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Nakamura TMC-15 Cutting out of round


navsENG
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OK, so I got the new/old nakamura running. Everything seems to work pretty good , but on my initial test part I saw the parts seemed to be about .0007-.001" out of round.. I thought it was odd, and did more testing on it tonight. Swapped the chuck for a collet nose. Tried two different size collets and material (aluminum, stainless) and they all seem to repeat this out of round.

Only had the stock sticking out 1.5" or so from the collet. 1" diameter  stock 

One other thing I noticed is that I could tell where the high spot would be on every part by the orientation of the lettering on the collet ( not rotating the collet between test parts, just putting a new one in). That would lead me to believe it is something related to the spindle ?? I just cannot see how this is possible. It sounds good, feels nice and smooth, makes a pretty good cut..

Any ideas? I indicated the register diameter on the spindle and the face and it only seems to be maybe .0001-.0002" (didnt have my .0001" indicator with me)?...

 

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Put a 1" known round bar in the collet....stick it out6-8 inches.....

Indicate the free end, indicate the end closest to the spindle nose.......note the readings

with the indicator on the bar, nearest the spindle nose, grab the free end of the bar and apply pressure...

Against the collet nose, you should not get any movement.....if you do, the rear spindle bearings might be toast...it'll be time to dig deeper

 

Really, the only way a lathe should even be able to cut an out of round condition  as you describe is  a bad set of bearings..

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I have seen lathes that have suffered a hard crash into a stationary spindle produce an out of round condition like this.

The impact damages the spindle bearings in one spot, that may be hard to detect when spinning the spindle by hand.

You can chuck something you know is round and indicate it to see if there is a deviation that corresponds to the

out of round condition your are seeing on turned diameters

I'm guessing you will need some new spindle bearing at the very least. Hopefully, the spindle itself is not damaged.

You may get lucky and find that it is only the ID wear surface of the collet nose that is damaged, which is a much cheaper

repair.

edit… a cracked collet chuck can also cause this condition

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It is a 1" solid 303 stainless bar sticking out maybe 2" from the collet nose. I have tried with 2 different collets and the chuck and seeing the same results..

I think we definitely will need to change the bearings, im not sure loose bearings would cause this issue. Hell my 1930s southbend lathe is far from tight and it makes a rounder part.... lol

I believe G code may be right in the one area of the bearing race is damaged or deformed which causes the spindle to "bump" everytime around... 

 

Has anyone ever replaced bearings on a machine like this ? 

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1 hour ago, navsENG said:

It is a 1" solid 303 stainless bar sticking out maybe 2" from the collet nose. I have tried with 2 different collets and the chuck and seeing the same results..

I think we definitely will need to change the bearings, im not sure loose bearings would cause this issue. Hell my 1930s southbend lathe is far from tight and it makes a rounder part.... lol

I believe G code may be right in the one area of the bearing race is damaged or deformed which causes the spindle to "bump" everytime around... 

 

Has anyone ever replaced bearings on a machine like this ? 

Nope and I would never attempt to. I call in the experts for situations like this. One wrong preload setting or not broken in correctly and you worse off than when you started.

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