NEMA 23 High Torque motors on Lead 1515

It very much depends on the motors and your machine really. High torque motors have a higher holding torque and higher torque at low speeds, but it is not uncommon for the torque characteristics of both low and high torque motors to be very similar at high rotation speeds. I have even had motors where the low torque motor had a higher torque at 1200rpm than one twice the size.

For 3D work, it is usually the acceleration of the machine that is the dominant factor in how long the cut takes. That is, I can run my machine cutting at 6m/minute (100mm/s) in all axes for a long straight cut, but the acceleration of my machine is only 400mm/s/s. My machine has to move 12.5mm from stationary before it is up to the full 6m/min cutting speed (that takes 0.25s). Therefore when there are lots of changes of direction over a short distance for 3D carving, the higher top speeds are never reached and the carvings take a long time to complete.

For a stepper itself that is not bolted into any machine, or if you CNC machine is very small/low mass (3D printers are often in this category) the motor inductance and the power supply voltage to the stepper driver play a large part in determining acceleration, with lower inductance motors/ higher voltage supplies allowing the motor to accelerate faster. With a CNC machine that has more mass and in particular, if you have a heavy spindle, the behaviour of the motor torque curve plotted as rpm in relation to the pitch on your driving belts/screws and machine mass tends to be more dominant (the power supply is still usually a factor too). I am not sure about the mass distribution on the Lead 1515, but a light-weight router could still be accelerated faster than a heavy water cooled spindle for example.

A higher torque motor will in general allow you to accelerate faster from stationary, so for 3D carving, can provide an advantage. The larger motor may not actually be able to sustain any higher maximum cutting speed than a smaller motor though, and in practice, it could even be less.

One key question is have you tried to see how far you can push your acceleration yet before you start losing steps or stalling a stepper? It is best to test the limits of X and Z rather than risk racking the Y axis!
By knowing what the maximum acceleration your machine can tolerate now, and if you find a torque curve plot of the motors you have, you can get some idea as to how much extra performance you may be able to get (or not) with a bigger motor.

It is also worth putting a volt meter across the powersupply to the stepper drivers too. When you want to accelerate hard, the motors draw a high current and if the power supply voltage dips, then the driver cannot push enough current into the motor (especially if you have a high inductance motor). If you have big dips in the power supply voltage, then just changing the power supply may help you increase your maximum acceleration rate, and you may need to change the power supply anyway if you put bigger (i.e. lower resistance, higher current) steppers on. Beware though, not all power supplies are made the same and there are many poor quality/ fake ones out there (the openbuilds store sells a good one).

 

Ah, yes, a change to a water cooled spindle may well change the dynamics of the machine from what you have now. I have a 3kW water cooled spindle on my machine that weighs 10.5kg when filled with water!

There is another 'game' that can also be played with the motor torque, depending on what you most use your machine for. My machine is optimised for profile cuts mostly, so I have traded the acceleration I need for the 3D work against being able to cut deep and fast when profiling. My Z is limited to 6m/minute, which essentially sets the max rate for 3D work, but when profiling I can cut in X and Y up to 10m/minute, with fast travels in X at 20m/minute. To acheive the faster profile cuts, I opted for a coarse-pitch screw on the X and Y drive, and a finer pitch on the Z. I have 1605 ballscrews on the Z, but 2010's on the X and Y rather than 2005 pitch. I also have a 1:2 drive on the X and Y (i.e. one rotation of the stepper gives 2 rotations of the screw). That way, although I get lower forces from standstill on the X and Y, limiting acceleration, the motors are not spinning to such high revs for the fast travel moves, where torque on steppers really falls off. If you look at the torque curves of many high-torque steppers, in the lower speed regions, halving the rotation rate may actually more than double the torque too, which means it is only the very slow speeds and holding torque that is cut in practice (i.e. if you are already moving slowly, accelerating to faster is just as easy with the gearing used).

For the 1:2 drive, I use timing belts, I am not sure if the openbuilds "reduction-stand-off-plate-set" will fit on the lead 1515, but it is that sort of configuration (with appropriate gears and belts!).