MT3 Collet Chuck In lathe - runout

[Ben17484]

My Smart and Brown SAB has an MT3 spindle taper, so I just tried out a collet chuck in it to try holding some work in the collet chuck for the first time.

Out of interest, I wanted to see what the runout was at the collet. I can see no measurable runout on the inside of the spindle itself, but when I measure it in the collet chuck, I get about 0.08mm runout:

I’ve tried with three seperate collet chucks and they all get around the same runout. I’ve double and triple checked that the internal runout of the spindle, and I still see no movement at all:

I’ve checked the spindle runout at various depths to see if there was any burrs on the inside and I can find nothing.

So my question is this - is that runout at the end of the collet chuck expected?


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[WobblyHand]

Seems it ought to be better. Is any surface on the collet chuck true? Does it fully register with the shoulder on your spindle? If it doesn't, that's the problem. The chuck cannot bottom out on any other surface except the spindle register surface.

 

[Ben17484]

Seems it ought to be better. Is any surface on the collet chuck true? Does it fully register with the shoulder on your spindle? If it doesn't, that's the problem. The chuck cannot bottom out on any other surface except the spindle register surface.

I’ve tried with three different collet chucks and they all act the same. I can’t feel any issues on the inside of the spindle, but that’s all I can guess at the moment - somewhere along the inside of the spindle there’s a rough spot of burr that’s not allowing any of the collet chucks to seat properly.


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[WobblyHand]

Can you show a complete picture of the chuck installed on your spindle? The first one doesn't fully show all of your spindle. Take a view like #2, but with the chuck installed. Also a picture of the inside of the chuck, the side which mates to the spindle.

It might be that this kind of chuck is using the flat face of the cylinder end where your spindle threads are. I don't think that is a true reference for the spindle. You could try indicating the surface. For my threaded spindle, only the flat register, which is perpendicular to the spindle axis, is the reference surface. If my back plates register to that surface, they run true. Otherwise they do not run true.

I found a document from Shars that explains how to machine their back plates, and they emphasize not letting the spindle bottom out on anything besides the register surface. Using that bit of information, might suggest your chuck internal threads are not deep enough. It may be that your chucks are made for a shorter spindle thread. If so, then you will need to modify your chucks to fit your spindle. Do not modify your spindle!

This is all conjecture, pending photos and measurements of your spindle and chuck.

 

[markba633csi]

If those are import chucks then they typically have runout- they are made cheaply and the tolerances are loose

 

[WobblyHand]

If those are import chucks then they typically have runout- they are made cheaply and the tolerances are loose

True enough, but perhaps if we know where the problem is, maybe the chuck could be modified. If the spindle nose bottoms in the chuck, perhaps that is something that can be fixed. Still have not heard from the OP if the chuck actually touches and seats on the spindle register.

 

[mksj]

If you have a MT3 dead or live center, try it in the spindle and see if there is any runout. It is hard to measure the MT3 taper deep enough to detect runout if it is slightly off deeper in the spindle. If you tried 3 different collet chucks and they all measure about the same then most likely it is a spindle problem. I am assuming this is an ER chuck, you might try it with a collet and ground pin and then look at the TIR with it indexed at say 90 degree increments. If the runout is always in the same place relative to the spindle then it is a spindle taper issue.

 

[pontiac428]

Are you using a draw bar, or just friction to hold the collet holder in place? You can gently deburr surface nicks with a stone on all mating surfaces and try again. If the collet taper isn't concentric, nothing you chuck up will be, either.

 

[Shotgun]

To add to what @pontiac428 said, put something six inches or so long and reliably round in a collet, and see if the runout increases as you move away from the spindle. The taper inside the spindle could be perfectly round at every station, but still point whatever is in it off to the side.

 

[WobblyHand]

From the pictures, it doesn't appear there is a taper that goes in the spindle. The chuck seems to engage the spindle nose threads. But perhaps that is just my interpretation of the picture.