Domino & Cutting On An Arc

[kenkrawford]

I'm making a bench that has an arched back.  I want to use my Domino to cut the mortices for the slats (show in red) on the back.  The straight bottom won't be a problem but the arched top is.  I'm concerned about cutting the mortices that will be in the region of the blue arrow.  Any ideas on how to accomplish this?

Thanks in advance.

https://flic.kr/p/o7zrd4

 

[Birdhunter]

Are all the slats perpendicular to the straight base as are the red lines or are they angled as is the blue line?

 

[kenkrawford]

Yes they are.

Are all the slats perpendicular to the straight base as are the red lines or are they angled as is the blue line?

 

[stvrowe]

This is the method I used; 

 

[pugilato]

The hard part will be matching the curve on the slat. The domino mortise will likely be less deep that the setting, which will result in dominos that are too long for the holes, and you will need to trim the domino.

 

[Birdhunter]

Your arc is much deeper that that shown in the video. You might have to first stair step the top piece to cut the mortises and then cut the curve. Otherwise, the mortises won't be deep enough. Each step will be where a mortise will be centered. I don't know how to create a drawing for this forum so I hope the written description will suffice.

 

[kenkrawford]

I forgot to mention that I've already cut the curve  [mad].  It wasn't raining when Noah built the arc.  My bad.

 

[epicxt]

With the curve already cut, my instinct would be to create some wedge-shaped pieces that would match the part of the curve where you want a mortise on one side and a flat on the other and glue/clamp to the bench top rail so the domino has a flat part to register against. Not an ideal situation.  Good luck!

 

[Birdhunter]

If this were my project, I'd cut the mortises in both ends of the slats and in the straight base. Id dry fit the slats into the straight base. I'd place the curved top over the slats to mark the top's curve on the slats and the mortise center on the top. Tag the slat's top and bottom so you know where they will end up on the straight bottom and curve top. I'd trim the tops of the slats to the marked curve. Save the cut offs. I'd use double sided tape to affix the cut offs, one by one, to the curved top so that the mortise marks line up. The cut offs will provide a reference surface for the Domino. Kind of like recreating a stair step. I'd cut the mortises into the tops one by one removing the slat's cutoff after cutting the mortise. Wish I could think of a better explanation, but I'd do it this way. By the way, I always cut one mortise the exact size of the tenon and the other end wider so I've got some wiggle room.

 

[dltflt]

This is not ideal but if you rip the board down so you have narrow strips.  Then cut your mortises from the flat side.  Then once you have cut all of the mortises, glue the strips back together.

 

[kenkrawford]

Thanks everyone for your thoughts and input.  Today I re-created part of the arc on a piece of scrap.  I was able to get about 3/4 depth on the mortice.  Since the slats will only be 12" or less in length,  I think that will be enough. 
Birdhunter, I followed you post up until the part where you suggested using the slat cutoffs when cutting the top.  Can't seem to picture what you were describing.
Don, I consider ripping the piece but since this is my first Domino project, I was bound and determined to use it.

 

[NineFingers]

My first major use of the Domino was on a crib project which I believe is similar to what you're undertaking.

http://festoolownersgroup.com/member-projects/first-large-project-crib-for-granddaughter/

The method I finally used was to simply cut the mortises as close to perpendicular to the insertion point as I could on both the slats and the rails they were to insert into.  In the end it worked quite well, and was plenty strong.  As another poster noted, the arch of the top piece did keep the Domino from cutting as deeply as the machine was set, and I did have to trim nearly every domino for a custom fit.  Still, it was easier and faster than any other method I could come up with.

I've no doubt my description will confuse... if that's the case, let me know and I'll try to clarify.

 

[Birdhunter]

Ken,

I'm glad you didn't do what I described. The cutoff idea would not work. This realization came to me in the middle of the night. It was one of those concepts that I always model in cardboard before I cut any wood. I would have seen the error.

Please post a photo when you get the piece done.