Circle cutting on a band saw. Ideas needed

[Art Bianconi]

Designing and building telescopes is one of my passions. Until now a good router and a circle cutter have been indispensable for cutting light baffles and structural bulkheads for optical tubes, some as large as 26" in diameter.

The router does a nice job and would still be needed for inside cuts but fixturing the materials to a sacrificial surface is awkward. (pink home foam insulating panels keep the bench top from getting gauged)

My acquisition of a new Gorilla 17" Band saw suggests that it might prove even better.

I've experimented with a right angle aluminum extrusion bolted perpendicular to the saw table.

I slotted the top plane of the extrusion so I can adjust the cut diameter by sliding a 1/4" hardened pin as much as 18" from the blade. It's apparent however, that a wider support plate and a flat surface is needed on the other side of the pin for large pieces.

The materials of choice are usually foam cored fiberglass and carbon fibered composites, tempered Masonite or thin plywood. It would be great if I could cut fragile, unbonded laminates like Formica which don't take kindly to jig saws, even when clamped down.

Has anyone used such a method? I'm sure there are better ideas than this for curring large circles.

Thanks!

Art

[Brad Shipton]

For a few ideas you might want to take a look at the Aigner Catalog. They have quite a few accessories for cutting circles on the BS. Martin distributes the Aigner parts, so you can find them on the martin-usa dot com website. You can buy them direct from simantech too.

Brad

[Lee Schierer]

You have the right idea. The trick to cutting circles on an bandsaw is to get the center of the pivot aligned exactly with the tips of the teeth on the blade, Too far ahead or behind that will cause the blade to walk toward or away from the desired cut line and eventually will bind the blade.

[Dave Sharpe]

I've used a simple 3/4" plywood auxilary table with a dado cut at right angles to the blade. A sliding arm in the dado can be adjusted to the proper radius of the circle, then secured with screws set flush into the arm. A small pin at the end of the arm povides the pivot point. The table is simply clamped to the bandsaw table with the center of the dado even with the front teeth of the blade(as mentioned by another poster). I can either drill a small hole in the workpiece for the pin, or attach a thin pice of masonite to both the center point of the workpiece (with a hole for the pin) and a matching piece just beside the blade attached to the auxillary table to keep the workpiece flat and even. This has worked quite well for me - it's cheap, reusable, and works well for pieces up to about 30-36" diameter. Anything bigger gets too heavy and unwieldy so I use a router with a trammel.

[Prashun Patel]

Try this:
http://www.taunton.com/finewoodworking/Workshop/WorkshopArticle.aspx?id=30943

Basically, you have a sliding platform with a pin that holds your piece at the proper radius. You slide the platform with the piece into the blade and then when it makes contact, you lock down the platform and then rotate your piece.

I've also cut large circles on a 'router table'. I mounted the router to the bottom of a large piece of 3/4" ply with a small hole cut in the center for a spiral bit. Then you measure the radius from the bit center and screw a pin at the pivot. Make a corresponding hole in the bottom of your piece.

You should rough cut the circle to within 1/8" of the desired radius. Start with the bit below the table. Clamp the cutting piece in place and then plunge the router UP through the hole. Then you gently release the tension on the clamp and rotate the piece into the bit as you hold the piece securely. If the piece gets away from you, the bit will run along the edge and you'll have a spinning top and the very least some nasty tearout when the bit encounters a any edges.