of the headstock, and table... and this is a brand-new live center with essentially no runout.

The dilemma is that I have an inconsistency in my parts without touching anything on the control and just running parts back-to-back. I have gotten 3 parts in tolerance, and then the 4th part will jump out of tolerance and be +0.0015" bigger. This seems like a large jump in size to be "wheel wear". This happens very often, I will grind a part in tolerance, and then run another part without any adjustment and it jumps size and is not in tolerance. Usually within +.002. What is going on here? Any advice or testing would be appreciated, because I am at a loss for what could be causing this.

-KGRINDE
 
You might grind your machine center point to 61* / 62* if you suspect that the parts center is not consistent. Getting a few parts good and then a bad one, I would suspect the part's center is not good...you might blue up 10 parts and rub them on a 60* center.
Plus knowing your part and machine centers are clean..

It would really seem odd to get a .001 error part occasionally from any decent grinder machine.
Expect the run out of the live center to be double on the part. a .0002 error live center is a junk one.
A dead center would be much better.

Keep your wheel flat with the travel direction of the grind, so the following is the high of the wheel, For some parts you go two passes in one direction to one pass in the other direction.. but that is not related to one out of three going bad.
 
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Thirty thousandths grind stock on a one anda half inch diameter ? For me, I wouldn't be too happy if I had to grind ten :(

If you're stuck with it, I'd rough a batch then come back and finish from a more reasonable size.
Good point..and you might run the rough grind with a live center, and the finish grind with a dead.
One scrap part would likely more than equal the time loss
 
Update for you all.

So I ran 10 parts and left 0.005" stock, then came back and did a 0.005" finish grind. I still had an issue on the size jumping around with no consistency, all +/- 0.002". I tried different wheels, and grinding approaches and got nowhere.

Then I decided to follow @michiganbuck advice, and I ground a dead center to 65* because I felt like 61/62* would not be enough. I ran 10 more parts taking off all the stock amount (0.030") and I finally got consistent results... All +/- 0.00005"

So, I think that this validates where the error is coming from, these carbide parts are getting the centerhole put in them before firing, leaving them very inconsistent.

My next question out of curiosity: Is there any improvements that could be made to grinding these? Obviously the part cannot be altered, so the only thing that comes to my mind is different Workholding that does not utilize a dead center... Would a dead center at say 80* be better than 65*?

-KGRINDE
 
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My next question out of curiosity: Is there any improvements that could be made to grinding these? Obviously the part cannot be altered, so the only thing that comes to my mind is different Workholding
You could try using those centers that are radiussed instead of angled .... next step after that is expensive: they make center grinders but even used they are in the tens of thousands. Someone made a cheapskate version called the Cen-T-Lap that was just an abrasive cone, you could try something like that - anything to improve the quality of the center before you ground off it.

No magic mushrooms there but some stuff you could try, if this is an ongoing thing. Regular ol' grinding between centers is pretty predictable after all these years, most likely you're not going to invent any magic new way to do it better.
 
.I used a Norton center lap to clean/restore /relocate the center in steel parts. When you had to go back on a used or abused bearing fit diameter on an abused part the center is a mess and it will take longer to re-locater the center than grinding the part,
Carbide-sintered parts are bugger and you could only use a diamond center hone a few times before it needed regrinding..and likely a harder diamond would be needed to grind the stone, ( I don't know if anyone makes a diamond 60* point mounted wheel.)
Changing the included angle was the easiest way to make a to-size part with a sintered center...but the inspector with a 60* inspection may get a different run-out test.
RE: stock amount (0.030") and I finally got consistent results... All +/- 0.00005"
I'm glad that it worked.
Buck
 
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What is your workholding method at the headstock end? You didn't make that very clear, only said "faceplate." So are you grinding between two centers? Or just a center jammed in the end pinching the part between itself and the faceplate? Or?

Lapping the center hole might be a right bear with the part being carbide, but I'm with Buck, that's what I'd try.
 
You could try using those centers that are radiussed instead of angled .... next step after that is expensive: they make center grinders but even used they are in the tens of thousands. Someone made a cheapskate version called the Cen-T-Lap that was just an abrasive cone, you could try something like that - anything to improve the quality of the center before you ground off it.

No magic mushrooms there but some stuff you could try, if this is an ongoing thing. Regular ol' grinding between centers is pretty predictable after all these years, most likely you're not going to invent any magic new way to do it better.
One just went at auction for $500 last month in Minnesota. 20 years ago dealers listed them at 20 to 30k.
 
