# Cutting and drilling Acrylic 1/4" sheet for windows.



## zz4gta (Aug 15, 2007)

How can I get a smooth edge on acrylic and drill for screw holes w/o worrying about the sheet cracking? Any special techniques? Thanks.


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## arf145 (Jul 25, 2007)

I've run a fine flat file lengthwise over the edge to round it out.


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## Northeaster (Jan 13, 2007)

You can use a belt sander to easily shape / even out the edge, etc.

Have you considered using something like 3M VHB tape, as an adhesive, instaed of drilling / using screws / bolts that will leak later. Also use something like Dow Corning 795 or 791 adhesive sealant to keep the water out.

I redid my deadlights using this method "sexy windows" thread on link below.

Very happy with it. They use the 3M tape to hold big windows in skycrapers, etc.
My old frames were through bolted to the cabin, but the acrlyic was just siliconed to the outside of the frame!!! So, now I have the acrylic sealed to the cabin, with no frame, no holes to leak. I tapered the black Dow 795 sealant along the edges, to give it about a 45 degree taper.

Was not able to upload pics here!!

Dscn0808 - Cruisers & Sailing Photo Gallery

new "sexy" windows - Cruisers & Sailing Forums

HOW TO HAVE SEXY WINDOWS!? - Cruisers & Sailing Forums


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## Faster (Sep 13, 2005)

Wet sanding after cutting with a fine tooth blade on a table saw works, but is a bit labour intensive. There are special drill bits available for drilling plastics, they are worthwhile picking up, also be sure to drill the hole oversize for the fasteners so that you don't stress the hole during expanion/contraction.

We have also had good luck with Dow Corning 795.


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## tommays (Sep 9, 2008)

We scrape it by hand with a knife to get a final clear finish


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## jarcher (Jul 29, 2008)

Northeaster said:


> Have you considered using something like 3M VHB tape, as an adhesive, instaed of drilling / using screws / bolts that will leak later. Also use something like Dow Corning 795 or 791 adhesive sealant to keep the water out.


Dow 795 and Dow 791 are both silicon based sealents, so if you are planning to seal the acrylic against the fiberglass you may want to consider a different adhesive. As has been discussed here, silicon is bad for fiberglass.


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## zz4gta (Aug 15, 2007)

How fine should the blade be? I only have a skill saw with a blade I used to put my flooring in. It's fine toothed........for wood.


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## jarcher (Jul 29, 2008)

Lots of information about 3M VHB Tape here:

http://multimedia.3m.com/mws/mediawebserver?66666UuZjcFSLXTtnxMtLXs6EVuQEcuZgVs6EVs6E666666--


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## jarcher (Jul 29, 2008)

Im in the process of the same job, and I am considering (have not decided yet) using 3M 560 or 540. 3M says:



> 3M™ Scotch-Seal™ Polyurethane Sealant 540 and 3M™ Scotch-Seal™ Polyurethane Adhesive Sealant 560 are one component, moisture curing products which form permanent elastic bonds. They bond to a wide variety of materials including plastics, FRP, SMC, aluminum, steel, coated metal, and wood.


Here is all the data:

http://multimedia.3m.com/mws/mediawebserver?66666UuZjcFSLXTtnxf25XF6EVuQEcuZgVs6EVs6E666666--

The only catch is I only see 540 on sale on their web site, can't find 560. 3M says that 560 has a tensile strength of 580psi and an elongation before break of > 300%. For windows that might be a good strength, although if you want less than the 540 has 250psi and > 800%.

I can't say I know just how strong a dead light bond should be, hopefully someone who knows will chime in


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## tommays (Sep 9, 2008)

One problem is that the saw will MELT the plastic as you cut SO you need to keep it a bit wet and work slow

Then you can use a sanding block to clean up the edges


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## zz4gta (Aug 15, 2007)

Would 4200 work as a sealant? Or does it not strech enough?


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## zz4gta (Aug 15, 2007)

tommays said:


> One problem is that the saw will MELT the plastic as you cut SO you need to keep it a bit wet and work slow
> 
> Then you can use a sanding block to clean up the edges


Thanks for the tip tommays.


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## jarcher (Jul 29, 2008)

zz4gta said:


> Would 4200 work as a sealant? Or does it not strech enough?


