Suggestions for Tight Bakelite Knobs?

[Dubis7]

I finally picked up some replacement knobs for my Zenith FM radio. These have been a bear to find. Unfortunately, the new knobs appear to have shrunk a bit, and won't fit over the power/volume pot shaft. I don't want to try and force them and risk damaging one. Does anyone have any suggestions to make the plastic flex enough to fit over the shaft? Last resort I could probably sand away some of the interior shaft, but I don't know if there are any other approaches that wouldn't risk damaging the bakelite.

[Electronic M]

I'm pretty sure bakelite don't shrink though some other plastics do.

If it is half round sanding the shaft may be the best option. If solid core round with set screw you may be able to gently drill out the knob with a bit a hair bigger (I did this with a metal knob once), if it is slotted core full round shaft pinch the halves together till it fits...If that don't work drill.

[Chip Chester]

This screed is valid for 1/4" shafts...

Hardware stores will carry a 1/4" rotary file that should do a good job incrementally, without the potential grabbing and cracking of a drill. Or the out-of-round situation of using a Dremel to nibble away at the hole.

Here's a link to one:
https://www.stinehome.com/forney-rot...d-plastic.html

( If you have to go the other way, try these:
https://www.ebay.com/i/323825291229?chn=ps )

Now, how to actually do it...

If you can hold it in a vise without scratching it all up, do that.
If you can't, get one of those kevlar kitchen gloves that prevent you from getting cut. Then, hold the knob in one hand and your varispeed drill in the other, and keep it slow and straight. Verify alignment often, and make sure it's cutting and making chips rather than just heating stuff up. The file doesn't have grabby teeth, but still make sure your glove doesn't get all wrapped up.

You could even do the 'no power' option and just twist the knob onto the file as if it were the shaft. Safer but slower.

A hand reamer would also do the trick. Sharper, more precise sizing if you need, non-powered, and more expensive. But it still beats filing away on a perfectly good volume control shaft, and having to deal with concentricity, filing debris, and danger of scratching faceplates.

[MadMan]

Even if bakelite does shrink (which I doubt) it's certainly not going to flex no matter what you do to it. It's a hard material with almost no give.

Assuming your pot shafts are not damaged... ie, gouged or mushroomed (which might be very hard to see with the naked eye) filing away some of the inside of the knob is probably the only option. Have you tried all the knobs on all the shafts? Have you tried grease?

[old_coot88]

Is the shaft (where the knob goes on): Round? Half round? 3/4 round? or knurled/slotted?

[Dubis7]

Okay, well this turned into more of a project than I had anticipated.

My immediate reaction was to use a dremel tool and sand away a bit of the inside of the shaft. Worked for a bit, but it ended up being too loose, and the knob would slip instead of turning the set on and off.

I had to step back a bit, and looked over my parts chassis. It turns out that the shaft on my set is a replacement that's slightly too big. That explains the old knob splitting so badly, and why the new knob wouldn't fit. Now that my knob was slightly too wide to work, though, I decided to go ahead and use some heat shrink tubing on the control shaft to give it a little more diameter. Sure enough, it fits snugly and works perfectly now.

The design of the old shaft has a bit of a bump on it, sort of a spring I guess, that will actually still work with the wider knob. I may replace the replacement on mine someday, though the work I did should prevent any splitting going forward.

Thanks for all the advice, folks.