Thursday, April 19, 2012

Adapting Carbon Fiber Composite Tube To Moonlite Telescope's Blocks

I mentioned previously that Moonlite Telescope's truss tube blocks won't work with tubes with an ID smaller than .625".  There are both cost and weight reasons not to go to a .625" ID tube.  Prices and weight rise rapidly, and finding something that is 58" long (or that I can cut down to that size) limits what is available off the shelf.  (Custom carbon fiber composite of course, is priced accordingly--start with expensive, and multiply.)

The obvious solution is to machine an adapter that uses the standard tubing dimensions that Moonlite Telescope's truss tube blocks expect (1" OD, .050" wall), and adapts to the size of carbon fiber composite tube that I need to get the required lightness and stiffness.  Going with a .625" OD, .553" ID tube gives me a maximum sag for a single truss of .0037" max, or about .0024" for all three working together (plus whatever stiffness is contributed by the top and bottom tubes).

The adapters will look like this:

It starts with a piece of 1" diameter aluminum rod.  I'll bore one end that it has the dimensions for the truss tube blocks.  The other end will be a .63" ID hole.  I can't just screw the set screw to the carbon fiber composite tube, because it will either crack or puncture it.  Instead, there needs to be something to support the tube on the other side.

I will drill a 0.25" hole through the 1" piece of aluminum.  The green piece on the drawing above is a 0.55" diameter aluminum cylinder, which will be drilled and tapped 1/4"-20.  Because the holes in both cases are carefully centered, the center cylinder will provide a space that will accept the carbon fiber composite tube.  The set screw thus locks the tube into the adapter, without putting any great stress on the tube itself.

At the same time, it's only two inches long, so it won't add much weight.

UPDATE: A commenter suggests epoxy to bind the tube to the 0.63" hole.  That has the advantage that it eliminates the need to drill and tap a hole for the set screw, and makes the interior cylinder unnecessary.  The more I think about this, the more I like it.  I might even simplify this by starting from a .625" ID, 1" OD piece of aluminum tubing.  Then all I have to do is bore one end out to .90" ID, and the other to .63" ID.

5 comments:

Marcus said...

I machined parts for satellites from titanium. they were basically really expensive elbows for carbon fiber tubes. The engineer told me the elbow was joined to the tubes with epoxy.

Sendarius said...

You might need to go a little larger on the boring - you need SOME space for the epoxy.

Anonymous said...

Epoxy bonding hints:

If you have a boring bar, you can put grooves around the circumference at few locations down the hole to give a hard lock for the epoxy into the aluminum.

Clean the bore scrupulously with fairly dilute detergent, then abrade with sandpaper, rinse several times, then wipe down with alcohol. You want a textured surface and zero oily residue.

Abrade (80 grit) the end of the carbon fiber tube to give it tooth and ridging (circumferential orientation), and to remove any surface residues from its original fabrication.

Use freshly bought epoxy and mix a small proof batch to test for competent cure, before you commit your work pieces.

Rorschach said...

Agreed. Epoxy a 1" od x 3/16" wall extruded seamless tube onto the end of the tube and call it a day. You probably don't even need to turn the I.d. because you'll need to sand the tube to roughen it a bit anyway.for that matter. Use a 5/32" or 1/8" wall tube and you'll have room for the epoxy. I doubt concentricity for this will be that big of a deal.

Jim Seargeant said...

Be careful of corrosion when using graphite and aluminum. See the wiki on graphite and search for "aluminium".