What size hole should I specify be drilled in Nylon to get a press fit
for a 0.195 inch part? And what sort of tolerance range would I need? I
want anything between a really light press fit that just makes enough
contact to positively locate the part from side to side with no force on
it, to a press fit that requires maybe ten pounds of force (i.e., that
won't break anything on assembly).

For that matter, if you specify a hole size in something like nylon that
tends to spring back, are you specifying the size of drill (and expecting
an undersized hole), or are you specifying the holes size (and expecting
the shop to compensate for the material)?

Here's the part: http://www.fairchildsemi.com/ds/QE/QED123.pdf.

This is probably going to boil down to giving the machine shop a sample
or two and saying "Make work. Make work good". But I'm curious...

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