Cutting two mating curves

[jeff phillips]

I am building a dining room table, and one of the things I want to do is put a border around it. The top will be red oak and the border will be 2"wide walnut. The catch is, the tables corners are rounded with an 18" radius. I am not up for bending the wood, which leaves me with cutting it. I'm not sure the best way to do this.

A router seems to be the sensible tool choice, however, since both arcs need to be exactly the same, the fact that the router bit needs to be offset by it's diameter between the table top and the edging is problematic, as this offset needs to be exact.

I'm considering simply using a router circle cutter base, but I can't figure out how to get the offset perfect, and there is also the problem of how easy it will be to overshoot on the table cut and cut too far.

My second option is to build a template, and use 1" and 1/2" guide bushings in conjuction with a 1/4" straight bit to get my exact offset. This is the option I am leaning towards, however never having tried this before, I was hoping for some input or alternate suggestions.

[Van Huskey]

A similar issue came up and was discussed a month or so back. The template/bushing approach is where I ended up. I think it is the most accurate and fool-proof. You will have to experiment to make sure everything actually works in practice since much depends on the tolerances of the bit and bushings.

[mreza Salav]

I had to do the exact same thing on a recent project.
There was a long discussion in two threads about how to do it (and I was in the both discussions). You should be able to find it.
I did it without the bushings. Briefly, the steps were:

- first cut the inner circle (the one with smaller radius)
- put that on a piece of 1/4 MDF or anything from which you want to cut the pattern from
- Us a flush trim (with top bearing) bit to cut into the pattern.
- the outer segment of the cut has a radius larger than the inner circle by exactly the *diameter* of the flush trim bit you used.
- Use that as a pattern to flush trim the real outer segment you want to cut.

Of course, this can be done with bushing too, but the above approach has worked perfectly for me.

[Steve Schoene]

Just to be sure. The red oak center of your table is plywood or veneer on MDF isn't it.

[harry boyer]

I just read an article at American Woodworker mag site yesterday that gave specific instructions about this. I think it's in the techniques section. Also free plans for a table with a border. I just double checked. "curved corner edging". I didn't sign in, just clicked techniques

Good luck

[john bateman]

If you think about it, all you need your circle cutting jig to do is make the first cut using the inner edge of the bit, and the second cut using the outer edge, set to the same radius as the first cut.

If you use a Jasper type jig, it just involves moving the pivot pin to account for the difference of the bit diameter.
http://www.parts-express.com/images/item_large/365-250_L.jpg

I don't think Jasper's actually go up to 36" diameter. You might have to DIY one for this purpose.

[jeff phillips]

I've never used a Jasper type jig, I've seen them at woodcraft; however, I was unsure of how much slop there was in the numbers. I was envisioning they would have a tolerance of somewhere around 1/32"+/-, which would be a bit much. If I'm wrong, maybe I should pick one up to play with it.

As far as the red oak center, it was going to be solid QS red oak. It has been nagging at me that the wood movement could create a problem. I've kinda been going back and forth on weather this will be a deal breaker. Living up in the Seattle area, there isn't a whole lot of large humidity swings, so I've been trying to figure out how to do this and allow for a little movement.

[Joe Chritz]

Without a doubt the best way to account for wood movement on a table like this would be QS Red oak veneer on a suitable substrate. I would likely use shop sawn veneer and Baltic birch plywood for the substrate. You could build it up to any thickness you needed pretty easy with a vacuum press.

Might be a good reason to invest in a Joe woodworker vacuum press.

Short of a CNC Mreza's route is probably the easiest and most accurate.

Joe

[Doug Shepard]

You ought to take a look at the MicroFence circle jig.
http://microfence.com/circles-c-23.html
Somewhere on their site they had a gallery entry with some sort of seamless giant S panelled project but I cant seem to find it.

Adjustability to 1/1000" will let you get a seamless joint. Measure the router bit with a digital caliper, then adjust the jig offset to that exact measure.

I did the corian and cherry frame on this table (and also all the bending forms for the bent lam pieces)
http://www.sawmillcreek.org/showthread.php?t=15693
with the MF jig and it was fairly straight forward. Just had to locate the pieces correctly on the same working board.

[Gerry Grzadzinski]

We cut them with a CNC now, but in the old days, we'd bandsaw them and sand to fit with a spindle sander.