16 vs 15-tooth pulleys



  • I'd like to ask, is there any reason that 15-tooth pulleys would be less appropriate than 16-tooth? I'd like to order the pulleys from SDP/SI but the salesperson with whom I spoke indicated that it may be up to 8 weeks before they have the 16-tooth x 6mm x 5mm pulleys back in stock.

    Thanks!


  • administrators

    Here are two possible reasons why 15 tooth pulleys might be a bad idea:

    1. If you are following an existing design that was intended for 16 tooth pulleys, then if you substitute smaller 15 tooth pulleys the sections of be!t that the carriage is attached to may no longer remain parallel to each other. This would result in inaccurate motion.

    2. You will get an odd steps/mm value. On a delta printer, or the Z axis of any other printer, this would mean that that you need to choose strange layer heights to make each layer an exact number of full steps. On the X and Y axes of Cartesian and CoreXY printers, it doesn't matter.



  • Because of step unit error. With a 15 tooth pulley, 16 microstepping and 1.8º motor your steps per mm would be of 106.666¨ instead of just 100 with a 16 tooth pulley.

    Check out this article, it's well explained and interesting reading:
    https://nutz95.wordpress.com/2014/12/02/reprap-how-to-choose-your-pulleys-for-your-3d-printer/



  • Oh… I took too long.
    David beat me 😛



  • Cool, just what I needed to know as well as learning a bit more about the printer tech. Thanks guys!



  • David, do I understand that the belts are to remain strictly parallel through matching pulley and idler diameters? If so, that sounds like a pretty important little fact about which I was also unaware.


  • administrators

    The sections of belt that get longer/shorter as the carriage moves must remain parallel. The idle sections of constant length don't have to be parallel to them.



  • Hmmm … I'm not yet certain that I follow David, no failure in your explanation surely but it's not quite clicking for me. Are you referring to parallelism of these sections of belts of the three towers in relation to each other?

    In any case, I received notification this morning that the 16-tooth pulleys from SDP/SI are back in stock early and have already been shipped to me. So at this point I'd just like to be sure that I'm not missing what sounds like an important point in delta kinematics. Thanks!



  • No what David means is the the "working side" of each individual belt, the side to which the carriage is attached should be straight from the carriage to the drive pulley and straight to the idler pulley. The non-working side which just goes from one pulley to the other can go around whatever path you want.

    This is especially important on a corexy or objects at the edge of the bed go from square to parallelogram. On a delta a few tenths of a degree of angle from carriage to pulley won't make much difference as the carriage mainly functions in the middle of the travel where any angle off straight is very small, when you get near the wrong sized pulley the angle becomes greater and the distortion it causes increases, corexys carriages often operate near the ends of the travel.

    EDIT as I chose to just fix my belts on the Xl with an m3 screw in 1 of the 4 holes in an mgn12h carriage they are slightly off straight but as they work in the middle of the travel I can't say the effect is noticeable in the real world.



  • Ah, thanks Simon – glad at least that I had it right the first time, thought there was something going on here which was beyond my grasp!


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