Lead screw pitch any thoughts?



  • I am changing my machine to all leadscrew positioning, I am using .9 deg steppers. does anyone have advice on screw lead vs accuracy? I was thinking single thread, 2mm per turn, but most of the screws I see are 4 thread, 8mm per turn. can I get the accuracy along with the added speed of the 8mm lead?
    Thanks


  • Moderator

    1,8 degree motor and 2mm lead is an almost ideal combination as you can get 0.01mm full step positional accuracy, speed is more than adequate for the Z axis, torque and holding is good so it's easy to lift heavy beds and hard to drop them when power is removed.

    It shouldn't be hard to find 2mm pitch. Where are you looking/located?



  • @Phaedrux He said "all lead screw positioning". My take on that is that he wants to have screw driven X and Y as well as Z.



  • @deckingman Thats correct, I already had the Z axis set up with two 1.8deg steppers with 2mm lead, I now am converting the y axis to 2 screws/motors and the x to 1 non captive screw "flying Motor"



  • @Phaedrux so if I use a 4mm lead with a .9 stepper I could double the available speed while maintaining the same accuracy?



  • @DallasH said in Lead screw pitch any thoughts?:

    @Phaedrux so if I use a 4mm lead with a .9 stepper I could double the available speed while maintaining the same accuracy?

    Not necessarily. I take it that you are assuming that if you use a lead which is twice as course, but use a stepper with twice as many steps per revolution, that one will cancel out the other. That may be true but can the motor deliver the higher torque required without skipping steps. ? The biggest challenge to positional accuracy will be eliminating back lash. As a general rule, that becomes more difficult as lead increases. There are other factors too. Will the lead screws start "whipping" at the higher speed?

    I'm assuming this is for a CNC machine with relatively large moving mass, rather than a 3D printer yes?



  • Also think carefully about matching the screw to the motor and desired speed. A 2mm lead at 100mm/s is 50r/sec or 3000rpm which is quite fast for most stepper motors. Stepper motor torque drops off pretty quickly at higher rpm, although running 24V rather than 12V will help at the higher speeds.



  • Also, if I was building a CNC machine with high moving mass, I'd give serious consideration to using rack and pinion, rather than screws for X and Y. The reason is that one can use split pinions or dual pinions to eliminate backlash, which creates a lot less friction than anti- backlash nuts on lead screws. That means more of the motor torque can be employed accelerating the mass, rather than overcoming friction. Racks can be joined too, so there aren't the length limitations that screws have, nor are there any issues with whipping at higher speeds.



  • or ball screws if friction and/or backlash is a concern; doesn't really solve any lenght issues though.



  • @bearer That's another option - (if you have very deep pockets ) 🙂



  • @deckingman said in Lead screw pitch any thoughts?:

    if you have very deep pockets

    chinesium is surprisingly affordable and accurate for for most hobbyist applications in this area; and if the chinesium specs aren't good enough then you'll need deep pockets either way.



  • @bearer I'll take your word for that. It's a long time since I last compared prices. The last time I looked, ball screws were a lot more expensive than "conventional" trapezoidal lead screws but things may well have changed.



  • @deckingman said in Lead screw pitch any thoughts?:

    @bearer I'll take your word for that. It's a long time since I last compared prices. The last time I looked, ball screws were a lot more expensive than "conventional" trapezoidal lead screws but things may well have changed.

    you'll get a kit with ball screw, ball nut, bearing blocks and coupler for under $50-60 in most common dimmensions. Guessing the lead screw comes in at a third or half, given the orders of magnitude difference in quality I'd say its affordable but its still money.

    (I've had some 16 and 12mm screws on a small cnc router for a decade with light use and so-so care, no discernible backlash yet)



  • @Phaedrux said in Lead screw pitch any thoughts?:

    1,8 degree motor and 2mm lead is an almost ideal combination

    I am working now on my Z axis and considered to switch from 8mm lead to 2mm lead. Is this reasonable ? https://www.amazon.com/Iverntech-Stepper-Integrated-Printers-Machine/dp/B07Q2RR6SC

    Also, do you use anti backlash nuts? I have a 300x300x6 heavy bed and 8mm lead, so without the backlash nuts the bed drops down.



  • @zapta said in Lead screw pitch any thoughts?:

    I am working now on my Z axis and considered to switch from 8mm lead to 2mm lead. Is this reasonable ?

