Thanks to all who've posted so far. If this is something more/different than an 'Introduction', apologies and please advise - I haven't got my head round all the forum chapter-categories yet, but felt I need to explain myself, ..for the reasons people explain themselves, etc.
Time for a reality check. I foresee this as being solely a design activity for some time, rather than a 'some design, then enter a concurrent design & manufacture phase + ongoing problem solving & upgrading (i.e. the proverbial "building machine number 2")' experience. The latter is not my style, for one, however the reasons are: current family circumstances - 3 small boys, garage not big enough and the concept of commiting to the costs associated with the genre / spec of a machine I'd like to build. The remaining pressing issues: we need to move into a bigger house & I would need a dedicated workshop, or at least a bigger (double) garage to get going. A bigger income wouldn't go amiss. What I do have is a healthy amount of thinking time, but not quite enough doing (design & CAD) time to match against it. I'm already finding it a balancing act to make enough time to get up to speed on the wealth of forum info. - it's a full course dinner, and some. Being prone to Displacement Activity, I've probably done some of my best stuff when I was supposed to be doing something else, ...so, the best I can make of that little lot is to work on the design and figure it all out such that when the doing comes along it'll go smoother and be pretty much spec'd. I think I've figured also that the objective has migrated to the substance of the rewards of just designing & making a CNC machine, but not dreaming about what I can do with it, just yet - the former is enough of a draw, and when the workshop with CNC machine has manifested, the ideas, projects and jobs will also come, but based on the idea that I'm probably doing enough bits and bobs now to justify it and my wife is an ex Design Tech teacher and is always raving about "laser cutter this and that" I think we'll be OK for dreaming up some future shed time. ...that's the theory.
So, for a while at least this is going to be a good look at the philosophy of CNC router design for a 1220x2440 vertical (Y upright) cutting bed, for wood and aluminium; the process of doing that design; generating some design output, fixing part spec's (incl. costs), & specific component sourcing options. It's for me, but I'm happy for it to be as generic (as possible) and open source - then it's repeatable, with all info in one place - a spec/design doc of some description.
I'd put my initial ideas down, but I'm wary of entering into detail whilst I have unanswered concept-type questions, ...because there's no sense in folks poring over stuff where I haven't thought about it myself, and especially given how wrong the initial concepts might be - so until concepts have passed an internal credibility / sanity check, against the background of reasonable research one could do on here or elsewhere, I'm better not going public.
Once again, thanks for a great forum, and the staggering talent.
Ok, here's the initial budget for a 1250x2500 bed, gantry router. I would be interested in lumping all the rails, screws and spindle together in one China order, with someone like solar jean. Sorry, still not sure where to put this on here - it's not a build thread, nor design - it just gives an idea of the big spend points and the overall project delivery philosophy. What I'm interested in is where it's right/wrong, needs adjusting...? I'm hoping this is a mid-term realisation and not hypothetical, but I figure the order of things is fix design/spec first, then purchase the big lumps all together, and weld together frame and get parts made and assemble near the end, when all components have arrived and any dimensional differences can be accounted for without waste/rework. Bearing in mind it's early days on design / spec., and spec relates to cost, I've tried my best to write something down, ...this is mainly a budgeting exercise.
X; 2 pairs of: screws (25 dia. - non rotating) + end housings & nuts; rails + brgs (hiwin profile 20mm)£1200
Y; 20mm dia rotating screw + end brgs & nut, pair of rails + brgs (hiwin profile 20mm) £450
Z; 16mm rotating screw + end brgs & nut, pair of rails + brgs (hiwin profile 20mm) £250
2.2kW spindle + vfd + collet/s £250
4 (Nema 23) motors + leadshine drivers (UK supply) £600 Control panel + cabling + energy chain £250
Steel + weld consumables £350
Other tools; epoxy, taps, straight edge, square, spirit level £200
Bespoke machined parts/assy's: motor mounts, Z plates, rotating ballnuts, pulleys & belts £500
You might get away with 20mm screws on X if using rotating nuts. The only reason for using bigger screws is if you cant tension them sufficiently to avoid too much sag over the length. Smaller screws also means less inertia in the (smaller) ballnuts, but this is not so critical. The tension you apply will depend on the amount of distortion in the frame ends you're prepared to tolerate and that will depend on the cross-section of the frame material. What do you plan to use for the frame?
Thanks Irving. I can make provision for screw tension - how's tension measured? Natural frequency or so many nut turns? 80, 90, or 100 box for frame, 3mm thick. I think torsional (cross corner) vibration mode is important given it'll be vertical mounted. It will be braced back to a brick or block wall.
if the X screws are vertical then sag is less an issue. Tension is measured in Newtons and the tension applied can be calculated from the pitch of the thread used for the tensioning nut and the torque applied to the nut. Google nut torque and bolt preload for the formulae.
X screws will be horizontal and probably about 2900+ long. I forgot about nut torque and bolt stretch, doh! I can't imagine screws get particularly hot, but it would be temperature differential between the steel and frame steel screw that might reduce tension, and then vibration potential if the nut is near the ends, I.e. not supporting near the middle. I don't have a feel for temp differential, but I read a figure of 3 degrees the other day, gives a large change in tension, so maybe 3 degrees is the number to base a nut torque calc on, but that still leaves 1st bending natural frequency, which I also don't have a feel for the permissible lowest natural frequency for a screw. It's not going to be much different between a 25 dia or a 20 - they are both long and slender at nearly 3 metres.
I'd be surprised if you got that much a temp difference between screw and bed rail given they're only going to be a couple of 100mm apart or so.I'd turn the screw end down to say 15 or 12mm depending on root depth and put a similar clearance hole through the end box section with an anti-crush tube inside and thread the last 20mm of the screw for the tensioning nut and thrust washer. You could look at back to back Bellevue washers to get a defined preload.
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