3D Printing Missing Thumb Nuts to Refurbish a Telescope

This post documents a recent project about 3D printing missing thumb nuts that we used to refurbish a telescope.

I recently had the opportunity to pick up an old Orion SkyQuest Intelliscope XT6 Classic Dobsonian Telescope for free. It looked like it was in pretty good shape but it was missing a few screws and nuts. I was able to replace most of the missing screws and nuts at my local hardware store. I figured it would be a good project to refurbish the telescope with my son so we could use it and then when we got tired of it we could sell it on eBay in good working order.

Unfortunately it was missing four thumb nuts needed to hold the secondary mirror mounting vane assembly in place within the main optical tube. The design was such that the screw threads attached to the ends of the vanes didn’t protrude through the holes on the optical tube. This meant that standard nuts or thumb nuts, wing nuts, etc. wouldn’t fit because they wouldn’t reach the screw threads through the holes in the optical tube.

3D Printing Missing Thumb Nuts

We needed a shouldered thumbscrew that had a narrow enough shoulder to fit through the holes on the optical tube in order to reach the screw threads. After quite a bit of searching online I was unable to locate a suitable replacement. I contacted the manufacturer, Orion, to see if they had any replacement parts or information about how I could locate a suitable replacement. They didn’t.

I first thought about buying the closest shouldered thumb nuts I could find and then grinding them down enough to fit through the optical tube holes. But that would probably be more trouble than it was worth. I then thought about designing my own replacement part in a CAD program and then using a 3D printer to generate the custom replacement parts myself. Our local library had a Cube 3 3D Printer (now discontinued) available so I figured I’d see if it would be capable of 3D printing the parts we needed. This would also give my son the opportunity to learn about 3D CAD and 3D Printing.

I started by designing a thumb nut based on measurements I took from the telescope. I used The Foundry’s Modo to design the 3D model as accurately as possible. Because the resolution of the consumer level 3D printer isn’t the highest I didn’t think the threads would print well. So rather than printing the threads I just made the interior diameter of the thumb nut slightly smaller than the diameter of the threads. This way after the thumb nut was printed I’d be able to drive a screw into the nut letting the metal screw threads just cut their own matching threads into the plastic of the nut. This approach works well for simple non-load bearing parts that don’t need to support any weight. The secondary mirror mount is pretty light and would be supported by four of these thumb nuts so the plastic should be sufficient to hold it in place using this approach for 3D printing missing thumb nuts.

Using the Cube 3 for 3D Printing Missing Thumb Nuts

After the initial test model was designed I exported it to a Stereolithography (STL) format file set to use millimeters as the export unit. I then imported the STL file into the Cube 3 Cube Print software. Once in the library I prepped the file for printing by selecting the Dark grey PLA plastic cartridge and selecting print parameters. For these tests I used the default setting for resolution of 200 microns and mostly solid construction. The printer software automatically creates a crosshatched lattice inside the object so that it doesn’t waste too much material. The mostly solid setting should be sufficiently strong to hold the screw threads. I then sent the resulting Cube file to the printer, applied glue to the plate and watched it print out the first test part.

The 3D printing was completed in about ten minutes. I then used an exacto knife to remove any extra bumps left from the 3D printing process.The threads on the ends of the vanes were 5mm so I initially made the inside diameter of the thumb nut about 4mm. After printing I took a 5mm screw and tried to drive it into the thumb nut to cut the threads, but it wouldn’t fit. I then tried a 4mm screw and that worked fine.

3D Printing Missing Thumb Nuts and Cutting Threads into the Plastic

I drove the screw in from the back of the thumb screw rather than the front that would grab the threads of the the vane mounts. This way any slipping while the screw started making the threads wouldn’t affect the more important front side of the thumb nut. I used a set of locking pliers to hold the thumb nut in place while driving the screw in. Once the threads were cut into the thumb nut I then backed the screw out and screwed it in and out of the front a few times to open the threads up a little. I didn’t want them to be too tight on the secondary mirror mount or they would bend the vanes. Even though this 4mm screw meant the thumb nut wouldn’t fit the 5mm threads I’d need for the mount it proved that the concept would work. The holes in the optical tube were about 9mm so I initially made the collar of the nut taper from about 8mm to 10mm. I tested the fit of the thumb nut’s collar on the holes in the optical tube and found that the fit was a bit too tight.

Refining the Dimensions for 3D Printing Missing Thumb Nuts

I then redesigned the part increasing the inner diameter to about 4.5mm and reducing the outer tapered diameter of the collar to about 7mm to 9mm and 3D printed a new test. This second version had a collar wall that was too thin for the 3D printer so it didn’t print well. There were gaps along the thin wall. This version had a perfect inside diameter that fit the 5mm screw so I kept the inside diameter the same and progressively increased the outer diameter of the collar area to increase the thickness of the wall and printed a few more tests. Once I got to the fourth version it almost printed completely without gaps. So I then held the wide part of the tapered collar where it was and just increased the top diameter of the tapered collar area letting it taper less so that the collar would be thick enough to print completely.

This fifth version printed successfully and fit both the 5mm screw and the holes in the optical tube so I printed six of them all at once on the 3D printer. They took about an hour to print all six. I printed six so I would have a couple of extras just in case they were ever needed. So we had success 3D printing missing thumb nuts.

Refurbishing the Telescope with 3D Printed Missing Thumb Nuts

After all the thumb nuts had threads cut into them using the 5mm screw and locking pliers they were ready to test on the actual secondary mirror mount vanes. They all fit so I then used them to attach the secondary mirror mount in the telescope’s optical tube.

Once it was tightly secured I performed the collimation procedure to align the primary and secondary mirrors of the telescope and we were ready to take it outside for our first test. We took a quick test snapshot of the moon. Success!

I hope you enjoyed this post! If anyone else with this telescope needs to replace the thumb nuts and would like to 3D print their own, here is the STL file that is free to use. Enjoy!