    I switched E5 from 8mm lead to 2mm lead as bed was dropping like a stone after the modifications (arms under the bed, glass on top of it with printbite) and I gave up on the enable signal driving the relay the short the Z motor so switched to 2mm lead. Bed is not dropping any more but while the motor should now move the bed "easier" the max speed of the bed movement halved. (not sure if the motor have issues rotating 4x faster or the issue is that new T8 single start 2mm lead is just lower quality than original lead screw. I did order 4 of them to find one that's at least decently straight that can be almost straightened)

    Also, do you use anti backlash nuts?

    On bed I never use them, the weight of the bed will make sure you are always riding the same edge of the screw, on XY I usually do but they are questionable little things, I just finish one mini mill conversion and did some measurements, to get the anti-backlash nut on the ACME thread to be really useful it increases the friction so much that you need order of magnitude bigger motors, and is questionable what the work-life of the nuts under those conditions be.

    Ball-screws or rack&pinion are imo much better solution. You can sometimes find second hand one that are good for hobby use, they are removed from high tolerance machines and are sold for the fraction of cost while still providing more precision than most hobby frames can handle.

    You might find this video useful:
    https://youtu.be/AZzV2_H9GRY



  • @arhi said in Lead screw pitch any thoughts?:

    On bed I never use them, the weight of the bed will make sure you are always riding the same edge of the screw

    I use them on my 8mm lead to avoid the bed dropping when power is off. With 2mm lead, the bed is not expected to drop without backlash nuts?


  • Moderator

    The 2mm lead is very flat compared to the 8mm. There is a lot more resistance to auto-rotate. The backlash nut is just adding friction. That friction is already there to some degree with the flatter lead, so you may not need it.



  • @zapta said in Lead screw pitch any thoughts?:

    With 2mm lead, the bed is not expected to drop without backlash nuts?

    There is some math that I forgot, some formula where diameter and pitch are input and if the result is bigger than 10 the force to "unscrew" is stupendeous, and that's called e.g. "self locking" or something like that. It is basically the angle of the thread, the force that is acting on the normal of the thread splits into force that acts along the thread and in the direction of screw/unscrew. The shallower the angle the bigger % of that force (in case of pressing down) is in the screw axis so force that is "unscrewing" is very small. I can't remember the exact formula but for e.g. if you have more than 10 threads for the diameter of the screw (so for 8mm screw that "self locking" would be .8mm pitch)...

    From experience, you need to press really hard on the table to unscrew 2mm lead T8 acme, more than 5kg I'd say



  • @Phaedrux said in Lead screw pitch any thoughts?:

    1,8 degree motor and 2mm lead is an almost ideal combination as you can get 0.01mm full step positional accuracy, speed is more than adequate for the Z axis, torque and holding is good so it's easy to lift heavy beds and hard to drop them when power is removed.

    It shouldn't be hard to find 2mm pitch. Where are you looking/located?

    Sorry, can I throw a quick semi related question in here please?

    I'm going with a belt driven triple Z axis for my 400x400x600 V-Core build, and I'm using your same Z stepper.

    Should I be asking them for 2mm pitch lead screws? They seems to supply an anti backlash (the one with an allen screw adjustment) for all 3 screws. Is this an issue, or should I just wind the adjustment right back?

    I should add that I was also going to go with your 40T/20T pulley/stepper systen for that 2:1 reduction gearing as well, as my plate with be quite heavy I'm tipping.



  • @Corexy 2mm lead is fine for Z. I actually use 1mn lead screws (single motor, no gearing, 400x400x10 bed). What lead are they supplying? You don't need anti backlash nuts on Z because gravity takes care of backlash. If they are the adjustable type, then you can use them but back off the adjustment because otherwise you are just creating additional friction.
    This thread seems to have veared away from the OPs question which was related to using screw driven X and Y and which have completely different speed and backlash requirements.



  • @deckingman said in Lead screw pitch any thoughts?:

    @Corexy 2mm lead is fine for Z. I actually use 1mn lead screws (single motor, no gearing, 400x400x10 bed). What lead are they supplying? You don't need anti backlash nuts on Z because gravity takes care of backlash. If they are the adjustable type, then you can use them but back off the adjustment because otherwise you are just creating additional friction.
    This thread seems to have veared away from the OPs question which was related to using screw driven X and Y and which have completely different speed and backlash requirements.

    Yes I did veer off there, my apologies.

    I'll start a new thread about my specific lead screw application when my kit arrives.


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