I am getting into this scratch building stuff. Now I have a problem. I am trying to cut 2 1/4 inch half circles into sheet plastic. I have tried an exacto knife, small very sharp scissors and a drimel tool with a small mill tool in it. HELP I cant get a smooth even cut. What works here?

a semi circle? Hmm. A scroll saw may work well, but I doubt you have one of those. I would try cutting it a little large, and as smooth as you can, and then find a way to clean it up afterwards by sanding the edge. If you need it to be really precise, the only way is to pivot around the center against some kind of sanding or cutting apparatus.

How thick is your plastic?

Micro Mark sells a thing called a "Circle Cutter." It is like a drafting compass, but one leg is set up to hold a knife or a pencil. Put an Exacto knife in it and you can cut a circle up to 16 inches in diameter according to their write up on the tool.

Hi Les,

I use an OLFA compass cutter it is much the same as the one Russ descirbes with a centre pin and blade fitted cut from either side a few passes which will givee an even good cut, any bits that need smoothing some wet and dry paper sorts it.

I think you are trying to cut two 1/4" diameter semicircles in a sheet of styrene.

Here is one way:

Get a 6" piece of 1/4" O.D. steel tubing. Sharpen the tubing as sharp as possible
on the inside with a tapered reamer and a jeweler's file. With a dremel, remove
half the tubing diameter about 1/2 " length from the end so you have a sharp tool looking like
a sort of gouge. Clean upand get sharp as you can then tap this into the sheet plastic.
When you get it cut through then remove the semicircle with a straight cut, Xacto, etc.
If you have all this on hand it is a very easy tool to make. The tubing could be a piece
of steel brakeline or...? Any hydraulic shop would give you a scrap piece.

Here's a method called "jigsaw puzzle"


Drill two holes for the size you need, where you need them. Go slow when drilling through plastic.
Cut the plastic in a straight line through the diameter of the circles.
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Take another sheet of plastic, and splice it to the edge of the cut plastic. Make sure you re-enforce the splice line with scrap plastic.
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Hope this helps.

I have a bit of practice cutting round and irregular shapes.

You can see some of it here:


Borrow a sewing needle from the Mrs. (actually, in our house, I'm the one that can sew... )
Chuck it up in your pin vise.
Print out or draw your semicircle. Use the needle to transfer the circle to the styrene. Connect the dots and smooth with an emery board or file (I find emery boards to work better).