Drilling acrylic without cracking the new windows.

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GeeW

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OK so what do I need to know so as to not foul up this job? Just about ready to order the replacement small windows.

6 mm thick acrylic, and clearance holes for 5 mm screws. Hole centres will be 20 mm from edge.
48 holes to drill.

Is 20 mm far enough from the edge?
Drill speed?
Drill pressure?
Anything special drill bit type?

Thanks in advance

Gordon
 
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I'm sure there will be lots of different views on this, but the important thing is to cut it and not melt it.

I would use a slow speed, not too much pressure and a wood bit. But almost any sharp bit would do.
 
Just done this job in Wellington, NZ.

I ground the drill bit quite flat and without much lead on the cutting flutes.

Always works, even when drillin double curvature motorbike racing screens which are very thin.

I used .5mm clearance and bedded the new windows on butyl tape.
 
rotrax said:
Just done this job in Wellington, NZ.

I ground the drill bit quite flat and without much lead on the cutting flutes.

Always works, even when drillin double curvature motorbike racing screens which are very thin.

I used .5mm clearance and bedded the new windows on butyl tape.
Click to expand...

+1 to all that**, plus lodesman's comment about overheating, which can cause stress cracking later. It's possible to buy bits designed for use on acrylic on eBay/Amazon.

** except that it's not strictly necessary to go to NZ to do it.
 
GeeW said:
OK so what do I need to know so as to not foul up this job? Just about ready to order the replacement small windows.

6 mm thick acrylic, and clearance holes for 5 mm screws. Hole centres will be 20 mm from edge.
48 holes to drill.

Is 20 mm far enough from the edge?
Drill speed?
Drill pressure?
Anything special drill bit type?

Thanks in advance

Gordon
Click to expand...

According to the little booklets I have from a couple of the drill bit manufacturers special drill bits ground to a more acute tip ( 60° instead of 118°) and an increased lip clearance angle ( 20° instead of 10° to 12°) should be used.

The safe/ smart way is to get them drilled by the supplier. They will also polish the edges if required
 
I am contemplating replacing windows this off season so have a serious interest in this topic.
My question is why drill holes at all? I have heard that Butyl tape alone is sufficient.
Comments anyone?

My boat is glassed over plywood construction.
My plan was to remove the windows, fill the holes and sand back and epoxy undercoat and 2 pack polyurethane top coat the window frames. (I have been using the 2 pot system on other areas of the boat with complete sucess.)
This means that if relying on butyl alone to bond the windows, it also means relying on the quality of paint bond to the subsurface.
Is this a problem or should i do like OP and refasten with lots of screws?
 
John the kiwi said:
I am contemplating replacing windows this off season so have a serious interest in this topic.
My question is why drill holes at all? I have heard that Butyl tape alone is sufficient.
Comments anyone?

My boat is glassed over plywood construction.
My plan was to remove the windows, fill the holes and sand back and epoxy undercoat and 2 pack polyurethane top coat the window frames. (I have been using the 2 pot system on other areas of the boat with complete sucess.)
This means that if relying on butyl alone to bond the windows, it also means relying on the quality of paint bond to the subsurface.
Is this a problem or should i do like OP and refasten with lots of screws?
Click to expand...

Not sure the Butyl tape would be enough if the windows were fixed externally.

Our steel Hartley had 42 screws,84 washers and 42 dome nuts fixing each of the larger windows-there were 4 of them, and ditto 22 screws washers and nuts for the portlights, x6. They were fitted internally, leaving a lip open to the elements which, of course, corroded.

I think a Sikaflex type product would be more suitable for screwless fixing. Butyl tape was used in our application for ease of use and cleaning up, as well as it sealing properties.

I reused the portlights and their fastnings but filled and re-drilled the larger window apertures to take enlarged windows-+4inches-and half the fastners. This allowed the coroded apertures to be enlarged, removing much of the corrosion.

The windows were fitted externally.
 
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Like rotrax, I'm a bit sceptical about using butyl tape and nothing else. I regard the tape as a sealant rather than a bonding medium and wouldn't trust it alone on a boat without fasteners or some sort of retaining trim.

