question about knobs

[Bill Raymond]

knob.jpg (20.96 KiB) Viewed 12975 times

I've been casting some knobs for my Standel 25L15 copy. I've cast one out of polyurethane resin and was wondering if it is necessary or desirable to put a metal sleeve inside. The knob I cast is solid, and I just drilled a hole to fit the shaft of the potentiometer (I'll drill and tap for a 6-62 srt screw too). So far. it seems that there is no need for a metal sleeve, but what do some of the rest of you think?

[Bob Francis]

I can't answer your question but want to say " That casting looks great!".

[Barry Daniels]

Without question, the metal insert would improve the longevity, but is it necessary? I don't know.

[Chuck Tweedy]

Bill, if you put it on a splined/split shaft pot, then I'de expect it will do okay without the sleeve. PU is pretty tough - and many knobs are done this way.
However, the 6-32 set screw makes me think that you plan on putting it on a straight shaft pot. I don't think the plastic will hold the threads well, and will likely strip very easily.

[Jason Rodgers]

That's cool! Can you post a few more pics to show your process?

I'll let David King give the definitive word on knobs, but I wonder if you could just incorporate a plug in the mold for the shaft.

[Bill Raymond]

I don't have any other photos of the process, just one of my pattern which isn't that interesting: I simply took a 1" diameter maple sphere, drilled part way thru the center, mounted it on a piece of scrap plywood with a dowel and cut it in half, leaving a half sphere with a hole part way thru from the flat surface. Then I drilled the cutaway portion with a 1" forstner bit, glued on the "skirt" from a birch disk and slotted the knob for the pointer made from a short piece cut from a tongue depressor. From there I pretty much followed David Robinson's tutorial in the library, made a 2 part silicone rubber mold, cast the knob in Alumilite polyurethane resin and drilled the hole for the potentiometer shaft locating said hole by a small countersunk "divot" I had cast into the bottom of the knob. A lengthy process, but not difficult. I'm happy with the way it turned out.

Yes, Chuck, I'm using smooth shafted potentiometers, but am annoyed to find that I have 3 with 6mm shafts and one with a 1/4" shaft. I'm not going to replace the potentiometer just for that, though, so I'll either drill the knobs to fit or simply shim up 1/4" holes! Alumilite, according to their website, apparently will hold threads and I can put in a 1/4" long set screw which should distribute the load on the threads over a long enough distance, I was thinking, but that's one of the reasons I'm asking for the collective wisdom of MIMF.

[David King]

Absolutely no need to put metal collars inside there if you got the mix right and mixed it well. Urethane does have some potential to stretch and a very tight set screw may deform it over time.
You can get brass tubing shims from Allparts that accommodate 1/4" id holes over 6mm pots.
https://www.allparts.com/EP-0220-B08-Bu ... _1291.html
You can also buy the tubing from any hobbyshop that sells K&S metals. You want the 1/4" o.d. x 15/64" i.d. stuff.
If you ever want to go to a metal insert I can send you some as I make them by the hundreds (or my apprentice does it for me if he's unlucky).

[Bill Raymond]

Thanks for your comments David; thus reassured I shall continue to go commando--insertless, that is. Should I decide I need inserts sometime in the future, they're easily made on my metal lathe, but I thank you for your offer, as well as for the link to the bushings. Unfortunately, living in the rural outskirts of a half-alive cowboy town makes it difficult to just go to the store to purchase brass tubing--if our undersized Home Depot or Walmart doesn't have it, I have to shop via internet.

[Jim McConkey]

Anyone interested in molding or casting their own parts needs to check out this oldie, but goodie: David Robinson's photo-essay on molding your own plastic pickup covers and other small parts

[Bill Raymond]

David Robinson's tutorial is excellent. However, he mentions that no release agent is needed when pouring the second half of a 2 part mold. That may be the case with the silicone rubber he is using, but generally, silicone will stick to silicone and a release agent is definitely needed. I used Oomoo 30 silicone rubber from Smooth-on and petroleum jelly (Vaseline) warmed by putting a small quantity in a little plastic bowl floating in warm water brushed on the surface for a release agent. Also, though a release agent is not required when casting urethane in a silicone mold, the manufacturers recommend it to prolong the life of the mold. David shows the use of a vacuum chamber, but I found it unnecessary to cast my knobs. I did use a release agent as recommended by the Alumilite co. and lightly dusted the preheated mold with talcum powder, which helps to eliminate bubbles from forming on the surface of the casting and is absorbed into the plastic without leaving a trace in the fiinished casting.

[David Robinson]

Bill- I can't believe I said that about no release on the second half. Not doing that has caught me several times. You don't need release on the plastic part, but you do need it on the silicone to silicone part.
The vacuum chamber is used with the slower setting alumilite. There is no way you can mix the material - pour a mold - get it in the chamber without sloshing it - and down to pressure in the 90 seconds before a set.
I am using a pressure pot now because of clear and translucent materials I am using. the pressure pot is better at crushing the bubbles.
I have made tons (a bunch) of alumilite based splined knobs that hold on better than other production molded knobs.