Cutting curved acrylic sheet.

[Aja]

I am making a new curved windscreen section for a motorboat. I used the old one as a template, make a buck using ali sheet and grp, with a hot air gun I mould the new sheet over the buck.

So far so good.

Now I need to cut out the exact shape of the screen from the big sheet I've moulded.

A multi tool with an abrasive cutter makes a gooey mess, as does a toothed cutter.

Jigsaw makes a gooey mess with coarse, medium and fine blades.

A hacksaw blade by hand works, but will take me days.

A grinder with a 1mm slitting disc makes a sort of hot melting plastic candyfloss fly everywhere.

A grinder with a 36 grit flexible disc works on the edge, which might be ok for final trimming. But I have a lot to remove.

Any suggestions?

You haven't actually mentioned what material you are cutting?

 

[[2574]]

You haven't actually mentioned what material you are cutting?

Thread title.

 

[VicS]

Thread title.

People never read the thread titles properly!

 

[sarabande]

We found that it is better to make a template in card, or heavy paper, first and then cut to shape when the acrylic is flat. Once you have the acrylic cut, its still flat - then form. Cut slightly over size and grind back (and then, bull nose ? and polish) the edges). We sere working with 10mm acrylic - smaller/thicker might be different.

BB is working in 3 dimensions draping the acrylic over a former and heating to the required shape. As this is a one off, and not using data off a CAD system in a windscreen factory, it's hard to estimate how difficult it might be to translate the compound curvatures to a flat piece of card. One has only to look at sewing patterns to realise how difficult the process is.

 

[burgundyben]

Here's a pic of the screen.

 

[Neeves]

BB is working in 3 dimensions draping the acrylic over a former and heating to the required shape. As this is a one off, and not using data off a CAD system in a windscreen factory, it's hard to estimate how difficult it might be to translate the compound curvatures to a flat piece of card. One has only to look at sewing patterns to realise how difficult the process is.

I'm not sure that Ben is making a 3 dimensional curve - it looks to be a simple bend to me.

But you are almost correct. He is not draping over a curved former and heating, but heating so that the acrylic, in our case 10mm thick, drapes over the former. It is very difficult to subsequently cut. We found it was easier to cut roughly to size (almost accurate), then heat and drape and finally grind to size. 10mm acrylic is heavy, its better to get it as close to the final shape before heating. Its not easy, or to be more accurate - its easy but needs some attention to detail making sure the cut acrylic is draped accurately. There is a bit of wriggle room - but not much.

Ben's picture looks to to use thinner acrylic - maybe he can simple bend and adhere.

I suspect if its thinner and the window not large other methodology might work.

Been there, done that

We tried bending cold and leaving under tension, did not work. If the curve is not too large then cutting to shape, using paper or card, then bending to shape and adhering works well. But you do need to have battens and some pressure to keep the 'bend'. Beyond a certain size, 2 of our (10mm) windows bend through almost 90 degrees, you need to pre-form.

I'd send a pic but I'm offshore on the way to the Lofotens and I don't have decent internet access - and I did try.

What is your experience based on?

Jonathan

 

[burgundyben]

Its 4mm thick. They are usually moulded.

Mine is now cracked. Humph.

 

[sarabande]

Aw, shucks, sorry to hear that.

When you took the old one off, were there any gaps or 'loose' areas between the acrylic and the frame to accommodate thermal stresses ?

Which sealant (if any, did you use ?

 

[chris-s]

If not already mentioned, you can get jigsaw blades purposely designed for plastics like acrylic. I’ve cut quite a bit of acrylic 4mm to 12mm and no goo.

 

[Neeves]

Its 4mm thick. They are usually moulded.

Mine is now cracked. Humph.

Sika and 3M make adhesives sealants (for archectural glazing) specifically for 'your' application. The curves in your pic seem fairly gentle and if the frame is sufficiently strong? the adhesive would probably be sufficiently strong to hold any induced curve without the need to use heat. I'm not entirely sure as your frame looks 'thin' - not much area for adhesive. (Our windows have a 100mm frame/overlap - so lots of bond). I'd speak to the people at 3M or Sika for advice. Its not impossible to bend gentle curves with 4mm acrylic - and then there are other materials (which might be better). But you have already heat moulded - so not really useful.

