Dave M. .......
I can buy recently made glass door knobs and spindle in a set from
local hardware stores that are identical to those sold by
architectural antique hardware suppliers, for around the same price.
I'll buy a new set, screw on the knob making sure that it's in the
right position so the set screw will be in contact with a flat side of
the spindle, and tighten the set screw. After a few weeks, the knob
rattles around in the door and after another week, if I pull on the
knob, it will come off in my hands leaving the spindle and knob on the
other side, in place. So I'll screw the knob back on, tighten the set
screw and two weeks later the knob slides off the spindle again.
I don't think the threads are failing, it looks as if there is a
fraction of a millimetre too much space between the threads of the
spindle and those inside the knob.
Loctite sounds like a great idea. I'll give it a try before epoxy
resin as a last resort.
Many thanks too all for their help, I do appreciate it.