[Amps] RE : amp hardware

[Ian White GM3SEK]

Peter Chadwick wrote:
>Hank bushes, PEM nuts, rivet nuts - whatever, they're all variations on 
>a theme - are real good for sheet metal. Not always applicable, though, 
>to spacers and the like, where internal threads are needed, or even for 
>thick ( say 1/4 inch upwards) metal or whatever.. I even have hank nuts 
>with a nyloc insert, so that they're self locking. Countersunk screws 
>have a tendency to be self locking, too, if they're the conventional 
>type with a screw driver slot - the head gets slightly compressed, and 
>so there's pressure against the sides of the countersink.
>You do need to get the hole the right size though. With hank bushes, I 
>always drill undersize and then ream to get the final fit. Then use the 
>vice to press it in and slightly deform things so it's solid.


Be very cautious about using hank/rivet/PEM nuts as a repair for 
existing equipment. They are the kind of technique that works best in a 
factory situation, where everything can be set up just right, where you 
can easily get to both sides of the sheet, and where there's a big 
hydraulic press handy.

A repair situation is the exact opposite. You will always be working in 
an awkward space, underneath a narrow flange, and probably you don't 
even have the right hand tools. (Steve's method with a big vice and a BB 
is about the best way to do it without specialist tools, but it's 
difficult to use for repairing a whole amplifier.)

But the very worst thing about using these bushes for repairs is that 
you have to drill out a much larger hole for them - and then you're 
totally committed. If it doesn't come out right, you are in big trouble.

(Also, it's also good to remember why we're doing this - not only to 
hold the cover on, but also to make a solid, RF-tight seam. 
Hank/rivet/PEM nuts can very easily make a small standoff between the 
cover and the case. This will open up a nice little slot antenna that 
wasn't there before.)

For situations where only a few self-tapping holes are stripped, it is 
much better to keep it simple. Leave the holes exactly as they are, and 
epoxy a new piece of metal over the inside of the hole. Then you can 
simply re-use the same screws.

Use a decent thickness of mild steel, so the screw won't ever strip 
again. Drill exactly the right size of pilot hole (not too big, not too 
small)  and start the screw into the metal *before* you glue the piece 
into place. Then use the strongest epoxy you can find, be very serious 
about de-greasing... and it'll be fine.



-- 

73 from Ian GM3SEK