Blades for cutting foam boards?

[Ian White]

We all know the problem about cutting foam insulation board with a saw - the kerf is converted into showers of "snow".

The best tool I've found for this job is a bread-knife with a scalloped edge. Having no teeth, it doesn't make a kerf, while the scalloping helps with the sawing action. Second best is a fully extended snap-off knife (trying to avoid the 'snap' feature). But in both cases the narrow blade makes it difficult to follow a long straight line.

Is there such a thing as a broader blade that has no teeth but a plain sharp edge, preferably scalloped like the bread-knife - either looking like a small hand saw, or maybe a blade for the circular saw?

[Andrew Gabriel]

I used an old break knife. I think you'll find a broader knife gets difficult because of the extra friction.

Wash it very well after use. Many of these foam insulation boards are expanded using nasty cyanide compounds (you can smell them as you cut through). I don't know what risk that brings, but I didn't risk it and haven't used the knife on foodstuffs since.

[Lobster]

I just use the "score'n snap" technique with a normal Stanley knife, running the blade along a straight edge. Works fine for me.

David

[Stuart Noble]

I noticed a double glazing installer using a broken hacksaw blade to cut back the foam round the edges of the window. Nice clean cut, and no sweeping up. I think you need a kerf, but a small one. There used to be a broad hacksaw blade on a handle, rather like a padsaw, but I haven't seen them recently

[The Medway Handyman]

The shops that cut foam into cushion size pieces use a jigsaw with a long smooth blade. Don't know where they get them though.

I have also seen it done with an electric carving knife.

[Colin Stamp]

Last time I needed to do loads of polystyrene, I rigged up a long hot wire with a biggish battery charger and 0.8mm MIG wire with a sheet of chipboard as a back-plate, bowed to act as a spring.

Well worth the effort if you have a lot to do. Dead straight cuts along the whole length of a sheet in about 2 seconds and no mess at-all.

Cheers,

Colin.

[Guy King]

The message from Ian White contains these words:

I made a hot-wire cutter. Much easier.

[Frank Erskine]

On Sat, 15 Jul 2006 19:38:31 +0100, Guy King had this to say:

So did I, many years ago. I made a frame a bit like a fretsaw out of a bit of plywood and a handle of a table-tennis bat (!), fitted two screw terminals and a bit of flex wired to a 4V ex-wireless-set transformer. Worked a treat, as long as you didn't stop the "cut" for more than a second or two - if you did, you'd develop a hole.

[Frank Erskine]

On Sat, 15 Jul 2006 19:56:33 +0100, Frank Erskine had this to say:

Oh - and a bit of element wire from a nelectric fire!

[John Rumm]

I can see that would work for jablite (i.e. polystyrene), but would it work for foil covered PIR foam like celotex or kingspan etc?

[Guy King]

The message from John Rumm contains these words:

No. You'd have to slit the foil first.

[The Natural Philosopher]

I used a small 'kitchen devil' serrated steak knife for that..

[Ian White]

Thanks for all the suggestions.

Normally I would score and snap the boards too, but further into the job it will be necessary to piece together some of the large offcuts, so it would be nice to have straight, flat, full-depth cut edges.

Based on all the other suggestions have led to a cunning plan:

1. Use score-and-snap, the breadknife and possibly the tenon saw, but only to line the workshop walls where the benches are going to be.

1. Build some workbenches (at last!)

2. Find wherever I packed the big low-voltage transformer and variac before the house move... and then have some fun with a hot wire cutter.

Thanks again!

[The Natural Philosopher]

Don't hot wire cut Celotex stuff. NASTY fumes.

[Roly]

I needed to cut and shape some polystyrene and made a very simple hot wire cutter from a steel guitar string ( the thinnest one you can easily get ). The transformer was one from a low-voltage lighting system ( it was about 120W @ 12 V, but didn't need to be that powerful ). Fne control was with a variac controlling the voltage into the transformer primary, just like you mentioned that you already have. The wire was held under tension in a gadget that looked like an oversized fret saw. The metal parts from electrical choc-block connecters made good connections to the guitar string and also acted as a mechanical anchor point to keep it under tension.

The fine control made all the difference to how efficiently I could make it cut, but for anybody without a variac, they might find that using bulldog clips and sections of a wire-wrapped 1KW bar element in series with the cutting wire, it's possible to regulate the current as desired.

Safety police saying the bleeding obvious ... Beware that the wire and regulator may get pretty hot.

[PeTe33]

I modded a jigsaw blade with an angle grinder to remove the teeth (metal cutting blade)providing just a sharp edge. It was just long enough to all but pierce the opposite layer of foil, so a follow-up with a stanley knife sorted that out.