Re: Servo controllers for table top machines.

> I was under the impression I could just bolt on the brush type servo
> motors directly to the ball screw end, similar to the way my stepper
> motors are connected currently.

No... You can't (unless you're looking to burn your brand-new
servomotor down in 5 minutes or less).

Rule #1: Servomotors are not step motors.

--------------------------
Typical NEMA-34 step motor: Develops 700 in-oz of torque out to 500
RPM for a power output of 260 Watts. The motor gets too hot to touch
(perfectly normal) at 7A and 80VDC.

Characteristics: This step motor's torque beyond 500 RPM is the
inverse of speed:

1,000 RPM @ 350 in-oz
2,000 RPM @ 175 in-oz
3,000 RPM @ 117 in-oz
etc.

--------------------------
Typical NEMA-34 servomotor: Develops 120 in-oz of torque at 5,000 RPM
for a power output of 445 Watts. The motor is very warm (a hot motor
is a motor in trouble) while drawing 7A at 80VDC.

Characteristics: The 120 in-oz torque is a limit. Go beyond that for a
prolonged period of time and you burn the motor down.

0001 RPM @ 120 in-oz
1000 RPM @ 120 in-oz
3000 RPM @ 120 in-oz
5000 RPM @ 120 in-oz (limit, cant't go any faster with this motor)

---------------------------

OK. So what can you take away from this?

1) NEMA-34 Servomotor has a lot more power.
2) NEMA-34 Servomotor has little torque (equal to a small 23 stepper)
3) Servomotors go real fast, steppers have lots of torque.

1) Power gets things done. 445W gets things done quicker than 225W.

2) Servos need to be geared down to match most loads. Power is torque
times speed. Trade in uneeded speed for badly needed torque. The motor
MUST be geared so that it's turning at 80% of it's no-load speed when
your mechanism is moving at its max design speed.

3) Steppers need little or no gearing to match the load. The above
servo MUST be geared 5.833:1 (700 in-oz/120 in-oz) to get the step
motor's 700 in-oz. Let's see what this means:

Geared 5.833:1, the servomotor's reduced output RPM becomes 857 RPM.
Let's say the application is a mill and you have a 5 TPI screw.

Step motor max IPM at full torque is 100 IPM.
Servomotor max IPM (period) is 171 IPM. It can't go faster.

Advantage to servomotor.

But machining involves relatively low feed-rates at high torque and
low-torque high-speed rapids. Rapids require only enough torque from
the motor to overcome machine friction and nothing else. Let's say
your machine has 100 in-oz of friction and see how things turn out now:

Servomotor max IPM (period) is 171 IPM because it can't go faster.
Step motor max IPM is 700 IPM!

Advantage to stepper.

The reason is the stepper's inverse speed-torque curve is ideally
suited to mixed-mode big torque @ low speed, little torque @ high
speed applications. Don't count steppers out.:-)

Mariss

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "enytned1" <alexkcpr@...> wrote:
>
> "It trades speed you can't use for torque you can." What??????
>
> I was under the impression I could just bolt on the brush type servo
> motors directly to the ball screw end, similar to the way my stepper
> motors are connected currently.
> How do DC-servo motors with 500CPR encoders with Gecko 320drives
> compare to a micro stepping motor and driver in terms of
> repeatability, precision etc?
>
> The old techno controller I use was originally setup for full step
> mode I have changed this to half step mode and have seen an immediate
> improvement. How reliable are these controllers you mentioned? I
> know I can get a 4-axis upgrade from Techno for about $3500.00 but I'm
> not sure if I can convert it to run of Mach3?
>