Servo selection for mesa cards

I'm starting down the path of planning to replace the motors on my Bridgeport Boss 5 mill. Leaning more to the side of installing servos on the mill if Im going down the path of making adapters and adapter shafts to fit a motor other than Nema 42. After this weekend I will be changing the gearing from 1:1 to 2:1 on the mill. Parts are in hand for that project already. I may have missed a good thread on servo motors in my search of the board. Been racking my brain for a few days on understanding what is good for a mesa setup.

Controller
5i25 +7i76 + 7i84

Easy difficulty,
Mount Adapters: none needed
Shaft Adapters: None needed

Nema 42 2830oz motor $169
www.automationtechnologiesinc.com/produc...r-motor-2830-oz-in-2

Driver $0 I own Gecko 203V already

This would be a joke to install. Would it have the repeatability I need later on?

Medium difficulty
Mount Adapters: Nema 42 mount to Nema 34
Shaft adapters: Shaft adapters to extend shaft

Nema 34 1200oz motor $99 each
www.automationtechnologiesinc.com/produc...tkl34h2120-60-4a-key

Driver $0 I own Gecko 203V already

Total cost: $300 + misc material

I have a hard time justifying the work required to make adapters to just put steppers on the mill again and not servos with a closed loop system.


Hard difficulty
Mount adapters: Yes
Shaft adapters: shaft adapters to extend shaft

There is where I fall on my face when it comes to the servo systems. Multiple different type of systems I can buy. Just will take a little more time and configuration changes compared to stepper but doable for me.

What is the recommended setup with a mesa controller system. Sounds like a 7i77 card but what is used to drive the motors and what brand of motor is recommmended.



If upgrading I only have once according to the wife so needs to be what I would use for a long time. Also planning on running some production parts on the mill for a friend. Not alot but a few a month.

One of the nicer things about servos is that because they can usually spin so much faster than a stepper, you can gear them down a lot compared to a stepper motor, so you usually don't need to have as high a torque motor. As an example your high torque nema 42 stepper probably loses about 2/3 of it's torque by about 60rpm, the nema 34 makes it to about 1000rpm before it's lost the same percentage, but most servos will still make 90% of their rated torque at 3000rpm (or what ever their max speed is). For that reason you might even want to consider even more gear reduction than 2:1 on a stepper to servo conversion. So a 750w servo with a 4:1 gear reduction would have comparable tourque numbers to your nema 42 at 1:1 but would probably be able to move almost 10x as fast (600 rpm vs 60).

Since you already have a 7i76, why not just buy digital servo drives that can accept step/dir inputs. The drives may cost a little more but it will save you having to buy more hard ware, and you won't need to do quite as much wiring.

My biggest suggestion, is to buy matched sets of motors and drives. Preferably that come with or you can get premade cabling for them. (building encoder cables can be a pain)

Todd wrote:

One of the nicer things about servos is that because they can usually spin so much faster than a stepper, you can gear them down a lot compared to a stepper motor, so you usually don't need to have as high a torque motor. As an example your high torque nema 42 stepper probably loses about 2/3 of it's torque by about 60rpm, the nema 34 makes it to about 1000rpm before it's lost the same percentage, but most servos will still make 90% of their rated torque at 3000rpm (or what ever their max speed is). For that reason you might even want to consider even more gear reduction than 2:1 on a stepper to servo conversion. So a 750w servo with a 4:1 gear reduction would have comparable tourque numbers to your nema 42 at 1:1 but would probably be able to move almost 10x as fast (600 rpm vs 60).

Since you already have a 7i76, why not just buy digital servo drives that can accept step/dir inputs. The drives may cost a little more but it will save you having to buy more hard ware, and you won't need to do quite as much wiring.

My biggest suggestion, is to buy matched sets of motors and drives. Preferably that come with or you can get premade cabling for them. (building encoder cables can be a pain)

With the digital servo drives that accept step/dir inputs wouldn't I lose the closed loop benefit of linuxcnc?

2:1 might be as far down as I can go due to mill setup.


So far I have looked at:

DMM-tech dyn4 servo motor with drive 750W
Automationtechnologiesinc DC servo motor with G320x 1125oz/in
Automationtechnologiesinc DC Bushless motor with drive
SimDrive AC servo motor and driver 750W

What benefits would those be that you are hoping to take advantage of?

You can add encoder feedback to a step/dir servo system, if you want to. My personal opinion is that it usually isn't worth the added expense and effort.

I've set up one such machine. it had digital servos and was controlled with software stepping at first. Later I added a Mesa 5i25+7i85S to get hardware stepping and encoder feedback. The biggest advantages to this over what I had before were, the increased step rates from the hardware step generation (by far the biggest improvement) and in a distant second place was simplified drive tuning using the encoder feedback into hal scope instead of having to drag out an o-scope and hook it up to the drives to tune them.