I'm actually on hold with 3M right now trying to get that very answer 

3M's web site seems says 4200 is "medium strength for disassembly." I a trying to get the tensile strength and elasticity.


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## Northeaster (Jan 13, 2007)

jarcher - I have heard lots of people advise against old style silicon, for various reasons( such as temporary adhesion only, the niusance or removing it from gelcoat, in order to repaint, etc), but I have never read that it would harm the fiberglass / gelcoat. The "sexy windows" thread, in it's original link to Bob Norson's article, suggests the dow 791. I found Dow 795 locally, and was extremely happy with it. Others have suggested Sika 295"??, which sounds ver ygood as well.

ZZ- I would not use 4200. You want something that is able to stretch much more, as the acrylic has a high elongation rate with temoerature changes.


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## Northeaster (Jan 13, 2007)

jarcher - I believe I used VHB 5952. It was black for sure, and I remember looking for one of the thicker tapes, as I put a good bead of Dow 759 a cm away, that went between the acrylic and the gelcoat (providing more adhesion, and a watertight seal), and spread out around the edges as well.

I wanted thick VHB tape, as the thinner it would be, the thinner the bead of Dow 795 would have to be. And you want it thicker, so it will elongate more.

Here's a link to the tape comparisons, and 3m store:

http://multimedia.3m.com/mws/mediawebserver?66666UuZjcFSLXTtnxMtLXs6EVuQEcuZgVs6EVs6E666666--

Shop 3M: 3M VHB Heavy Duty Mounting Tape 5952 Black, 3/4 in x 15 yd 45 mil


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## jarcher (Jul 29, 2008)

Northeaster said:


> ... but I have never read that it would harm the fiberglass / gelcoat.


I misspoke (mistyped). The issue with silicon on gel is that (according to the books and threads I have read) gel does get porous, and the silicon gets in there and is very hard to get out.

Once its there, nothing else, including more silicon, will stick. So if you ever need to rebed or replace your dead light, you may have trouble sealing it again.

Of course I never argue with results, and if it works well for people good deal.

3M just emailed me all the relevant data sheets and I am waiting for them to arrive. The customer support person suggested 4000UV is the best choice for a dead light but she said that you still need mechanical fasteners, which apparently cause other problems.

I'll post the info when I receive it, which should be any second...


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## Faster (Sep 13, 2005)

Though DC 795 is silicon based, it is not of the "vinegar smelling" variety, which, I believe, is the "problem" silicon to use on boats in general.

We have done several boats with it, and years later it still stands up OK and has not leaked.


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## Northeaster (Jan 13, 2007)

there have been many threads about the difference between fastening and sealing (ie if you have mechanical fasteners - screws / bolts, then you can get away more with a sealant, from a gun or something like butyl tape - Mainsail / Halekai has lots of nice pics and comments in his posts ) 

I choose, as many are doing now, to use the VHB tape, as the mechanical fastener, instead of screws / bolts that are prone to leak. I added the Dow 795, which is both a strong adhesive (mechanical fastener / glazing silicon) as well as a "stretchable" sealer, that will stay adhered to both surfaces as they elongated or shrink at different rates.

I can tell you that I put in one portlight / deadlight by mistake, and had to remove it seconds later. The Dow 795 was obviously not cured yet, so the only adhesion was the VHB tape, which I had not even pressed on hard. It was all I could do to use my whole body weight to push it out. Now that they have cured, I doubt they could be pushed out - maybe after many hard kicks!!


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## jarcher (Jul 29, 2008)

Faster said:


> Though DC 795 is silicon based, it is not of the "vinegar smelling" variety, which, I believe, is the "problem" silicon to use on boats in general.
> 
> We have done several boats with it, and years later it still stands up OK and has not leaked.


Here is the data sheet for Dow 795:

http://www.meiglobalsolutions.com/images/dowcorning795.pdf

I should point out that Tony at Select Plastics highly recommends this stuff for adhering dead lights to gel coat, and recommends against using any mechanical fasteners.

If I read the data sheet correctly, the elasticity is 50% and the strength at 50% elasticity is 50psi. If that is all that is required to secure a dead light, then there are probably many other options in adhesives.

