optimal layer vs steps/mm?

I'm sorry if this is a duplicate post, but I've been wondering about this thing for quite a while now. Prusa mk2 uses tr8*8 acme, while one of my printer uses the same acme screws; has 8mm lead per revolution.

The setting of steps/mm in the firmware config should be at 400 steps.

I checked the prusa calculator for optimal z layer height it only requires 25 steps to have a full mm travel.

the optimal layer height for 8mm lead acme screw is a multiplication of 40 microns. However, prusa mk2 able to go with a multiplication of about 25 or 50 microns.

If I were to take the number of steps from the config which is 400 steps/mm, to get 0.1mm should be at 40 steps while at 0.05 at 20 steps and lastly 0.025 at 10 steps.

Currently I'm using a 1/16th stepper driver.

I'm thinking that something is odd here especially on the firmware configuration or was it that the steps/mm in the configuration is only required to generate a full steps which is 1.8 degree turn?

Now I'm confused

My question was, if my optimal layer height should be a multiplication of 40microns, can I go to a multiplication of 25microns?

Edited 1 time(s). Last edit at 01/24/2017 02:21AM by sarf2k4.

My understanding of it is you want to use multiples of a full step as layer heights when using leadscrews. So if you have 1.8 deg motors then you get 1 revolution for 200 steps, which moves the axis 8mm. 8/200 = 0.04mm or 40 microns.

So you can use 0.04, 0.08, 0.12, 0.16, 0.20, 0.24, 0.28, 0.32, 0.36mm layer heights with a 0.4mm nozzle. That's a fair range of options. Can you use 25micron multiples, yes but you might get less than ideal prints if you have twin screws and twin motors which might go out of sync when they stop between full steps.

Why not try 0.9 deg motors then you get 20 micron multiples which gives you a wider range of optimal layer heights.

I used to have an i3 rework with tr8 leadscrews and I was quite surprised at the 8mm/rev pitch, surely designed more with a CNC axis in mind for slightly faster travel, a 2mm pitch leadscrew must make more sense for a z axis, gear it down increase resolution. Increase accuracy, not unless those leadscrews are very well made.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

I see, let's say my optimal layer height is 0.04mm = 8/200, can I achieve 0.02 multiplier if I were to use a 1/32 driver?

Don't be fooled by micro-stepping

The first rule of steppers is they can really only do 1/4 steps, when you select something higher it wont actually move every step. But it does result in quieter moves
(Some steppers will attempt 1/8 positions, but the positions will not be uniformly distributed)

Eg In general if you have 1/8th micro stepping

position, action
1 doesnt move
2 moves to 2/8 position
3 doesnt move
4 moves to 4/8 position
5 doesnt move
6 moves to 6/8 position
7 doesn’t move
8 moves to 8/8 position.

Edited 1 time(s). Last edit at 01/24/2017 05:43AM by Dust.

Microstepping is irrelevant because were talking about full steps. You'd need 400 steps/rev motors or finer pitch leadscrews to change this multiple. However all of this is "conventional wisdom", it would be interesting to see the sides of a few prints done at each of the layer heights you're interested in, I think slic3r can slice a set of cubes each with different layer heights.It would also be very interesting to do this weekly and see how it changes over time.

However the two motors still come to a stop when powered off on full steps and one motor can go in one direction and the other motor the other direction, hence they can become 1 step out of balance at each power off. Before I decommissioned my i3 I was in the process of using a closed loop belt and pulleys at the top of the two leadscrews to tie them together and see if I could run them both from one motor at the bottom. I was going to use a pillow bearing at the base of the non-motor side.

Edited 2 time(s). Last edit at 01/24/2017 05:48AM by DjDemonD.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

When you connect both Z steppers in series, is it likely they'd move in a different direction when powered off?

Say, you home the printer and unfortunately the Z steppers zero at the worst point between full steps. Now you print with a layer height that is a multiple of the 40 microns. Does it mean, the stepper will stay at this worst position , or will it vary layer height up/down randomly?
In other words: does it make sense to chose layer height, based on the 40 microns when you don't know the starting point?

It would probably make sense to home the printer in full step config, then you'd know your starting point and use microstepping later to make it run quiet.

I wonder how much the full steps wisdom applies if you use a single stepper and don't let it turn off during the print.

I know David is working on having the ability to park independently controlled Z motors on full steps to keep them synchronised, maybe even a power off residual current parking facility like a hard drive. Also moving that forward to be able to have 3 motor control of bed levelling screws for true auto bed levelling. On reprap firmware you change microstepping on the fly, so homing using full steps and then maintaining power by using partial current and not having a stepper power timeout makes all that possible. I have to wonder though for most conventional scale printers whether its easier to just use one motor for z with a bit of gearing to help it.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions

Quote
JamesK
I wonder how much the full steps wisdom applies if you use a single stepper and don't let it turn off during the print.

I think it depends on the pitch of the leadscrew. Usually switching off z steppers will not drop the X-carrier and the driver knows, what was the last microstep position and counts on from there. ( I hope ) The steppers will follow and jump to the position they had have before.

Two+ Years ago, I asked the community about the z-rod couplers of my Prusa. They were pretty soft and compressed in z-direction, depending whether the heavy MK8 extruder was on the left or right side.
A degree of freedom, we don't want to see. I helped myself with a 4.5mm steel ball inserted in the coupler. Now the stepper shaft and the lead screw have no vertical play, but the coupler can still do it's job.

Edited 1 time(s). Last edit at 01/25/2017 02:03AM by o_lampe.

I was quite convinced when I had an i3 that the fairly awful z banding issues were caused by the flexible couplers.

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Simon Khoury

Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions