On Wed, 12 Oct 2011 18:42:15 +0800, Dennis wrote:
"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?
Whoa -- good question! It's being used as a photogate (there's two
sensor/LED pairs stacked vertically), which is pretty much on/off.
But that's the sort of thing that you need to ask. I once went to a
presentation by Litton of their fiber optic gyro product. The
engineering rep gave a technical talk, starting with the pitch "This
wants to be a gyro, an accelerometer, a thermometer, a microphone, and
many other sensors all rolled into one -- we had to do some work at
compensation." (For those of you scratching your heads -- it's a
system's engineer's in-joke. Every sensor you try to make responds to
_everything_ -- it's your job to try to figure out how to build it to be
inherently insensitive to everything else, or to independently measure
the other factors and null them out. It's why dedicated systems
engineers are bald).
--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com