Concave Oval

[Cliff Rohrabacher]

Concave oval: Pretty little scalloped out ovals cut into a surface.

Every year I approach this problem again and ever year I end up telling myself that it's done with either a tracing router or a CNC.

I've seen 'em and they look real nice.

And every year I try some cockamamie idea like suspending a router from a gimbal of sorts and every year my rig of the year proves too sloppy and unworkable to produce the fine crisp edges that make the conceit work.
And every year I haven't got a reliable way to cut that geometry into a panel

And Again I am raising this as a Puzzler for the "How to Do That" wizards.

So wizards of the craft: Any ideas?

[harry strasil]

the right diameter sanding disk set at the right angle will clean up the routed surface, that's how machinist cut large diameter shallow radius's on a Bridgeport Mill.

[David DeCristoforo]

How big is the workpiece? Do you have a lathe?

[Jamie Buxton]

Are you looking for a smooth concave surface, or do you want fluting in there?

I did something related for drawer pulls. I chopped out oval depressions into the faces of the drawers, and put a bar across the depression. The bar became the pull, and the depression allows fingers to get behind the bar. I wanted texture inside the hole, so I made the depressions by hand with a chisel. It took a long time. While I was doing it, I was thinking about a gimbeled router, too. I'd put a bull-nose bit in it, and include detents in the jig so that the fluting from the bit forms a regular pattern. The scheme I was thinking about would get me a circular depression, not oval. That would be a much much fancier gimble.

[Cliff Rohrabacher]

How big is the workpiece? Do you have a lathe?Drawer fronts, flat faces of table legs it's a decorative feature. Yah I have a little Walker Turner lathe I bought from A Creeker.

I see where you are going. Form a large enough diameter piece into the right geometry on the lathe so it'll make the depression profile I want and then affix abrasive to it and then figure a way to raise the work to the spinning abrasive wheel I just made.
Very Creative David, Very creative, direct, and simple. (the best ideas often are) That may be the ticket.



Are you looking for a smooth concave surface, or do you want fluting in there?Dead perfect smooth like a CNC machine did it.

The gimbal idea isn't dead yet I just ain't solved it.
I need two axis of movement one shorter than the other.


the right diameter sanding disk set at the right angle will clean up the routed surface, I don't think that's the geometry I'm after.

If I were going to do a production run of something like this in steel, I'd either us a CNC or a plunge oil bath EDM. It'd be child's play to form the graphite electrode.

[Jamie Buxton]

You could use a round-nose bit and a one-axis gimbal for a router. The radius of the round-nose would be radius along the minor axis of the elliptical depression. The radius of the gimbal would be the radius along the major axis of the depression. The surface left by the tool should require only minimal sanding.

The challenge might be finding the router bit with the radius that you want. There are round-nose bits, with radii up to 1" or so. There are also larger-tip-radius bits intended for making the cove part of cove molding -- for instance http://www.eagleamerica.com/product/v130-2815/ea_-_molding_bits. Would 2 3/4" be enough for your minor axis? And is the mouth of your router able to handle this?

[Cliff Rohrabacher]

You could use a round-nose bit and a one-axis gimbal for a router. The radius of the round-nose would be radius along the minor axis of the elliptical depression. The radius of the gimbal would be the radius along the major axis of the depression. The surface left by the tool should require only minimal sanding.

That's an excellent idea and one I hadn't imagined.
I was unaware of huge radius router bits.
I wonder how hard it'd be to control?

[Wes Grass]

I think a radius milling cutter is the way to generate this. Held at the proper angle to the long axis, like cutting a cove on a table saw, gives you the ability to adjust the radius to suit your needs.

Mounting and driving it, in some easy fashion, is another matter. Maybe on an arbor in a drill press or (preferably) a milling machine. And then you need some sort of a swing arm jig, or a cam follower, to generate the long radius. Not simple. I'm sure my brain will hurt if I keep thinking about it.

Depending on the actual dimensions, maybe a custom (hand) ground shaper cutter in a table saw would work. Cutter diameter would equal the long axis, cutter radius the short. Then set up stops on the table and lay the piece down on it.

Sounds like you've got access to metal working equipment. Should be trivial to make up a cutter like this out of an aluminum disc and an old file.

[Cliff Rohrabacher]

I haven't had access to a machine shop since I left Grad school.

[William Powell]

If the ovals aren't large you could use this
http://www.sears.com/shc/s/p_10153_12605_00903217000P?vName=Tools&keyword=molding
on the table saw with the proper tooling and raise the head into the workpiece a little at a time.

Bill

[Jamie Buxton]

That's an excellent idea and one I hadn't imagined.
I was unaware of huge radius router bits.
I wonder how hard it'd be to control?

Well, I'd want to clamp the base of the gimbel securely to the workpiece.

I think the gimbel would fasten to the router's base. That is, you get to adjust the depth of cut into the workpiece by using the router's height adjustment. You could make multiple passes in a very controlled way to cut down to your final depth.

[Cliff Rohrabacher]

Well, I'd want to clamp the base of the gimbel securely to the workpiece.[...] multiple passes in a very controlled way to cut down to your final depth.

Indeed. ....