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I am currently trying to attach a small flat piece of plastic (no bigger than 2", could be smaller) to a 1.9" hollow aluminum tube bent into a 2 foot radius torus, spinning at 2Hz. My first and normal approach would be to drill a tapped hole in the hollow tube; however this is not an option in this situation, as the inside of the cylindrical tube is kept in a vacuum, and has to be in order for the project we are working on to work. My second approach would be to mill the plastic to match the diameter of the tube, but the problem remains; how would I attach the resulting plastic to the tube?

I have access to a machine shop, so I can do some basic mill and lathe stuff in order to accomplish this. How should I do this?

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One way would be to clamp the outside of the pipe with a two-piece shaft collar. Properly tightened, a shaft collar will resist pretty heavy loads before it slips. You could tap the shaft collar and attach your plastic part as you normally would before attaching the collar to the pipe under vacuum. Note that a shaft collar will compress the pipe, so there is some risk of crushing the pipe, especially since it is already under inwards pressure. I doubt this will be a practical concern, but it is worth doing at least a rough check if you are using a thinner pipe wall.

One problem might be that shaft collars are typically manufactured for shaft sizes not pipe sizes, so you probably won't be able to find a perfectly sized collar. Since you have access to a shop, you could machine your own from scratch just for the pipe, or you could buy an 1 7/8" collar (like McMaster Carr 6436K26) and machine the ID slightly larger. In a pinch you could open up the ID with a grinder too, but turning or milling would be a much better option.

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  • $\begingroup$ Thanks for the suggestion! I added in a few details I omitted when I originally posted in the OP, and found out that the thickness is ~3/16 in. My intuition tells me that the tube should be able to handle the collar being clamped on pretty tightly, but I just want to double check this before I test this out and realize that my intuition was wrong. $\endgroup$ – VoidShredder Oct 19 '16 at 20:52
  • $\begingroup$ I would guess that it will be fine too, I think you have 1 1/2" Schedule 80 aluminum pipe which according to this ( sapagroup.com/pages/522736/Brochures/… ) random PDF I found on the internet would be able to handle around 5,000 PSI before collapsing. Since a vacuum is only about 15 PSI lower than ambient air, you should have plenty of available strength to apply the shaft collar. All that said, only you know the consequences of failure and other circumstances. $\endgroup$ – Ethan48 Oct 19 '16 at 21:01

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