Kerfing and Bending Curved Stair Risers

      Woodworkers describe in detail their techniques for fabricating curved stair risers. July 27, 2008

Question
I've been making round risers for quite a few years by kerfing them and bending them around end blocks with limited sucess. I've been using oak plywood that the contractor supplies lately and have been having a heck of a time. I know there’s a formula as to the radius and the spacing of the kerfs but don't remember it. Any help on the subject would be appreciated.

Forum Responses
(Architectural Woodworking Forum)
From contributor J:
I never used a formula but I use about the blade kerf for the cut and leave about 1/8 to 1/4 between kerfs depending on the radius. The key to success is very consistent spacing and very consistent depth. I also run two dados in the back of the rise along the length about 3/4 in from each edge. My radius "form" blocks go in the dado. I then usually smear some adhesive in the kerfs.



From contributor M:
If you have a light weight circular saw use it with a large homemade wood t-square that you cut through on the first pass. Spacing should not be a whole lot wider than the saw kerf. This method is actually fairly quick and there is no danger of going through the face such as a radial arm saw or miter saw as the piece gets flexible. Put a mark on the t-fence to automatically slide to the next kerf. If you are on a good work surface there is no need to clamp just get a good hold and go for it. From there everything Contributor J said is spot on.


From contributor H:
We have been doing it exactly the way Contributor J has been for many years with almost no problems. Dado first (1/2 the thickness) then kerf about every 1/4 ". Consistent spacing is very important. We do use our radial arm and it has to be set up right.


From contributor T:
Why not just build them up with thin laminates, form to the proper radius, and glue it up?


From the original questioner:
Thanks for the input. As usual your advice is helpful. I use a sliding miter saw and I did notice a kerf or two was just the least bit deeper and that’s where I get the flat spot. Must be as you've said it lifted up a bit. What is the reason for the dados? To keep the end blocks aligned? What thickness - end blocks 3/4"?


From contributor B:
We cut radius plates and then wrap them with bending luan and then use wood veneer of the desired species. We got away from kerfing a long time ago because of the same problems you have mentioned.


From contributor C:
Ditto Contributor T. When time permits make a nice reusable jig. 6 or 8 inches tall and 6" radius or what you need - 5 plys from the band saw and then wide belt for a decent glue surface. Wet the plys for flexibility and a slower glue dry. Paper towel dry them, we use titebond II, and clamp over night. We do the ripping when we need a few and always re-saw a few extra. Glue up one a day for week or so.


From contributor R:
I also make my own starter risers with kerfed plywood. I have an old Delta sawbuck (for the older guys, this is the 4' wide , basically a circular saw that slides on 2 rails, mine is one of the last models, and the motor is height adjustable for dadoing). I put a wood sub-fence on it and adjusted it to make a down to a light 1/8" kerf, and leave it that way as a dedicated kerfing machine, the fence is marked to make a kerf at 3/16 spacing.

After making a end block, upper and lower ply with blocking in middle (to screw in the shure-tite bolt, I use) to the height I need less 1/2", I tack on a 1/4" ply on bottom, squeegee some construction adhesive in kerfs of ply, and on a flat table , wrap it (on edge) with a couple tacks at each end, doesn't take too long to make any size or configuration I need. Then pop off the 1/4" ply spacer, which leaves me with a 1/4" spacing at top and bottom of block, for any scribing.

I can't see any purpose or the time it would take to make a starter out of laminations (waste of time). If I need one out of a different wood, then standard oak or birch or an available ply, then I make it and just veneer over. It's a starter riser, no need to make it a difficult process.



From contributor O:
We do a lot of these. We cheat a little though, we have our CNC run the two dados, whatever the desired length, then either use and aggregate to put saw blade kerfs every 3/16", or a 1/2" tool to put a kerf, with about 3/8" spacing, and we take it down to about 0.080 and then use 1/2" PB for the inserts that snap into the dado grooves for the proper shape. If the kerfs vary in depth, you'll snap them all the time. If they're a good consistent depth, you shouldn't need to wet them for a standard 6" radius. We haven’t, and we run a lot of exotics, such as cumaru, ipe etc. as well as oaks, maple, hickory etc.



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