Upgrading a PRS standard 48x96" to Alpha

[kodiak]

I currently cut foam and plastics at 3.5 ips on my PRS standard, with a 1" round over bit at a depth of .095" ... if i go over that speed the table shakes too much and i feel like i'm pushing it to the limit. My problem is I need to cut faster for production to make sense. If i upgrade to the alpha will it be worth the cost? Will my table handle 9 ips? and do i have to change my ramping numbers? It just seems the table can't handle over 4 ips.

And are these advanced stepper motors better as they say then servos?

Any thought would be appreciated.

Thanks in advance

[thewoodcrafter]

The PRS standard and the PRS Alpha use the same table.
It is your steppers that can't handle the speed. They start to chatter and will miss steps.
I think the upgrade to an Alpha is quite expensive.
You need to change all the steppers and the control box.
Call Shopot in the morning for a price but I would think it is over $4000.

[richards]

You've asked some interesting questions.

Let's do a little math to see how the standard motors compare to the Alpha motors.

An Oriental Motor's ASM98AAP stepper motor has 17 lb*in of torque. That particular model number is similar to the ASM98AAP-T7.2 shipped as part of the Alpha package. The difference between the motors is that the ASM98AAP motor has no gearbox. 17 X 16 = 272 oz, so that motor is rated at 272 oz*in. The 7.2:1 gearbox version is rated at 79 lb*in or 1,264 oz*in.

The PK296-03AA non-geared motor is rated at 310 oz*in or 440 oz*in, depending on how it is wired. The 3.6:1 geared motor, (either the PK96A1A-SG3.6 or the PK296A2A-SG3.6) is rated at 22 lb*in or 352 oz*in (because of the components used in the gearbox).

If you disregard the gearboxes for a moment, you can see that the PK296 motor actually produces more torque than the ASM98 motor. So, with the correct power supply and the correct stepper driver, the PK296 motor should perform very well at the speeds used on a CNC router.

Now, if we do the math for speed, a stepper motor with 3.6:1 gearbox and a 25 tooth spur gear would move an axis 1.09 inches per motor shaft rotation. ((25/20) X pi / 3.6) = 1.09. (A 20 tooth gear has a pitch diameter of 1-inch, so a 25 tooth gear has a pitch diameter of 25/20 or 1.25-inches). Going one step further, 9-inches / 1.09 = 8.257 and 8.257 X 60 = 495 RPM.

If you look at the torque chart for the PK296-03AA motor wired unipolar, 500 RPM gives about 175 oz*in of torque. 500 RPM is also the shoulder portion of torque curve, meaning that when you spin the motor faster than 500 RPM, the torque falls off rapidly. So, for normal purposes, 500 RPM would be a good maximum working speed for that motor when cutting.

To get maximum torque with that motor, you need to verify that the power supply is ideal. When using Gecko G202 or G203v stepper drivers, the formula to determine the power maximum voltage needed is 32 X SQRT( Inductance). The PK296A1A-SG3.6 motor has a half-coil inductance rating of 7.7mH, so 32 X SQRT(7.7) = 88V. The maximum voltage that you can use with a Gecko G202 or G203v stepper driver is 80V, so the maximum voltage for that motor is 80V. If you build your own power supply, a 56VAC toroidal transformer generates about 79VDC after rectification and filtering with a large capacitor.

The PK296A2A-SG3.6 motor, when wired half-coil, has an inductance rating of 1.5mH, so 32 X SQRT(1.5) = 39V. A 27VAC toroidal transformer produces 38VDC after rectification and filtering.

(For practical purposes, you can buy off-the-shelf toroidal transformers that are 25VAC or 50VAC.)

The ASM98 motors come with a matching power supply directly from Oriental Motor. You don't have to build a special power supply, because Oriental Motor has done all of that for you.

That's a lot of beating around the bush to say that a properly configured PK296 geared motor should work very well when making light cuts when compared to the ASM98 geared motor. The gearbox on the ASM98AA-T7.2 motor gives you about 4X the usable torque as the gearbox on the PK296A2A-SG3.6 motor. That is why I mentioned light cuts with the PK296 motor.

However, unless the gearbox is binding, the stalling/missed steps that you're getting is more related to the power supply than to the gearbox.

You must adjust the ramping to get the maximum efficiency out of a stepper motor. If you ramp too fast, the motor is going to stall. If you ramp too slowly, the motor may pass through its "harmonic" speeds too slowly and stall. Some experimentation will probably be necessary.

Of course, the Alpha motors have have the no-stall circuitry to help prevent stalling. That is both a blessing and a curse. On a single axis move, when only a single motor is driving the axis, the anti-stalling feature is a blessing. On a multi-axis move or on an axis with multi-motors (X-axis), if one motor momentarily stalls and then recovers, you'll probably find a "divot" in the piece. Depending on how bad that "divot" is, the piece may be ruined.

Even with a perfect set of stepper motors, you may be trying to do something that is not easily done. Although a cut 0.095-inches deep would seem to be very light, when cutting plastic that might be a very heavy cut. I don't have the figures in front of me, but when I cut Delrin on my PRT-Alpha, I used a slow cut speed, probably around 2-ips. I don't cut a lot of plastic, but I have cut enough acrylic, polycarbonate, Delrin, and polyethylene to have given up on trying to get high speed cutting.

When I cut any material, the material dictates the cut speed - at least if I want quality. I always test the material at various cut speeds and at various spindle RPMs to see what combination of spindle speed/cut speed gives acceptable results. I also try various hold-down methods, especially when cutting plastics. (Usually I resort to screwing the plastic to a 24" X 48" piece of MDF, letting the vacuum hold the MDF as the screws hold the plastic.)

[erik_f]

I have cut MDF at 4 inches per second. Granted I was using 2 passes...but I would think cutting foam would be no problem at those speeds. Guess it depends on what type of foam.

[kodiak]

Thanks for the info!

[Buckeye Don]

Big J
I might be interested in your old motors if you upgrade. Let me know. Don