Endless trapezoidal T8 screw Ø8 mm pitch 2 mm 1/2 principles?



  • I have a question seeing that my Lead screws from motedis are no perfectly staright I was going to change them.

    I found some endless trapezoidal T8 screw Ø8 mm pitch 2 mm 1 or 2 principles 50 cm

    My lead screws are 1 principale with a pitch of 1.5

    What is a better choice for a 2mm pitch to go with, 1 principale or 2 principales?

    Thanks



  • the lower the pitch the less torque required and the better accuracy you will get high pitch is not good as is true also of multi start threads.

    IMHO The best would be T8x1 single start but there not that easy to find after that I would look for T8x2 single or even T10x2 single.



  • Ok thanks for the info I have found a site where they offer the single or double start at the same price.
    Will go for the single then like the ones I have now but they are a little woobley.



  • Be aware that 1mm pitch lead screws won't be very fast for motion. They don't require as much torque though. I'm using 3 of them on the Z axis of my D-Bot with a single 1.8 motor with no reduction. The motor has to spin very fast to get it to home at an acceptable speed, and any binding is enough to stall it out, so it must be set true.



  • Yeah I haven't started testing for speed yet on my new leadscrew delta but it's a good kinematic to test since it has to be fast, all three axes are in constant motion and they all need to be precise.

    I've used T8 screws which move 8mm for one rotation (400 steps/mm with 1.8 deg motors). These should enable reasonable speed however you use them if you can achieve 40rpm from your steppers (not that difficult) you can go 320mm/s and resolution is 5 times higher technically than 20t pulleys and belts on 1.8 deg motors (80steps/mm). However you have to contend with the backlash unless using antibacklash nuts which I have on order, and the alignment which can be difficult to get right. I am not relying on the screws to structurally support anything they are attached to linear rails.

    I am using 1.8 deg, 48mm motors 1.68A rated at 1.2amps driver current and currently 1/16th microstepping with interpolation. I will experiment with other microstepping settings in time.



  • @Phaedrux:

    Be aware that 1mm pitch lead screws won't be very fast for motion. They don't require as much torque though. I'm using 3 of them on the Z axis of my D-Bot with a single 1.8 motor with no reduction. The motor has to spin very fast to get it to home at an acceptable speed, and any binding is enough to stall it out, so it must be set true.

    Thats why when i tried moving it fast it would bind up but they are 1.5mm ! Good to know.

    What about 4 pricipales are they precise right now the 1 priciplaes should arrive this week as they ran out.

    Overall what is a good compromise between speed and precision?



  • For a Z axis, I have found that 1mm pitch is fast enough and more than precise enough. 2mm would probably be fine too. For other axis I have no experience using lead screws there.



  • Just want to step in here and air one of my favourite hates.

    When talking about lead screws, the important thing is lead, not pitch. Pitch is the distance between thread peaks. Lead is the distance a nut would travel in one revolution of the screw. It just so happens that with single start screws, pitch and lead happen to be the same value which is why many people incorrectly use the wrong term. With a 2 start screw, the lead is double the pitch and with a 4 start screw, the lead is 4 times the pitch.

    So, taking single start screw with a pitch of 1mm, in 1 revolution a nut (or in our case the print bed) would travel the lead which happens to also be 1mm. With a 2 start screw and 1mm pitch, the lead is 2mm so the bed will travel 2mm for 1 revolution. With a 4 start screw and 1mm pitch the lead will be 4mm so that's how far the bed will travel in 1 revolution of the screw.

    So a 2 start screw will give twice the linear movement for a given motor step but will require roughly twice the torque, and a 4 start screw will give 4 times the linear movement but will require 4 times the torque.

    It's generally best practice to select the screw lead such that the desired layer heights are achievable by using multiples of full steps and not have to rely on micro stepping for positional accuracy.

    IMO, multi start screws are best used in applications for which they designed to be used. i.e. to give large (or rapid) linear movement for a smaller circular movement, which is not what we want for Z axis which only moves a fraction of a mm at a time during printing (unless ones criteria is for fast homing of the Z axis rather than good quality prints that is).

    HTH



  • Thank you Ian that is more or less what I was trying to advise but you explain it much more succinctly then I could

    Doug



  • Thanks to all for the very helpful info.



  • I will leave as they are I perfer the precision over the speed I have no need for the plate to rush 40 cm down in 5 secs.
    🙂


 

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