How do I make a 2D elliptical dome?

[david kramer]

I'm making the base of a stand-up mirror. It could just be a flat board, but I'd like to round it over in two dimensions as shown in the drawing. Any ideas on how to do it? I'm thinking a router on a tray or something, but the 2D aspect is messing with my head. I guess I could do one dimension and then the other dimension.

For reference, the board is 8/4 cherry and the dimensions are roughly 24"x20".

[David DeCristoforo]

"...router...I guess I could do one dimension and then the other dimension."

You just answered your own question!

[Bruce Page]

If it were a square convex shape you could shape it in a lathe but being that it’s rectangular I think your best bet would be to layout the contour lines on the edge and hand plane it or use a handheld belt sander to shape it.

[Joe Scharle]

Angle grinder comes to mind.

[george wilson]

Glue on some side strips to prevent the main piece from splintering off,and contour it on the outboard spindle of a lathe. You could also build up the sides so that you had a continuous cut on the lathe,and saw them off later.

On those vaulted back 18th.C. guitars I used to make,if you saw the 17th.C.marquetry guitar I put up in the Neanderthal page,those were made in pine moulds which had the shape of the guirar sawn out.then,the sawn out piece was put back into the mould,and the arch was hand chiseled and hand planed into the final shape. I'd spring a steel rule over the arches I was planing to make sure they were smooth,round arches.

[Bruce Page]

If he wants an equal thickness on all four edges as his model suggests, then the pillow shape would have to be formed by hand with a good eye, or done with a 3-axis CNC.
If done on a lathe, the longer axis edge would be noticeably thinner.

[Wes Grass]

Arched guide rails along the long axis, and an arched base to span them with the router attached in the middle. You'll have to eyeball the router base 'square' with the side rails as you cut to get an accurate contour.

Oh, and BTW, that's considered a 3D surface in the CNC world.

[Ben Cadotte]

A router and a double slide jig. Like using a router to route out a chair bottom. But instead of having the slides arch down in the middle. You want them to arch up in the middle. Thus leaving a crown in the center and lower edges around the outsides.

But basically you want to make a jig that has 3 parts. 1st part is the lower slide. You make a simple rectangle of wood with the 2 long boards with a crown that matches what you want on your piece. Then you make a second part that rides on top of the first. It is designed so that the crowned rails are 90 deg to the lower ones it rides on. 3rd piece is a base for the router that rides on the upper crown boards.

Set the router for the height you want while it is centered. Since you now have a jig that allows the router to slide in any direction. And arches down evenly no matter which way you move it. You get a nice 3d dome cut on your project.

And obviously you need to make the jig larger than the item your working on to account for the width of the jigs and router base so the bit can reach all the edges.

http://www.cadottephotography.net/sitebuilder/images/Lower_slide_view-600x450.jpg

[David DeCristoforo]

"A router and a double slide jig."

That's what I'm talkin' about! Perfect!

[Bruce Page]

Ben, how would that be different from doing it on the lathe? The end result would be the same wouldn't it?

[Ben Cadotte]

Ben, how would that be different from doing it on the lathe? The end result would be the same wouldn't it?

Ah, there is an error in my logic. Yes, this would be the same as a lathe.
There is no size limit to doing it this way though. With a lathe there is a size limit (and if he has one). Since he said it was a base for a stand up mirror 20x24 it would have to be outboard turned on a big lathe.

This could also be done with a sander if you had a way to secure the sander body.

His picture shows the edges uneven so the jig above would perform what is in the picture (a domed piece).

I was also thinking this jig would allow the edges to be even as well but that would only work on a round piece (same as what a lathe would do as mentioned above). With a square or any other shape other than round, this jig would only dome the work piece not allow the edges to be same thickness.

[Ben Cadotte]

I have a solution, see if I can draw it up. It will be a 1d tool using a router that will do a 3d dome on a rectanular shape and have consistant edge thicknesses!!! :D

[Bruce Page]

I have a solution, see if I can draw it up. It will be a 1d tool using a router that will do a 3d dome on a rectanular shape and have consistant edge thicknesses!!! :D

I'm looking forward to seeing it!
(tomorrow...):D

[Ben Cadotte]

I'm looking forward to seeing it!
(tomorrow...):D

Hahaha, I am overseas for work (real job) so my day has just started. :D

Ok, still can't do the full 3D without adjusting bit height. But did figure out another jig that will allow consistant edge thickness and a arched or if you want long sloped top (think of very large door panel). The edge thickness would be the same and a nice consistan arch on a round piece. On a square or odd shaped piece the edges would be a consistant thickness, but there will be a flat area centerd along the lines of the longer length. But what this will also allow is a large taper on a large piece (only limited by the size of the jig). Just deppends on how you make the rail guides.

http://www.cadottephotography.net/sitebuilder/images/3D_dome_routing_jig_base-758x575.jpg

Basically you turn the piece. Run the router up the slides. Turn the piece / route, etc... until you have turned the piece 360 degrees. Now if the piece is round you keep the piece centered in both axis the entire time. Just rotate it. But if its odd shaped. What you do is move the edge of the piece to the cut start point on the reference circle (keeping the center point of the piece aligned on the cutting center line). You do a cut, turn the piece slightly, again move the edge to the cut start point on the reference circle. By doing this you get a nice even height edge around the entire piece, no matter the shape. You keep the center of the piece centered on the cut line, but it will be off center from the jig center line.

Now if you made the router carriage so it slides left or right, you could use this design to make very large door panels. With curved or straight slopes deppending on how you cut the lower guides. This design allows you to attach different guides for different arches or slopes so the jig would not be a one trick pony.

[David DeCristoforo]

"...how would that be different from doing it on the lathe? The end result would be the same wouldn't it?..."

Only if you assume that the radius of the arch on the "long side" of the rectangle has to exactly match the arch on the "short" side. It does not. Yes there will be slight difference in the edge profile between the "long side" and the "short side". But it should be minimal and unnoticeable except when viewed in exact elevation which will never happen.

[Bruce Page]

David, I agree that if you just turned a convex on the lathe the difference between the short leg & long leg of the rectangle would not be very noticeable. I was taking the OP’s question literally, more as an exercise. To that end, I still think the geometry would have to be generated by hand with a keen eye or on a 3 axis cnc. I think Ben’s second jig would get you close but you would still have to work the drop off where the invisible “miter” would meet.
It’s hard to describe….:rolleyes:
I have machined this type of geometry back in my mold making days and there’s more to it than meets the eye.

[david kramer]

Thanks everyone!

David

[Dino Drosas]

Here is an illustration of the jig you need to do this. From the spring 1995 issue of "Home Furniture".