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Old 11-04-2013
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Drilling/Tapping Acrylic Lenses

A slip neighbor of ours has purchased replacement Acrylic hatch lenses, one of which has a cut-out for a vent. The vent itself is held in place with 3 machine screws that, in the old lens, screw into threaded holes in the lens that the replacement lens is not furnished with. (Note that the screw holes do not go all the way through the lens and are tapped for 6mm machine screws.) So he needs to drill and tap new screw holes and has asked my help. I am not certain how one goes about doing that and I surely don't want to goof up his $100+ lens. I will appreciate any information/instruction from anyone that has previously done a project like this successfully. (I did tap Google for info on the process but thought the collective here might offer some helpful insights.)
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Last edited by svHyLyte; 11-04-2013 at 02:52 PM. Reason: Add addendum; Correct typo
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Old 11-04-2013
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re: Drilling/Tapping Acrylic Lenses

I've not done this exactly, but generally: there are drill bits specifically meant for drilling plastics rather than metals, they will probably make a cleaner hole. A drill press with a depth stop will ensure no 'overdrilling'..

To tap a blind hole like that requires a bottoming tap.. taps run from tapered 'starting taps', to plug taps and then bottoming taps that will thread right down to the bottom of the hole. In a thin lens a tapered tap might hit bottom before it even gets started, so you may need to start with the plug tap and move to the bottoming tap for the last few threads. I do not know if there are any specialty taps for plastics...
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Old 11-04-2013
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re: Drilling/Tapping Acrylic Lenses

Plexiglas drills and taps with metal cutting bits just very low rpm, perhaps hand turned.
Acrillic iv very brittle go easy.
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Last edited by desert rat; 11-04-2013 at 04:44 PM.
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Old 11-04-2013
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re: Drilling/Tapping Acrylic Lenses

A good response by faster, and he himself knows he left some details out.

Having tapped many holes in metals and plastics...
* This will be tricky, since the plastic is thin (1/4"?).
* This requires bottoming taps, which you will have to buy.
* You will need to practice on a scrap. I would.

After practicing he may decide to through tap (easier, safer, and stronger) or through bolt. I might consider a cemented-on reinforcement ring and tap through that; it could look very professional, if the ring was well cut and edge polished, and the port would be stronger for it.
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Old 11-04-2013
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re: Drilling/Tapping Acrylic Lenses

If you tap a slightly larger size, it will prevent the plastic from cracking when the screw gets tightened. Went through a couple of pieces of plastic in the learning process....
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Old 11-04-2013
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re: Drilling/Tapping Acrylic Lenses

Whatever you decide to do, let the owner do the actual work, that way he cracks his own $100 lens
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Re: Drilling/Tapping Acrylic Lenses

Yes, let the owner do it himself.

Beware of through tapping. The original was not through tapped probably to avoid leaks. A through tap might leak he will need a creative solution. Goop on the threads to seal the leak will eventually swell up and crack the lens.
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Old 11-04-2013
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Re: Drilling/Tapping Acrylic Lenses

Personally, I wouldn't go that route. I would drill through and bolt it. Acrylic and polycarbonate expand and contract differently than stainless and you could end up with cracks if the threads happen to be just a wee bit too tight (yep, voice of experience talking). No downside to through bolting that I know of, seal everything with Dow Corning 795 (hat tip to Faster for recommending it when I resealed the pilothouse windows).
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Re: Drilling/Tapping Acrylic Lenses

Ditto what PDQ sez....
Thru-drill larger than bolt and bush w/ either rubber or preferd sealant. General indications are specialty silicones formulated for plastics. If that,s the method to bed, it should hold that it *should* be OK to "bed" thru bolts.
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Old 11-04-2013
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Re: Drilling/Tapping Acrylic Lenses

I'm thinking a plexi shop would have the right taps, the drill press, and wouldn't charge a whole lot to tap three holes.

Or a machine shop.
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