What's a recommended way to hold a cylindrical printed part in a lathe?

[WobblyHand]

For diameters less than or equal to 32mm, I can use a collet. What do you do for larger diameters?

I really want to face off the ends, but at the moment I can't hold the cylinder true. The chuck jaws dig into the soft material and even if I bump the cylinder true, any loading force due to a slight cutting cutting force throws if off true.

I tried a live center, but the center is in the way of where I want to cut. I then thought about a steady rest, but mine has bronze fingers and would eat through the plastic. My steady rest also seems to need repairs. I tried it and it didn't operate easily. There was paint in places that shouldn't be there (from the factory!) so it would need some work to move smoothly.

Do you recommend a sleeve to spread out the clamp force at the chuck? The object I want to true is 40mm in diameter and about 165mm long. Would a bored out tube with a slit hold this "well enough" to get a light facing pass on the end? Is about 50mm long enough to work? Any hints would be helpful. It was disheartening to see this tube having 2-3mm of run out at the end I wanted to clean up. It's not off that far, by a long shot.

 

[whitmore]

It might sound elaborate, but have you considered making wooden jaws for a four-jaw chuck, and
turning the jaws to suit the object? Doesn't need four, could just be two bits of 2x4 lumber
bolted into opposite jaw positions.

 

[WobblyHand]

It might sound elaborate, but have you considered making wooden jaws for a four-jaw chuck, and
turning the jaws to suit the object? Doesn't need four, could just be two bits of 2x4 lumber
bolted into opposite jaw positions.

Sounds more elaborate than I had hoped, but what I'm hearing is that yes, some way of spreading the clamp force is the right approach.

A split ring would be simplest, if I have stock that large. I just found a 2" diameter piece of aluminum which could be used if I bore it out. Being aluminum, it wouldn't take that long.

Had a short piece of thick wall tubing, but alas, the ID is about 50mm, so no luck with that.

Spit ring with a four jaw would work I guess. Maybe I could partially cut it with four slots. I just don't want to deal with four separate pieces. Did that once, it worked, but it was like a three ring circus for a while.

Thanks for your ideas, maybe I can do a variation of it.

 

[Jim F]

Part does not need to be true to the z axis to face.

 

[WobblyHand]

Part does not need to be true to the z axis to face.

I need to create a flat face that is square (and flat) to the z axis and the cylinder walls. Doesn't that mean the part has to have minimal TIR on the cylinder wall? If it is wobbling, then the face created wouldn't be flat and square, I would think. I'm confused.

 

[rabler]

I believe what @Jim F is saying is that the part's centerline needs to be parallel to the lathe's Z axis, but doesn't need to be on the Z axis. So you don't need to indicate for TIR, you can run an indicator along the Z axis by moving the lathe carriage and just get a constant reading there.

 

[WobblyHand]

I believe what @Jim F is saying is that the part's centerline needs to be parallel to the lathe's Z axis, but doesn't need to be on the Z axis. So you don't need to indicate for TIR, you can run an indicator along the Z axis by moving the lathe carriage and just get a constant reading there.

Ah, that makes sense. In my case the TIR was near 0 at the chuck and 3 mm at 165mm. So the part wasn't parallel to Z axis. Facing in my case would have created a non straight surface.

I don't know why it was so bad, but it can happen if a jaw lags on closing. The chuck is over due for a cleaning.

 

[RaisedByWolves]

Ah, that makes sense. In my case the TIR was near 0 at the chuck and 3 mm at 165mm. So the part wasn't parallel to Z axis. Facing in my case would have created a non straight surface.

I don't know why it was so bad, but it can happen if a jaw lags on closing. The chuck is over due for a cleaning.

Pics might help.

 

[PNW_Guy]

Printed part?

Can you print a set of soft jaws, with less infill than the original part? The "squishier" soft jaws should adapt to the lathe jaws, but not distort the main part.

 

[WobblyHand]

Pics might help.

Of a slightly tilted cylinder in a three jaw chuck? I can get one tomorrow if you'd like.
The part is a right circular printed cylinder made of PETG. For some reason, it canted in the chuck. I know from experience, you can't just crank down on the chuck key and have it made perfect again. The material is softer than delrin. I thought it was in straight, but maybe one of the jaws dug in, or the part got wedged somehow. It's clearly operator error. Maybe tomorrow, I'll figure it all out. Sometimes, that's all it takes.