Very large radius cove cut

[Mike MacGregor]

I need to mount a square 5x5x3/4" block on a 18" diameter cylinder. Any suggestions on how I can get a very large cove cut? I am thinking I might have to stand it on end, use the bandsaw to rough it in, then tape sandpaper to the cylinder and finish it by hand.

thanks

[Prashun Patel]

You might be able to do this at the tablesaw. I made coves using the technique of clamping a fence diagonally across the top and then running the stock over the blade in shallow passes. There's some math to it, and definitely do your research before attempting it; I'd say it's a technique that if not done with full understanding, can be dangerous. But when done right, is safe and effective (albeit dusty).

edit: I can't get my head around it, but the largest diameter you might be able to muster with this might be the diameter of the saw blade itself, 10".

Come to think of it: if you have access to a lathe with > 18" swing, you can tape sandpaper to the cylinder itself and grind away your block after having rough cut it on the bandsaw.

[Metod Alif]

Mike,
On a table saw you get elliptical profile unless you run the stock perpendicularly to the blade ((probably not a good idea - safety plus a strain on the blade).
A suggestion: Trace the (circular) profile on your stock, the excavate most of the material with a sequence of kerfs on the table saw. Adjust the height of the blade close to the profile for each pass, and finalize with sanding.
Best wishes,
Metod

[Jerry Miner]

You can use the tablesaw to cut a cove, but getting an 18" diameter cove out of a 10" blade would be challenging. You would have to slide the work side-to-side to get a wide enough cut.

Your bandsaw/sandpaper method might be your best option. By my calculations, you will need to remove about 3/8" from the back of your 5" block in order to fully seat on the column.

If you do try the tablesaw, start with a longer board and trim to 5" after--too dangerous, IMHO, to cove a small block. And remember that the tablesaw method will not produce a circular section cove (unless you run the stock perpendicular to the blade) but an elliptical section.

[Tipton Lum]

If the cylinder is wood or some other solid material, cut a flat spot into the cylinder and mount the block to that. Flat to flat will be way easier and more solid.

[Mel Fulks]

Something that small, can't you scribe and gouge ?

[Shawn Pixley]

I'd mark the radius on an oversized piece of stock. I'd then kerf out much of the hollow by ripping on the table saw. Clean up with chisels and spokeshave.

[Lornie McCullough]

I would probably use the cove cut on the table saw. If my calculations are correct, you are only taking off 0.354 inch at the deepest part. I would make a template as Shawn mentioned, and use it as my gauge while setting and repeatedly adjusting the cove cut angles, and saw blade height. I would also use a piece of stock at least 15 inches long, as making a cove cut on 5x5 would scare me. I still have all ten fingers, and I intend to keep them intact.

Be careful.

Lornie

[Dave Zellers]

I've done this with a router and jig. I made a rectangular base plate that extended a couple inches past the router. Then screw on a 3" x 3/4" board on each end that extends up past the pivot point, in this case 9" minus the bit protrusion. Drill a 1" hole at the pivot point in each board and hang the router on a 1" dowel. Then the swing of the router cuts the desired cove and you slowly slide the workpiece (use a long piece) left to right taking small cuts with a 1" bit. Fine tune by adjusting the depth of the bit.

My project was a 16" radius, 9" is getting kind of small perhaps. The router might be too tall for a 9" radius and still fit under the dowel but you could cut that middle part as long as the two separate pivot dowels were well secured.

A lot of set up for one piece but if you needed a bunch of them it might be worth it. If you are precise in making the jig, the results are nearly perfect.

[Mark Bolton]

Circle cutting jig on the band saw with a couple aux. fence/jigs would do it in a single pass.

[Mike MacGregor]

Thanks for all the ideas gang. I scribed the curve on the two edges of the plate, then clamped it to a right angle plate (from my machine mill). I then cut kerfs on the band saw. I then sanded it curved by taping sandpaper to the cylinder (shell).