machining stainless steel tubing

[tobnpr]

Straightness tolerance on Stainless Steel Tubing; Round, Square and Rectangle
.075" in 7 feet

This tells you it ain't the stock. You're measuring more than ten times the allowable runout.

Yes chuck is new.

What make is the chuck?

There's not much to look at here. If it's not the spindle, it's the chuck. There is no other cause I can think of for straight stock to indicate completely warped only 8" from the chuck face.

Take any other piece of stock- and chuck it up. I'll bet you see the same problem.

 

[tobnpr]

That’s not uncommon, chuck jaws are not at true to the bore as people think , tubing flexes, etc

Any chuck worth mounting on a spindle wouldn't give you .035 runout only 8" from the chuck.

Tubing (and everything else) flexes when you turn it from tool pressure. Nothing causes it to flex, just sitting in the chuck.

 

[strobes]

This tells you it ain't the stock. You're measuring more than ten times the allowable runout.

.075" in 7 feet is 0.010 in 12", this is straightness. The run out is double of that, about .020"

 

[Dusty Stevens]

Start by machining the backplate alone on your machine then mount your chuck

 

[akajun]

Any chuck worth mounting on a spindle wouldn't give you .035 runout only 8" from the chuck.

Tubing (and everything else) flexes when you turn it from tool pressure. Nothing causes it to flex, just sitting in the chuck.

Stacked error ever come across your mind? This may be a symphony of errors he needs to check his whole setup and material.
And unless your turning precision tubing it will not be true and yes you can deform / flex it in a chuck Especially a 4 jaw as you’ll have uneven pressure if you overtughten. 6 jaw chucks were made for tubing and you can still deform it.

 

[jackieschmidt]

Come on, a piece of tubing is not truly straight. What they publish as straightness tolerances have little to do with what you actually get. And I doubt the ID runs true with the OD. I would be shocked if it did.

If you want things truly straight and round, you have to Machine them to the acceptable tolerance.

 

[tobnpr]

Roll it on a surface plate.
Hell, even a straightedge will show your eyeballs if it's out by that much.

This isn't complicated to narrow down. All that's needed is a verifiably straight piece of stock chucked up instead of the tubing. Absent that, turn any stock between centers then chuck it up and see what you have.

 

[strobes]

Here you are. The clamp is there just keep the tube from rolling, very little pressure applied to that end.

 

[Dusty Stevens]

Lets say you threaded it anyway, both ends perfect. What kind of baffle strike are you going to have halfway down there? Baffle strikes do not have to be the bullet itself think of the shockwave around the bullet and you disrupt one side of that. Lots of muzzle brakes have ruined barrel potential from that with no physical evidence ever

 

[strobes]

It's not an application in this case, but I totally agree. The point is to make people aware that tubing is not straight. As I see it it is a surprise for many as some where questioning the setup and lathe.