Let’s move on to the next part of building the CNC vacuum plate: machining the base. You may recall from the last post that the base was to be machined from a 1 inch thick piece of cast acrylic material. The base will have channels that allow air to be drawn through a top plate that has an array of holes. From there, the air will be drawn out of the vacuum plate and into a vacuum pump that has adequate suction to hold the work piece in place.
Below is a photo of the base being machined. I’m using a flat end mill to carve out the channels and the area at the end where a vacuum connection will be made.
These small desktop CNC machines usually handle bits that have a 1/8 inch shank, and as it turns out, just about all of the bits I’ve found that have a 1/8 inch shank have a cutting width of no more than 1/8 inch. I did manage to find a 1/4 inch drill at one site, and I’ve found some “deep reach” bits that have a longer than typical bit length and cutting depth. It just means that it’ll take a bit longer than it would with larger bits because more passes will need to be made.
Finished base part, all cleaned up. Beautiful!
Next, we’ll make a top plate which will have all of the holes in it, connect vacuum, and see how it works!
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!
Or: How to double throughput on an ethernet cable. This trick could come in handy if you find yourself in a position of needing to run another 100 Mb/Sec ethernet cable alongside an existing one.
As an example, I found myself needing another 100 Mbit cable in my house when reconfiguring my home network when I got UVerse. UVerse transmits TV over TCP/IP, so it can (in theory) be sent through routers, switches, etc. Unfortunately I found that the TV service would die occasionally if I did not have a dedicated ethernet cable running directly from the UVerse “Residential Gateway” to the set top box. For some reason it just didn’t like my home network, and would just randomly fail. So I was forced to run another long cable dedicated to my set top box to make it work flawlessly. Bummer.
Then it occurred to me that I didn’t really need to run a separate physical cable, I just needed to get 2 cables out of the one that was already run. How could I do that? Well as it turns out, CAT5/5e cables only use 2 of the 4 pairs of wires inside them, leaving the other 2 pairs available for… you guessed it…. the other cable I needed. All that was necessary was to cut off the ends of the cables and install new ends so that there would effectively be 2 cables inside one jacket. That was a lot better solution than having to run 100 feet of new cable.
Here’s a drawing of an ethernet cable using EIA-TIA-568A wiring:
The table below shows a connection pinout that can be applied to both ends of the cable in order to get (2) 100 Mbit cables in one.
As you can see, connector 1 uses regular 568A wiring, and connector 2 uses the same thing, except that we substituted colors. Two pairs are used for each 100 Mbit connection.
Both ends of the cable will now have 2 connectors, yielding 2 100 Mbit Cat5 cables from 1 physical cable.
Unforunately you can’t use this trick with gigabit ethernet since all 4 pairs are used for the gigabit connection. If you’ve got 100 Mbit though, you can use this technique to “run another line” without pulling any more cable. If you have a complicated cable run going through walls, which may be the case with pre installed home wiring, this may be useful since running a cable through the same path can be quite difficult.
More information on Cat5 cabling can be found here as well as many other resources on the web.
If you find this handy I’d love to hear how you applied it to your situation