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

On 10/11/2011 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...



That part lists a +/- 10 thou tolerance on it's diameter, so it's going
to be very hard to get a consistent press-fit.

Most light press fits are 1/4 to 1/2 thou.

The drawing would spec a hole size & the machinist would use their own
judgement & the spec'ed tolerance to determine if he can drill, ream or
bore to hold the spec.


MikeB

--
Email is valid but not checked often

On Tue, 11 Oct 2011 17:33:45 -0400, BQ340 wrote:

On 10/11/2011 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...



That part lists a +/- 10 thou tolerance on it's diameter, so it's going
to be very hard to get a consistent press-fit.

Most light press fits are 1/4 to 1/2 thou.

The drawing would spec a hole size & the machinist would use their own
judgement & the spec'ed tolerance to determine if he can drill, ream or
bore to hold the spec.

In my experience a press fit into pliable plastics like nylon is much
easier to achieve that into metal, and can have a lot more interference
than 1/2 thou.

It's the other side of the "hard to get an accurate hole" coin.

--
www.wescottdesign.com

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...

One of my assemblies has a 1/2" Delrin plug push
fit into an AL hole.

Delrin plug diameter is spec'ed .503" +.000" / -.001"
AL hole diameter is .500" +.001 / -.000 with edge broken

Worked good for me in production, plug can be pushed
into hole with the back end of a screwdriver and hand
pressure.

Tim Wescott wrote in news:htydndfv-
:

On Tue, 11 Oct 2011 17:33:45 -0400, BQ340 wrote:

On 10/11/2011 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...



That part lists a +/- 10 thou tolerance on it's diameter, so it's
going
to be very hard to get a consistent press-fit.

Most light press fits are 1/4 to 1/2 thou.

The drawing would spec a hole size & the machinist would use their own
judgement & the spec'ed tolerance to determine if he can drill, ream
or
bore to hold the spec.

In my experience a press fit into pliable plastics like nylon is much
easier to achieve that into metal, and can have a lot more interference
than 1/2 thou.

It's the other side of the "hard to get an accurate hole" coin.

A lot of parts designed to be press fit into plastic are knurled, which
gives a bit more leeway.

Doug White

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.

"rangerssuck" wrote in message
...
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.

-------------------------------------------------------------------------------------

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.

On Oct 11, 8:09*pm, "anorton"
wrote:
"rangerssuck" wrote in message

...
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.

---------------------------------------------------------------------------*----------

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.

That threading is a good trick. Have to file it away in the memory
bank. Sort of like knurling in reverse.

what ever happened to the little snap-in plastic LED holders?

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.


-------------------------------------------------------------------------------------

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

"Doug White" wrote in message
. ..
Tim Wescott wrote in news:htydndfv-
:

On Tue, 11 Oct 2011 17:33:45 -0400, BQ340 wrote:

On 10/11/2011 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...



That part lists a +/- 10 thou tolerance on it's diameter, so it's
going
to be very hard to get a consistent press-fit.

Most light press fits are 1/4 to 1/2 thou.

The drawing would spec a hole size & the machinist would use their own
judgement & the spec'ed tolerance to determine if he can drill, ream
or
bore to hold the spec.

In my experience a press fit into pliable plastics like nylon is much
easier to achieve that into metal, and can have a lot more interference
than 1/2 thou.

It's the other side of the "hard to get an accurate hole" coin.

A lot of parts designed to be press fit into plastic are knurled, which
gives a bit more leeway.

Doug White


Yep.

Maybe drill the hole to slightly oversize and make up a rough and ready tool
to hand press into the hole to "knurl" it.

On 10/11/2011 9:13 PM, Tim wrote:



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).


Here...


http://parts.digikey.com/1/parts/690...hlmp-0103.html

"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.


-------------------------------------------------------------------------------------

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?

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.



-------------------------------------------------------------------------------------

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

On Tue, 11 Oct 2011 16:02:44 -0500, 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...


Any chance that the Nylon thickness might allow the use of a regular
rubber or soft plastic grommet as the mount? Might allow for
tolerance issues.
Bob
rgentry at oz dot net

On Tue, 11 Oct 2011 17:09:35 -0700, "anorton"
wrote:


"rangerssuck" wrote in message
...
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.

-------------------------------------------------------------------------------------

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.
The idea of threading a hole sure sounds good. I wish I'd thought of
it. I could have used it several times. I suggest that a roll formed
thread would work better than a cut thread because the deformed
material making up the thread would probably require a little more
pressure to deform by the pressed in part and so may grab a little
better. Thanks for the great idea!
Eric

"Tim" wrote in message
news
...
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

I encountered that problem when I built a fast picoammeter to measure the
millisecond-scale relaxation time constant of dielectric absorption in FR4,
wire and reed relay insulation. It detected any sounds in the room including
the ultrasonic alarm system, which we didn't know about and anyway should
have been well above its bandwidth. The Feyndishly brilliant engineer
calculated that the cover panel had to move only about the diameter of an
atom to produce the capacitive coupling we measured. I fixed it by creasing
the panels to increase their fundamental resonance.

jsw

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.

Threading the hole for 1/4-20 seems to work really well -- thanks. I
tried this with a #7 bit and a 1/4-20 tap in some plastic I have on hand
(not Nylon, but with similar mechanical characteristics). It works
great, even though the bit is oversized. I'd try it with a 3/16" bit,
but I can't find any at the moment!!! (Life is great in a well-equipped
shop, ain't it?).

--
www.wescottdesign.com