shaping upholstery foam

[AlanT]

I needed two pads to get a better finish on a leather car-seat:
https://www.flickr.com/gp/90670218@N04/3Z1hY2

They are shaped like this:
https://www.flickr.com/gp/90670218@N04/y321u5

This stuff can be easily cut with a scalpel but getting a smooth round shape is hard with a blade.

Strangely you do it with a steel angle-grinder. Use a gentle touch and mind your fingers!
https://www.flickr.com/gp/90670218@N04/6C2gy3

I am NOT joking!

[petertheterrible]

Probably the most useful tool in any toolbox.

There are commercial available blades that do that sort of work, they look like panel shrinking blades.

My personal favorite is the hot wire method, thanks for sharing.

Ever tried cutting a perfect circle in about anything with a baby grinder, works perfectly, no add-ons needed.

[vtsteam]

One more possible method -- I remember someone saying a few decades ago that they used an electric carving knife to cut through sponge material, like mattresses.

[Sid_Vicious]

I have used sandpaper on the grinder with good results.

[Arbalist]

One more possible method -- I remember someone saying a few decades ago that they used an electric carving knife to cut through sponge material, like mattresses.

Yes I've used that method. Another is a smooth bladed band saw.

[steampunkpete]

On a few occasions we machined rubber for certain strange applications (no, not what you're thinking) but putting it in the freezer overnight. I wonder if that would work, or help, with foam materiel? 

[petertheterrible]

I believe it depends on the type of foam, with some high density foams freezing them would make them a breeze to machine.  (Especially the types made from soy-bean would stiffen when friezed).

I am skeptical if it would work on the foam commonly used in the upper parts of auto seats.

[petertheterrible]

Problem solved, soak in water, then freeze and machine very quickly after freezing.

Beware, very messy!

[AlanT]

Thanks for all the replies chaps.

The carving knife method is good for big straight cuts but you really want a double-blade type.

I'd never done upholstery before and had to work out a whole load of new tricks.

[hopefuldave]

For sculpting motorbike seat foam (I suffer from Jack Russel syndrome - long body, short legs) I've always used a router motor with a 150 grit abrasive flap wheel, leaves a nice smooth flat finish.
Wear goggles and a good dust mask though, first time I felt like I had Something Serious for about 3 days afterwards...

[millwright]

Electric carving Knife worked very well for me for cutting upholstery foam, and cuts tight radii or circles.

John