hi ppl, im about to start my first build, it'll be a fixed gantry router with travel in x and y bein 500 x 500, it'll be mainly made from mdf for now (or whatever i can swipe from work lol) and used for cutting 3d into wood blocks, my buget is real tight so would m16 studding work instead of ballscrews (accuracy sacrifice i know but hey), an has any1 used rails made of ally angle with sealed bearings bolted to sides?
You could use M14 at the same pitch or MarchantDice sell M10x2 or M12x3 reasonably cheaply
With all of these you need to consider how it will be supported bearing wise... usual low cost solutin is to turn it down to 8mm and use the 608 series skate bearings
You can use the same bearings on ally angle, there are several designs base on this approach.. another is to use 3 bearings on a round rail...
Last edited by irving2008; 01-08-2009 at 09:03 AM.
It's heavy which counts against fast reversals, probably zinc plated which is high friction, dubiously straight and bends under it's own weight if you bridge any kind of distance.
Apart from that it's fine, but if I was thinking studding I'd probably go for a much smaller diameter, hold it in tension and use pairs of brass nuts to counter the backlash
FWIW I started out with M8 (but if you buy off the net, be prepared to discard warped rod that doesn't come up to scratch - best go somewhere to buy where you can eyeball)...but have just bought some M10 A2 Stainless (which works out at about £3.87 a metre)....which looks like it'll be much better. I intend turning the ends down to 8mm to fit the skate bearings I have supporting the rod (which in turn are held by SK16 mounts).
Eveyone normally takes a sharp intake of breath when you propose using threaded rod, but it's cheap as chips...coupled with a homemade delrin nut & small backlash compensation in Mach3, I'm getting very decent results.
"effingcheap" technology©®™ - I can't find that in my Yorkshire-English dictionarylol (yes I really have one!), but silk purse, sow's ear come to mind (a Yorkshire saying I believe?)
Yeah... M8/M10 can be a little slow...especially with underpowered motors that I previously had! But hey, this is just a hobby (not my day job thank heavens - my family would be perpetually like a nest of newborn starlings) & I justify it by saying "Hey, I can resolve down to 1.5mm/800 steps = 0.0001875mm (as if lol). Also, I'm typically making stuff that's about under 100mm x 50mm (so it's not as if I need to cover a lot of real estate fast!)
That's a good 'bang per buck' suggestion though - 3mm pitch is starting to get a little more like it ...but I guess then there might be a bit of a battle making your own delrin nuts? (then the cost starts taking off)
Last edited by HankMcSpank; 01-08-2009 at 12:04 PM.
M10? I'd go for around M4 :whistling:
Okay you need to gear it up a tadge and hold it in tension but the frictional losses will be so much lower and no way could you pull M10 straight.
Real engineers from days gone by had no fear of disadvantaged gear systems.
You can't get 100% straight with anything but the thinner you go the better the approximation. There's a wonderful piece of accidental verse in the Oxford Dictionary or Quotations...
...and so no force however great
can stretch a cord, however fine
in to a horizontal line
that shall be absolutely straight
One point to watch out for is that the pitch tends to be slightly greater than standard on rolled threads (rather than averaging out), so a 100 turns on a 1.5mm pitch may well be greater than 150mm.
You'll need to calibrate the exact pitch when you set-up the axis params in mach or emc.
 tip: if you are buying mail order, buy three or more lengths - they'll almost certainly be taped together when packaged so will be less prone to being bent in the post
[edit2] I don't see the point of using M10x1.5 over M8x1.25 - it'll have larger drag (because of radius & circumference) than M8 for little gain in pitch. Irving's suggestion of 12mm x 3 acme/trapezoidal from Marchant dice sounds a better option.
[edit3] As soon as I get my travelling steady finished, I intend making a 5/16" 11TPI with new roller nut idea, for my engraver .
Last edited by BillTodd; 01-08-2009 at 05:54 PM.
I've just gone through this very procedure .....I have 200 step steppers but send 'em 8 pulses per step. With M8 rod (1.25mm pitch), I worked out that it should take 1280 pulses to move the axis 1mm - I then compared this to real world situation using Mach's funky little 'calibrate axis' utility on the setup page ...it transpires my steppers need 1286 pulses to move the axis 1mm. (not that I think I'd have noticed!)
All valid points & I must admit I am wavering a little (esp since you guys know what you're on about & I remain firmly in the 'haven't got a clue' camp!)
However, the other problem I have is that M8 rods are a Gnat's reproductive organ smaller in diameter than the skate bearings they are held in place on my machine - this means a bit of rod wobble (particularly noticeable at higher speeds) - at least going M10, I get a bit more erhm 'straightness', a bit more pitch (ok, miniscule but it all helps!) & can turn the M10 end down to mate with the bearing exactly so there's no slop. And more of an irritation than anything (ie if I was to stay with M8), I bought some oldham couplers with an 8mm bore - it's not ideal to have M8 threaded rod feed into those bores. (if I stay with m8, I'd have to buy some other Oldham couplers...they aint particularly cheap, & this is just a hobby and my 'discarded bits & bobs box' is starting to get full as I've made so many mistakes & bad purchases!) ...far better to turn M10 down to mate with these Oldham couplers with a smooth shaft.
I could stay with the M8 if someone can come up with a cunning way to remove this slop where the M8 rod feeds through the skate bearings (we're probably talking 0.4mm or so in total - but nevertheless that potentially 0.8mm discrepancy between each end which in the CNC world is a monster amount!)
Also...those TR12 with the 3mm pitch - bet you can't roll your own nuts with them that easily ...which starts making them less attractive to someone like me (who's CNC kit budget is miniscule!)
Last edited by HankMcSpank; 01-08-2009 at 06:24 PM.
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