DigiComp I actuator bearing boss repair

Repair part and instructions to fix broken DigiComp I crank boss with a roller bearing


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DigiComp I is an educational working computer in plastic, sold in the mid 1960s.

My DigiComp I hadn't worked in decades because the plastic boss that holds the crank in place had split and broken - this is a part that takes a lot of mechanical stress.

I built this part to hold a 2x5x2.5mm roller bearing (Amazon ASIN: B00TVPSCVO) and make the crank stand up and turn smoothly, transfer force efficiently to the cranked parts.

Materials and methods

The bearing is a press-fit into the base. I used my Panavise and its standard jaws to press the bearing into place.

I had to judiciously sand a little off the crank end where it fit the bearing, using 400-grit sand paper, then finishing with, I think, 1000 grit. The crank will sit fixed in the interior bearing hole - the bearing is now doing all the rotating. Since tolerances can be tight, don't get your crank permanently jammed into your bearing before you've finished the repair and it's ready to stay there.

You can follow the repair process with the pictures posted here.

The halves of the crank boss had completely come off the base (base inverted and closup views).

I taped around the raised remains of the crank boss, and wrapped a small block of wood in sandpaper. IIRC I used 200 grit.

After sanding, the boss was completely removed and the base was flush.

I then placed the printed part on the underside of the base, and put the crank through the base and into the bearing. I found what felt like the right orientation to mount the bearing plate, taped it in place, and got ready to drill mounting holes.

I drilled the base from the underside, to the top, using a handheld "pin-drill" (pictured), and drilled through the holes in the taped-down bearing plate to get the right alignment of the mounting holes in the base.

Finally pictured are the crank mounted in the base (2 views) and the crank in operating position.

Works pretty well and looks almost stock! I did have to make a couple bend-adjustments in the crank to get it just right, but I'm very happy with the result.



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982d0d4da3ab804d086c3580624d8def?default=blank&size=40dburum commented ago

Nice work!!

8181d65d73a57fc78fb4a6c7bf96f17d?default=blank&size=40jddj published this design ago