machining stainless steel tubing

[strobes]

I need to thread both ends of a 1.5" OD SS tubing, 8" long. Put it in 4 jaw chuck set it at about .0005 and guess what far end is about .035 of center?! The tubing from respectable manufacture. What you would do if you need +/- .005 concentricity?

 

[Terry]

Turn it ?

 

[strobes]

the wall is .15", have to keep it uniform

 

[Terry]

Threading ID or OD?

 

[Dusty Stevens]

Threading ID use a 6 jaw set tru chuck which is designed for tubing. Threading the OD use a long nose live center shimming or offsetting the tailstock to true it up

 

[Toolbreaker]

OD, I'd use an expanding mandrel between centers. ID, a 4 jaw with a split collar over the tubing and a steady rest. Both will give you room to sweep in both ends of the part.

 

[strobes]

It is ID., I look in to getting these tools, but for now what if I get a tube with double wall thickness, chuck it in the middle in 4 jaw chuck and bore out to correct wall thickness?
Reading documentation for these tubes, the straightness +/- .020 on 12" is in specification.

 

[carlsbad]

6 jaw set tru. one end at a time. Not sure what you mean by set up one end and the other end is 35 off center...

 

[hogpatrol]

I'm with carlsbad, don't understand the other end is off center. Also what's the needed length of thread?
Having said that, if a short length of thread, let one end hang out enough to clear the tooling, dial it in, cut the thread, turn the tubing around, repeat.

 

[rickiesrevenge]

Sounds like a form1 suppressor to me. If thats the case then it'll need about .5" of thread length

 

[ridgeway]

I'd turn soft jaws to hold it.

 

[akajun]

That’s not uncommon, chuck jaws are not at true to the bore as people think , tubing flexes, etc
If your doing tubing with fairly thin walls like a small hydraulic cylinder or suppressor your gonna need to use a cat head like a viper chambering fixture or a 4 jaw and a steady rest and indicate it in at two points like a barrel. Even then your gonna have to average out the bumps and Dips you’ll find. If it’s thicker tubing or solid, common practice is to center up the chuck end then tap in the tailstock end of the material.

 

[GenePoole]

A bit of a different end goal, but the setup is the same:

 

[tobnpr]

"Far end is .035 off center"
I suspect the 4-jaw is worn and bell-mouthed, not making contact with the workpiece except at the front of the jaws. It ain't the tubing that's out by that much.

Chuck up the stock, tighten the jaws and try to slide a .001 feeler gauge between the stock and the back of each jaw. My bet is you've got clearance there.

 

[Dusty Stevens]

6-1/4" 6 Jaw Set-Tru Chuck - Grizzly Industrial

<h1>T10420 Bison 6-1/4" 6-Jaw Self-Centering SET-TRU Chuck</h1> <h2>We are proud to offer Bison Lathe chucks.</h2> <p>Bison lathe chucks deliver precision and accuracy you can count on, making them a trusted choice for professionals.</p> <p>This is a 6" self-centering chuck with fine adjustment...

www.grizzly.com

 

[carlsbad]

6-1/4" 6 Jaw Set-Tru Chuck - Grizzly Industrial

<h1>T10420 Bison 6-1/4" 6-Jaw Self-Centering SET-TRU Chuck</h1> <h2>We are proud to offer Bison Lathe chucks.</h2> <p>Bison lathe chucks deliver precision and accuracy you can count on, making them a trusted choice for professionals.</p> <p>This is a 6" self-centering chuck with fine adjustment...

www.grizzly.com

That's exactly the chuck I have.

 

[strobes]

Here is the set-up, it is about +/-.001 at A point and run out .035" at B point.
I'm reading for this type product that +/-.010 out of round and +/- .010" deviation is straightness is still in spec. In my mind if tube has a curve in it, there is little magic can be done by a cat head or a special chuck, only boring will make it straight with acceptable tolerances.

The manufacture where I purchased the tube does not publish the spec for straightness.

With tolerance stuck-up the tube from this place may have about +/- .020 run out in 12".

Speedy Metals Information for 304/304L Stainless Steel Tubing

Speedy Metals Material Library, get Information, Analysis, Mechanical Properties, Applications, Machineability and Weldability, Heat Treating, Tolerances, Speedy Metals items. Goto www.speedymetals.com to purchase.

www.speedymetals.com

TOLERANCES​

Wall Tolerance on Stainless Steel Tubing; Round, Square and Rectangle
Plus or minus 10% of nominal wall

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

 

[carlsbad]

Runout is likely in your chuck. Take a straight mandrel and check your chuck.

 

[boltfluter]

Having done several form 1 builds. I would bet, if your Chuck is good, you are deforming the tubing with your chuck. If the tubing will fit in your head stock, thread both ends, after putting a plug inside the tube to clamp on. Go easy on the pressure. When all finished, put the suppressor on a mandrel through the baffle holes. Take a light clean up pass on the mounting surface and you will be good to go. Perpendicularity is your friend. Have fun and keep us posted.

Paul

 

[strobes]

Yes chuck is new. There is no pressure on tube in the setup, just to keep it firm for measurements.
I'll dial both ends of the tube in a cat head straight. Let you know what reading I'll get from the middle.
My bet it has curvature