Building a table on the CNC without using any hardware.
A project by:
Gabriel B. Soares
I recently became certified to use the ShopBot® CNC machine at the Techshop and have been looking for a simple, yet useful project to start getting familiar with the machine and software tools. In this effort, I decided to make a table out of furniture-grade plywood that would not use any metal hardware (only dowels, glue, and fitted joints).
So I began by looking online for work that others have done with CNCs and quickly stumbled upon the website OpenDesk.cc which has an entire selection of open source furniture designs that can be built on a CNC (chairs, tables, storage, etc). Most of the designs that are posted on the site use only glue and fitted joints, which is exactly what I wanted to do.
After browsing and reviewing some of the posted designs, I settled on the "Olivia Desk" designed by Joni Steiner for its simplicity and ability of being easily editable to suit the dimensions that I needed. At the time of writing, the design files were posted under the Creative Commons - Attribution-NonCommercial license, which allows me to adapt and share the designs (giving proper credit to the author and stating changes made to the original design).
Table Design & Generating Toolpaths:
The original dimensions of the table are 1180mm(46.46in) length; 500mm(19.69in) width; 720mm(28.35in) height. For the location where I was going to put this table, I needed the length to be reduced to 36in (914mm), which meant I had to make a small modification to the original file. The edits to the pieces were fairly straightforward to make (only the top of the table and the beam that runs along the length had to be changed). Once I had the dimensions that I wanted saved on the .dxf file, I exported the design to the software that would generate the CNC toolpath.
For generating the toolpath G-code I decided to use the software VCarve Pro, which is already installed on all the machines at the Techshop. This was the first time I was using this software so it took a little bit of patience and research online to get the hang of the interface, but once I knew the basics, it was very easy to use. The software is nothing more than an interface in which you select the line or spline that you want to generate the toolpath for (e.g. the contour of the table), and then specify parameters for the cut (depth of cut, router bit/end mill type, direction of cut, etc). The software will then calculate and generate the toolpath and give you the ability to preview what the cuts are going to look like.
The images below show the preview of the generated toolpath for the different pieces of the table. You can see in these the 45º chamfer around the edge of the table, pockets for where the pieces mate, and holes for the dowels. This preview tool allowed me to make sure that I had everything set up correctly before generating the G-code and committing to cutting the plywood.
Then came time for the fun part: spinning the bit at 12,000rpm and letting the machine cut through 3/4" 12-ply plywood like a hot knife through butter! For this table, only two bits were necessary: a 45º chamfer router bit and a 1/4" 2-flute end-mill.
The main challenge of operating the CNC machine is the calibration (X, Y and Z positioning), and in this case converting the machine's settings from the default Imperial units to Metric (this is due to the fact that all measurements in the CAD design were done in millimeters). But once those tweaks and settings were set, it was just a matter of sitting back and letting the machine do its job.
After about 2 1/2 hours of cutting, I had all of the pieces perfectly cut, and it was just a matter of assembling the whole thing.
The assembly of the table goes as follows: the legs (which are two sheets thick) consist of four pieces of plywood glued together (with dowel pieces for alignment and strength), a horizontal piece fits (by friction) into the two leg assemblies, and the table top fits (also by friction) to the legs.
There are almost no tolerances in the pocket cuts in this design so the plywood fits very, very, very tightly into the slots (for some joints, I was having difficulty closing the joint and had to use a sander to slightly increase the tolerance). The assembly essentially required the use of heavy duty clamps and a large mallet. Once all the pieces were put together and were properly seated, the table felt extremely sturdy!
All in all, the table cost around $120 in raw materials to make (includes the price of the CNC bits). If I were to do this project again, it would cost roughly $90 just for the plywood.