CNC Cut a Family Heirloom Step Stool

Measure the thickness of your material throughout the entire board for accuracy (Figure A). Open the OnShape model, double click the “#Thickness” variable, input the measured thickness (Figure B), and click the checkbox; it will adjust all the joints for press fit construction (although I do suggest using glue).

Right-click on a face of each part in the model and select “Export as DXF/DWG” (Figure C) to create a 2D vector. You must click on the bottom surface of the stool top and the cross brace to export the joint holes. Click the other two parts on the sides with the largest surface area.

Input your board’s dimensions in VCarve (I cut in millimeters). Set your home origin to the bottom left of the part and the zero as the top of the board you are cutting. Import each DXF one at a time and use the Join tool to close up any vectors. Lay out your parts based on the size board you are using, leaving plenty of clearance on the corners. I was able to fit all the parts for the stool on a single 44″×11″ board (Figure D).

The stool is cut in three operations: a pocket (Figure E), an inner cut (Figure F), and an outer cut (Figure G). A pocket removes all the material inside a selected boundary line, leaving a hole — select the Profile cut tool, and in the “Machine Vectors” section select “Pocket” for the dog-boned holes first. I like to use a ¼” two-flute straight bit. You only need to cut down 10mm deep for all the pockets in this design. If you want to go any deeper (or shallower), be sure to modify the “#PD” variable in the OnShape document before exporting it.

Next select “Inside” in the Profile cut tool to cut the inner sections of the legs. Vectors can be cut either inside the line, outside the line, or on the line. This cut should go all the way through your work piece so set the cut depth to the board’s measured thickness. Before finalizing this operation be sure to add a few tabs to your part to prevent the piece from coming loose after it’s cut and possibly damaging your job, your machine, or you.

Finally, setup the outside cut — this will be the majority of the parts. Choose “Outside” in the Profile cut tool and again add tabs. This job also needs to cut the entire way through. When all your paths are set up (Figure H), export each job type as the G-code that matches up to your machine type.

Affix your material to the bed, and be sure to square it to the machine. Important: If you use screws to hold down your material, triple-check your paths to make sure the bit doesn’t run into them and ruin your tooling. If you separated the G-code into individual files for each operation when you exported the files, start with the pocket job first.

Safety first: Make sure you have proper eye and ear protection before running your CNC router.

After the pocket job (Figure I) is done, run the inner cut (Figure J) then the outer cuts (Figure K) to complete the milling.

Remove the board from the bed and cut the tabs free (Figure L). Use sandpaper to clean up the edges. Add a little wood glue to each of the holes in the top of the stool, making sure it evenly coats the bottom of the hole and up the sidewalls. Now press the legs into place. You might need to use a mallet for this as the parts should fit tightly together, being careful not to damage the wood. Work from side to side on each leg until it is fully seated.

Put a little wood glue on the side of each leg coming up about an inch to help bond the sidewalls (Figure M) to the legs and stiffen up your stool a little. Again hammer the sides into the top, working from side to side until they are fully seated. Finally, add some wood glue to the holes in the cross brace and with the stool sitting right side up, slide the brace through the legs and down onto the pegs. Gently hammer if necessary (Figure N). Let the glue dry for a few hours.

Since it can see years of heavy use, finish the stool with some kind of protective coating. I like tung oil, which can be brushed on with a rag and worked into the wood. It will seal the surface of the wood helping it live up to the stresses a stool is put under.