Re-fitting stripped plastic knobs

danalog · June 13

I am in the process of restoring a 60s Magnavox radio, on which the two plastic knobs on the clock dial have become detached.

One of the shafts these screw onto is threaded. For fixing this one, I was thinking of filling the knob with a bit of epoxy and sticking it onto the shaft so that, as it dries, it molds to fit the screw threads.

For the other, it is not threaded - looks like the shaft had a part that broke off inside the knob. For this one, I am not quite as sure what to do because the metal shaft it needs to attach back to is completely smooth. I could rub the metal down with sandpaper to rough the surface and then use the same epoxy strategy, I suppose?

These knobs do not simply flip a little electronic switch, they turn springloaded mechanical parts that require a considerable amount of force to move.

Advice would be appreciated!

CasualExtremist · June 13

Do you have any photos to aid your description? Also are you trying to restore in a way where the part(s) are still removable as originally designed or are you okay with a more "brute force" solution?

danalog · June 13

33 minutes ago, CasualExtremist said:

Do you have any photos to aid your description? Also are you trying to restore in a way where the part(s) are still removable as originally designed or are you okay with a more "brute force" solution?

They don't need to be removable, the knobs should be quite firmly secured.

There are the described left and right shafts.

Kilrah · June 13

If the diameter is right there are off the shelf knobs that squeeze a "fork" as you tighten from the end, might work...

manikyath · June 13

some form of universal glue (as long as the glue says it bonds to metal and the type of plastic you're dealing with, it should be good) will probably have enough strength to operate a knob as long as you make sure the metal is properly clean and just a but scuffed up. you want to sand the shaft length-wise so the score marks go along the length, not around, so they can provide additional friction.

OhioYJ · June 13

What about using a set screw? You could always carefully put a flat spot on the shafts so the setscrew has a spot to sit. They make knobs with set screws, or you might be able to modify the existing knobs to add them.

CasualExtremist · June 13

If the goal is to preserve the original knobs:

It would take a decent amount of precision, but my first thought is to drill a 1/16th inch hole in the shafts that also goes through the body of the knobs and then put a roll pin through that drilled hole. You might still add epoxy for added strength/rigidity, but that roll pin should help distribute the load into the aged plastic. (Do you have photo of the original knob?)