Hovering over "Buy"... care to sanity check my thinking?!

[brumster]

Right then!

Dismissing the idea of a large, steel-constructed machine with serious aluminium milling capability. (1) I don't have the space to do it justice, (2) I strongly suspect a converted Bridgeport would be a far more sensible option.

So I've downgraded to a desktop-sized machine, and I'll focus on doing PCBs with that. When I move house and have more space, I'll contemplate the bigger build. So I'll use the desktop as a learning exercise. Which leaves 3 flavours to go for :
1) The aforementioned "bugger it and just get a Chinese 6040" for £900.
2) Build something completely on par with the 6040 myself, like-for-like.
3) Build an overspec 6040-like machine, mainly to learn on construction but also invested in the parts so they can be re-used on a bigger machine in the future.

I've been bouncing around CNC4YOU and eBay today, making a spreadsheet up to price these.

Option 1 we all know will do a minimal job, but I'll be replacing the electronics more than likely fairly soon. But it's a baseline, £900 for a rather average machine.
Option 2 I've worked out, CNC4YOU don't come out that well in this regard, and going eBay for the majority of it I've got a very high level ballpark price of £768... that's with the same spec ballscrews, supported rails, 1.2Nm NEMA23s, cheapo TB6560 drivers (the motor/drivers come in a 3-axis pack for £95). £175 on a spindle, 1605-C7 ballscrews for a 600x500mm working area. That price includes £130 on aluminium extrusion but there would undoubtedly be more cost on getting the relevant plates/etc for the Z-axis and so forth. So, to cut a long story short, I see no point in trying to build a 6040-like machine yourself for less money; I don't think it can be realistically done. They are great value for what they are.
Option 3, then, comes out at a £1262 plus the frame material, so if I bank on £1400-1500 I think that would cover it. This would be same size as a 6040, but like I said overspecced on the components - HGR15 linear rails, 4Nm NEMA23's, Leadshine DM556 digital drivers, the same old 2.2kw ER20 spindle and VFD, a 48v PSU, same 1605-C7 ballscrews (contemplating C5s, they're not much more), and then some guesswork on the Z-axis, cable, rails and so forth. Most of it UK sourced seems just as cheap, if not cheaper, than China. I guess the quality stuff is the same price wherever you source it from, and China has no interest flogging it!

Option 2 to me seems pointless. So it's a choice between option 1 and just get started playing with it, or option 3 and long-term investment.

I understand that £1500 buys a much larger machine but the reality is the costs are kind of irrespective of size; I know aspects of that don't make sense (do you really need 4Nm motors to mill PCBs?!) but I'm thinking re-use in the long term on the next project... and doing the construction with the components I'd use on a large build, so I can get savvy with them.

Because if I don't do that, then I really might as well just buy a 6040 and be done with it, given my size limits.

Question - by my calculations a 5mm pitch ballscrew, 200 step motor and no microstepping would give me a 0.025mm per step movement, which is just under 1 thou... and with microstepping obviously could chop that in half a few more times if need be. There would be no reason for me to gear this down would there, given worst (toughest) case the most resolution I'd need on a PCB would be half a thou... (excluding backlash and rigidity issues, I know).

Am I right in thinking that to maximise accuracy, I need to keep the thing as rigid as possible, run it at appropriate speeds, but ultimately the ballscrews will define that resolution. So the quality of drive, the ballscrews, that's all just a case of how deep your pockets are - the bit that *I* can f**k up is not making the machine square/rigid.... anything else? I know accuracy is more than just resolution, it's about repeatability as well, but go easy on me, I'm learning... :D

Basically if I go NEMA23 4Nm, Leadshine digital drivers, a capable 48V 600W power supply, C5 (let's say that for now, given the price difference to C7's) 1605 ballscrews with anti-backlash nuts... the only other bits that are going to limit the accuracy/repeatability of the machine are my fudged-up design, or bad software/CAM....?

If I can accurately mill SMD boards (typically a pad spacing of 0.25mm between pads) then I'd be happy as larry.
If that's pie in the sky then you're really falling back to through-hold DIP stuff, which really could be 0.5mm or worse, and I then wonder whether it's worth all the bother, apart from as a learning exercise and investment for future upgrades. I guess it could probably do *some* aluminium work if I was careful/slow, but I'm kind of dropping the focus on that now.

Basically, £1500 on an over-specced DIY build the same size as a 6040 (but obviously better quality).... or spend £900 on a 6040, use it for a year or two, accept it's limitations, fix it as needed, then flog it on for £400 and consider it a learning exercise... or salvage it for the spindle if nothing else :)

I'm thinking the DIY route but happy for someone to tell me I'm mad for doing it/it makes no sense :)

P.S. Sorry for the clearly indecisive warblings... I do value your feedback though :)