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Saturday, March 30, 2013

Plywood Kerf Test (CNC)

Same piece, different method. I am using the CNC machine to cut everything here. This time with a better quality piece of plywood. The kerfs are cut with an eigth-inch spiral bit down to a depth of 11/16ths. On 3/4 inch plywood this leaves 1/16th inch material to hold everything together. Instead of 12 kerfs, I cut 20.
Kerfs cut by CNC
In the end, I cut a bit too deep. A small dip in the CNC gantry rails caused the bit to dig into the facing ply. You can see the kerfs from the outside. So this test is a bust. Otherwise, the piece fit just fine. The 20 kerfs were a bit excessive for this radius. The piece can be bent much beyond the desired angle.

I still think this can work by CNC. Next time I will try leaving a little more material and less kerfs.

CNC Process

I don't recall writing much about operating the CNC machine. For the curious geeks, here is how this kerf test was cut.
Planning the piece in CAD

First step: plan the piece in CAD using measurements carefully taken from the physical space in the boat. Layout the kerf lines (evenly spaced), boxes for the dadoes, and the outer cutout rectangle. For dadoes aligned on the edge of the piece, I usually expand the limits beyond the edge up to a quarter inch. This makes for a cleaner cutout in the end.
Generating toolpaths in CAM

Second step: Export the CAD drawing to a DXF file an import into your CAM software. In this software you plan the toolpath for the router using the lines of the drawing. Various types of operations give the desired result: 'engraving' cuts the kerfs, clearing 'pockets' cuts the dadoes, and an outside 'profile' makes the final cutout. There is one tool change from 1/8 inch endmill (engraving) to 1/4 inch endmill (remaining operations).
Simulated cuts in CAM

Third step: Let the CAM software simulate the operations as a "test cut" and verify it will all work as planned.
The "bits" that are the cutting instructions.

Fourth step: Generate the "GCODE" file, this is the file that contains instructions for toolhead movements. The CNC controller software interprets this file to control the machine.
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