Potentiometer with 3/8" shaft?

[moosapotamus]

I found some really cool knobs, but they require a pot with a 3/8" shaft. The usual sources do not seem to carry anything like that. Does anyone have a source for pots with 3/8" shafts?

Alternatively, maybe, any ideas for how to make a 1/4" pot shaft fit in a 3/8" knob hole?

Thanks
~ Charlie

[armdnrdy]

Off the top of my head.....a reducing bushing with a set screw.
Look into possibly using the brass insert found in so many knobs. Brass would be fairly easy to file down to the correct dimensions needed.

Update:
I examined one of the small MXR type knobs. The brass insert looks to be usable once the splines that hold it in the plastic are filed down.
The set screw will have to be filed as well so that it's flush. You can turn the setscrew around so that the slotted side is inside the threads. This will act as a holder as you file the non slotted end.

[davent]

I've used 1/4" solid brass rod to create bushings for 1/8" pot shafts, you could do the same with 3/8" brass rod. Clamp a piece of hardwood to the drillpress table. Chuck a 3/8" bit drill a hole into the hardwood deep enough to accept the brass plug. Stick the brass plug into the just drilled and still clamped down piece of hardwood. Replace the  3/8" drillbit with a 1/8", drill plug then step it up with bits until you've got a 1/4" hole bored through the center of the brass plug. Remove  the plug and drill a hole through the side big enough to allow the set screw to pass through.



dave

[PRR]

It is trivial with a lathe. Have not seen a lathe in years, except in catalogs.

If you can find 1/16" sheet (16 gauge), cut it say 1/2"x0.6:, and wrap it around a 1/4" drillbit, you got it. Tip: cut 1/2" by 2", bend around the drill, finish the bend with a small vise; then trim the excess.

Hobby shops *used* to carry small brass tubing. It ran 1/16" to 1/2" and each size telescoped into the next. Buy the 3/8" OD, and the next and the next, stop at 1/4".

It may take too much set-screw force to deform this thick wad for a good clamp. You can side-drill through and use a longer set-screw.

[moosapotamus]

Nice ideas! Ideally, I think my first preference would be to find some 1M log pots with 3/8" shafts. But, I like those custom inserts, Dave. Unfortunately, I don't have access to a lathe, or even a drill press. So, I'll probably look into the other ideas from Paul and Larry.

Thanks!
~ Charlie

[zombiwoof]

If you have 3/8" holes for the pots, isn't that to accept the bushing of the pot, and not the shaft?.  Many pots have 3/8" bushings.  Maybe I'm misunderstanding your needs.

Al

[defaced]

If you can find suitable 3/8 material, you can do this with a hand drill and a vice.  Cut the material to length, chuck the soon to be bushing in the drill, and hold the drill bit in a vice.  With a little touchy feely finesse, the bit will center on the work and then you just drill as normal.  

Or buy a stock spacer with the OD you need and open the ID with a drill bit.  Select 3/8" OD from the boxes on the side, then select the material and length spacer you need: http://www.mcmaster.com/#unthreaded-spacerss
I'd probably go with aluminum (no plating) and the smallest size ID hole (more stable drilling).

[duck_arse]

I dunno my aighth from my 16th, I only think millimeters. small diameter brass tubing can be got from rabbit-ear, telescopic antennas, tv's and the like.

[moosapotamus]

Thanks for that McMaster-Carr link, defaced. That looks like the ticket for making a 1/4" pot shaft fit into a 3/8" knob hole, with a minimum amount of machining.

If you have 3/8" holes for the pots, isn't that to accept the bushing of the pot, and not the shaft?.  Many pots have 3/8" bushings.  Maybe I'm misunderstanding your needs.

Al

I have some knobs that accept pots with 3/8" shafts. So, I'm looking for either 1M log pots with 3/8" shafts, or some way to use a pot with a standard 1/4" shaft.

Thanks
~ Charlie

[PRR]

Ordinary pots were ALL 1/4" (or 1/8"), until the orientals rounded-down the sizes.

There have been 3/8" shaft pots but only in HUGE sizes, like 100W; and these don't usually go to 1Meg (that would be 10,000V; you don't want your finger on it).

Bush it.

[duck_arse]

not only posting in an old thread, but I'm going to quote myself as well.

I dunno my aighth from my 16th, I only think millimeters. small diameter brass tubing can be got from rabbit-ear, telescopic antennas, tv's and the like.

I did this today, and it worked about as perfect as it could.

"tayda/alpha splined pots" are this week 5.8mm shaft dia. the bottom section of my telescopic antenna is brass tube, 6.0mm OD and ~0.3 thick walls. wrap with some masking tape, mark yr length, 12~13mm, cut through with a triangle needle file. clean and square the cut ends, slit the length with the triangle file on the inside of the tube, dress the slit w/ sandpaper, remove the tape.

block-up the shaft-split as you do, I use off-cut cable tie. back off the grub screw, push the sleeve up into the knob, then slide it onto the pot shaft. done. tight the screw. even more done. you want to keep the end of your new shim above the end of the threaded bush of the pot, it'll bind and scrape. maybe adjust w/ jeweller's screwdriver.

this stops your knob nosing. nobody really wants that. you can also extend a too-short shaft (there's spam for this, isn't?) by a small amount if needed. I tried it on some tayda knobs, some ancient sato's and some chicken heads, and they all sat nice and straight, although I didn't have them on a panel. one solid machined Al knob didn't go straight, it had a slightly larger (6.6mm) internal dia. all the rest were 6.4mm.

this method is obviously not for the drinking straw or heatshrink fixers, it's too time wasting.