It's time for a quick sanity check before modifying my PRT-Alpha. Basically, I have two choices. (1). Buy the 7.2 Alpha motor upgrade from Shopbot. (2). Install Oriental PK299-F4.5A motors and Gecko G203v stepper drivers.
From the surface, it looks like a no-brainer. Shopbot is offering an upgrade at the world's greatest price, about $250 per motor when the old motors and drivers are returned. Those new Alpha motors with gearbox do more than I would ever expect my PRT-Alpha to do. With those new motors and new 30-tooth spur gears, I would have excellent resolution, with about 0.000655 inches per step. (1.5 spur gear diameter * 3.14 / 7.2 gear ratio / 1000 steps per revolution). Brady and others have reported cut speeds up to 12-ips and jog speeds up to the limit of the gear box. (I'm sorry, but no matter how fast the machine will jog, I feel safer jogging at a maximum of 12 to 15 ips. A geared motor has just too much torque and can cause too much damage if it gets away from you.) The only fly in the ointment is the replacement cost of the stepper and stepper driver. I haven't checked with Shopbot, but Oriental Motor sells the motor and driver for about $2,500 per one-motor set. Stepper motors and stepper drivers, especially those from Oriental Motors seem to last forever, so that's a fear that I might actually outlive.
On the other hand, if I install Oriental Motor's PK299-F4.5A stepper motors and Gecko G203v stepper drivers at about $380 per motor, driver and spur gear, what would I gain and what would I lose? Actually, in my case there would be almost no price difference if I raided my spare parts cabinet. I would end up buying, at the most, three stepper motors and four spur gears. Everything else is on hand. Some would say that I'm cheating by not including the cost of the the G203v drivers and the motors that I have on hand. That's true, but I would end up retaining all four AS911 motors and drivers, which cost $1,100 each if I bought them new from Oriental Motor. So, I think I'm being fair. Secondly, by gearing the motors 3:1 or 3.6:1 with a belt-drive transmission, I would end up with similar per-step resolution of 0.000785 inches per step for the 3:1 gear box or 0.000655 inches per step with the 3.6:1 gear box. The main factor that makes a 3.6:1 gearbox give the same resolution as the 7.2:1 gearbox is the number of steps per shaft revolution. A Gecko driven motor requires 2,000 pulses per revolution and an Oriental Motor Alpha motor can be set to 500, 1,000, 5,000 or 10,000 steps per revolution, with 1,000 being the default. So 3.6 X 2,000 = 7.2 X 1,000.
The real difference is holding torque when an axis is in reduced current mode, which automatically happens about one second after an axis stops. The G203v stepper driver automatically reduces current to about 50% of the normal current. The Alpha stepper driver can be adjusted to give more or less than 50%, depending on the amount of heat that can be tolerated. Now, this is the crux of my dilema. The PK299-F4.5A motor puts out 880 oz*in of holding torque. With a 3:1 gear box, that is multiplied to 3 X 880 or 2,640 oz*in of torque. Even at 50% when the motor is in reduced current mode, the motor has 1,320 oz*in or 82 lb*in of holding torque. The figures are even more impressive with the 3.6:1 gear box.
Cutting speed is about the same. The amber light comes on at 8.5-ips with a 25.6V power supply. If I used a 35V power supply, the amber light would come on at 11.6-ips. The amber light means that the stepper drive has reached full output. The motor will turn much faster than 8.5-ips, but torque drops off rapidly as the speed increases. According to Oriental Motor's torque charts, when the PK299-F4.5A motor is running at 325 RPM, which is the speed needed to move an axis at 8.5-ips with a 3:1 gearbox, the motor is still putting out more than 800 oz*in of torque. Multiply 800 oz*in by 3:1 and that motor cuts with 2,400 oz*in or 150 lb*in of torque. Can anyone say "impressive"? Also, the motor has no trouble jogging at 20-ips, which is too fast for me.
So, my choice is whether to replace the four orignal Oriental Motor Alpha stepper drivers and motors with three PK299-F4.5A motors and 3.x:1 belt-drive gearbox on the X and Y axes, and one PK296A2A-SG3.6 motor on the Z-axis, or whether to buy the upgrade from Shopbot and get four new 7.2:1 motors.
Right now, with the original alpha motors, 20-tooth gears, and 500 steps per revolution, the machine is running very smooth with almost no chatter. Even though resolution is about 0.0063 inches per step or about 10 X worse than either upgrade would give, my cuts look good. What I am seeing is what I call torque induced chatter. When I do have chatter, it usually happens after an axis goes into standby mode. I'm thinking that replacing my current drives and stepper drivers with either the 7.2:1 Alpha upgrade or the PK299-F4.5A motor upgrade would multiply the holding torque during standby by a factor for 2X or more. Does anyone have an opinion?