New Chuck, excessive runout

AR1911

AR1911

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I recently bought a new chuck and backing plate for my SB9A. It's a 6" 3-jaw, with 2-pc jaws, from Shars.
The specs online and in the included inspection sheet show TIR to be .003" maximum.
Mine reads .008" right out of the box.

So this week I did the following to try to remedy that:

Checked the spindle runout at the register surface. .0002"
Deburred the backing plate, then trued the mounting surface on the lathe. Checked it after dismounting and remounting, zero runout
Disassembled the chuck. Cleaned, deburred all parts, lubed,
Reassembled carefully. Much smoother, like a swisswatch.
Still .008" runout.

I walked over to my much-abused and oft-used 10" Enco. This is a 28-year old lathe with original 8" chuck.
.003" runout.

The only thing left that I know of is truing the jaws with a toolpost-mounted grinder.
I really don't want to do that.

I have not yet contacted the seller because I really don't want to go through the hassle of returning, especially if I am going to wnd up with the same issue on the replacement. I'd like to hear from anyone else who has run into this and how you resolved it.

Any suggestions welcome
 
Try checking RO with different diameter proof bars. This will tell you if it's a scroll problem.

Disassemble the chuck and carefully clean and debur the jaws and the slots/grooves they ride in. If there's crud in the scroll, it could throw RO off considerably.

Make sure your spindle threads are clean and the shoulder is making square contact with corresponding shoulder in the backplate.




AR1911 said:
I recently bought a new chuck and backing plate for my SB9A. It's a 6" 3-jaw, with 2-pc jaws, from Shars.
The specs online and in the included inspection sheet show TIR to be .003" maximum.
Mine reads .008" right out of the box.

So this week I did the following to try to remedy that:

Checked the spindle runout at the register surface. .0002"
Deburred the backing plate, then trued the mounting surface on the lathe. Checked it after dismounting and remounting, zero runout
Disassembled the chuck. Cleaned, deburred all parts, lubed,
Reassembled carefully. Much smoother, like a swisswatch.
Still .008" runout.

I walked over to my much-abused and oft-used 10" Enco. This is a 28-year old lathe with original 8" chuck.
.003" runout.

The only thing left that I know of is truing the jaws with a toolpost-mounted grinder.
I really don't want to do that.

I have not yet contacted the seller because I really don't want to go through the hassle of returning, especially if I am going to wnd up with the same issue on the replacement. I'd like to hear from anyone else who has run into this and how you resolved it.

Any suggestions welcome
Click to expand...
 
Not much of a problem to me with 2 pc jaws. I'd just rig up some soft jaws and not look back. Use the enco for stuff .003 is good enough for, use the 4 jaw for what .003 isn't acceptable for, and soft jaws for accurate repetitive work.

Really inaccuracy is just a matter of degree and it isn't likely you'll be making parts from stock that is only .006" larger than the finished dimension, making the enco suitable but the shars unsuitable:)). generally speaking .003 or .008 isn't good enough for repair work and you'd need the 4 jaw or collets any how, but now you can true up the softjaws and do the work.

Steve
 
AR1911 said:
I recently bought a new chuck and backing plate for my SB9A. It's a 6" 3-jaw, with 2-pc jaws, from Shars.
The specs online and in the included inspection sheet show TIR to be .003" maximum.
Mine reads .008" right out of the box.

So this week I did the following to try to remedy that:

Checked the spindle runout at the register surface. .0002"
Deburred the backing plate, then trued the mounting surface on the lathe. Checked it after dismounting and remounting, zero runout
Disassembled the chuck. Cleaned, deburred all parts, lubed,
Reassembled carefully. Much smoother, like a swisswatch.
Still .008" runout.

I walked over to my much-abused and oft-used 10" Enco. This is a 28-year old lathe with original 8" chuck.
.003" runout.

The only thing left that I know of is truing the jaws with a toolpost-mounted grinder.
I really don't want to do that.

I have not yet contacted the seller because I really don't want to go through the hassle of returning, especially if I am going to wnd up with the same issue on the replacement. I'd like to hear from anyone else who has run into this and how you resolved it.

Any suggestions welcome
Click to expand...
There is a video on this site, although is in Russian, on how to rebuild your chuck to high precision. Someone posted it December last year.
Regards.
 
I would put the chuck on the lathe and indicate the chuck body to see if it runs true.
If it is near perfect, then you should just grind the jaws.
If the chuck body has runout, I would remove it from the backing plate and check the shoulder on the backing plate where the chuck registers. See if that is true.

A step at a time when problem solving.

Charlie W.
 
Dumb question. Do you have acceptable runout with your previous chuck and what does it show under the same circumstances using the same test bars?
 
With a 3 jaw, the jaws must be inserted in the correct order (1, 2, 3) but you can also rotate their relative position in different slots to see if that helps. I have a 3 J that is very true but one day, I remove the jaws and didn't put them in the same slots and RO was terrible. Put them back in their preferred slots and it was back to normal.
 
Ray C said:
Try checking RO with different diameter proof bars. This will tell you if it's a scroll problem.

Disassemble the chuck and carefully clean and debur the jaws and the slots/grooves they ride in. If there's crud in the scroll, it could throw RO off considerably.

Make sure your spindle threads are clean and the shoulder is making square contact with corresponding shoulder in the backplate.
Click to expand...

I have done all that except trying different diameters
I use one inch which is about where most of my projects run
 
CharlieW said:
If the chuck body has runout, I would remove it from the backing plate and check the shoulder on the backing plate where the chuck registers. See if that is true.
Click to expand...

I trued up the shoulder on the backing plate. that is the outer 1/2" of diameter that bears against the chuck body.
Running the backing plate alone now shows zero runout, and that is repeatable

There is a stepped center portion, but it does not touch the chuck body except at the sides.
It may be worth trying to reduce that diameter and see if there is enough slack in the screw holes to get it aligned.
 
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