Such parts might be better manufactured by targeting 58 or 59 degrees so just a little honing would make a 60* center see a circle...or even with no honing the machine center would see a larger circle to intersect.
 
Good Morning All,

Turns out, I just got a lucky streak (4 parts in a row), because the problem is still happening even with a 65* center. I went to run 30 parts this week, and only 7/30 were in tolerance.

@eKretz My workholding method at the headstock is a flat faceplate... your second statement is correct, I am sandwiching this part against the faceplate with the tailstock center. Attached should be an image of the setup.

1696346899907.png

I am also attaching a portion of the measurements along with the offset (X Offset) to hopefully show that if my part is measured -0.001" then I add 0.001" to my 'X Offset' and the next measurement should reflect -- However, it does not. So, there must be something causing the inconsistency... I assume it is the Center like you all have suggested. But I have had no luck trying to make the center more consistent. I am looking at different Workholding options currently.

1696346500227.png
 
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I am sandwiching this part against the faceplate with the tailstock center. Attached should be an image of the setup.

View attachment 410893
You've got to be kidding ... that's never going to work. Bad idea, back to drawing board.

What is going on here?
The part is moving. Figure out some way to actually hold the workpiece.
 
QT (And this is a brand-new live center with essentially no runout.)
Royal or the like center? with a solid point, not changeable tips..
The center point is not bottoming out.
The part back is flat?
The part is assured to not be turning on its face plate position.
Might you try a face plate surface that would decrease the chance of the part turning?
Might you start each part at the same face plate location radially.. the same degree location?
A smooth carbide face on a faceplate is like a bearing.
*Perhaps a flat rubber washer glued in place on the face plate, A .010 or .015 thick rubber washer may be the right choice.
+- .0003 seems an easy target.
*Might mark parts to see that they are not turning from the mounted spot.
Might cut your infeed rate 20%
Might dwell at +.005 and slow feed the .005.

RE: you might put a wrench on the held part and see how much hand-torque it takes to turn the part. Two fingers is about 15 pounds of pressure. Grinding .03 you may get pretty close to 14lbs.

Plus wheel grit and concentration can help...use the coursest grit your surface finish spec allows.
7/30 that is terrable.

RE: a live center for grinding should be <50 millionths error IMHO
 
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You've got to be kidding ... that's never going to work. Bad idea, back to drawing board.


The part is moving. Figure out some way to actually hold the workpiece.

Thank you for the advice @EmGo -- I am trying to figure out some better workholding.

Just because I am curious and not that experienced, can you explain why it is never going to work / how you know the part is moving?
 
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can you explain why it is never going to work /
Because there is nothing holding the workpiece in position except hope and maybe a little prayer.

Friction on one side and about a .030" point pushing it on the other ? That's not going to hold anything harder than a piece of uhmw ... and even that would move all over.

how you know the part is moving?
You've been measuring, are they the same ? How do you think they get to be different sizes ? The wheel grew .010" by magic ?
 
Because there is nothing holding the workpiece in position except hope and maybe a little prayer.

Friction on one side and about a .030" point pushing it on the other ? That's not going to hold anything harder than a piece of uhmw ... and even that would move all over.


You've been measuring, are they the same ? How do you think they get to be different sizes ? The wheel grew .010" by magic ?
Agree the part is likely turning/moving.

With a .0001 or .0002 error live center that would eat up most of your tolerance, and a little turning or moving, and you're out of spec.

Carbide on a steel face plate is like skates on ice.
The thin rubber washer would/might give some holding force on that side of the part.
 
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Because there is nothing holding the workpiece in position except hope and maybe a little prayer.

Friction on one side and about a .030" point pushing it on the other ? That's not going to hold anything harder than a piece of uhmw ... and even that would move all over.


You've been measuring, are they the same ? How do you think they get to be different sizes ? The wheel grew .010" by magic ?

I am not oblivious to the part moving. I am aware the wheel isn't growing. I just wasn't sure if the part is moving due to the Workholding or due to the poorly made center holes.

@michiganbuck I have changed to a dead center, so there should be no error being introduced because of the center.
 
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I have changed to a dead center, so there should be error being introduced because of the center.
So you're using a dead center to exert enough pressure on the workpiece to try to drive it ?

Like, wow.

Make something like a very low profile 4 jaw that will locate and drive the workpiece, then use the center maybe as a safety because with such a small area to grip you can't get a really good hold, indicate each piece then grind.
 








 
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