Milling Steel Tubing

I'd use a shaft that closely fits the ID of the tube. Make it longer than your part and clamp the ends down in the t-slot. It's easier to cut larger sections out with a small tool. A picture of the project showing the size or sizes of the slots would help. The more slots and indexing involved the harder it will get to find good clamping points.
 
If it were me and there are many to do I would put a 2 1/2" aluminum round say 4" long in a lathe and bore a 1.510" diameter hole through it.
Put it in a mill and mill flats on the OD 180 Deg. apart, slot it along it's length 90 Deg. from the flats leaving 1/8" of material or so on the far side this makes it easier to clamp. The flats do not need to be this large, I drew them that way for clarity, if the slot were large enough then the work could be milled right in the fixture slot.
I would also install a 1/6" roll pin as close as possible to one end of a flat so as not to push it through the vice when changing parts.

If you don't have a lathe bore it in the mill on end.

Like so
tubeclamp_zpskoh7olsh.jpg

tubeclamp_zpskoh7olsh.jpg
 
I mill the side off of 3/8" square steel tube all the time. I know its smaller than you have but I slide a piece of 1/4" round bar inside so when I clamp it there is no smashing. I also put a small length of 1/8" thick metal between the tube and the screws of my tube holder so they don't leave marks. I have also ran the bar past the ends of the tube in a fixture to bolt it down tight. http://www.hobby-machinist.com/gallery/tube-milling.94237/
 
Clamp the tube right down on the table, right over the center T-slot. Sometimes I use a piece of shafting through the tube as part of the clamping system.

Thanks everyone, I really appreciate all the suggestions. I'm thinking Jim's suggestion is probably the most viable for my setup...
Although I probably should have thought about using a V-block, I can see where three clamping edges (and the aluminum insert) could have provided enough force to keep the paper from tearing.

I finished this tube prior to asking my question, but I ended up just spotting and drilling out the corners of the sections I had to remove, then cutting between the drill spots and filing to fit. That worked fine for what I had to do this time, but I'll eventually need to mill out straight, thin slots as well.

Here's a pic of what was dealing with:
9979FC26-8EAA-4AF1-AA33-0179E18E6F22_zpsfzaqhkyj.jpg
As you can see, that slot ended up wandering quite a bit (and I wasn't getting a very clean cut at all. Switching to just drilling out the corners and connecting the dots with a cutoff wheel ended up being much easier (but won't really work well when I need to cut a thin slot).

For the slot in the pic above, this turned out to be the culprit:
A749E355-214E-403D-9071-48C5DB017DE7_zpsugfwihs3.jpg

9979FC26-8EAA-4AF1-AA33-0179E18E6F22_zpsfzaqhkyj.jpg

A749E355-214E-403D-9071-48C5DB017DE7_zpsugfwihs3.jpg
 
I'd get rid of the stencil paper and just paint on some dykem and scribe my points/lines directly on the tubing. That might solve the clamping problem all by itself.
 
I'd get rid of the stencil paper and just paint on some dykem and scribe my points/lines directly on the tubing. That might solve the clamping problem all by itself.

That is also a very good point! Next time. Thanks!
 
Yupp use the paper to center punch then remove apply dykem if you need to have the lines do your scribing then mill it will work without the core Probally but it wont hurt to leave it in there, I would .
 
I'd get rid of the stencil paper and just paint on some dykem and scribe my points/lines directly on the tubing. That might solve the clamping problem all by itself.

You could chain drill or plunge mill a series of smaller holes first and clean up with a size mill. That would reduce milling forces and avoid wandering.
 
V blocks, but also turn out a mandrel to support the tube. Another way I've done them is to weld on a couple of small pieces of bar stock at the ends ,of an over length tube. Do your setup, cut the slots, then cut the tubing to finished length on the lathe.

Also, never trust those templates, always use them as a rough guide, no more. Blueprints are all over the place for stuff like this. Dykem and scribe is your best bet. CZ or l2a3? ;)
 
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