About to build CNC miller, need design critique please

[m.marino]

Adding to some good points already given,

1) rotate your rails so that they are opposing each other (one facing up and one down) That will bring your Z plate closer to Y as well as be a better transfer of force when cutting. Boxed section to mount them on with index plates to insure alignment would also help. Given the limit on space you might look at different options how you are mounting the X axis motor as that is costing you 60 or more mm off Y movement and if you plan on using precision profile rail that means you will be needing around 200mm width of the z plate to mount them. Also making a "C" with box section and plate for the axis as suggested above will add a good bit to the rigidity you will need to cut steel. Good luck and will keep an eye out and let others add their much greater depth of knowledge to this.

Michael

[Jonathan]

If you want to cut steel efficiently then I'm afraid the machine will need to be significantly stronger and consequentially more expensive. My subsequent comments are based on the assumption that the majority of the parts you want to cut are steel.

Do you actually need 200x200, or would about 140x300 be acceptable? If so you could just convert a milling machine to CNC. Get one second hand and it could well be cheaper than making the CNC router strong enough. I got my milling machine on eBay for £320 (add to that price of ballscrews etc), compared to about £1k new...search by distance nearest and if you're prepared to wait something's bound to come up.

if you plan on using precision profile rail that means you will be needing around 200mm width of the z plate to mount them

Which linear rails are you planning on using? Profile rails are pretty much a must to cut steel. You might get away with SBR25 supported round rail, but it will be mediocre by comparison. You should be able to fit two 15mm profile rails in a 150mm wide Z-axis quite easily so long as you don't use flanged bearing blocks. Profile rails can have fairly good seals on the bearings, but it's still best to protect them particularly as the only way to get rails for a good price is second hand where you can't specify the correct seals/scrapers.

'500+ oz-inch motors'

Common misconception that you'll need big motors. You wont need more than 3Nm. Compared to say woods and plastics steel is cut at very low feedrates and the machine is small, so high rapid speeds are irrelevant. Most motors greater than 3Nm are Nema 34, so they're unlikely to go faster anyway. There's a lot to work out first before considering buying motors!

What spindle are you thinking of using? The standard spindles for CNC routers are no good for steel as they are optimised for high rpm. On mild steel I doubt you'll ever need more than about 3000rpm. Making your own spindle is probably the best bet.

Do post a drawing when you've considered the changes that have been suggested.