Background and design spec.
I'd seen a lot of CNC builds where the second one was just used to remedy the faults of the first one. That wasn't going to happen to me ! lol... So in this section I'll try and explain some of the decisions I took and why. I should also add that at this time, not only did I not have all the answers, I didn't even have some of the questions!
Although I had a good idea of what I wanted to achieve and some ideas I'd not seen used before that I thought would improve the build or make it easier. This CNC was not going to be an exercise in what was theoretically possible, just a no frills, very VERY strong, quick and easy to build structure that a tank could drive over! It had to be built without recourse to another CNC or exotic measuring equipment, just the equipment I had, could borrow or was prepared to buy. I did have some milling done on the frame, (making the 26x 22mm holes for the conduits, we had a jig on the mill and just knocked them out but that could as easily have been done with a pillar drill but would have taken a lot longer. Apart from that milling job, it was built with power tools and a pillar drill.
It seems like a long time ago I asked the question "what do I want to cut with it?" "Don't know yet" was the unhelpful conclusion as I recall. But after much thought about what I needed, opposed to what I wanted. I came up with a basic spec; a very strong, fast CNC that could work anything I threw at it including metals and portable enough to fit through a standard door. The ability to use the addressable 3d space with other implements was also seen as a bonus.
Design criteria (draft 5)
A few additional criteria added during the build, the spec mainly stayed the same and was met.
1. Portability; Self contained, Electronics contained within the design, able to fit in the back of the car / through a doorway.
Overall, successful. although a hydraulic trolley, lifting jig and 2 people are required to move it. it can be moved by one person but its a swine. It fits into the back of my car and would go through a doorway. It is also capable of creating its own doorways if required.
2. Able to handle any material I was likely to throw at it. With the best accuracy I could build in a shed.
Successful. (Probably... really need to fit the slower motor to test it with SS.)
I couldn't see me needing to work anything harder than stainless steel, so that was where the bar was set. The easy part of the design was discarding wood and alloy extrusion for the build. Converting an existing mill was explored and discounted. So, metal was going to be the material, box section would give me strength to weight advantages as well as simplify the construction process.
2a. Strong and rigid, lots of rigid.
I realised fairly early on that for the performance I was wanting, a hybrid of bolted and welded steel construction and profiled rail were the right option. So large quantities of 80mm thick wall box section steel were calculated. The Hiwin catalog and price lists took a beating too.
I looked at various existing designs but was also aware of the limitations they created. There was a lot of sketchup time invested in looking for the solution and one variation kept winning over others, one I'd not seen before. The description sounds harder than it is so bear with it. A box made of box section wrapped around another box made of box section wrapped around another box (made of 20mm alloy plate). That describes the X, Y and Z axis.
While getting a rigid frame was IMHO critical, getting all the levers as short as possible was just as important. More hours and iterations invested doing that..
I also spent a lot of time getting the loads generated by the milling process contained within the bearing footprint. So the spindle works inside the Z axis bearing locations, the whole of the Z sits inside the Y etc. (as an aside, this can limit the vertical travel you can get from the spindle - if you want it to remain inside the footprint, so...
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