Re: [CAD_CAM_EDM_DRO] Servomotors & Minimill - do these need speed reduction or not?

grantfair2001 wrote:

>I want to CNC my minimill. I have Electrocraft E543 servos and Gecko
>320's. I have received conflicting advice about whether speed
>reduction (& torque increase) is necessary with these servos.
>
>A listmember who is using these servos on his Sherline? mill stated:
>
>"My Mill is much smaller than a grizzly but then again my servos are
>way overkill for my applications. I have them limited at 10a (geckos
>can put out 20) and I still have WAY more power than I need. Alot of
>people seem to be useing those huge surplus amtek (may be spelled
>wrong) servos for those size mills but I'm thinking the E543 would
>still do a good job on that size machine. Youll defenatly need a
>reduction.
>
>These motors have a Kt of 14.9oz/a and a continious current of 3.7a.
>This works out to a continious torque of 55 oz-in. This might seem a
>little low but the peak torque is 420 Oz-in and combine these specs
>with a belt reduction and the torque isnt so bad.
>

But, 420 is not applicable. With the 10 A current limit, you get a peak
torque of 149 Oz-In. Are you sure these motors can really deliver 420
Oz-In?
(Closest spec I have is for the E-542.)

I am using MUCH smaller motors from a Calcomp plotter with a 4:1 reduction.
That is plenty of power. These motors are 2" dia, 4" long. A 420 Oz-In
motor with some belt reduction would be sufficient to damage things on
a Bridgeport! I'm using motors with a Cont. rating of 73 Oz-In on my
Bridgeport,
with a 2.5:1 reduction. Using 3x that as the peak rating, I can deliver
over
1000 Lbs of linear force to the table.

Using 149 Oz-In from the motor x 4 for belt reduction, you get 596
Oz-In. Assuming 100%
efficiency of the screw, and 16 TPI, the equivalent diameter of the
screw is .00995"
596 Oz-In / .00995 = 59916 Oz of linear force, or 3745 Lbs. Assuming 50%
screw efficiency, that's still 1872 Lbs of force. I think that would
break parts
on the machine for sure!

Jon