Avid Benchtop Pro 2424 Clearpath Servos

I just placed an order for an Avid Benchtop Pro 2424 and am looking for which Clearpath servos to add to it. Since there are only minor differences in price among the servos, I'm leaning towards going for a more powerful model. The question for me now is whether to go for a high torque model in the lower max RPM range or a lower torque model with a higher max RPM. I will be cutting mainly wood, ABS plastic and aluminum with the machine. Speed is not too much of a concern, but having it is nice. The servo models I'm looking at right now are:

CPM-SDSK-3411S-ELNd (638 oz-in peak, 150 oz-in cont, 2320RPM) https://www.teknic.com/model-info/CP..._voltage=75VDC

CPM-SDSK-3421P-ELN (547 oz-in peak, 164 oz-in cont, 3070RPM) CPM-SDSK-3421P-RLN https://www.teknic.com/model-info/CP...del_voltage=75

CPM-SDSK-3421S-ELN (1,094 oz-in peak, 288 oz-in cont, 1410RPM) https://www.teknic.com/model-info/CP..._voltage=75VDC

Any insights as to which of these would fit best would be greatly appreciated!

I'm using a CPM-SDSK-2321S-RLN servo on all the axes of my Benchtop PRO 24x24. Works great for X and Y, but I'm having intermittent issues with my Z axis currently.

The z motor randomly gets RMS torque exceptions, and when looking at the Scope feature in the MSP software, it appears to be struggling to hold the weight of the spindle (2.2kw plug n play spindle from AVID) against gravity. I'm still trying to figure out if that is because this particular motor is undersized for the application, or if I just have it incorrectly tuned.

Regardless, I think any of those motors you listed will work fine. Z axis is probably the only one that need extra torque compared to the X and Y motors.

Sent from my Pixel 3 XL using Tapatalk

Hm, the torque exceptions are strange, especially just holding the spindle against gravity... I'd think these servos should all have more than enough power for that.

First thing I'd do is ask Avid what the pitch of the screws are, and why they list the cutting and rapid speeds at only 200ipm?
Probably a 5mm pitch screw, with a 1000rpm max from the steppers?
If that's the case, than probably the 3421P? That's just a guess, though.
You probably should ask Clearpath, once you have the info from Avid.

Howdy! I have a benchtop pro 3x2 with clearpaths and I'm using 4 CPM-SDSK-3411S-ELS motors. It works great and I don't have any issues. Ballscrews are all 10mm pitch btw so speed isn't a problem.
I don't know but avid may limit the max speed to avoid ball whip. That said I run 400 to 500 ipm on rapids and have no issues.

Keep in mind on the benchtop pro the 2 y motors don't move (as they do on the avid full size machines) so it makes no difference how heavy they are. Obviously the x and z motors do have to be moved around which is why I went with the smaller of the nema 34s.

Sent from my SM-G960U using Tapatalk

Thanks for all of the responses. I am pretty new to CNC (electrical/computer engineer by trade), so I am still figuring a lot of this out. As I understand it, the maximum theoretical rapid rate is calculated by the ball screw pitch x max rpm of the servo. So for example for the CPM-SDSK-3421S-ELN (1,094 oz-in peak, 288 oz-in cont, 1410rpm) and a 10mm ball screw, this would be:

1410rpm x 10mm = 14100mm per minute or 555.12 inches per minute

The acceleration should be proportional to the torque, so I'd think that the CPM-SDSK-3421S-ELN would be more than adequate in terms of max rapid rate and would provide the highest acceleration and strongest cutting force of the motors I listed. Please correct me if I'm wrong here.

@Richmaple I see you got the shaft seal option on your servos. Is that useful for the Avid? I'd think it may only be of use on the Z axis since that seems to be the only axis where the motor shaft is exposed?

Actually correction: The Z axis also seems to be completely sealed, so the shaft seals don't seem too useful?

Quote:

---

As I understand it, the maximum theoretical rapid rate is calculated by the ball screw pitch x max rpm of the servo.

---

Yes and No. You have to include the mass that you are moving and the rate you are accelerating at.
If you need 250oz of torque to accelerate at a given rate, the servos will fault when they fall below 250oz of torque (due to position error), which may be well below their max rated speed.

Quote:

---

The acceleration should be proportional to the torque,

---

Acceleration is linear, so you need the same amount of torque when you start accelerating, all the way until you stop accelerating. So if you want to accelerate to 500 ipm, and the motor only has 200 oz-in of torque at that speed (~1270rpm), then you really won't ever use more than 200 oz of torque.

Good thread, team. :)
First off, maxmeaker, are you sure your drive is set to 100% torque limit in the clearpath firmware? I'm also surprised it's having issues keeping the z axis up, partcularly because it shoudl REALLY be having problems moving it at speed (accel/decel) if it can't even hold it up. Also, did you run autotune for that Z axis, in position? You really should if you haven't.

Second, yeah, I went for shaft seal just for safety. Though it seems the motors are "sealed" against the axis assemblies, those axis assemblies are actually rather open. There's just a reinforced close thing that covers it up but although it's rather effective, the inside of that assembly can still definitely get dust and stuff in it. That said, the shaft seal is cheap insurance (though it does add some rotational resistance).

Buy your motors for torque, not speed, because in most CNC operations, torque will determine the rate your machine can accel and decel without faulting and that will determine how fast your cut is. Put another way, if our machines were full size (8 or ten foot long, etc) beds, you'd want fast rapids (like 1000ipm+) to avoid waiting around for rapids from the other end of the table, partilcularly if you're making signs or cabinet parts all day. But on a 2 or 3' long machine, even 300 ipm is fine (in my opinion) and your machine is going way slower in the turns anyway (depending on your smoothing settings). You want all of the torque you can get so your machine can make fast turns and accells and decels just to keep up with whatever rapids it can do on straight lines. PLUS, these are ballscrew machines so we're trying to avoid ballwhip anyway (hence me limiting my machine around 400ipm even though it can go faster).

Just some random thoughts and answers :)

@Richmaple, or really anyone familiar, can you link to or show an example of setting up an Avid Benchtop Pro with Clearpath Servos? Do you use Avid's own control box minus the drivers or something else?

I meet Avid on Monday; are they likely to have this kind of info and be willing to share?