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

Several people have reported that it takes 80 oz/in just to move the
X or Y axis on a well set up minimill with stock leadscrews and ball
bearing screw end mounts. This can be determined by adjusting and
oiling everything and then wrapping some cord around the 4"
handwheel and adding weights until the axis seems to move reliably.

With this in mind, I'd gear down the servos. I used 420 oz/in
steppers direct drive on all 3 axis of mine and it works fine, but
the steppers peak out RPM wise at about 40 IPM rapids.

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--- In CAD_CAM_EDM_DRO@yahoogroups.com, "grantfair2001"
<grant.fair@s...> 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. If I'm not

mistaking

> people use 600 oz steppers on these size machines (better confirm
> this!). And steppers are rated in terms of holding torque, as in

when

> stopped. The actual torque is much lower.
>
> If you were to use these motors with say a 4:1 reduction you would
> have 220 oz-in continious at the screw with a peak torque of 1680
> oz-in. I think this would be more then sufficient. These servos
> running from geckos (assume 70v supply) will run at 4000 RPM and

still

> provide full torque. You could run them at 5000 RPM or possibly

even

> higher with a slight drop in torque. Should be fine for rapids.

Lead

> of these machines is 0.0625 right? Assuming 4000 RPM and 4:1

reduction

> (lets be conservative) this would give you a rapid speed of 62.5

IPM.

> 3:1 reduction would give 83 IPM rapids."
>
> Another source who has experience with CNCing the minimills

suggested

> I could direct couple these servos for the X and Y axes. He drives
> these axes with 150 oz-in steppers.
>
> What do listmembers think? I have no experience with servos and

need

> some sound advice.
>
> Grant