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  1. #1
    Hello all, being trying to work out what motors i require and work back from there.

    Used the Motorcalc sheet but the numbers do not look right or i can not make sense of the output. Can some help?
    It will be a dual screw machine made from steel. I have inputted the Worst case data. I think.

    My question is, how do I interpret this into the correct motor? since i was going to have two motors for the longest axis do i factor this in by halfing the max load?

    Looking forward to getting some parts and starting a build log.

    INPUTS
    Load Calculations
    Mass of Load 25 kg
    Friction co-efficient 0.01
    Cutting forces 5 N

    Leadscrew calculations

    Screw pitch 6 mm
    Screw dia 20 mm
    Screw length 1500 mm
    Screw minor dia 18 mm
    Screw efficiency 30% %
    Screw Fixing Fixed-free

    Max linear speed 1200 mm/min
    Acceleration time 0.02 sec

    Screw mass 3.676 kg
    Screw inertia 1.838E-04 kgm^2

    Total Inertial load 2.066E-04 kgm^2

    Critical Speed 1486 rpm

    Motor revs 200 rpm
    20.944 rads/sec


    Acceleration torque 0.216 Nm

    Total Torque 0.240 Nm
    at 667 steps/sec

    Motor Power 5.030 W

    Required Motor 7.55 W
    per phase 3.77 W
    (bipolar halfstep)

    rotor inertia > 2066 gcm^2

  2. #2
    I should have also said that my budget for the electronic side of things is somewhere around the 400 - 450 mark. does my budget allow me to over engineer it, so i might be able to expand in the future? Would you recommend Saving more cash? The electronic side of this is my week point. Willing to learn though.

  3. Hi Bob,

    A couple of thoughts... for 1500mm span, 1200mm/s is far too low, thats over a minute to get from one end to the other. Since screw inertia is the major factor on a screw that large you need to use a higher linear speed to ensure your motors can give you reaonable rapids... say 3000mm/min, you wouldnt cut at that speed but you need some headroom, you can reduce the acceleration to 0.1 for rapids calculation. Secondly, although it won't affect your motor choice, use supported-supported on a screw of that length and diameter - always worth spending more to get the mechanics right.

    This gives you a motor needing about 10W output.. A suitable motor would be SY60STH86-3008BF (3Nm NEMA23, 3 stack) on 40v/4A drivers, wired bipolar parallel. You could go for a smaller motor on Y and Z

  4. #4
    Thank you for your comments irving. Looks like i did not set the speed and left it at the default value. Sill me. The one thing I had not even considered, is the speed I wanted it to move at. LOL

    I will be supporting the lead screw but just wanted to put in a worst case. However, from your spread sheet it makes a massive difference to the power requirements. :surprised:

    So, for 4 motors comes in at about 130. It is amazing at this point how many people go for either a driver board or PSU that can not drive them to their full potential.

    So, i guess the next logical steps is to research the best buy when it comes down to the drivers and PSU. So much to learn!!

    Thank you irving.

  5. #5
    I also notice that you have selected a leadscrew with an efficiency of 30%.
    Have you not considered a ball screw? A ball screw will be far more efficient and also will not wear as quick.
    As an indication of the efficiency, a ball screw can be between 80-90% efficient, so for the same motor you will get a lot more thrust with a ball screw.
    Allot of people buy a three or four axis driver board then the motors and find that the drivers just dont cut it on larger motors,
    and in some cases they just start to smoke.
    Look at the torque curves of the sy60 motor and you will see two torque curves at two different voltages.
    This shows how important voltage is.

  6. #6
    since my design uses dual screws for the X axis cost would be my reason for using lead screws.

    In the future i would also like to have a play with rack and pinion.

    However, depending on the old cash flow i might be able to afford ball screws for the Y and Z axis.

    Main thing for me now is to get some stuff ordered to get the project rolling.

  7. Quote Originally Posted by Bob_series1 View Post
    I will be supporting the lead screw but just wanted to put in a worst case. However, from your spread sheet it makes a massive difference to the power requirements. :surprised:
    With big leadscrews the key thing is screw inertia and acceleration... a big screw operating at high cutting speeds needs high acceleration (short acceleration time) to be able to follow a cutting line.. and that needs torque AT THE REVS IN QUESTION... this is the thing that most people miss... its not bottom end torque you need, its torque at the top end... and as you say, you need to drive them optimally to get that.

