One of the fun things I’ve been using my desktop CNC machine for is to make printed circuit boards using the isolation routing technique, where essentially you start with a copper clad board and remove material from it to isolate areas of copper to make pads, traces, etc. Once I had the initial setup done and made a few boards I became more familiar with some of the challenges one faces in making boards with finer features. One of those challenges is keeping the board really flat and also parallel with the tool plane so that consistent depth cuts can be made. This is the first of several entries showing how I’ll build a vacuum plate to improve the quality of my routed boards.
My first jig was pretty simple and worked well enough for crude circuit boards. It consisted if some MDF particle board that was clamped to the work table, and then an area of the board was machined to be parallel to the plane that the tool moves in at a given z-depth (vertical position of the tool). I held the board only around the edges with masking tape (not shown). The square area accommodates the 6×6 copper clad board I was using. The various shapes carved into the area are from previous pcb outline routing where the bit went a little too deep.
There are a couple of problems with this approach. If the board is warped at all, it can cause the depth of the cut to change, and since the pcb engraving bits are usually “v” shaped in order to get down to a really fine point, changing the depth also changes the width of the cut. This is not a good situation if you’re trying to make very fine features like traces less than .010 inches. Another problem is vibration that can occur when the tool contacts the surface to be cut.
Professional grade pcb routing machines use a vacuum plate to hold the board in a fixed position. A very flat plate has holes drilled in it (think air hockey table) and a vacuum applied to the other side. Since the force is fairly evenly distributed over the board via the hundreds of tiny holes sucking the board down, the board stays in place and very flat since it’s drawn down to the machined surface.
I will endeavor to build a working vacuum plate that I can securely mount to my desktop CNC machine to improve the quality of my boards. To do this, I’ll be using some scrap material from a local plastics shop, my CNC machine, a vacuum pump that I happen to have handy, and some custom made fittings I’ll make on my 3D printer.
The above photo shows the piece of stock material I’ll turn into the base of the vacuum plate. This was purchased as scrap from a local plastics shop for $1 a pound. A pretty good deal considering what it would cost if you wanted to buy a 1 inch thick larger piece and have it cut down.
One very important note! There are 2 general types of acrylic materials. One is extruded and one is cast. If you try to machine extruded acrylic, you’re gonna have a bad time Extruded acrylic will melt and gunk up your cutting tools. Cast acrylic on the other hand has different properties and is pretty friendly to the machining process. It chips away rather than melting. So be sure to get cast acrylic instead of extruded if you’re going to machine it.
The following picture shows the plan for machining this part. There will be another part on top of this later which will be where the actual PCB will rest and where the array of holes will be drilled. This first part will be where the vacuum source is connected and where the channels will run under the top part allowing the vacuum to be evenly distributed.
Come on back later to see how this turns out!