CNC Efficiency: Tenon Cutting

Over the last decade or so I've been cutting joinery on my CNC for college student furniture projects.  Often the job is simple straight end tenons with the part clamped vertically.  It took just a few experiments to find the quickest and cleanest strategy show in the video below.

For a clean shoulder I used a spiral upcut router bit, running clockwise for a climb cut. This way the fibers are sheared off rather than pushed out. 

The 1/4"diameter router bit has 1" of cutting height and in normal profile or pocket cuts uses 1/2 of the diameter (1/8") for each pass.  That leaves 7/8" of  sharp edges that are rarely used.  A little math to calculate the area being cut in a normal profile pass and you get .25" x .125" =  1/32sq.in.  For a stepover pass that uses 40% of the bit width that area is even smaller at  1/80sq.in. So long as the area being cut off remains between those two the depth of cut and stepover width can be changed. 

For a 3/4" tall tenon 2 passes at 3/8" depth (rather than 6 passes at 1/8" depth)  reduces the time spent running around the tenon by 2/3. To keep the cut area under 1/32sq.in the stepover should be no more than 1/3 of the bit width. 1/16" width is a safe amount for a 3/8" deep pass.  Roughly 1/43sq.in. per pass.  The whole 3/4" depth could be cut in one pass if the stepover was only 1/32". In that case the chips produced tend to be 3/4" long fibers and are more challenging for a dust collector. That is the reason I make the pass depth 3/8".   If your CNC has some backlash/play you can run a conventional counter-clockwise final cut around the tenon to trim off any not cut by the climb cut.    

Another variable is feed speed.  The short lengths of the tenon sides self limits the speed of the bit travel. Acceleration/deceleration between nodes never reaches the feed speed set. This is true for the small Probotix CNC I use. For beefier and quicker CNCs I'd set feed speed to 100ipm. 

If you are using a larger diameter bit then the stepover and pass depth can be even larger. Do the math to verify you are still getting good chip removal. A 3/8" bit, taking 3/16" deep passes for a profile cut removes 1/14sq.in. as it moves forward. Making a single 3/4" deep pass taking off 1/16" is 1/22sq.in. removed.  Easily managed by the 3/8" bit. 

This strategy also works when the tenon is being cut at any angle up to 20 degrees or so from vertical. For woods that tend to split easily a smaller angle limit is recommended. 

Questions and comments are encouraged!

4D