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By "better" I mean "more precise"...

With respect to a RepRap P3Steel or Wilson II, I am getting some 330 mm T8 leadscrews for the Z-axis movement.

There seems to be a choice between a lead of 1, 2 or 8 mm - the pitch is 1 mm in the first case and 2 mm in the last two cases1. However, there is precious little information about the advantages and disadvantages of each lead size in RepRapWiki - Threaded Rod - Leadscrew.

It seems to me that using a leadscrew with a 1 or 2 mm lead could result in a more precise Z-axis movement, as one rotation of the stepper results in a smaller increment in height. Therefore the layer thicknesses could be smaller.

However, is the minimum layer thickness not, also, dictated by the thickness of the filament, horizontal speed of the print head, nozzle size, etc.? Thus, at some point there would be no need for a super fine vertical resolution from the leadscrew, as it may be constrained by other limiting factors.

Of course, conversely, the use of a 2 mm lead would mean that the stepper would need to "work" four times as hard, than when using 8 mm, in order to raise the print head the same distance, as well as making the movement, during a "home", take four times as long (or, if you will, four times slower). In the case of a 1 mm lead that would become eight times...


TL;DR

Is it worth getting a leadscrew with a 2 mm lead, or is 8 mm sufficient?

By extension, would a 1 mm lead be even better, or just overkill?


Footnote

1 Nomenclature:

  • Pitch is the distance between the adjacent threads;
  • Lead is the distance that a nut will turn with one rotation, and;
  • Start is how many starting (or thread entry) points at either end.

So, for a leadscrew, with a pitch of 2 mm, if there is only one start to the screw then the lead is the same as the pitch. However, if there are four starts to the screw, then the lead will be 8 mm. If there are two starts to the screw, then the lead will be 4 mm. And so on.

For more information, see Wikipedia - Lead, pitch and starts.

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Based on what I've learned over 8 months of printing:

What matters the most is getting a very accurate zero relative to the bed surface. An offset of 30 or 40 microns can strongly affect first-layer adhesion. Now, unless you put in some of the advanced auto-levelling sensors, the repeatability of the Z-axis limit switch may outweigh the precision of the lead screw itself. But if you've got a good limit sensor, then the more precision in the lead screw, the better you can set (and repeat) the position of the first layer.

I don't see a significant difference in strain on the drive motors if they have to run longer per mm of z-drive - just make sure the lead screws are clean :-)

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From Accuracy vs Precision and Threaded Rod vs Leadscrews in 3D Printers. I have highlighted the relevant parts:

In general, FFF/FDM printers use relatively infrequent, small, precise movements on the z-axis and consistent, fast movements on the x and y axes. A single start leadscrew with the tightest pitch possible (highest thread density, smallest pitch) is generally going to be your best bet for the z-axis, while you may or may not need something a little steeper to get the speeds you'd like from your x and y axes. While this may seem somewhat arbitrary given the precision of movement you can get from a stepper motor, an important factor to remember here is torque.

A more aggressive leadscrew will require more torque to drive. We have one kit printer we bought a couple years ago that has an overly aggressive multi-start leadscrew for the z-axis. The small motors included in the kit do not have the torque required to reliably start upward movement of the carriage, leaving it sitting there skipping steps until the carriage is given a little upward nudge to get it going (no, it's not a lubrication issue or a driver that needs turning up).

So, a 2 mm lead is preferable to a 8 mm lead, as not only is less torque required, but also more precise movements are obtainable.


With respect to the layer heights, I found this nugget of information, from Ditch the threaded rod in your RepRap 3D printer and upgrade to a lead screw z-axis

It's always better to use layer heights that are a multiple of your full step. If you trust in your micro-stepping you will get poor results because torque is very poor and the motor won't stop very precisely. Best approach is to use a lead screw (whose longer step allows the gravity to work for you and eliminate backlash) and a stepper driver configured to 1/4 micro-step for low noise, but not for micro-layering. I always use multiples of my full step and have printed 0.040 (1 step), 0.080 (2 steps), 0.120 (3 steps and so on), 0.200, 0.600 and 0.800 mm (of course using 2 different nozzles). Do you need more than that?

See also Reddit - Lead screw opinions

  • the general consensus is that a 2 mm lead is preferable to a 8 mm lead:
  • Lower the lead the better for an Z-Axis screw to allow for more precision.
  • No need for anti-backlash nuts for a Z-Axis if it has a decent amount of mass/weight to it.
  • 1 mm will slow the printer down, but provide 0.005 mm (5μm) vertical movement:

That's going to be awfully slow when homing. While you really don't need speed for the z-axis, there's not much point in going with such a low pitch... you won't be printing anywhere near the layer heights that can achieve. In the end I guess it's not a big deal, but I don't see any reason to go below about a 4 mm pitch. A 1 mm pitch with a 1.8° stepper gives you 0.005 mm increments. Anything under 0.02 mm (which is sort of ridiculous anyways) is just unnecessary. Just my thoughts.

  • (As Tom points out) An 8 mm lead can result in the weight of the X-axis gantry (especially in a P3Steel) overcoming the idle torque of the stepper. As a result of this, the X-axis gantry can end up sliding down the leadscrew, in particular at power down:

I have lead screws on my Z and it readily falls down if you kill the power. Regular threaded rod is much better at keeping it in place without power.

and

That's one of the problems with 8mm lead ;) With 2mm lead it will hold itself. The stepper motor provides a fair amount of resistance even when powered down.


TL;DR

Be wary of leadscrews with a lead, as opposed to a pitch, of 8 mm, as there are disadvantages, when compared to a lesser lead of, say, 4 mm, or 2 mm (or 1 mm for leadscrew with a pitch of 1 mm):

  • Higher torque may be required
  • Less accuracy
  • Possibility of slippage, of the X-axis, once power is removed, or stepper is disabled1
  • Anti backlash nuts required

However, their major (if you can call it that) advantage is a higher maximum movement speed.

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    $\begingroup$ With regards to the "less torque" part: with an 8mm leadscrew, the weight of the printbed or carriage can overcome the (idle) torque of the stepper, causing the bed or carriage to drop like a rock when the stepper is disabled. This can be rather annoying. $\endgroup$ – Tom van der Zanden May 11 '17 at 5:47
  • $\begingroup$ @TomvanderZanden - Yes, that is a very good point. Some users in the Reddit thread have made mention of that... especially at power down. I have updated my answer. $\endgroup$ – Greenonline May 11 '17 at 6:01
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Longer lead

Pros:

  1. Faster movement (mostly benefits homing)
  2. More standard
  3. Weak motor friendly
  4. Might be slightly more stable (sideways) than shorter leads because of more starts creating more contact with nut (?)

Cons

  1. Backlash
  2. Less resolution (unless using 0.9 degree motor but see note1) although this is ignorable since it's still very high. It's already around 5x higher than xy axes using the longest leads2.

Shorter leads

Pros

  1. More accuracy if you turn off microstepping. it can give you more resolution, but it's useless resolution unless you print with < 0.01 mm layer heights3. Turning off microstepping trades away this useless resolution for increased accuracy. But this comes at the cost of (much) louder drivers.
  2. Less backlash

Cons

  1. Stronger motors maybe needed
  2. Slower homing

The reason why companies prefer longer leads is because shorter leads simply add useless resolution and slows down movement.

Footnotes

1 It makes more sense to use higher degree motors to drive shorter leads than vice versa because of higher torque.

2 0.0025 mm vs 0.0125 mm

3 or using a probe to autolevel, which causes the z axis to go up and down in very small increments

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