But I may have read it wrong, I didn't get much sleep last night and I am starting to fade 

Still no email from 3M


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## Northeaster (Jan 13, 2007)

In my case, most of the adhesion is provided by the VHB tape. As the two surfaces expand / contact at different rates, the Dow 795 doesn't have to keep the windows on the boat, it just has to stay adhered to both surfaces, so it remains sealed. I know they expand alot, but in real life, I doubt if the 795 is being expanded to the full 50%, unless you have very long or wide deadlights.


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## jarcher (Jul 29, 2008)

Well the data sheets never did arrive by email, so some Googleing finally revealed the magic incantation to conjur them up. So:

_Adhesives_

3M 5200 (polyurethane)

http://multimedia.3m.com/mws/mediawebserver?66666UuZjcFSLXTtlx&EO8TVEVuQEcuZgVs6EVs6E666666--

3M 4200 (polyurethane)

http://www.bindingsource.com/pdf/Fast Cure 4200 Data Sheet.pdf

3M 4000UV (polyether)

http://www.bindingsource.com/pdf/Fast Cure 4000 UV Data Sheet.pdf

_Sealent_

3M 101 (Polysulfide)

http://multimedia.3m.com/mws/mediawebserver?66666UuZjcFSLXTt48TcnxM_EVuQEcuZgVs6EVs6E666666--

*Readers Digest, (test results from dumb bell samples as listed on data sheets):*

5200 Tensile Strength 638psi 802% elongation
4200 Tensile Strength 300psi 750% elongation
4000UV Tensile Strength 253psi 790% elongation
101 Tensile Strength 139psu 416% elongation

Those numbers can be used for direct comparison, but the strength varies grately depending upon what is being bonded.

5200 says fiberglass 362, gel coat 519 and acrylic 217.

4200 says fiberglass 258, gel coat 254 and acrylic 22.

4000UV says fiberglass 251, gelcoat 424, ABS 279

So what does all this mean? You need to be a chemist to fix your boat :laugher

Im not sure I like any of these for bonding acrylic windows to gel coat. I will look further but I want that goldilox adhesive that holds firm but not so firm I can't separate it later.

I'll take a closer look at that VHB (very high bond) tape and post those numbers, but I'm afraid it won't come off.

Hm, sorry for taking the thread so far off topic...


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## JeffBurright (Apr 22, 2009)

*acrylic holes*

Have you thought about using a wood burner or soldering iron to make fastener holes in the acrylic? I used this trick on a thin sheet that I used to hold my boat's registration numbers, and it worked like a charm.


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## Maine Sail (Jan 6, 2003)

*Guys..*

Guy you don't need to take my word for this. Here are some quotes from Tony D'andrea who is the Acrylic & Polycarb guru in the marine industry. He is the Nigel Calder of portlights and hatches and owns the largest hatch warranty & repair facility in the world..

*
Tony on Acrylic vs. Polycarbonate:*

_*"Consider the Following:

*_
_*All major hatch, portlight and window manufacturers use Acrylic in offshore / bluewater marine products.*_
_*Acrylic is more scratch resistant than standard (9034) polycarbonate.*_
_*Acrylic is significantly more durable when exposed to Ultra Violet radiation (sunlight).*_
_*Acrylic is less expensive than Polycarbonate.*_
_*Don't misunderstand my preference for acrylic. I buy, use and sell a significant amount of both products and each has its application. In my humble opinion Acrylic is more durable, versatile and cost effective in the hands of a skilled craftsman than polycarbonate.

Additional considerations may include polycarbonate with UV and scratch resistant coatings. While these products are heavily promoted by several manufacturers and carry 5, 10 even 15 year warranties the following information has been reported in "real life"applications:
*_
_*Polycarbonate is impact resistant. When its new it is almost impossible to break.*_
_*Small quantities (less than a 4 by 8) in gauges over 1/8th inch are difficult to to find in the uv/scratch resistant grades.*_
_*Colors are limited. Only two standards (gray and bronze). Try and find anything thicker than 1/4 in UV/ scratch resistant!*_
_*Polycarbonate foreshortens when subject to static or dynamic loads. What this means is if you replace your hatch lens with polycarb, seal it and then step on it the ductile material will deflect (bow) in the center. One of two things may happen. 1st you will surely break the watertight seal, 2nd you may end up with a leg in your galley.*_
_*As for the warranty: The original owner is warranted against failure subject to the material being submitted to the distributor for evaluation with the original invoice subject to actual replacement cost at the time of purchase. I guess this means they sell you a new square of material and apply the old payment to the new cost. How about the labor to fabricate the part, install it and sealant? Why take the chance?*_
_*Polycarbonate is a great material, The US Air force uses it for fighter canopies! I sell Polycarb to the USCG and US Navy. Remember they don't* *mind using it because we are paying to replace it every three years.*_
_*Both Acrylic and Polycarbonate have specific uses and installation requirements.

