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Old as Dirt!
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Discussion Starter #1 (Edited)
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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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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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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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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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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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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Courtney the Dancer
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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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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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██▓▓▒▒░&
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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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Don't over tighten the bolts, no matter how you do it. Even if the plexy doesn't crack right away, it may crack over time. Acrylic (lexan) is much more forgiving and does not crack. It's literally "bullet proof".
 

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The problem with a tap that you want to do, assuming you don't make any mistakes drilling and tapping, is you can't use any sealant (should not need any, I know) or loctite. You may create a hydrolock and crack or blow out the bottom of the hole. This could also happen with just air being bound. I would suggest through drilling and using Seaduction's suggestion of a sex nut or well nut... Well Nuts from Estco Enterprises
 

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Old as Dirt!
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Discussion Starter #14
Thank you all for your insights and observations. After considering your collective answers and suggestions and reviewing several on-line tutorials including a good one by Tap-Plastics, I have come to the conclusion that going to a local plastic's fabricator is the only sensible solution. Neither I nor Kurt have a drill press necessary to control the angle or depth of the penetrations nor the drill bits or proper taps for cutting threads in plastics. Nor do I want to accept responsibility for ruining someone else's equipment if I err (which is more likely than not). Frankly, I can't afford the learning experience (although I will see if I can get the old lenses for some experimentation in my garage). As Clint Eastwood once intoned: A man's GOT to know his limitations!

Thanks...
 

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Take a look at Outwater Plastics. They have just about everything that has to do with plastics and it's all on line.

And I'll have to correct AllThumbs. Lexan is a brand name of polycarbonate sheet. It's the material of choice for hockey rinks, helmets and the like because it can take a blow without fracturing. It can even be brake formed like sheet metal. Acrylic can't be used that way. But there is a product called impact modified acrylic which is slightly softer and can deflect appreciably without fractures.
 

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Because this has come up in the thread; can anyone here recommend an online source for UNEXPIRED Dow 795?
 

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eh-
Dow Corning has (or at least had) great phone support. If you call them to complain about expired product, they should be able to direct you to authorized distributors and if you get the right rep on the right day, they may just send you a tube. Free, gratis, on the house, because that tends to make loyal customers out of folks who have problems.
 

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And I'll have to correct AllThumbs. Lexan is a brand name of polycarbonate sheet. It's the material of choice for hockey rinks, helmets and the like because it can take a blow without fracturing. It can even be brake formed like sheet metal. Acrylic can't be used that way. But there is a product called impact modified acrylic which is slightly softer and can deflect appreciably without fractures.
My bad. You are correct. I was thinking polycarbonate but typing acrylic. Lexan is indeed polycarbonate. Plexy is acrylic.
 
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