This is the second CNC machine that I have designed and built. My first machine was based off of oomlout’s instructable “How to make a Three Axis CNC Machine (Cheaply and Easily)” (by far my favorite instructable and the one that got me hooked on the site). It was moderately successful, cutting a number of parts from foam (a summary of parts made can be found on my abandoned blog here along with some build photos). The lack of overall stiffness and play in the linear mechanisms meant that plywood and plastics could not be cut effectively. The biggest downfall of the machine was the difficulty to setup and square the axes and lacked the ability to make fine adjustments once set up. The drive pulleys were sandwiched between the gantry sides and if a pulley loosened the entire gantry structure had to be disassembled and put back together and squared all over again (a couple evenings of work).
In reviewing published designs for a 2nd generation machine I revisited Joe’s CNC, a popular design but I questioned my ability to produce so many duplicate parts with enough accuracy. I came across buildyourcnc.com and their blueChick design . What caught my attention was their use of V-groove bearings and how it simplified the design and the ease of aligning the axes. I had previously discounted V-groove bearings due to their cost ($150/ set vs. $12 for skate bearings) but after my first build I had enough experience to fully understand their benefits and to realize they were well worth the investment. The blueChick was simpler than the Joe’s CNC design but was still a bit too intricate for my tastes so I set out to design a new machine based off of the new bearings. I came up with a new design with three main design features that solved shortcomings of my first machine:
1) All of the drive mechanics are exposed. If anything requires adjustment or tightening you can walk up with an Allen key, screw driver or wrench and access everything allowing the machine to be up and running again in a matter of minutes. The axes are easy to setup with the V-groove bearings and can be micro adjusted once installed.
2) The design has a low number of fabricated components and allows for low build tolerances. The precision is based off of the flatness of the plywood and the straightness of the aluminum extrusions. All of the fabricated components can be roughly cut (except two edges detailed in Step 3) and all holes are oversized to allow for slight inaccuracies in drilling. This allows for any inaccuracies in the building stage to be taken up during assembly without loosing any precision.
3) Low operational noise. The machine had to be quiet enough to use in an apartment or I couldn't use it. The rotary tool I used on my last machine worked well but when running at 20k rpm, it screamed too loudly for me to use in my new home. A custom spindle was built as a low noise solution with negligible reduction in performance.
Step 1: Terminology & Specs
Cutting volume 22 1/2" x 18 1/4" x 2 1/4"
X&Y: MXL timing belts w/ 40 groove pulley (pitch dia 1.019") maximum resolution 0.004 inch at 1/4 micro stepping
Z: 1/4" threaded rod. Theoretical resolution 0.00006 inches at 1/4 micro stepping
All axes powered by 130 oz-in stepper motors.
Cutting speed/depth are dependant on the material being cut and are limited by home made spindle power and router bit.
The terminology for the different components as I will refer to them is shown in the picture. I have the X&Y axes oriented as shown so that when sitting in front of the machine, the axes match a 3D CAD drawing as seen in a top view (X axis horizontal, Y vertical and Z out of the page/screen).