Cast Acrylic (of a specific thickness) is in accordance with CE and ABYC guidelines, and installed on virtually all of the big blue water sail boats produced on both sides of the pond. Polycarbonate is commonly used as a replacement due to its ease of fabrication and incredible initial strength. The USCG and USN require Polycarbonate on their vessels but they also have a PM cycle of 36 to 42 months for change out. My Tax dollars at work...

Due to its ductility Polycarbonate it is more challenging to install. I have seen Sika Flex 295UV with primer and Dow 795 both mentioned. I use and recommend both. Dont go over 4 ft continuous length with a fixed portlight. Remember the coefficiant of thermal expansion for Acrylic and Polycarbonate is in the neiborhood of .000039 per inch per degree F. That means an 8ft plastic port will expand and contract up to 1/2 of an inch from the coldest day in Feb to the hottest day in summer. WOW!! Compartmentalize the job. It will be easier to install and less prone to leaks.

Never ever bolt a plastic portlight in place. Screws are fine to hold a lens till the adhesive cures. Take them out asap and fill the holes with the afformentioned products. Both of these products are rated at 700 + percent elongation before tear. Strong flexible and UV resistant.

Been to a boat show lately? Seen any screws? Glass is good so long as your boat does not twist or torque. Show me a fiberglass boat that does not twist and I will show you a cocktail barge tied to the dock.*_"

*Tony on Sealants:*

_*"I hear the question as to which sealant to use when bonding Acrylic, or Polycarbonate to aluminum, stainless or FRP over and over and over.....*_
_*Well here goes... The only three adhesives I would consider using are Sika Flex 295 UV with the primer, GE SG-4000, and Dow 795. Using the correct adhesive is only 1/2 the battle. Do not apply the sealants below 50 degrees F. The temperature must maintain at least 50F during the entire 21 day cure cycle. Cut this corner and your finished before you start. Preparation of the bond area is also very/ very important. DO NOT TOUCH THE BOND AREA WITH YOUR BARE HANDS! Contamination from the dirt and skin oils will make a solid cure impossible. You may clean the FRP and metal with acetone to prep the area but if you touch the Lexan or Plexi with harsh solvents you will ruin the portlight. A 50/50 mix of isopropal alcohol and distilled water will work well to clean the plastic if needed. Remember that clean enough is not clean enough."

*_Those are not my words they are the words of one of the most experienced marine plastics guys on the planet..

So this means DO NOT screw dead lights into place and DO NOT use standard sealants like 4200, 5200 or any of the others. If there were a better sealant for cast acrylic Tony would mention it. Use Dow 795, GE SSG 4000 or Sikaflex 295 UV with the special primer. Oh and acrylic not polycrabonate but the OP chose the right material anyway..

*
To answer the OP:* In order to get good clean cuts make 1/4 plywood templates and then have the cast acrylic CNC cut by a plastics fabricator using your templates. You made the right choice with acrylic but please don't try and through bolt them or they can most definitely split..


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## timebandit (Sep 18, 2002)

Cut it with a tile saw and drill with a step bit.


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## jarcher (Jul 29, 2008)

Maine Sail said:


> In order to get good clean cuts make 1/4 plywood templates and then have the cast acrylic CNC cut by a plastics fabricator using your templates. You made the right choice with acrylic but please don't try and through bolt them or they can most definitely split..


Or you could ask Tony at Select Plastics to make them for you. You can send him the old dead lights or a template.


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## AllThumbs (Jul 12, 2008)

I cut mine out with a band saw, then sanded the edges.