An alternative might be Arbomast BR, which is butyl-based but claimed to have some sealant properties. It's had excellent reports on here as a sealant. But I'd ask the manufacturer before going screw-less (and then not do so ;))
http://www.arbo.co.uk/arbo-sealants/arbomast-br/
 
To add to the above I always put some masking tape over where I wish to drill the hole to reduce the chipping around the hole and helps to stop the drill running away from the required position.

Also hold the perspex down to stop it jumping up when the drill breaks through.
 
I happily drill 6mm clearance holes centered about 8mm from the edge of 5mm acrylic. The dangerous bit is when the drill breaks out at the back. My method is to start with a smaller hole, say 2mm, then 4mm, then 5mm and then finish with 6.2mm. The sizes are approximate but the bigger the hole the smaller the increment should be. Always drill with the acrylic supported by scrap wood at the back, and make sure that the drill will break out into an undrilled bit of wood. If possible use a drill press to keep everything at right angles and use the slowest speed.
 
John the kiwi said:
I am contemplating replacing windows this off season so have a serious interest in this topic.
My question is why drill holes at all? I have heard that Butyl tape alone is sufficient.
Comments anyone?

My boat is glassed over plywood construction.
My plan was to remove the windows, fill the holes and sand back and epoxy undercoat and 2 pack polyurethane top coat the window frames. (I have been using the 2 pot system on other areas of the boat with complete sucess.)
This means that if relying on butyl alone to bond the windows, it also means relying on the quality of paint bond to the subsurface.
Is this a problem or should i do like OP and refasten with lots of screws?
Click to expand...

Butyl tape is an excellent sealant, clean, effective and easy to use. It is not an adhesive
 
Pirx said:
I happily drill 6mm clearance holes centered about 8mm from the edge of 5mm acrylic. The dangerous bit is when the drill breaks out at the back. My method is to start with a smaller hole, say 2mm, then 4mm, then 5mm and then finish with 6.2mm. The sizes are approximate but the bigger the hole the smaller the increment should be. Always drill with the acrylic supported by scrap wood at the back, and make sure that the drill will break out into an undrilled bit of wood. If possible use a drill press to keep everything at right angles and use the slowest speed.
Click to expand...

Ditto. I drilled a sheet of 12mm acrylic for a replacement companionway hatch and some much finer sheet and rods for a couple of drawers. I used HSS drills, working up from a small pilot hole. Masking tape (or the protective film the acrylic comes with) is a good idea. Use low pressure, slow speed, a dribble of water to lubricate. Take your time. You can buy special acrylic drill bits which have a different head angle, but I seemed to manage without.

One big tip is that if the drill stops within the sheet you’re drilling, reverse it out and start again - don’t start the drill when it’s jammed in the acrylic or you create much greater forces which can crack it - don’t ask me how I know this.
 
macd said:
Like rotrax, I'm a bit sceptical about using butyl tape and nothing else. I regard the tape as a sealant rather than a bonding medium and wouldn't trust it alone on a boat without fasteners or some sort of retaining trim.

An alternative might be Arbomast BR, which is butyl-based but claimed to have some sealant properties. It's had excellent reports on here as a sealant. But I'd ask the manufacturer before going screw-less (and then not do so ;))
http://www.arbo.co.uk/arbo-sealants/arbomast-br/
Click to expand...

I wouldn't trust arbomast br as an adhesive. As a sealant it is very good (I normally have a tube on board) but the item does need something to hold it in place.
 
GeeW said:
Many thanks for the replies. Planning to obtain a sample to practice on.
Gordon
Click to expand...

Best tip yet. Try the advice on a scrap!!

Although I am not a huge fan of silicone, DOW 795 is the preferred sealant for acrylic windows in the US. ONLY DOW 795 (most silicones do not work). If you use Sika 295, remember that it does NOT resist UV unless you use the primer (the primer blocks UV from the bond area).
 
I cut my teeth on perspex with aircraft screens. Getting them wrong was seriously expensive :( The low angle drills are good and lube/cooling help. Control of the pressure when drilling is critical. Breaking though quickly is likely to start a crack, so best drilled with a pillar drill and care.
Bonne chance..
DW
 
GeeW, if you'd like a couple of small offcuts to play with I can happily post 'em to you ?
 
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