4' x 4' x 8" Router Build

[JonnyFive]

Hi all, thought it was about time I broke my silence and made myself known on here. Building my own CNC router is something I've wanted to do for a long time. Over 10 years ago whilst pondering the best way to tackle a woodworking project I came up with the idea of a cross vise on a drill press using a router bit to create a low cost but reasonably accurate solution. It then occurred to me that I could put motors on each axis and voila my own CNC machine. A short amount of Googling later revealed I wasn't the first to have this idea and that using a drill press in this manor wasn't a good idea and that is when I stumbled across the buildyourcnc website. Over the years I have collected project ideas and developed my machine requirements to a point well beyond the limitations of an MDF machine which is when I settled on a design based on Joe's Evo machine.

A decade goes by and I'm now in a position where I have the space to build a machine, more importantly I have approval from the boss!! The only thing in short supply is time, a young family takes up the majority of time and energy that I would usually commit to hobbies but I've set myself goals to make this happen. I'm determined to start building this summer and have spent the last 6 months or so learning Fusion 360 and designing my machine. I'm a long term (18 years+) Pro/Engineer / Creo user and initially found Fusion really wasn't intuitive but I'm getting there slowly.

So on to my machine, as mentioned it started as Joe's Evo but I have made tweaks along the way and have ended up with something that doesn't have much in common with this starting point. I've settled on a cutting area of 4' x 4' x 8" with an increased travel (an extra 12") on the y-axis (still confused about which is x and y but it makes most sense to me for the long axis to be y not the gantry) to allow dovetailing or a 4th / 5th axis to be added at some point in the future. I'll be working mainly with wood, plastic and with the occasional bit of aluminium.

The base frame consists of 2 welded side frames (made from 50x50x3 box section with 100x50x4 for the top beams), a welded top frame acting as the machine bed, and lower cross beams. These are all bolted together and then the top beams levelled using epoxy.

The x & y axes uses 20mm Hiwin rails, the z uses 15mm. The x rails are mounted top and bottom of a length of 160 x 80mm aluminium extrusion. The z axis has the carriages fixed to the x axis and the rails on the moving spindle mount plate. I believe this will give me the stiffest setup.

Up until this point I am pretty happy with the design although there is a lot more detail to be added before I start building. I have a number of questions that I would really appreciate the benefit of everyone else's superior knowledge and experience:

1. How much over travel should each axis have, e.g. for the 4' x-axis how long should the travel really be? Is an extra 1" each side about right?
2. What should the clearance between the collet and the spoilboard be at maximum (lowest) travel of the z-axis?
3. What is the best transmission method for this size machine? A lot of people on here use ballscrews but there seems to be a move away from this within the Joe's CNC community for bigger machines (over 1000m). I'm currently using the CNC router parts rack and pinion unit but wondering if this is the best solution?
4. I'm a little stuck on how I maintain the parallel relationship between the y-axis rails and the racks. Once the frame has been levelled with epoxy it is straightforward to align the rails, but my assumption is that the epoxy is the only reliable datum and the side of the frame could be distorted / twisted due to the welding. The best I can come up with is to either mount the rack on a piece of angle fixed to the top of the epoxy or mount the rail and rack to a piece of aluminium extrusion to keep them both parallel?

I'm sure there will be hundreds of questions to come, thanks all for looking and any suggestions you have.

Cheers,

Jon

P.S. Ignore the left hand side in these pics, I have only concentrated on the right hand side for now.