Instrument Panel Knobs

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Instrument Panel Knobs

Postby brian.olson » Tue Sep 14, 2021 11:34 pm

I know it is generally impossible to find replacement instrument panel knobs. Thus, another opportunity for an interesting experiment presented itself this past week. There are plenty of quality videos on YouTube showing how individuals restoring old radios duplicate impossible-to-find knobs. I won't bore you with all of the details, but a summary might prove inspiring if you want to try the process yourself.

Step 1: I don't smoke, and if I did I certainly wouldn't do it in the cockpit, so after my last flight in 62C I took the cigarette lighter home. 62C won't miss it for the few days that it sits on my desk. I was delighted to discover that the knob easily unscrews from body of the lighter.

IMG_7826.JPG


Step 2: With the knob off, it was an easy process to create a silicone mold from it, exactly duplicating the size, shape, and (with imperfection lending a bit of nostalgia) also duplicating a tiny blemish that was on the knob. I used a disposable plastic cub as the mold container, securing the knob with a 10-32 bolt through the bottom of the cup and a large, thick washer serving as a stand-off between the bottom of the cup and the knob. The molding material is bi-colored two-part liquid and easily mixes in a 1:1 ratio. Just measure it out, mix it up thoroughly, and pour it in. After the mold has cured (about 4 hours, though you can purchase faster-curing material) the plastic cup can be cut away. The silicone mold is tough but flexible enough that the original part can be easily removed.

IMG_7827.JPG


Step 3: Once the mold is ready it is a snap to create a resin duplicate of the knob. The resin material is specific to mold-making and comes in a two-part liquid. Just mix it up - also in a 1:1 ratio - and carefully pour it in. For my draft copies I chose a relatively fast-curing material - it is ready to de-mold after 10 minutes. While you are at it, why not pour a couple of knobs at the same time, assuming you made two or more molds of the same knob (which I happened to do).

IMG_7829.JPG


Step 4: I used a 15/64" drill bit to drill a pilot hole, into which I press-fit (i.e., pounded in with a hammer) a 10-32 brass threaded insert from McMaster-Carr. A curious fact that the 10-32 insert is the same size as the threaded adapter on a knob-less replacement instrument push-pull cable from Univar (but who is checking, anyway). I was initially concerned that the insert might pull out, but after install I am quite convinced the knob would probably disintegrate before the insert moves.

IMG_7830.JPG


Step 5: And, simply because I had a can of paint just sitting around, it was easy to confirm that yes, indeed - paint will stick well to the knob. Note that this is the color I used for the backing of the new plexi overlay panels I am engraving and not necessarily the matched color of the knobs currently in the panel; but it does give a nice idea of what the finish looks like.

IMG_E7832.JPG



This was just a draft of the process, and for the final work I probably would custom-match epoxy automobile paint with clear coat, much like the paint on my yokes (or, exactly the same paint). And would probably use a higher-strength resin designed for high-impact tooling, less susceptible to heat and vibration. And the threaded insert would also be permanently epoxied in along with the press-fit installation so it had no possibility of movement over time. All of which I may or may not have done already.

If only I could find a couple of knobs somewhere ....


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Product Details:

Silicone Mold Material: https://www.amazon.com/gp/product/B00AT ... UTF8&psc=1
Resin Material: https://www.amazon.com/gp/product/B00AT ... UTF8&psc=1 (note that in the final casting I used a higher-strength material, though it's likely I didn't need to - the white stuff shown above was really tough)
Mold Release: https://www.amazon.com/gp/product/B002Y ... UTF8&psc=1
10-32 Threaded Inserts: https://www.mcmaster.com/93738A210

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AN IMPORTANT NOTE and DISCLAIMER: All kidding aside, I am sharing a process that was both interesting and enjoyable, and I am not advocating that you begin mass-producing control knobs. Understand I do not make improvements to my aircraft without the supervision and approval of an A&P. Please consume the above information at your own risk.
Last edited by brian.olson on Wed Sep 15, 2021 10:51 am, edited 4 times in total.
Brian
1950 170A
N5762C s/n 19716
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Re: Instrument Panel Knobs

Postby voorheesh » Wed Sep 15, 2021 4:53 am

This is an example of basic ingenuity. It’s also perfectly legal (owner produced part). Removal and installation is likely preventive maintenance which can be accomplished by an owner without getting an A&P involved. But you probably know all that. Good Job!
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Re: Instrument Panel Knobs

Postby wabuchanan » Wed Sep 15, 2021 5:43 am

This is a great example of an owner produced part. Well done.
1950 170A N5776C SN:19730
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Re: Instrument Panel Knobs

Postby GAHorn » Wed Sep 15, 2021 2:52 pm

Do you have any scars from having done it on your wifes’ granite counter-top? :lol:

EXCELLENT WORK , and THANKS for SHARING!
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight.
An originality nut (mostly) for the right reasons. ;)
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Re: Instrument Panel Knobs

Postby DaveF » Wed Sep 15, 2021 7:25 pm

Great work, Brian. Thanks for the write-up!
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