PK296 vers PK299 steppers

[thewoodcrafter]

I am thinking of going with the 4G board.
Will I see the same upgraded performance with my geared PK296 steppers that you guys have seem with the PK299 steppers?
My Z stepper is very small (don't remmember the model#) and it most likely should be upgraded.
This machine is an old PR that was upgraded to a PRT before I bought it. I need faster and stronger performance. Do I need to replace all the steppers to get it?

[Brady Watson]

Roger,
If you have geared motors on your PR, then keep them. They are more than adequate for a 4G conversion. You'll get better resolution & torque using a geared motor than a 1:1 drive. As far as your Z axis goes, I would look into getting a larger stepper with more torque & adapting it to your Z, OR look for a used PRT Z axis and convert the whole deal to rack & pinion.

-B

[richards]

Roger, I agree with Brady. The PK299 motors have at least 2X the torque of the PK296 motors, but you'll have to build some kind of gearbox to get the resolution from a PK299 motor to equal the cut quality of the PK296 geared motors.

With a PK299 motor driving a 3.6 gearbox, you would end up with about 1,800 oz*in of holding torque compared to the PK296A1A-SG3.6 torque of about 450 oz*in (the torque is limited by Oriental Motor's gearbox design - not by the motor). However, from a practical standpoint, you would probably only increase the move speed by about 1 ips or possibly 1-1/2 ips. The cost of buying new PK299 motors is about $200 each (if bought directly from Oriental Motor) and the cost of adding a belt-driven gearbox for each of the X-axis and the Y-axis motors will easily be another $500 to $800, depending on whether you do the work or hire it done.

If my PRT-Alpha motors die, I'll probably replace them with some PK296A2B-SG3.6 motors and Gecko G203v stepper drivers that I have on hand, rather than building new gearboxes for the PK299 motors that are on my test bench.

[thewoodcrafter]

Thanks for the replys guys.
Glad to here I don't have to replace the steppers.
That was more than I wanted to spend.

[Brady Watson]

Roger,
If you keep the original Z axis with ballscrew, then you can run ANY direct drive stepper. I don't remember off the top of my head what pitch the screw is, but let's say it's a .2" pitch. That equates to 5 turns per inch...and I'm not sure what that equates to in gear ratio. Maybe Mike can chime in on that one. If you keep the screw & just make a different mount for the larger stepper, then you're good to go without much work or expense besides maybe a shaft coupling from the stepper shaft to the screw, and a new stepper mount.

-B

[richards]

A 0.2" per turn ball screw would be a 5:1 gear ratio. With a Gecko stepper driver on a 4G upgrade, the Unit Value would be 10,000 (it takes 2,000 steps to turn the motor shaft one turn and 5 X 2,000 = 10,000 - so each pulse to the motor would move the Z-axis 0.0001 inches).

In comparison, a PK motor with 3.6:1 gearbox and a 30 tooth pinion gear, assuming that a 20 tooth pinion gear has a 1" pitch diameter and that a 30 tooth gear would move the axis 1.5 X pi, or 4.71 inches along the rack, each pulse to the motor would move the motor 0.000654 inches - which would equate to a Unit Value of around 1529.

If you're planning on doing a lot of 3D work, the Z-axis might be too slow with a ball screw (but it would be very precise).

[thewoodcrafter]

That tiny stepper for the Z is a little too small and it is just 2 sided tape and zip tied on.
I have 2 Z's so buying 2 PRT's Z could be a bit much.
I could make some sort of a mounting bracket.

Should I buy 2 PK296 geared for the Z's or the PK299?

[Brady Watson]

If you are going to keep the screw, get the direct drive motors. If you are planning to eventually kick up to PRT Z assemblies, then get the geared motors.

Mike's point was that if you choose the geared motors, the Z may not be able to get out of it's own way fast enough to run 3D files quickly...BUT it would be very precise.

It would be well worth it to rig up some inexpensive min/max limit switches when you move up to the larger motors. The amount of torque that will be applied to the screw is enough to twist things apart if you exceed the mechanical limits/travel of the screw & nut.

-B