Sizing Steppers / Servos for PM25MV Mill

[carbuthn]

Been working on my Precision Mathews PM25MV mill. I purchased it at the same time as a bought a Techno Lathe. At the time I worked with Technic to figure the size of the Clearpath servos for the lathe and purchased the motors for the Lathe and Mill at the same time (may have been a mistake).

While trying to auto tune the X and Y axis I am shutting down on torque (tried loosening gibbs). The servos are oz-in peak (SDSK-2310S-ELN) and have the 6400 resolution encoders.

Question #1 How many inch pound servos do I need?
Question #2 How many RPM do I need?
Question #3 What resolution encoders do I need? A post from Richards stated that he was using 800 resolution encoders which would exceed the tolerance of his machine (my PM25MV also).

I am looking at:
CPM-SDSK-2331S-RLN, 620 oz-in peak, 124 oz-in continuous, 2520 RPM and 800 resolution encoder or
CPM-SDSK-2341S-RLN, 828 oz-in peak, 166 oz-in continuous, 1860 RPM and 800 resolution encoder.

Thanks for the help,
Chuck

[Richards]

Many of us started our CNC journey using open-loop stepper motors. We soon learned that to be safe, we had to use motors that had "holding torque" values much higher than the "working torque" that we would need. We learned how to read speed/torque charts to see how much torque the motors produced at various speeds. We fiddled with gear ratio tables to see if a particular motor could spin fast enough at a desired torque to move the axis at the speed that we wanted. It was an inexact science.

Let's start with the basics. What is an ounce-inch? According to the Internet, it is the force required to lift one ounce one inch. ("a unit of moment equal to the moment of a force of one ounce acting at a distance of one inch from a center of moments.") I experimented with a spring scale connected one inch from the center of a shaft. By slowly pulling the scale away from the shaft, I could see how many ounces were required to start the shaft moving. I also observed that once the shaft was moving, that it took less force to keep it moving.

Now, translate that into the ratings of servo motors. The Teknic Clearpath motors have two ratings. The higher rating is the momentary torque that the motor can produce. the lower rating is the constant torque that the motor can produce without overheating. Also, keep in mind that slowly pulling on a spring scale is much different from a motor trying to quickly start a shaft turning. The oscilloscope function of Clearpath's setup shows that concept clearly.

Because I don't have a PM25V mill, I can't tell you what size motors to use. Personally, the smallest motor that I use from Teknic is the SDSK-2321S-RLN. I selected that motor because it has about 500 oz-in of torque to 500 RPM and 100 oz-in of torque to 2,500 RPM with lower torque at speeds up to 3,000 RPM - then the torque drops off quickly. On a machine with ball-screws and linear rails with ball bearings in the trucks, that size motor can do what I need. On a mill with ball screws but with metal on metal friction between the gibs and the ways, that size motor might be much too small.

I also have an SDSK-3421P-RLN, which has similar torque ratings at similar speeds at $50 more.

I based my decision to use the RLN model (800 counts per revolution) instead of the ELN (6,400 counts per revolution) on the ball screw that I will be using. The ball screw moves 5mm per revolution. 5mm / 800 is about 0.0002" per step. I'll never own a machine that can hold 2/10ths and I'll never be able to afford ball screws that don't have backlash.

As far as speeds go, 2,500 RPM X 5mm = 12,500mm or 492 inches per minute. Typical desired speeds would be more like 100 to 200 inches per minute.

[carbuthn]

Thanks Mike,

I know that I am not the only one that has purchased motors that were to light for moving the mill axises. I am sure this will help others as well as my self. I like to buy everything that I want for a project all at once (to save time) unfortunately with this hobby I have paid extra on ordering what I need verse what I think I need.

So many variables are involved in moving the X and Y axis.

The mounts that I have on the X and Y axis are NEMA 23 so that is easy to pick. The speed will be fast enough with RPMs > 1800. The Standard resolution (800) will be good enough for my equipment.

So with my current servos = 223 / 44 oz. inch (Max. torque / Cont. Torque) should I go with the:
2321S-RLN = 492 / 98 oz. inch @ $299 each
2331S-RLN = 620 / 124 oz. inch @ $319 each
2341S-RLN = 828 /165 oz. inch @ $359 each

Still don't know what the torque requirement is for my mill, I will do some searching through the Forum to see what motors others are using and look up their torque values. I am leaning towards the 2331S-RLNs that have about 3 times my current servos.

Chuck

[Richards]

Maybe someone who uses Clearpath motors can tell us which motors work on their Mill.

When the wrong motor has been purchased, you can still adapt them. It's relatively simple to add a belt drive transmission to a NEMA 23 motor. A 1:2 ratio will cut your speed from 4,000 RPM to 2,000 RPM, but it will double the torque. Also, by slowing down the ramp time from the default 0.5 second to 0.75 or longer might allow the motor to work without faulting. Be sure to re-run the Clearpath setup routine whenever you make changes.

[fusion]

If you haven't already, search franco cnc on youtube. He has many videos on his PM25 conversion to cnc.

[carbuthn]

Fusion,
I had gone through all of Franco's PM25 videos, He was using Leadshine clones. I re-checked them and the NEMA 23 motors that he purchased were 3 Newton-meter (about 425 oz. in.).

I will try to contact ProCutCNC on Tuesday (I purchased their conversion kit), they sell ClearPath motor kits and see which SDSKs they recommend/sell.

Mike,
I thought about adding a belt drive but want to minimize the size that is hanging off the table. Plus I have other uses for the motors that I have (so not a loss), small rotary table, and a small CNC router/plate cutter.

Thanks for the help,
Chuck

[carbuthn]

I have decided on trying the 2331S-RLN = 620 / 124 oz. inch (Max. torque / Cont. Torque) and placed an order for two servos this afternoon.

Hopefully when I will get them quickly so I can try them next week (they have a 90 day free return policy).

iS there a way (and worth while) to add/change the wizard fields to record motor model numbers and machine information etc.?

I think that this would help new people trying to build machines as well as individuals trying to help those that are having problems.

Thanks,
Chuck

[DrLocke]

Just got 3 axis running on my Grizzly G0760 mill. Don't know how its size compares to yours, it is a 800 pound "desk top" mill. I used Clearpath motors on all 3 axis. The X & Y axis I used 3432S-ELN, 1400 oz-in; and on the Z axis I used 3432P-ELN, 700 oz-in. I guesstimated the size based on my estimate of the torque required to turn the axis by hand and doubled that. My estimate was several ft-lbs on the X & Y and 10 ft-lbs on the Z. The X & Y are directly connected to the screw and the Z axis has a cheap 6:1 gearbox (from amazon LOL). Hope this helps.

PS: I would recommend the ELN resolution because you probably need higher number of steps per .001 resolution you're interested in.

[tkbot47]

I used the ArizonaVideo99 conversion kit for my PM25MV, and pretty much the same motors as Franco - Hybrid closed loop Leadshine clones - Nema 23 425 oz in on X and Y, and Nema 34 900 oz in on Z, and it works smoothly and without issue. However, with the Leadshine clone drivers, you will have to run at 100KHz to avoid faults under high load. If you use the Clearpath ELN's (which I used on my ShopBot conversion), you will be able to run up to 400KHz with the Acorn without problem. The Clearpath's will yield much more torque than the hybrid steppers. When sizing your motors, something to keep in mind that for X and Y you are moving sliding loads with inertia, but with the Z on a mill, you are lifting dead weight (the milling head), requiring more torque and stress on the mechanics.

[carbuthn]

Thanks for the information, I should have two new servos on Thursday to try. I will know more after bench testing and fitting to the mill.
Chuck