Background and design spec evolution

A project log for Sweet Dreams CNC (multimachine)

There are solutions and there are elegant solutions.

Crum3leCrum3le 09/24/2015 at 13:300 Comments

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 you make the bed moveable! )

The Y axis has its bearings at the outer edges of the Z, and contrary to usual design ethos, I went away from the idea of having the profiled rail facing forward! Herecy! I put the rail at the top and bottom of the Y axis. There is some lateral loading but the main loads are vertical. Profiled rail has its best load carrying capacity when loaded from above, then from the side, then from underneath. By putting the rails back to back as it were, any vertical load is always hitting the strongest part of the rail. (Having the rail facing forwards means the load is always hitting the side of the rail)

3. Operational automation.

I was learning on several, make that many fronts and knew that when it was running I'd need to be watching everywhere at once until I could trust it to not try and kill me or self destruct. Various operations could be automated to alert for problems. Voltage levels, Stepper temperatures, coolant temperature could all be monitored independently.

Other staples were the standard 2.4kw water cooled spindle, NEMA 34 steppers, 8amp drivers, Omron switches, nothing unusual except the later addition of a 250 rpm single phase spindle to deal with any low speed spindle requirements, mounted parallel but alongside the main spindle.

4. Flexibility.

Not in the frame but in operation. I liked the idea of putting other tooling to work in this addressable 3d space, so what else would work in there?

I wanted a slower speed drill / quill for metal work. so that was in.

Pick an place was doable, 9 axis computer control, including 2 servos from the controller board and the hot swap spare Kinco 880n meant I had the control side sorted, but my electronics abilities don't stretch to a production requirement so shelved for the moment.

Plasma / Laser cutter, with a suitable bed change both were possible. Neither urgent as no requirement at the moment.

Soldering iron? not so much for soldering as for pyro-art

4th Axis, probably what I'll do next.

3D printer head. with the right ancillaries, very doable. might try this one after the 4th axis... 4 axis 3d printing??? lol.. GOTTA try that.

drag knife... hmm, all sorts of bits could use the space.. so the Z was drilled to accommodate various tool heads that could be swapped out in conjunction with swapping out the bed.

...But of course, the devil is in the detail and how you put it together.