Z axis configuration June 09, 2016 09:22AM |
Registered: 7 years ago Posts: 168 |
Re: Z axis configuration June 09, 2016 03:10PM |
Registered: 8 years ago Posts: 622 |
Re: Z axis configuration June 09, 2016 06:27PM |
Registered: 7 years ago Posts: 168 |
Re: Z axis configuration June 10, 2016 01:44PM |
Registered: 8 years ago Posts: 622 |
Quote
hobbymods
I must have got it wrong about the 1mm pitch being the norm....thought I'd done my homework.
I'm looking again at the Hiwin range:
[www.industrialdynamics.com.au]
And if I'm using a Duet which seems likely, I'd probably be looking at 0.9 deg steppers?
The simple/standard internal recirculating screws they have seem to come in a range with all sorts of pitches starting at 1mm, so maybe 4mm with 2:1 pulley ration would be good?
Obviously it must be a good system with many of the pros swearing by it, but I'm amazed it doesn't all just lock up.
Re: Z axis configuration June 10, 2016 05:25PM |
Registered: 7 years ago Posts: 168 |
Quote
deckingman
Quote
hobbymods
I must have got it wrong about the 1mm pitch being the norm....thought I'd done my homework.
I'm looking again at the Hiwin range:
[www.industrialdynamics.com.au]
And if I'm using a Duet which seems likely, I'd probably be looking at 0.9 deg steppers?
The simple/standard internal recirculating screws they have seem to come in a range with all sorts of pitches starting at 1mm, so maybe 4mm with 2:1 pulley ration would be good?
Obviously it must be a good system with many of the pros swearing by it, but I'm amazed it doesn't all just lock up.
AFAIK, 0.9 deg steppers are more expensive. Do you need double the resolution if 1.8 deg give a finer resolution than you need? It's easy enough to do the maths. e.g. 360 degrees in a circle, so a 1.8 degree per step motor takes 200 steps to do I revolution. If the pitch is 1mm then 0,1mm will take 20 steps. If the pitch is 8mm then 0.1mm will take 200/80 = 2.5 steps. That's not good 'cos it's not a whole number so you'd be relying on micro stepping for positional accuracy. Gearing an 8mm pitch screw by 1:2 would give 5 whole steps per mm which is better. Equally, using 0.9 degree steppers and no gearing will give you 5 whole steps. Personally I think gearing only comes into it's own if you need to lift a heavy load with a small motor (which we don't). There are various ways of doing this. I don't think there is a best way (but what do I know?). Just bear in mind that we don't need to lift a heavy load nor do we need to do it quickly. What we do need is to do it in very precise increments of (potentially) 0.1mm or less and for that we should really be looking at whole steps, not fractions of a step. Also, as I said before the typical step accuracy of a motor seems to be +_ 5% but it's non accumulative. However, the fewer the number of steps, the more significant this accuracy may become. I guess if 4mm pitch is significantly less costly than (say) 2mm pitch, then use that with 1:2 gearing (the motor shaft being the 1 and the lead screw pulley the 2). If there is hardly any difference in price, then why not use 2mm pitch (or even 1 mm)?. Either method would achieve the same end. Maybe another factor might be belt length. You'll need a closed loop belt. What's the nearset size you can get and does this mean that you have lots of belt length to lose? If so, then big pulleys might help.
Re: Z axis configuration June 11, 2016 04:04AM |
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Re: Z axis configuration June 11, 2016 05:11PM |
Registered: 7 years ago Posts: 168 |
Re: Z axis configuration June 13, 2016 12:59AM |
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Re: Z axis configuration June 13, 2016 04:40PM |
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Re: Z axis configuration June 13, 2016 05:16PM |
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Re: Z axis configuration June 13, 2016 05:24PM |
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Re: Z axis configuration June 14, 2016 11:38AM |
Registered: 7 years ago Posts: 1 |
awesome info!Quote
deckingman
What you have in mind is exactly what I'm proposing to do with a couple of minor differences. I've got (designed) 2 linear rails on the sides but at the front, two lead screws at the same position and a single one in the centre of the back. My bed will be 400x 400 (about 330 x 340 printable area) but a lot taller (about 800mm Z movement). As the bed will be constrained by the linear rails (I'm proposing to use Open Build V slot, Delrin wheels etc), then the screws are only providing lift, and I think they need to kind of "float around" otherwise they'll be fighting with the linear guides and perhaps cause binding issues. So, personally I can't think why 8mm diameter won't work. I've got 3 off 1metre lengths next to me and for the life of me, I can't think why I'd need anything bigger. Of course, if the screws are being use to constrain the bed from any sideways movement, then it would be a different matter.
Originaly, my plan had 4 linear guides - one in each corner more or less but I soon realised that alignment would be a real pain. The I went down to 3 guides, one near each screw but after a lot a research, found that this too would cause more problems than it solved. Two guides seem to be optimum. Take a look on OpenBuilds. From memory, I think there was something called a tripple D bot or some such that someone built. There is some stuff there that made sense (at least to me it did).
I keep getting blasted about thread pitch but I really don't get this "use 4 start / 8mm pitch" thing. With a 1.8 degree stepper, you get 200 (full) steps per mm with a 1mm pitch screw or 20 steps per 0.1mm. Maybe that's a bit too fine a resolution but what the heck, - the odd skipped microstep ain't gonna matter too much. If you have an 8mm pitch, you get 2.5 steps per 0.1mm. So, you have to rely on micro stepping for positional accuracy which ain't what microstepping is good for. AFAIK microstpepping is just a way of transitioning smoothly between full steps. It'll increase resolution, but it doesn't increase accuracy. Typically, positional accuracy of a stepper is +_ 5 % per step. Having 16 small steps between full steps, doesn't improve this. When I started my toolmaker apprencticeship back in the 1969, we had lathes with multi start lead screws. They were invented to give greater linear movement for a given angular movement - i.e designed for one thing only - speed. We don't need speed on the Z.
Now of course, you can use a course thread and gearing to get the resolution back. With an 8mm pitch screw 1:2 gearing will give you 5 full steps per 0.1mm, which is better than a fraction, like 2.5. So, I guess it's a case of what you can get hold of locally, at the best price. 4mm pitch would be the same as geared 8mm. I mm pitch is maybe finer than you need but if you can pick it up at a good price, why not?
I know I'll get blasted for this and I'm just an old guy who cuts and screws bits of wood together, but I've yet to hear anything which can convince me that I need to be using 4 start, 8mm pitch lead screws which is what everyone seem to be selling.