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The Myth of Z Speed... (or, at least, strange Z defaults)


illuminarti
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Posted · The Myth of Z Speed... (or, at least, strange Z defaults)

As a result of my dabbling in Marlin, I've noticed something odd about layer height changes on the Ultimaker: by default they happen very slowly.

I wrote about it on my newly-resurrected blog, here:

http://www.extrudable.me/2013/04/02/the-myth-of-z-speed/

That page has more explanation and pretty graphics but, long story short, because of the default Z-acceleration in Marlin of 100 mm/s², typical layer height changes happen at less than 2mm per second. This means they can take on the order of 0.1s to complete. Even if you have a much faster z-speed configured, you don't get it, because the small move distance and low acceleration doesn't give enough room to accelerate to any faster speed.

I was wondering if there are particular reasons for setting the defaults so low?

I have been printing quite successfully with the acceleration up closer to what we have for the other axes - I found 1500mm/s² to be quite workable. That results in moves that are about 4 times faster than the default - the z-changes look and sound noticeably different: the platform now snaps into place on each height change, rather than the whir/chirp noise it used to make. This reduces the time the head is kept stationary over the print, helps reduce blobbing further, and in my limited testing seems to be making 'hop on z moves' a more viable strategy.

 

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    Posted · The Myth of Z Speed... (or, at least, strange Z defaults)

    Hi illuminarti

    Thanks for your efforts. I read your blog, good one.

    I have a question

    Do you think it would be beneficial or not to skip the microstepping for the z motor as long as you chose suitable layer heights?

     

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    Posted · The Myth of Z Speed... (or, at least, strange Z defaults)

    Owen - I'm really not sure. I don't understand exactly what the rationale is for engineering the z drive system to be quite so finely controllable. Without the microstepping, it could still be moved in 0.015 mm increments - but I guess we would lose the ability to control the acceleration at all... moves would become more of an all-or-nothing kind of thing.

    Maybe there are benefits to being able to move it into place slowly... perhaps, indeed, there's a good reason why the default acceleration is so low... I just don't know what it is.

    And as to the electromechanical pro's and con's of micro stepping vs not doing it, I simply don't know enough about that.

     

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    Posted · The Myth of Z Speed... (or, at least, strange Z defaults)

    Not sure why the defaults are low, but it's important to never skip a step on it, and the platform full with PLA, can be quite heavy. And mass is slow.

     

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    Posted · The Myth of Z Speed... (or, at least, strange Z defaults)

    Good point, but the platform mainly only needs to move down. And gravity acceleration is 10Meters/sec or 10000mm/sec^2 so as long as you keep acceleration under 10000mm/sec^2 the greater mass will only help.

    Maybe there should be different accelerations for up and down but I think it's fine right now.

     

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    Posted · The Myth of Z Speed... (or, at least, strange Z defaults)

    Good point, but the platform mainly only needs to move down. And gravity acceleration is 10Meters/sec or 10000mm/sec^2 so as long as you keep acceleration under 10000mm/sec^2 the greater mass will only help.

    Maybe there should be different accelerations for up and down but I think it's fine right now.

     

    It is not always fine. Daid brings up a good point. The platform must move down (accelerate) only downward during a print, but then it must stop (decelerate). With the greater acceleration, I've found that it is prone to bouncing when it stops moving.

    I have a relatively heavy build surface (a 5/16" thick precision Mic-6 aluminium plate). It is extremely flat and stays flat during temperature swings (unlike the wooden arms of the Z-stage :() but it causes my Z-stage to bounce a little with the higher acceleration settings--so much so that it will trip the Z-stop limit warning on the initial move downward from the bounce!

    Be wary, if you print large objects (heavy) or use a heavy build plate. The current Z-stage design really doesn't seem to be up for significantly increasing both Z acceleration and mass, in my opinion. I'm going back the the standard settings.

     

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