Replacement motor choice for SEM servos on a Harrison M300 lathe

Hello all, first post! I’ve got hold of a Harrison M300 CNC machine in excellent mechanical condition but with an old Anilam Crusader controller so the electronics are all out now and I’m planning the rebuild with up to date electronics. I’m looking at motor options and am a bit confused…

It was running SEM DC servos (with tachos), models 30E4-30 for the X axis and 30M4-48 for the Z. They are plated as follows:
X Z
Stall torque 1.1Nm 3Nm
Volts 120V 144V
Pulse amps 22A 37A
Max RPM 3000 3000

I have found an old spec sheet on the internet and plotted the torque /rpm curves; my decision is whether to use conventional steppers, closed loop steppers or try some of the BST Automation 220V servos (which work out at similar cost to the closed loop steppers).

For the servos option I think that the 600W ‘110ST-M02030’ will suit the X axis (2Nm continuous / 6Nm max). For the Z axis, either 1200W ’110ST-M04030’ or 1500W ‘110ST-M05030’ (4/12Nm or 5/15Nm ratings). Can anyone advise if these look about right for this application and has anyone tried the BST units in anger (I found the test video posted on here and they look pretty good!).

Second question – if I go down the closed loop stepper route I was thinking of ES-M23440 & M23480 units. Again I found a few curves and plotted them as below. The ‘70V / rated current’ curves that I found suggest torque above the continuous rating of the servos up to 1500rpm, but then another curve marked ‘68V 40% holding torque’ makes things look more marginal marginal….or should I in fact be looking at the intermittent capability on the old servos? This is where I am confused – will these units be OK for this application?
Last question - are the closed loop steppers worth the additional cost over conventional steppers? They look impressive but if I get things sized right and the mechanics are good I’m struggling to convince myself that there will be huge benefits.

Attachment 17611Attachment 17612


I have estimated the moving masses of the carriage & cross slide to be 100Kg & 44Kg and the ball screws are 5mm pitch – I did a few quick calcs on that basis and it suggested that the steppers should be OK but comparing the curves has now led me to confusion!

Any guidance gratefully received before I start spending £s!

Hello guys, can anyone provide any guidance please? It's the servo / stepper comparison on this size of machine that I am really struggling with.

Thanks, Pete

Hi Pete,

The Video you watched will have been one I made.?
Believe it or not 2 x those motors are now fitted to Harrison M300. They are well OTT to be honest for this machine but lad was in hurry and I wasn't using them at time so let him have 2.

Don't pay too much attention to the speed curves of most steppers because they are done in unrealistic conditions and you will not get 3000rpm from them with 70Vdc. In fact you'll be lucky to get 1500rpm and have any usable torque.
The larger Closed loop motors are still 2 phase motors unlike there better smaller versions which are 3 phase and much smoother and stronger for there size.
Affectively they are just oridinery steppers with an encoder on the back connected to advanced drive which closes the loop much like a servo does. They are better than standard stepper system but not much more powerful and still have the same characteristics in that they start losing torque soon as they turn.

Servo's on the other hand give there rated torque across there full Rpm range and will give peak 3x that for several secs. They also spin much faster and are much smoother in operation. Esp at slow speeds.

Now for your needs you'll get away with 400W servos which are rated 1.27Nm continous and 3nm peak. But Personaly for the small price diffference I'd go for 750w which are rated 2.3nm Cont and 7nm Peak and give plenty of spare power.

The only small down side to Servo's is they require much higher pulse rate than steppers to get there full speed/resolution so you'll need an external motion control card or if using Linux Cnc Mesa card. This however is no big deal because the parallel port is dead and dieing technology so I'd advise you move away from it.
To be honest I'd also advise you to consider Linux cnc rather than Mach3 because Mach isn't great at Lathe. Esp threading.

I'll PM you my number and have chat if you like. (Not tonight thou it's gone 12pm . . Lol)

Hello I got 2 motors you saw in the video on my Harrison m300 and they work fine . Its a bit tight under the cross slide and you have to grind a little of the lead screw carrier but not a lot. They wire up and set up really easy and work just like that . Pulse maths are a little difficult because of the the gearing . I resorted to a readout temporarily fitted to fine tune the numbers. and it works . If you use these motors I will let you have the pulse setting's.