Centroid Community CNC Support Forum

The standard knee motor was a 3 phase 415v 1.125kw induction motor. I have replaced it with a 130ST-M15025 AC servo rated at 15nm torque or 3.8kw.
It sounds like it should easily do the job of the original induction motor, but it doesn't - it struggles with the current gearing. Which is why I am making a larger pulley to gear things down.
Here is a photo of the motor, the original pulley, and the larger pulley that I am adapting to fit the knee. The pulley on the LHS of the motor is the original pulley.

with all that weight on the knee you won't have to worry about any backlash from screw support bearings at least!

that motor should do it, what is your final overall turns ratio? i would shoot for at least 10 turns of the motor = 1 inch of knee travel.
i would guess somewhere in the 10-20 turns per inch range would work what is your pulley and screw overall ratio coming out to be?

To be honest I haven't taken the time to work out the ratio of turns of the motor to turns of the screw.
Very roughly though, I am able to see a reading on the servo controller that shows the percentage of power utilization from the controller while moving the knee.
With the smaller pulley which is 70mm in diameter and has 24 teeth, the the servo controller power utiliization is averaging around 95% (with occasional overloads).
The pulley that I am fitting is 90mm in diameter and has 30 teeth. I think fitting this will have the effect of increasing the effective motor torque by 20%.
I am hoping that the reduction in power consumption of the servo controller will be sufficient, but theory is one thing.....

The other thing that I haven't taken into account, is what happens when I have a heavy item on the bed of the mill? I don't know how much extra effort per kg (or lb) will be required from the motor.
This larger pulley, however, is one that I just happened to have, so if knee movement is still an issue, I can look at buying one that is 50% larger than standard.

most knee mills that i have used that have this setup have at least 10 turns of the axis motor = 1 inch of travel, and some of them had 20 turns of the axis motor = 1 " of travel. most of them had ballscrews, a few had acme thread. the ones that had acme thread where the 20 turns per inch units. before you go to all and hard work of pulleys and mounting etc.. you should figure out what the overall turns ratio is going to be.

15Nm with 1/10" travel per rev should give 23kN force with acme screws (efficency 0.4)

If this is sticking, something is wrong with the machine....

Uwe

I think the problem causing the overload errors, is the servo controller, rather than the motor. I agree Uwe, the motor's specs say that it should easily handle the torque required to drive the knee, but it doesn't in its current incarnation..

CNCKeith, you asked:
"with all that weight on the knee you won't have to worry about any backlash from screw support bearings at least!
that motor should do it, what is your final overall turns ratio? i would shoot for at least 10 turns of the motor = 1 inch of knee travel.
i would guess somewhere in the 10-20 turns per inch range would work what is your pulley and screw overall ratio coming out to be?"

Uwe, you said "If this is sticking, something is wrong with the machine...."

The Knee seems to be working well now. The knee is using about 60% of the servo drive's capacity when moving up. For posterity, it currently has 1.34 motor turns per mm, or 34.036 motor turns per inch. I think the large % of utilization, was caused mainly by an undersized servo drive, rather than an issue with the motor's capabilities.

I haven't yet done any "fine" tuning of movement amounts, just measured the distance the axis travels with a tape measure, and selected a "turns per mm{" setting that moves the axis the amount measured with the tape.

I am now 4/5 of the way through setting up a rotary axis, which I have called "A". The rotary axis is from Tomatech in China via Aliexpress. I had some initial issues getting the Adtech servo drive and QS7 servo controller turning on the DB25 connector of the Acorn board, but they seem to be sorted now.
I bought a chuck for the rotary axis at the same time, it looks to be of very good quality: I'll have to make a mandrel on the lathe to center the chuck, when it's getting installed on the rotary table / axis. Unless , of course, there's an easier way?

I prefer NOT to have any centering devices on my chucks preferring to indicate them in using a 0.0001" indicator.