    The method of supporting the leadscrew does not directly affect the motor requirement, it affects the critical speed above which the screw starts to whip, putting sideways force on the nut and adding friction (as well as a nasty vibration and a poor cutting finish). But adding a better support configuration means you could go for smaller screws, and since inertia varies as the fourth power of the diameter even a small reduction in screw size can have a dramatic variation in motor requirement... or for the same motor get better performance... I cannot find a spec for a Tr20x6 screw, only 16x4 or 20x4 - given those two I'd go for the 16x4. You might also look for a 16x4, 2 start. (same as a 16x4 but the lead is 8mm so you get twice the speed, or more importantly, you need half the motor revs for the same speed). Anyway, its worth playing with different screw sizes and types (1/2-10 2 and 5 start ACME is popular in the US).

    Note also if you are going for dual screws you need to consider how you will drive them... if you use one motor and a timing belt/pulley arrangement you need to add the inertia of two screws together plus that of the pulley arrangement to get the required power... in this case it will be more than twice the single screw arrangement.

  8. #8
    Quote Originally Posted by irving2008 View Post
    With big leadscrews the key thing is screw inertia and acceleration... a big screw operating at high cutting speeds needs high acceleration (short acceleration time) to be able to follow a cutting line.. and that needs torque AT THE REVS IN QUESTION... this is the thing that most people miss... its not bottom end torque you need, its torque at the top end... and as you say, you need to drive them optimally to get that.
    Wow, all the hours spend reading the forum and i did not know this. I now understand why you recommended wiring the motors up in a bipolar parallel configuration. Looking at LeeR graph in the sticky, the torque is preserved much better high up the Rev range.

    So, since my rails are going to use skate bearings i was going to use the skate bearings to support the lead screw. Turn the end down to 8mm on lathe, place skate bearing on, put thread on the end and lock with a nut+washer. Also, I have a plan at supporting the lead screw at the stepper end.
    Wow, playing with the diameter of the screw on the spreadsheet makes a large difference. Will put some more thought into the screw size.

    Quote Originally Posted by irving2008 View Post
    Note also if you are going for dual screws you need to consider how you will drive them... if you use one motor and a timing belt/pulley arrangement you need to add the inertia of two screws together plus that of the pulley arrangement to get the required power... in this case it will be more than twice the single screw arrangement.
    So, the plan was to have one motor per screw. I would have the extra motor running on its own driver board and slave it in mach3. From reading a thread on CNCZone it was discussed that if your gantry is rigid enough then the your unlikely to encounter the screws loosing sync.

    The build will be fully out of metal so it being rigid is not my worry it is the weight of the gantry.

    I can see why people spend money replacing parts on their CNC machine if they do not do the maths.

    P.S. That spread sheet is ace.

  9. Bob,

    Thanks for your kind words.

    With big screws running at speed you need to locate the leadscrew axially as well as radially, otherwise there is a lot of force on the motor. Skate bearings are pretty poor axially. You either need a pair of deep-groove bearings or thrust bearings, which are cheaper. The diagram shows a simple bearing block that will hold the skate bearing surrounded by two thrust bearings. The picture shows one of my motor mounts, you can just see the thrust bearing against the cover plate, which on mine is screwed in place though not strictly necessary.

    Thrust bearings can be got cheaply from TechnoBots who also have an eBay shop.
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  10. #10
    I understand what you mean, when you say there is alot of force on the motor. I did design a method to counter act this, however compared to your design it is over complex and thus more likely to fail. I like how yours deals with all the forces on all the axis in one neat solution. My solution was also bulky and thus not very good for use on the z-axis.

    Going to order the bits and have a go at building some of them.

    Is this your design or a common know configuration? I tip my hat to the designers.

    Well that is most items covered for my machine then. I am going to order the bearings, screws and motors, start a build thread and once it is all together I was going to buy the Driver boards etc.. at the end.

    Thanks for putting me in the right direction!!!!!!

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