Repairing broken knobs.
 

The Black and Chrome Ekco had two broken knobs that is to say the inside was broken where the screw fits , I had this idea ,got my friend to make me a solid block of aluminium with a 1/4" hole drilled down the centre , in the lathe .
Having cleaned all the broken parts out ,I was left with just a cup , I had some shaft extenders which I used part of ,and fixed them into the cup making sure they were centralized by using a piece of silicone sleeve , applied some body filler and assembled it as shown ,clamped together until set ,painted the coloured dots with some very useful pens bought off Ebay , Mick.

Re: Repairing broken knobs.
 

Good idea and good job, Mick!
Necessity is the mother of invention!

Re: Repairing broken knobs.
 

They also make a thread repair kit here in the U.S. Called Helicoil (R) for repairing damaged threads. One drills out the hole a bit larger then inserts the Helicoil. Once it is in place, use the original screw.
That being said, more than once I have drilled the holes a bit larger and re-tapped them to use a bit larger Allen screw aka grub screw.

Re: Repairing broken knobs.
 

Helicoils are pretty much globally available, but are limited in how small they are made.

For those who haven't come across them:

You drill out the damaged threads with a tapping-size drill to suit the outer of the helicoil, you then tap the hole the appropriate size and fit the helicoil. The helicoil is a helix of 4-sided quite hard wire with one end bent inwards as a lug. The fitting tool goes through the centre and has no threads. A notch at the end engages the lug. You screw the helicoil and tool into the tapped hole. By dragging the helix in, driven from the far end, it coils a little tighter and shrinks in diameter, easing the insertion. Once as far home as you need, you pull the tool out leaving the re-expanded helicoil gripping the outer threads. There is also a cutter tool which can cut off the lug if needed.

This leaves you with relatively hard steel threads which are good regarding wear compared to tapped holes directly in aluminium alloy or brass if things keep getting fitted and removed.

Their limitation is that you have to have the same pitch on the outside as on the inside, so the outer thread is a bit fine for the diameter, and the tap needed to cut the thread can be an unusual size. The outer threads are somewhat finer than you'd like in soft materials for best strength.

They are a standard engineering fix for stripped threads and can leave the repaired part taking the original size of screw. There still has to be enough 'meat' in the part being repaired to take the larger tapping of the helicoil outer, so you could have drilled, tapped and used a larger screw if the rest of the assembly would have suited the increased diameter.

Welding up the hole, drilling, tapping just doesn't work on plastic knobs :-)

David

Re: Repairing broken knobs.
 

Here are some I have repaired/fabricated recently

Chris