"Tim" wrote in message
news
On Tue, 11 Oct 2011 17:09:35 -0700, anorton wrote:

"rangerssuck" wrote in message
news:4fdee0fa-c53d-4417-9638-
...
On Oct 11, 5:02 pm, Tim Wescott wrote:
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...

--www.wescottdesign.com

If you make the hole any larger than 0.185, LEDs at the small end of the
tolerance will not be a press fit at all. What you may want to do is
drill the hole a tad smaller than .185 +0/-1 (and you ALWAYS want to
specify a finished hole size) and then see how it is to press in
something 0.205 diameter. My guess is that it's going to be too tight.

Your choices may be to buy and measure a batch of LEDs (they probably
won't vary much in a single batch) and drill the hole to suit, or, use
some sort of adhesive. Another option would be a small printed circuit
board to hold the LED and a screw to hold that to the plastic.


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I agree with the above. I don't think I have ever seen a press-fit LED.
The plastic encapsulant is somewhat brittle and there might be too much
risk of damage.

That said, there are a few ways to increase the lee-way of a press-fit.
One is to counterbore the hole so only a small rim is deformed during
pressing. Another is to thread the hole so during pressing the peak of
the threads can flow into the troughs.

I may suggest the threading trick, assuming that there's a standard size
that's close enough.

In case folks are wondering, the reason that I want to press them into
the holes is to maintain the angle: there's an LED, and about six inches
away there's a phototransistor (in the same type of case). I'm using
parts with about an 18 degree included angle. That's fairly generous,
but I'd still like to hold the aim.

Come to think of it, a fallback position would be to hold the LED flange
against a countersunk rim (this all is going into a holder that aims the
LEDs, provides convenient mounting features to a larger assembly, and
strain-relieves the cable to the LED).

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com


Tim - out of interest is the LED being used for sensitive measurement /
linear feedback?

I have no idea if it is an issue but does the characteristics or tempco of
the LED change when under mechanical stress?