Had a go using motorcalc sheet but does not look right.

[irving2008]

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

[Bob_series1]

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.

[Gary]

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.

[irving2008]

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.

[Bob_series1]

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.

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.