2x 2005 C7 Ballscrew 1450mm Length with Ballnuts + FK/FF mounts and motor coupler

[Jonathan]

Its not slight when your spinning them at 1000rpm+ At 1.45m I've got a 1610 on the Y axis and it just starts to show whip at 10m/min, at 14m/min its like jelly! You won't run at these speeds of course but for cutting wood you want in the range of 4-7m/min depending on the cutter dia and flutes along with DOC. Unless of course your spinning the nut but I'd still go 2010 at those lengths anyway. The cost difference is minimal for the added peace of mind.

The cutting force on a CNC router is a few tens of newtons, even when taking 'heavy' cuts in wood or aluminium. There's an additional force when acceleration so lets say 100N. The RM1610 ballscrews are rated for something like 7800N for one million revolutions, so operating at 100N they will last for a very long time. Hence you don't need a bigger screw unless critical speed is an issue, which according to your experiment and the calculation it isn't for this length (1450mm) and RM1610.

Got two RM2010 at 1450mm with end machining and ballnuts from chai for $218 including shipping. So 325 is very expensive where ever your buying those from.

I did not say that I had bought any at that, or any, price. That was the first seller I found on ali-express who has RM2010. Last time I checked Chai doesn't sell RM2010 and Jazz posted only a few days ago saying the same. So where did you get your RM2010, or are they actually RM1610?
Either way my point is valid - RM1610 or RM2010 is a much better choice for this length for the vast majority of machines built on this forum. If John S has got a use for them then good for him, since he's just got a good deal.

[Shinobiwan]

The cutting force on a CNC router is a few tens of newtons, even when taking 'heavy' cuts in wood or aluminium. There's an additional force when acceleration so lets say 100N. The RM1610 ballscrews are rated for something like 7800N for one million revolutions, so operating at 100N they will last for a very long time. Hence you don't need a bigger screw unless critical speed is an issue, which according to your experiment and the calculation it isn't for this length (1450mm) and RM1610.

Jonathan, your good with this maths stuff.

I have a gantry that weighs approximately 80kg with spindle. With a decent rate of acceleration let's assume its set to 1000 and 160 steps per in mach with dual RM2010 and 4Nm motors. How much kinetic energy is transferred to the motors when doing a direction change at say 1000rpm or 10m/min? You got to decelerate that 80kg in a very short space of time and I'd hate to think how big this number is going to be or if the motors can even handle that.

[Jonathan]

I have a gantry that weighs approximately 80kg with spindle. With a decent rate of acceleration let's assume its set to 1000 and 160 steps per in mach with dual RM2010 and 4Nm motors. How much kinetic energy is transferred to the motors when doing a direction change at say 1000rpm or 10m/min? You got to decelerate that 80kg in a very short space of time and I'd hate to think how big this number is going to be or if the motors can even handle that.

Right, so you have said let:
acceleration =1000mm/s^2=1m/s^2
velocity = 10m/min = 10/60= 0.167 m/s (not actually required for the initial calculation)
mass = 80kg

It's just newtons 2nd law, so F=ma=80*1=80 Newtons (equivalent to lifting about 8.2kg), so not far off my earlier estimate.

From the motor's point of view it's not 80N, since we have to consider the ballscrew in between. The ballscrew gives the motors a huge mechanical advantage. The relevant formula, which can be derived from considering the ballscrew as a ramp, is:

T=F*p/(2*pi*e)
Where T is the torque, F is the force applied to the nut, p is the ballscrew pitch (so 10mm=0.01m) and e is the efficiency (about 0.9). So chuck the numers in:
T=80*0.01/(2*pi*0.9)=0.14 Nm

So changing direction at 10m/min and 1m/s, as you describe, only translates to an extra 0.14Nm torque on the ballscrew, compared to going at a constant speed and 80N force on the ballnut.