CNC Router : X and Y Axis
Linear Bearings and Rails
The idea of the rails and linear bearings is to support the gantry structure and allow translational motion along one axis, my X and Y axes use 20mm aluminium shs as the rails and some homebrew slides I made up with skate bearings and some more shs. I came up with this design after experimenting a little with a box of skateboard bearings I bought of ebay for really cheap, I'm sure its not a unique design it was just what I ended up with. The design gives very low resistance sliding and great support was a lot cheaper than buying linear slides this size, one thing that I wish I considered before was that I could quite easily have used this design in a fully supported configuration and it would have eliminated the sag problem I discussed in the previous section. I chose skateboard bearings because they are available in bulk quantities cheap, I got a box of 30 for about $25 of ebay. Skate bearings are just standard 608 bearings, the ABEC rating is just related to the machining tolerances of the bearings, if you want some more detailed info on bearings see this link http://www.nmbtc.com/bearings/608-bearing.html
X Rail Support
You can see in the model below the final construction with the X and Y axes labeled, as I mentioned in the previous section my machine is a bit too big for my needs, with a large machine like this one thing to consider before construction is where it is possible to reduce unnecessary forces or moments (rotational force) as these can cause inaccuracies and put strain on the drive motors. For example placing the drive thread and linear rails on the same plane will eliminate a moment that can cause slight deflections in the gantry position. In the model you can see the two supports on the 45 degree angles which are not in some of my photos, I added these recently to help overcome a deflection problem I was having due to a moment I have shown in the image below.
Lead Threads and Nuts
It is quite possible to spend hundreds of dollars on some really nice ground ball screws and nuts and if you are looking for high speed and low backlash then this is the way to go. In my case I was working on the cheap so I used standard 3/8" zinc plated all thread for my lead screws and made my nuts with blocks of acetal (delrin), I used delrin blocks and tapped them with the 3/8th thread to create some cheap very low backlash nuts, I originally used some off the shelf nuts but they created a lot of noise and there was about 1mm of backlash which was just not good enough even for my machine. To tap the delrin block I undercut the hole slightly so the thread was a nice tight fit, It was safe enough as its hard to snap a 3/8" tap and even if I did I was only using offcuts of delrin that I got cheap from a wholesaler. The picture below is the best shot I have (I took some new ones but the machine is quite dirty after a year of use), it shows the homemade nut on the Y axis of the machine. One upgrade I plan to make when I get the time is to add a second lead screw on the X axis, duel drive will make the gantry much more rigid and help it resist unwanted rotations about the Z axis.
Y Axis Lead Screw & Nut
The lead screw used directly relates to the speed that the machine can achieve, I am using single start 3/8"- 16 tpi which has a lead of 1/16" or 1.58mm and I get a jog speed of about 2m/min and usually cut at about 1.2m/min, for higher speeds and better accuracy I would like to replace the lead screws with some trapezoidal thread http://en.wikipedia.org/wiki/Trapezoidal_thread_forms. The equations below let you calculate the lead and speed of your machine, a nice thing about some of the trapezoidal threads such as the acme thread is that you can get multi start lead screws which can increase the speed of your machine.
Lead Speed Equations
How to Make it Better
Probably the largest problem I have with my machine is deflection in the X axis lead screw which when driven causes whip that vibrates the entire machine. I have added several upgrades/hacks to try and overcome this problem, I have tensioned the X axis lead screw, adding tension reduced the static deflection of the rod but puts a fairly large load on the base of the machine, I have also added a third support/leg structure for the machine (in the image below) which I can move depending on the size of the work piece, and finally by experimentation I have found that the vibrations are lowest as speeds between 1200 and 1600 m/min. Acme threads are available in larger diameters which would help a lot to reduce the sag, using a larger diameter lead screw would require larger stepper motors and therefore larger power supply to overcome the larger inertia of the bigger lead screw.