Woodsmith CNC Router - Adapting it for Horizontal Milling #1: Milling Tenons

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Blog entry by Mike_190930 posted 06-10-2021 10:57 PM 439 reads 1 time favorited 0 comments Add to Favorites Watch
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Recently I’ve been learning through trial and error, mostly error, to make tenons with compound angles for building chairs. For me at least, this has turned out to be a skill that is not easily acquired. But, as a retired engineer, I retained other skills that have allowed me to sidestep the traditional methods of producing compound angle tenons (and mortices for that matter).

Since I have a working CNC, it occurred to me that I could build some adapters for the CNC (described elsewhere in this blog) that reduce the problem to a straightforward engineering task. The goal was to turn the CNC temporarily into a horizontal router table. If you follow Matthias Wandel on Youtube, you are likely aware of his horizontal pantarouter. If you haven’t followed, it may be worth a visit to see his engineering style approach to woodworking. The pantarouter build, while really useful and clever, requires a fair investment in time and materials, and when complete, it would take up yet more room in my crowded shop. It also needs to have scaled templates made for each different cut. A great solution though if you do not already have a CNC.

Unfortunately, I did not think far enough ahead during the original CNC build to leave a way to mill the ends of vertically clamped boards, and that modification was next to impossible without tearing the whole CNC apart and re-building it. My approach to adapting the CNC for milling tenons is to mount the board horizontally and re-mount the router into a horizontal holder. Of course, since the vertical travel on the CNC cannot allow the cutter to get to the bed level, a short table to hold the workpiece above the bed is also needed. Finally, the control software had to be changed to redefine the x and y-axes to be in a vertical plane, and the z-axis to move along a horizontal line.

Router Holder

I first had to design a horizontal holder for the router. This required that I change the original z-axis carrier plate to incorporate a removable router holder since the original construction just glued the holder to the plate. Since I had a working CNC now, making these parts was faster and easier than the original build which was done by hand. I also included in the new plate two locating pins (dowels) and four captured 10-24 T-nuts so I could mount and de-mount either the vertical or horizontal router holder. This photo shows the new horizontal holder attached to the plate with the router in place. All parts were cut with the CNC from ¾ plywood and glued up to form the thicker pieces.

Adaptor Table

The adapter table is a simple plywood bridge about 6” tall. Construction is unsophisticated edge glue up with wood screws added. There are feet at the bottom for clamping the table to the CNC bed. On the table top, I glued on a piece of ½ inch MDF that had been previously routed with an engraving bit to define several lines at 2 degree increments for assistance in aligning the stock. One group of lines was placed on either side. Angled tenons can be cut by aligning the stock with the desired angle. Compound angle cuts will need additional shims under one end of the table or stock. The next iteration of the table will include a hinge and locking mechanism eliminating the necessity of shims. Two dados were routed into the top to accept T-tracks for clamping the stock. The next two photos show the table top and the overall view of the table on the CNC bed.


My CNC is controlled by Mach4 software, and so far I have used the pre-packaged 4-axes CNC configuration that came with the software. To redefine the axes for horizontal operation all that is necessary is to change the motor assignments under “Control Configuration” in the “Axis Mapping” tab. The screen shot on the left shows the standard vertical CNC assignments, while the screen shot on the right shows how I reassigned the motors for horizontal operation. I’ve overlain text tabs because the screen shot blurred the motor numbers too much. Top to bottom the motors wer 0, 1, 2, 3 originally; for horizontal mode they are, from top to bottom, 1, 2, 0, 3. (Notice the A-axis remains unchanged. That motor is only used for rotary jobs). If you do this, be sure to use the Mach4 loader to define a new profile for horizontal operation so you do not overwrite your vertical profile. If you wish, you can also change the button labels in the Mach4 jog screen to truly represent the new axes definitions. I have not yet done that, and it was a little confusing not to have done so. Maybe later.

To test the scheme, I drew up a mortise sized to allow ¼ shoulders on a scrap piece of 2×4 planed down to 3-1/4×1-1/4 inch. I set the Vectric drawing origin at the center of the mortise, making zeroing X and Y to the work piece simple. This photo shows an alignment pin in the router as I zeroed the axes against a crude center mark on the end of the stock. After zeroing X and Y, I exchanged the pin for a ¼ inch end mill and zeroed Z to the end grain surface.

Using similar methods, I also milled a matching tenon. This technique will only work if the tenon sits at right angles to the tenon shoulders, no matter what angle the tenon is to the stock. In situations where the tenon is not at right angles to the shoulders, I would opt for floating tenons and angle a mortise into a face that is cut to the desired angle relative to the stock. Here are the first test mortise and tenon (a) side by side, (b) partially assembled, and ( c ) fully socketed.

Yes, a smarter person would have made the tenon slightly shorter than the mortise depth, but on this day that was not me. Anyway, this is just a test piece. I will also have to experiment on just how much undersized the tenon should be so it doesn’t have to be pounded into the mortise. This one is only 3 mils undersized, and took a mighty whack to seat it. Finally, there was some chip-out at the shoulder edges as the router bit milled through this redwood piece while making the tenon. I should have programmed an outside perimeter climb cut first to avoid chip out from happening.

This is just a proof of concept. There are several refinements in technique and some for the equipment to come. At this point I am confident that I can make the angled tenons I need for the chair project. If the need arises, say for a full set of 6 or 8 chairs, then I can batch the parts through the same setup and save a lot of time. Hopefully I will also avoid ruining some lumber as well. That is to say, if I can keep my head straight and cut the proper angles in the proper ends of the proper pieces.

Wonder if this works for dovetail joints too…

-- Huh? Whadaya mean it ain't "measure once cut twice"?

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