Need assistance calculating steps/mm for Z axis



  • Hi All. I need some help calculating the steps/mm on the Z axis of my printer. Here are the details.

    1 stepper motor (1.8deg/step) has a 20 tooth gear on it that drives a GT2 belt. The Z axis is moved up and down via M8 threaded lead screws. The lead screws have a 20 tooth gear on them that is driven by the GT2 belt. I am using 256 microstepping.

    Here is a picture of how my Z axis works.


  • Moderator

    @cyoung

    Based on the Prusa calculator you can use 2560 if using 16 microsteps.

    https://www.prusaprinters.org/calculator/#stepspermmlead

    And from the gcode wiki you can set the steps for 16 microsteps and then set the microsteps to whatever you like and the system will recalculate.

    a) Set Steps/mm correctly for the default 1/16 microstepping with M92, then set the microstepping to the desired amount using M350:

    M92 Z2560 ; set axis steps/mm
    M350 Z256 ; set microstepping



  • To elaborate - what we need to know is the lead (not pitch) of the screws. As it happens, if you use single start screws, the lead happens to be the same as the pitch but for multi start screws, it's different. The lead is how far a nut would travel in revolution of the screw, the pitch is the distance from peak to peak (or trough to trough) of the the thread. So for example if it's single start and 2mm pitch then the lead would be 2mm. But if it's 2 start, then the lead will be 4mm and if it's 4 start, the lead will be 8mm.

    So you are using 1.8 degree steppers which means that 1 revolution will be 200 full steps (because there are 360 degrees in a circle). You have 1:1 gearing so the screw will also need 200 full steps to turn 1 revolution. If you divide the steps per revolution by the lead in mm, you end up with the steps per mm. But these are full steps so you then multiply that by your chosen micro-stepping setting. In this case 256.

    There are two ways that you can enter these settings into your config.g. You can EITHER set the micro-stepping using M350, then set the steps per mm using M94 but M94 must come after M350. OR you can set the steps per mm at 16x micro-stepping using M92 which will calculate the micro-stepping at 16x first, then change the micro-stepping using M350 after and the firmware will re-calculate the steps per mm for you. If you are sure that you will never use anything other than 256x, then either method will work. If you think there is a possibility that you might want to change the micro-stepping at a later date, then the second method is preferable as you won't have to re-calculate the steps per mm again.
    HTH



  • I think he was not sure if the pulleys affect the steps/mm of the lead screw.
    Since the motor and lead screws rotate at the same rate, you do not need to.
    You only need to do that when you are translating tangential movement in to linear. And in this case you have axial to axial (not sure if I used the correct expression) with the ratio of 1:1.
    I hope this explains it.



  • @deckingman said in Need assistance calculating steps/mm for Z axis:

    what we need to know is the lead (not pitch) of the screws

    Printer started as a Folgertech FT-5. The website describes the lead screw this way: "T8 4 Start Lead Screw 8mm pitch "



  • If I remember correctly you need 400 steps/mm
    The lead screw has 2mm pitch and 4 leads. That means 8mm per rotation that is written in the description. Put that in to the calculator that @deckingman linked. You will get 400 steps/mm from there.



  • This is how I have my steps/mm set up in the config file.

    M350 X256 Y256 Z256 E256 I1 ; Configure microstepping without interpolation
    M92 X1280 Y1280 Z6366 E6582 ; Set steps per mm



  • This is better

    M92 X1280 Y1280 Z6400 E6582 ; Set steps per mm
    M350 X256 Y256 Z256 E256 I0 ; Configure microstepping without interpolation



  • @cyoung said in Need assistance calculating steps/mm for Z axis:

    @deckingman said in Need assistance calculating steps/mm for Z axis:

    what we need to know is the lead (not pitch) of the screws

    Printer started as a Folgertech FT-5. The website describes the lead screw this way: "T8 4 Start Lead Screw 8mm pitch "

    Very bad description and I'll bet a pound to a pinch of Chinese excrement that it isn't 8mm pitch. I'd wager that it's 2mm pitch and 8mm lead which is what I wanted to know.

    I see that everyone else has jumped in so I'll say no more, but you can re-read my post and do the maths base on 8mm lead if you want to.



  • @obeliks said in Need assistance calculating steps/mm for Z axis:

    This is better

    M92 X1280 Y1280 Z6400 E6582 ; Set steps per mm
    M350 X256 Y256 Z256 E256 I0 ; Configure microstepping without interpolation

    Erm, no it's not better. In fact it's very bad indeed.

    If you set steps per mm (M92) before the micro stepping (M350), then the steps per mm will be set for 16x micro stepping. Then when you put the M350 command in after, it will recalculate the steps per mm. So, if you use M92 before M350, the Z value needs to be 400 which get get converted to 6400 when the M350 is run. If you use 6400 to start with, it will get converted to 102,400 when micro-stepping is set to 256X.
    https://duet3d.dozuki.com/Wiki/GCode#Section_M350_Set_microstepping_mode



  • Ah, I missed this detail. Sorry.
    But the steps/mm values should be correct.



  • @obeliks said in Need assistance calculating steps/mm for Z axis:

    Ah, I missed this detail. Sorry.
    But the steps/mm values should be correct.

    No worries. Yes it's 400 steps per mm @ 16X or 6400 @ 256X. I wasn't go to tell the OP that as he asked for help in calculating it, not for someone to tell him what they should be☺

    As the saying goes, "Catch a man a fish and he'll feed himself for a day but show a man how to catch a fish and he'll feed himself for life". ☺



  • @deckingman said in Need assistance calculating steps/mm for Z axis:

    As the saying goes, "Catch a man a fish and he'll feed himself for a day but show a man how to catch a fish and he'll feed himself for life".

    I believe what you meant to say is "Give a man a fish and feed him for a day. Dont teach a man to fish...and feed yourself. He's a grown man and fishing's not that hard."

    Thanks for the help.



  • @cyoung There are multiple variations of that quote, attributable to multiple people. I prefer the version I used which basically means that it's better to teach someone how to do something, than to do it for then. Hopefully, if you need to calculate the steps per mm for anything in the future, you'll be able to do so without asking for someone to do it for you. Just trying to help, no offence meant or intended.



  • @deckingman I was just being silly.



  • @cyoung said in Need assistance calculating steps/mm for Z axis:

    @deckingman I was just being silly.

    Ahh OK. Add a ☺ next time, then I'll know. Always difficult to tell when you only see the words but not the facial expression.


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