z axis question/ idea

This is kind of a two part question.

First, has anyone ever tried to make a Z axis with just large diameter threaded rods?

What I'm talking about would be similar to a Mendel design, but with no smooth rod/linear bearing, and thicker threaded rods that are captive in bearings on both the top and bottom.

Also, how coarse is too coarse for the Z threads? If you were to try this, and the thicker rods ended up being a 2.00 pitch, would that be bad? I know that it would mean less steps/mm to move and therefore more stress on the steppers, but would it be possible, or is that pitch too steep for the motors to push?

Edited 1 time(s). Last edit at 06/11/2013 07:44PM by TheJones.

The problem is that the threaded rod would need to be perfectly straight and coupled to the motor with absolutely no eccentricity. In practice there is almost bound to be some wobble that affects the print, even with proper lead screws rather than studding.

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[www.hydraraptor.blogspot.com]

Yeah, that's what I was afraid of. In my mind I was imagining the rods flexing about half way up and making the whole assembly wiggle.

Oh well, it was a neat thought while it lasted.

What about the pitch? Let's say I'm looking to do a RepStrap type build, and I have some threaded rod, but it larger than what the standard kit calls for.

Is a pitch of 2.00 too coarse to be used to drive the Z axis?

Edited 1 time(s). Last edit at 06/11/2013 09:56PM by TheJones.

So I put some thought into it and came up with the following:

2.00 pitch = 2mm per full revolution
1.8 degree per step = 200 steps per revolution
That means at 2.0 pitch you have 100 steps per mm.
Compared to 1.25 pitch, which gives 166.6 steps per mm

I take that to mean that when printing at .100mm resolution you would have 10 steps per layer Instead of the standard 16. That seems like a significant difference.

Any idea what that would do the the accuracy? I was initially worried about wear on the motor, but doing this bar napkin math makes me question the effect it would have on print quality as well.

1.25 pitch gives 160 steps per mm, not 166.6.

If you are using 0.1mm layers it will make no difference to the accuracy if it moves 10 steps, or 16 steps per layer. It will only make a difference if the layer height was not a multiple of the step size. With 100 steps per mm it is more likely to be an exact number of steps than it is with 160 but when it isn't the error will be a bit more.

Micro stepping is normally used to multiply the resolution by 16 but the intermediate positions are not as accurate as the full steps so it is still best to use a layer height that is a multiple of full steps.

Edited 1 time(s). Last edit at 06/12/2013 06:20AM by nophead.

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[www.hydraraptor.blogspot.com]

nophead Wrote:
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> 1.25 pitch gives 160 steps per mm, not 166.6.

My fault, I missed the 5 when punching 1.25 into my calculator. (200 / 1.2 = 1.667)

So is there an advantage to using the 8-1.25 instead of something like 16-2.0, other than cost?

You would only have half the resolution!

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Bob Morrison
Wörth am Rhein, Germany
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BLOG - PHOTOS - Thingiverse

It probably needs considerably more torque to turn it because of both the steeper lead and the bigger diameter. However is doesn't need to turn as fast for the same speed and steppers have more torque at low speeds. I have used smaller lead screws and not been able to get them to go any faster but I don't know what happens with bigger ones.

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[www.hydraraptor.blogspot.com]

rhmorrison Wrote:
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> You would only have half the resolution!


I really don't think 3D printing is currently able to print at a level where it would make a difference.

one step at 2.0 pitch is .01mm of linear movement. Thats 10X smaller than we are currently able to print with standard machines. A pitch of 1.25 world give you roughly .006mm per step, and while that's remarkably better, I don't think we are pushing that envelope yet.

My point is, when I think about it, it doesn't seem like it would be a hindrance to printing at current resolutions, and it leaves you with an even and consistent number to divide your layers by, as well as being more staple and less prone to flex. The only down side I can see is that it's slightly more expensive, and I don't know how much extra stress it would put on the steppers.

It was just an interesting idea, and I was wondering if there was a reason that people didn't do it. ( like too much slop with big threads, or the motors not coping well with the additional stress.

nophead Wrote:
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> It probably needs considerably more torque to turn
> it because of both the steeper lead and the bigger
> diameter. However is doesn't need to turn as fast
> for the same speed and steppers have more torque
> at low speeds. I have used smaller lead screws and
> not been able to get them to go any faster but I
> don't know what happens with bigger ones.

I think I'm going to give it a try, unless I find something in the near future that convinces me otherwise.

Thanks for all your replies.

i us a bore grinder and it oscillates at 15000 mm/s but the diameter of the lead screw lol would be way too big for its about 150mm in diameter