TPI for Torch height axis

[edwardpic]

hey guys, Im about to order a Zaxis linear slide, it comes with a NEMA 23 470OZ motor, but they want to know the TPI I need so they can build it for me. Any help would be appreciated. Almost forgot this is for a Plasma table and will be using the MP1000 torch height controller.

Thanks
Ed

[Torchhead]

With that large a motor you could easily use 4 or 5 TPI. That multiples the torque by the length of the thread over one inch (not just the TPI number). The larger the dia the longer the thread. You can go higher in TPI but at 10TPI you need a higher pulse rate from MACH so if you want the IPM on Z to be 120 IPM you need 200*10*10*2 or 40,000 pps (pulses per second). Pretty high. 5TPI puts that same rate at 20,000 pps.
Since the max velocity for the THC is limited (THC Rate in MACH3 uses a percentage of the max velocity since acceleration curves are NOT used under THC control). you will be running the axis at 20 to 40% of the max velocity. That means if you can do 120 IPM with accel you will be at about 40 IPM. That means that if you are moving XY at about 3.3 IPS the Z will only move about 1.1 IPS. The max up angle then is slighlty more than 30 deg (1.1 inch in 3.3) rise. As long as you aren't trying to cut wavey sheet steel (siding) at fast feedrates then a 30 deg rise rate is good.

The tradeoff to faster speeds is lower resolution and that can effect the accuracy of the THC moves. If the smallest step you can do is .010 then the moves can get pretty saw toothed as the tip volts bounce back and forth.

Yep, 4 ot 5 TPI is the answer!


Tom Caudle
www.CandCNC.com

[edwardpic]

Thanks Tom,
Should I drop the motor size down to around 260Oz and if I do will that change the TPI you recomend from 4 or 5 to a different number? I see alot of control/motor packages use 260Oz for the Zaxis...I was looking at using 1060Oz motors for the dual X and Single Y axis..Im not sure now since reading your response to a previous post about motor sizing.

Thanks
Ed

[Torchhead]

You can downsize the Z motor to a 260 oz stepper and still keep the 4 or 5 TPI. The screw gives you the mechanical advantage and it's just about how fast you want the response to be. Personally I think 1000 oz motors are WAY overkill for a plasma only table. You will find that driving those size motors will be more of a challenge due to their low inductance. You will need a bigger power supply. It's kinda like putting a 600 hp engine in a go cart. You can get to the same torque by using toothed belt reduction with 300 to 400 oz in motors. Unless you plan on making this a real heavy gantry and a combo table with a large router you will just be buying torque/speed you can't use.

If you have that much cash to burn think about using servo's. Some nice big (new) servos can be had for about 250 each. Smaller ones are less. All it takes is bigger step down ratios and a change to Gecko 320/340 drives. The torque rating for servos's is taken at higher RPM and is "usable" torque. Stepper torque is rated at stall (where it is max) and drops off rapidly with RPM. A 400 oz-in stepper drops to less than 100 oz-in at the mid part of it RPM range. A 100 oz-in servo with a 6:1 reduction produces more usable torque than a much larger stepper.