Help Needed to Reconstruct 1950s Hewlett-Packard Knob

[Anthony]

Could someone with experience of 1950s ?plastic knobs suggest a solution to this problem please. I have refurbished a 1950s HP 'Adjustable Short' for J-Band Waveguide. The problem is that the original 'crank' for this knob broke off before my time. The knob has about 40 turns for full travel and now spins quite freely. It was originally frozen due to corrosion and I suspect this is what caused the original crank to snap off.
A knob similar to the original intact one is shown on the X920A image. There is also a photo of the broken knob.
It is now fully functional except for the crank (!) but it would be nice fully to restore it.
The obvious one of replacing the old knob is difficult. The set screws are rusted-in solid. Of course one might remove the knob casing destructively but the knob is carefully designed I suspect to provide a match to the threaded tube in which a threaded bolt runs. It all turns nicely at the moment and trauma may knock it all out of its nice alignment.
I know nothing about this area but would it be possible to glue on a rough blob of ?epoxy which could be filed and drilled to accommodate a crank? There are several variations on this theme. Ideas?
Very grateful for advice.

[Boater Sam]

If you can make a new crank, the whole thing, you could cut the old one off square to match up with the new one.
Then drill 2 or 3 small holes in the ends of both for short stainless steel pins to connect them in line.
Epoxy the 2 parts and the pins together. It should be well strong enough.

[Anthony]

Sam. Nice idea. Thanks, What sort of epoxy might be best?

[Boater Sam]

The old slow sort, never had much success with the quick stuff. Reputable makes, not Poundshop cheapies.
If you can get a good close fit, superglue takes some beating.

[Diabolical Artificer]

I've just had some experience of removing stubborn knob's and can sympathise. It's tricky to drill out the grub screws as they're made of hardened steel. Heats out as it will destroy said knob. I did get some off with force by prying them off, grub screw intact, but these were on small diameter ali shafts.

Andy.

[Anthony]

It's not perfect but I grafted a crank from a toolbox HP knob onto the waveguide knob. I just used traditional Araldite but chickened out from Sam's suggestion of adding stainless steel rods for fear of cracking. The torque is low. We'll see. Many thanks indeed for the help. It works very well.

[Diabolical Artificer]

Good to see you fettled it, it looks a preety decent fix.

Andy.

[high_vacuum_house]

Would this do? At £30 its a bit steep though probably ok as a one off. Probably find one cheaper if I looked deeper into finding one.

http://www.ebay.co.uk/itm/CRANK-HAND...QAAOSw4CFY5oEm

Christopher Capener

[emeritus]

I have used Plastic Padding car body filler (type hard) for this sort of thing, in conjunction with wire reinforcement as per #2. For small items I use some 0.7mm stainless steel dental wire that I got from a surplus stores many years ago, but steel or brass panel pins have served for larger items. Plastic Padding sets quite quickly and, once cured, can be filed or sanded or scraped to size and then painted. I did once make my own cranked knob using a 4BA round head chrome plated brass screw with a plated brass 4BA spacer that RS used to do that was the same diameter as the screw head.

[emeritus]

As a post script, petrol lighter fluid (e.g. Ronsonol) can work wonders with seized screws.

[Anthony]

All comments gratefully received. Not being a true perfectionist I think I am about there with this knob. The crank now works although I admit that the look is imperfect. The waveguide itself, at least, has now been sprayed to look a bit more like the original HP olive-green. One side-effect is that I have rediscovered the 'original' slow-cure Araldite.

[BulgingCap]

For info of anyone with similar problems:
I have had a lot of success with casting parts onto broken components using epoxy or casting resin, or glass-loaded filler resin. The moulds can be made from Plasticene if simple, or candle wax if more complicated. Any pins needed are thus easily drilled and fitted without problems with 'registration' if used to glue on a part.
BC

[Radio Wrangler]

Within the HP plant the darker olive green was affectionately known as ducks*** green.

It was so much used that I forget the official name.

David