# Cutting mortises on a curved surface



## gaviaimmer (Dec 6, 2007)

I would like to cut 3/8” wide by 3/4” deep mortises perpendicular the axis of a curved surface. On a flat plane it is very simple to cut a mortise like this with a router, jig, and 3/8” spiral bit. When the edge of the plank is not flat but curved, you have a problem. If you allow the router to bore with the base riding on the curved surface, the mortise will be perpendicular to the angle of the router base and not a common horizontal plane. If the mortises are cut prior to the arc, then a very long bit would be needed, possibly 5” or more, depending on the angle of the curve to get a net 3/4” mortise depth. I thought of re-sawing the a plank and cutting 3/8” channels in it prior to re-gluing the plank and cutting the arc, but the mortises would be square sided not round, and I could not conceal the seam as perfectly as I would like. Any tips, ideas, or tool suggestions would be welcome.


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## JON BELL (Nov 2, 2007)

A chisel? Are you asking how to cut it,or how to layout a straight mortise in a curved surface?For the layout,I usually have a template.Before I cut out the curved template from the square 1/4 Mdf sheet,I use the straight edge and a right angle guage to mark out straight mortise lines in the curve.Cut out the template,sand,transfer the marks from the template to the finish wood and chisel it out.If you want,you can toss a bevel guage in there somewhere:shifty:

By the way,a couple days ago I used a lam.trimmer,str.bit,and a bearing fence to rout out rabbits in the length of 4 legs curved on all sides.In curly maple no less.It was a little dangerous to say the least!Wouldn't reccomend it.


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## cabinetman (Jul 5, 2007)

For round mortises on a curved plane, I would use a hand drill (electric, or cordless, or crankwheel), and drill bit. Or, a brace and auger bit. For this, clamp onto the subject piece an extended piece of wood with a straight edge as a guide for the drilling angle.

For square mortises, I might drill out a major percentage, and chisel out the corners, checking for fit frequently. A corner chisel is very good for this procedure...it's called a "bruzz".

This pic is from the Woodcraft site as they are better photographers than me. The "bruzz's are corner chisels. The small "bruzz" on the left (*A*) is a cabinetmakers chisel. It's got 3/8" legs and is 10 1/4" long. The larger "bruzz" (*B)* is a TF'ers, and it's a 1" corner chisel, 18 1/2" long. I've got both of these, and I use them fairly often. 
.


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## gaviaimmer (Dec 6, 2007)

Thanks, but I should have been clearer in my posting. I need to cut a large number of these mortises. Too many to do a lot of hand work and they need to be highly precise. The "lam.trimmer,str.bit,and a bearing fence" sounds like an interesting possibility.


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## Knot Home (Feb 14, 2008)

I'm thinking a jig with a base that works like a V-block to convert the curved surface to a reliable flat plane for the router to work on. In other words, the jig would need a fence to pick up the edge of the work and a V-shaped base to find the curved surface a repeatable distance from the center of the arc. A slot for a guide bushing and you're there.

Roy


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## Al Killian (Aug 14, 2007)

Before you cut the curve, cut the mortise. This can be done with a plunge router and stop blocks or you can do it on a drill press. Either way you will need to use a chisel to clean them up. You can also buy a mortiseing attachment for the dp that will save you time.


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## TexasTimbers (Oct 17, 2006)

Gavi,

If you can picture my idea I think you will like it. I have never used it, but I like a challenge like this because I fancy myself as a good jig designer.

Fasten two lengths - say 3" or so - of small diameter (1/4") dowels, perfectly parallel to each other, to the base of the router on either side of the bit and far enough from the bit that you can get two electrical tie straps on each dowel but still far enough away the contraption would not be tipsy-turvy. bear with me a little more it don't make sense to you yet I am retyping this because it didn't make sense to me after i posted it.

the dowels should be stainless steel (or even a steel alloy for such a temp need) would be good for two reasons #1 stiffness at small diameters which are needed so not to detract too much from plunge depth and #2 SS is slick. Wood dowels _might_ work but I envision 1/4" wood dowels having slight flexibility. Maybe not though just have to try. You might even have to use 3/16" dowels if you can't get a bit with a long cutting depth. they can be ordered though I don't know ytour time frame for this.

You could fasten the dowels to a throw-away base plate and drill the holes where you need. It would not render the baseplate useless though so it could be the one on it now, just remove it and drill, as long as the ties don't make contact anywhere where there is metal casting once re-attached. I have a PC that has a base with plenty of holes already, but you could use large plastic tie-straps to bind the dowels to the base and place the tie straps strategically so that they just barely stradled the edge of the piece and could be used as on-board guide.

This idea is totally dependent on my perception that you are trying to place these mortices in the edge of a board and not on a wider flat curved surface. 

If you don't see what i am trying to explain i think i could throw this all together in 15 minutes or so and try it out for you. If it works I could post it on youtube. that would take an hour though. :laughing:


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## osweezea (Oct 16, 2007)

Sorry TT, but I surely didn't capture that thought. From what I have read, I would be tempted to try the v grove idea from Knot home. Just my .02


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## gaviaimmer (Dec 6, 2007)

Thanks for the suggestions. 

The issue with any approach seems to be the limited length of plunge router spiral bits. As the angle of the arc steepens it makes using a router, regardless of the jig, less and less workable because the router base radius and the arc angle can make for a sizable rise between the router base and the work piece, then I still need a 3/4" deep cut. This surpasses the length of any plunge bits I have seen. 

Using a jig does minimizes this depth issue compared to cutting the slots prior to cutting the arc but all in all it looks like its back to the drill press and a lot of hand cleanup.


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## TexasTimbers (Oct 17, 2006)

At it's apex in the center of the arc, how much wood is removed (not counting the plunge) from the board measured from a straight line from end to end?


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## gaviaimmer (Dec 6, 2007)

I have some flexibility here but about 3" at the center of a 50" piece.


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## TexasTimbers (Oct 17, 2006)

Maybe a dado blade in a Milwaukee 10 1/4" circular saw? :laughing:


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