A new knob from an old one, using a lathe

[Yorkie]

I've recently posted a restoration story on a littler Portadyne Princess TRF set. When I bought it, it was minus its three knobs, which are quite distinctive, and aren't easy to come by. I was lucky enough to find two on internet, and my son found another at the NVCF, though it was a push-on spring type of fixing rather than grub screw. As it's a live chassis set, a push-on knob was not a safe option and in any event, the Bakelite housing for the spring was crumbling away so the knob wouldn't have worked properly and would soon have fallen apart. I therefore decided to modify the knob to a grub screw fixing.

First I filled the hollow around the spindle housing with so called 'plastic steel' a two-part filler made by Plastic Padding. Then I wound one turn of masking tape around the knob to protect it from damage when held in the chuck on the lathe. Having mounted the knob in the chuck when the 'plastic steel' had hardened, I faced off the rear of the knob, then using a small 'boring bar' gently increased the diameter of the spindle hole to 3/8" so that it would accept a piece of 3/8" brass bar stock. This has to be done with care, because the boring bar could easily snatch the knob out of the chuck if too fierce a cut was attempted.

I then mounted a piece of 3/8" brass bar in the chuck and drilled it out with a 1/4" drill in the tail stock (having first used a centre bit of course). I parted off this 'sleeve' to the correct depth to fit the knob. I roughened up the 'sleeve' with a hacksaw blade so that Araldite would key to it, and gued the sleeve into the knob. When it had set, I drilled and tapped the knob with its integral brass sleeve to accept a 4BA grub screw.

I doubt I'll have been the first person to effect this type of repair, but I thought I'd post the info as an example of how a small lathe can be useful for little restoration jobs that only spring to mind when you've actually got a lathe. I've attached pics of the underside and upperside of the repaired knob, and the underside of an original grub-screw mounting knob for comparison.

Hope it's of interest.

[Retired]

Hi,

Thanks for adding this interesting thread David. A first class repair that should make the knob serviceable for many years to come and done very neatly indeed.

These are delightful little jobs for a lathe owner giving a great deal of personal satisfaction. I would recommend any member to buy a small lathe assuming Rob (Bushbaby) hasn't cornered the lathe market with his growing collection of lathes.

I've done similar work but mostly on home made wooden knobs; your explanation of the technique should help others wishing to have a go at such repairs.

Your technique ensures concentricity which is most important if a lop sided knob is to be prevented. It is always a gamble whilst chucking fragile items as it is very easy to crush and destroy them as a chuck can exert a tremendous amount of pressure; if not gripped tightly enough though as you say David damage can occur during drilling and using a boring bar but it is most interesting work.

I'm impressed and pleased to see you using the lathe; keep up the good work.

Kind regards, Col.

[Aerodyne]

Radio knobs are always a weak link and any methods of repairing or renewing them are always welcome. Thanks for this useful tip, David.
-Tony

[Geaney]

Excellent work David.

Note to self, must get a lathe.

Cheers, John

[Robert Darwent]

A super repair job David, with an excellent explanation and end result. Thanks for sharing!

Regards

[Yorkie]

Thanks for your very kind comments guys!

Knobs, dials, backs and speaker fabric - damaged or missing - are the story of our lives! I posted a thread some time back on repairs to the polystyrene knobs on a 'Fenman 1' which have a long fragile sleeve and are held on with a clip. They're notorious for crumbling away. I turned down the knobs so that only the front discs were left, and made new shanks from transparent acrylic, and glued the knob discs onto the new shanks to which I fitted grub screws instead of a spring clip. The initial attempt wasn't successful as the acrylic shanks cracked when the grub screws were tightened, so I added a sleeve of aluminium tube.

The lathe I use was one that was given to me by a friend. It was just a rusting heap of bits and lay under the workbench neglected for several years till I mentioned it to Colin. At that time, the make of lathe, it's origins, age and original purpose were a mystery to me. It was only when I mentioned it to Colin that he told me it was probably 100 years old and was a watchmakers' lathe by a German firm called Lorch Scmidt. Colin encouraged me to clean it up and get it into working order, since which time it's proved very useful for all sorts of little jobs I'd never imagined.

It came with a box of collets, which I hadn't really known the pupose of, but Colin put me wise and made a rather splendid brass nameplate for the collet box. I've mentioned this on the forum before, but another mention may not be amiss if it encourages others to acquire and use a little lathe. I'm never going to be into big stuff, and most of what we're likely to want to do can be done on a model makers' lathe.