Sizing a Hole

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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On 10/11/2011 5:02 PM, Tim Wescott wrote:

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

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.
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fit
need?

enough
sample
going
or
A lot of parts designed to be press fit into plastic are knurled, which gives a bit more leeway.
Doug White
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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.
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Tim Wescott wrote:

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.
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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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wrote:

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.
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wrote:

That threading is a good trick. Have to file it away in the memory bank. Sort of like knurling in reverse.
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what ever happened to the little snap-in plastic LED holders?
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On Tue, 11 Oct 2011 17:09:35 -0700, anorton wrote:

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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).
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Tim Wescott
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wrote:

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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?
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On Wed, 12 Oct 2011 18:42:15 +0800, Dennis wrote:

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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).
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Tim Wescott
Control system and signal processing consulting
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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
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On Tue, 11 Oct 2011 17:09:35 -0700, "anorton"

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

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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?).
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wrote:

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
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