Re: [CAD_CAM_EDM_DRO] I need some help with servo's PLZ!!!! RETROFIT

twoextrememachine wrote:

>
> I thought a 600in/oz servo with 900rpm would have enough power to run
> my Knee mill. The individual I want to purchase a drive kit from
> does not agree with me (AHEM Camtronics)

Might I add some comments:

First, it is 600 oz*in (ounces TIMES inches = torque). Second, if it is a servo,
then that 600 oz*in is most certainly PEAK, not to be occurring much more
than in about a 5% duty cycle. The CONTINUOUS rating might only be 1/5 or
1/10 of that rating, lets say 1/10 to be safe (ie 60 oz*in or the equivalent
of a <pretty small> single stack Nema23 stepper).

Contrast this to a stepper motor, where the torque rating is CONTINUOUS
(actually at stall, less torque at higher rpms). Steppers are much smaller
than servos at the same CONTINOUS torque at low rpms. Fortunately for
steppers in CNC applications, heavy cutting is at low rpms, while at higher
rpms (ie rapids) where torque falls off, high torque is less essential.

Back to servo motors...

So Dan (at Camtronics) is certainly correct if you are meaning to do heavy
machining in a direct drive (1:1) or slightly geared (2:1) or (3:1) drive
using that servo motor.

Note that gearing multiplies the torque, but decreases the speed, and
using a 3:1 gear reduction (ie 3*60 oz*in = 180 oz*in is still pretty small
potatoes for a big cnc mill) the 900 rpm = 15 rps goes down to 5 rps
on the leadscrew (ie 60 inch/min on a 5 TPI leadscrew is pretty darn
slow for rapids......).

Summary:
- the servo needs gearing down to raise the torque up, because 60 oz*in is
way too small
- 900 base rpm limits your gearing options and limits your rapids
(a 2000-6000 rpm motor would give you more options).
- the existing motor (600 oz*in peak, 900 rpm) motor can probably do
light duty machining at a 3:1 or 4:1 ratio, with limited rapid speeds.
That might be acceptable, considering the circumstances, but it will always
annoy you.
If you forced it to do heavier cuts (ie asking for higher torques
in a duty cycle of more than 5%), you might/will burn the motor up.

Doug Fortune
http://www.cncKITS.com














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