how do you cut circles in plastic sheets?

I use a circle template and very carefully cut around it with an exacto knife. I have never done this for a prop on a plane kit but I have done it for the tire mud plugs on sprint car kits. I'm sure it would work the same.
Good luck,
Pat

Some mathematical compasses can be fitted with specially made blades in place of the usual lead pieces.

I used to have a student grade Staedler drafting compass that I found blades for in a drafting supply shop. It worked very well for cutting circles out of sheet styrene that was up to .040" in thickness.

I know that Evergreen makes clear styrene sheets up to that thickness, so if you can find a drafting compass with blades available for it, I'd say very lightly score the outer diameter of your prop disk onto the plastic then gradually adjust your compass to smaller diameters and lightly score circles until you come to the centre of your disk. After that, set your compass back to the full diameter and cut the disk out of the sheet with a little extra pressure on the blade.

Once you have it cut out, indicate the positions and colors of the prop blades with a wash of their color and streak the paint in the direction the prop would turn.

Thanks for the advice, guys.

Just want to let you know that the manufacturer Olfa of Japan makes a very nice
circle cutter.
HTH,

Mark

You might also try a craft store and check in the framing department for mat cutters. you can find some that cut circles and elipses.

A drafting circle templet and a frisket knife. You can find the templets at drafting supply stores or Hobby Lobby. I bought my frisket knife at Walmart. The circle templet has marks that will let you mark dead center.

To expand on the drafting compass recommendation, find a compass that uses leads that are about 1/16" or so; this is pretty common, I found mine at Staples. Typically, look for the professional looking compass as opposed to the type where you insert a whole pencil.

Then go to the hobby shop and find the replacement blades for the x-acto retractable knife. This is a small blade with a while plastic holder molded onto it, with two steps. Cut the larger of the two steps off, and you will find that the second smaller step will fit nicely into the drafting compass.

I use this not only to cut circles in plastic, but also in sheets of solid color decal film, to duplicate the thin warning circles found around modern naval weapons.

Hope this helps a bit

Kurt

Expanding on the circle template idea use it and scribe your circle on a piece of sheet metal & cut your hole out of that & make you up a series of patterns & that way you won't cut into the drafting template. You can drill a smaller hole in the sheet metal & file up to your required dia

In another thread I mentioned using my Olfa circular cutting tool for jobs such as this. It will cut circles up to approx. 8 inches in diameter. Not bad. Comes with a spare blade as well. -Ed

Leather Craft stores and websites have huge assortments of "circle" punches in different sizes and shapes.

http://www.tandyleather.com/
http://www.leatherfactory.com/

QUOTE: Originally posted by Aurora-7 I want to simulate spinning props using clear plastic. I've read several ways of how to simulate the blur of the prop blades but it's how to cut a perfect circle that I'd like some advice on. What have some of you used to do this?

I've done exactly what you've trying to do. Two suggestions:
(A) If you're doing a multi-engine aircraft (e.g., a P-38 -- EVERYBODY does a P-38 sooner or later), use a circle cutter as described elsewhere in this topic, but cut the circles a little large and mount them NOT TOUCHING on a common axis. Now you can use a disk sander (I used my Shopsmith) to make them EXACTLY the same size. It also leaves a nice "blur" at the edge, which leads to ...
(2) [foolish consistancy being the hobgoblin of small minds!] If you take your prize(s), mask all but where you want the blurs to occur (nominally wedges in the same number and spacing as the prop blades that aren't going to be there) with Scotch Brand Magic Tape, spin them up (one at a time this time) on whatever you've got that passes for a lathe, and lightly touch with 0000 steel wool -- the result, after stripping the mask off, is a first rate prop blur.