Eric


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## US27inKS (Feb 6, 2005)

Great thread guys. When I built my windows, I cut them out with a router. Worked great, no worries about breakage during the cut, and was an easy edge to clean up. You can even use an angled bit to bevel the edge as you cut them out. I through bolted my windows, mostly because I had no idea how else to attach them. Drilling can be a problem though. A regular drill bit will try to screw itself into the acrylic. The result will be cracked acrylic or the backside of the hole blown out. There are 2 solutions, the very best way is to buy an acrylic drill bit, the other is to make one. To make one, simply take a new drill bit and dull the crap out of it in a grinder. You have to reduce the "angle of attack" on the leading edge so it can't screw itself into the acrylic.

I used Boatlife's LifeSeal silicone on mine, and made sure I didn't squeeze all of the silicone out. I soon found out that the surface of the window where the silicone goes must be roughed up with sandpaper or the silicone won't stick for very long. On the second go-round I used Permatex Ultra Black sealant from the auto parts store. Worked like a charm and looked better than the clear I used the first time.


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## jarcher (Jul 29, 2008)

Maine Sail said:


> ... and DO NOT use standard sealants like 4200, 5200 or any of the others. If there were a better sealant for cast acrylic Tony would mention it. Use Dow 795, GE SG 4000 or Sikaflex 295 UV with the special primer.


I'm not trying to start an argument, and I am new to this and only really know what I have read. I spoke with Tony at Select Plastics at length and he is clearly very knowledgeable. MainSail and SailingDog, and many others here, are also very knowledgeable.

So my question is, why the big disagreement on silicon, at least for this purpose. SD has said over and over (and he cites good reasons) that silicon has very few uses on a boat, those perhaps being limited to the tips of cotter pins and the underside of dishes.

Tony and Mainsail, at least for this purpose (bonding acrylic dead lights to gel coat) say Dow 795 (silicon) is the way to go. And, the Dow data sheet does say that 795 is suitable for bonding glass to buildings in high wind applications, but buildings are not made from gel coat.

Another poster here commented that perhaps the issue with silicon is limited to that which has the vinager smell.

And then there are the products that have some silicon in them.

Is this just a difference of opinion, or is using silicon to bond the windows one of those exceptions where it is appropriate? If the later, what happens when it is time to rebed or reseal them?

I don't mean to start a war or be a pain, but I am in the process of replacing dead lights. If I don't use fasteners of any type, and the dead light pops off while I'm on my ear, the boat could easily sink as water rushes through the 45 inch opening that is left behind. Hence my concern not just for the initial installation, but the quality of the bond should it need to be done again. So if the silicon makes it difficult to reseal later, I would prefer to use something else, even if just for my own mental calmness. If there is a reason the silicon is not an issue, then so much the better.


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## Northeaster (Jan 13, 2007)

jarcher - not all silicons are the same. The normal, cheap silicon would not be good for this application. The recommended products are the Dow 795, Sika 295, and maybe one other that Mainesail mentioned. Yo ucanneot go worong using one of these!!! I would also advise using the 3M VHB tape as the primary adhesive, as mentioned earlier. YOu can have as much tape and dow/sika as you want. (ie you can overlap your deadlight 2" if you want, which would leave enough room for 3/4" or 1" of VHB tape, and then a gap, then a nice bead of Dow or sika that extends out to the corner, to be faired against the cabinsides later.


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## Maine Sail (Jan 6, 2003)

jarcher said:


> Is this just a difference of opinion, or is using silicon to bond the windows one of those exceptions where it is appropriate? If the later, what happens when it is time to rebed or reseal them?


While there are few good uses on a boat for Silicone bedding acrylic, polycarbonate, Beckson plastic products and a few others are the appropriate uses for Silicone.

Please DO NOT confuse Dow 795 with your basic Silicones. Dow 795 is a structural silicone and it is what is used by most boat builders to bond dead lights to fiberglass.

This is NOT an application for bathtub Silicone or general purpose "marine" Silicone. Dow 795 is tough stuff with millions & millions of blue water miles behind it over many years of service life on thousands of vessels...

Before using ANY sealant the surface MUST be properly cleaned and this will require sanding to remove ALL previous Silicone contamination. Even Dow 795 will not stick to a surface previously contaminated with Silicone.

Polysulfides and many polyruethanes should not be used on certain plastics as they can leach the plasticizers out of them, over time, and leave them prematurely brittle and prone to failure.


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## tigerregis (Nov 24, 2006)

Note that Maine Sail spells silicone correctly. There is a big difference between silicon and silicone.


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