Other people will make similar adhesive products, as a manufacturer, Fix Tec, makes in Oz (which we used) - and I cannot believe they are an exception.

Jonathan

 

[DownWest]

I did quite a lot of this with 10mm, which is easier to cut than thin stuff, less likely to crack.
Jig saw with sharp wood blade and a squeezy bottle of water with some detergent in it. Stops the gumming up and makes it quicker. Belt sander to smooth edges, as visible. Wet and dry to polish.
To curve, formica buck and I fabbed up a big oven as the pieces were over a meter long. 120°C is right for it to droop over the buck. Surfaces must be spotless, as any dust makes a dent and shows up.
This was a staircase, so lots of the panels were spiral.

Going back, for really nerve wracking work, trimming a/craft windscreens to fit, slightest crack and junk at an eyewatering cost.

Now, I might use a router with the perspex on a table supporting it right to the cut. The heat of the cutter has a menlting effect that seals the edge.

Nice boat BB!

 

[Freebee]

I think we used kerosene as coolant when cutting acrylic, I cut my triana screens on a record bandsaw never had any trouble with melting.

 

[DownWest]

I think we used kerosene as coolant when cutting acrylic, I cut my triana screens on a record bandsaw never had any trouble with melting.

One thing I realised after 'experience' is that machines, with a constant cut rate are far better than hand sawing, as by hand you might put a sideload on the cut and... 'crack'!

 

[burgundyben]

I think we used kerosene as coolant when cutting acrylic, I cut my triana screens on a record bandsaw never had any trouble with melting.

Surely made a huge mess?

But, clever, my jig saw blade has maybe 1cm doing all the work, but if I used my bandsaw I'll have nearly 2 mtrs of blade doing the work.

Thank you Mike.

 

[Freebee]

using kerosene was in my toolmaking years would have been dripped on from a fairey bottle , it would have been a crime to use water as it would have made the machine bed rusty

 

[fredrussell]

Jigsaw with fine cut wood blade and wd 40, or any such spray can ‘oil’. Don’t skimp on the oil - buy cheap screwfix wd-40-alike, loads cheaper.

 

[burgundyben]

Update!

The belt went on my bandsaw, so I brought it home, new belt and new guide bearings now all fitted and set up.

Windscreen is at home, so I did a little trial, it works brilliantly!

Winscreen is quite big, so I'll have to set up a workmate outside.

Another challenge resolved!

 

[Daydream believer]

Possibly the OP already has the material. If not then I would suggest not using acrylic. Use polycarbonate instead. This can be cut with a bandsaw. or a tungsten toothed saw blade on a sawbench. I used to cut loads of it in my factory. Accepted, I had a purpose designed triple cut tungsten blade, for speed, but most tungsten blades will do.Lots of straight cuts to trim to the curve.
Alternatively, a portable saw with fine tungsten teeth. One might cut several straight lines then reduce to a curve by plane.
Regarding the acrylic- he has mentioned a hacksaw working but taking ages. As others have said , that demonstrates that speed of the sawblade is the solution. A slow running jigsaw pressed hard against the surface & not allowed to jump, or vibrate against the surface, with paraffin as a lubricant would be my choice

 

[jamie N]

I did mine with a jigsaw, deliberately cut to a 'few' millimetres oversize. This of course left a slightly serrated edge, which I used a traditional plane to smooth out, and reduce to the correct size.

 

[DownWest]

I did mine with a jigsaw, deliberately cut to a 'few' millimetres oversize. This of course left a slightly serrated edge, which I used a traditional plane to smooth out, and reduce to the correct size.

A plane would worry me a bit about starting a crack.
Just made some oval portlights out of perspex for my little cat boat. Used a jigsaw with a sharp wood blade, slightly oversize as you did, then a belt sander fixed to the bench to finish.