Parameterized hex nut holders for inside of fiberglass or cardboard tubes

When rebuilding a vintage telescope, I needed some interior hex nut holders to use when bolting some fairly heavy items to the outside of the tube.  Only some small washers had been used in the original design, and the resulting high stress had actually caused the tube to develop radiating cracks.

These nut holders are contoured to the inside diameter of the tube and provide considerable load spreading while still keeping a low profile inside the tube.  They are also useful in sport rocketry for anchoring rail button screws in fiberglass or even cardboard airframe tubes.

The dimensions of the nut holders are completely parameterized in the OnShape CAD file: https://cad.onshape.com/documents/d2c89c20ba8568edb15e9121/w/4595fab9ea90c9dc62ccb8c8/e/a073ebe5538b02b0213fb42d

You'll most likely need to customize the parameters for your own needs.  Have a look at the Feature Studio in the OnShape workspace to see how to do it.  All of the example .stl files are for a tube with a 9.375 ID that was used in an 8"/200mm aperture telescope.

The “hexdiam” parameter is the distance measured between two parallel sides of the hex nut, with a little allowance for clearance.  You can play with this by a few thousandths to adjust the press fit.

A tilt parameter is also provided - many old tube-mount scope accessories had mounting brackets with noticeable toe-in on the mounting bolts.

These were printed in black PETG with 0.25 layer height and 100% infill.  Once they were mounted inside the telescope tube, they were painted over with Black 3.0 to kill the plastic shine.