brush-head has brought this to us :
hello
Two of these chairs have one of the four holes that take these screws,
blocked with a piece of broken screw. Now I know that I would drill these
bits out but, not being a consummate DIYer, I thought I'd ask this group, the
best way of doing this and if there are any gadgets around that would make
this easier/safer. I obviously want to avoid damaging the thread that's been
cut in the tubes by the previously unbroken screws.

If they are self tappers, you will find these are hardened steel, much
harder than the material they self tap into - so impossible to drill
and even if you could the drill would wonder into the softer steel of
the chair frame. If you can't get them out with a self-grip wrench (or
mole grips), then I would be inclined to try using an electric welder
if you have access....

Deliberately get a welding rod to stick on the screw rather than form
an arc, allow the rod to cool then use it to turn the screw out.


The other question I have is how do I go about determining the size of the
thread for these screws? Or do I just go trying various packets in B&Q and
Homebase until I find the right ones - it's difficult on sites like Screwfix
and can work out expensive if you get the wrong ones.
Thanks for any help anyone can give.

Find a local fixings supply company or a nut, bolt and screw specialist
and show him a sample of what you require.

--

Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.org