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Tuesday, March 26, 2013

Plywood Kerf Test (Table Saw)

The interior design calls for a number of 4 inch radius vertical corners. The idea is to save your leg from getting gashed on a hard square corner edge when getting tossed in the boat. This means many of the plywood kick panels will need to be "kerfed" to enable the 4-inch radius bend. I have never done this before, so an experiment is in order (google "plywood kerf" to find lots of tips and guides). I picked a corner and used some cheap C/C grade 5-layer plywood I had in the shop. Here is how it went...

Kerf planning in CAD

Since the settee surfaces were cut in CNC, I had the original CAD drawing to help plot where to cut the kerfs and how much arc length to cover. 5.5 inches of arc-length at half inch spacing mean 12 cuts.
Hand cutting kerfs and dadoes on the table saw
After rough cutting the plywood to fit the space (cut a little larger than needed), I cut the kerfs on the table saw with a blade height of 5/8's inch, spaced every half inch. On 3/4 inch plywood, this leaves 1/8-inch of material to hold things together. The top and bottom edges were also dadoed to joint into the top settee surface and lower cleat attached to the floor. The piece was then trimmed test fitted to the settee.

Glue and clamp
Next came the bending and gluing of the plywood. From the same CAD drawing, I had the CNC machine cut some clamping blocks (from MDF) that match the radius (adjusted down for material thickness). The piece was clamped to the block and kerfs filled with polyurethane wood glue.
Fitting the panel. Note the vertical splits on the radius
Last, I did the final fitment. It looks like crap. But this is a test, not final finish. The facing surface started to split on the radius, which I attribute to the cheap plywood (and the kerfs were a bit too thin).

So, I will claim "not bad" for my first try, and note what to do different next time:

  • Use a thicker (standard) blade, or space the cuts less than a half inch.
  • Cut the clamping blocks out of plywood and glue them into the final piece. This will help the panel keep it's shape (and provide a hidden cleat to secure the piece in place!).
  • Use better quality plywood (which would happen in final finish work anyway). The cheap stuff is prone to splitting when bent.
  • No polyurethane glue. Use thickened epoxy.

It also occurred that this entire piece could be cut in CNC. I might try that next.


Rhys said...

Have you considered gluing a veneer over the pieces that require the kerfed corners? It's nice work, even in "test mode".

comsa said...

You need to left 1/16 of material to bend it without splits.

Robert Sutton said...

Rhys, we have not decided if the surfaces will be veneered or painted.

Veneering is another thing I must experiment with.

comsa, thanks, I will try 1/16 next time.

Dan said...

I agree that it's "Not Bad" Could steaming have helped?

I'm thinking steaming would require building a steam box and mean more work.

Robert Sutton said...

Dan, I have read about the technique, but never tried it.

I may give it a try depending on how things go.