Re: servo or stepper size selection

With 20/20 hindsight having just gone through the exercise of
converting a BP look alike using servos, if I were to do it again I
think I would still use servos for X&Y but would use a stepper for
Z. Knee mills don't move very fast in the Z axis but require a lot
of torque. (My mill required 56lb/in, 784oz/in to start moving.)
Steppers have high torque at low speeds which drops off at the higher
speeds, servos on the other hand have very little low end torque and
require to move fast to bring it up, this is fine for X&Y.

A lot of the hobby people use Mariss's magic 3xx boxes to drive
servos as they can then use Step/Dir; I think you're into much more
serious $$ if you are using closed loop.

Bernard

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "ftomazz <filipetomaz@p...>"
<filipetomaz@p...> wrote:

> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "sparkness2001 <mark@c...>"
> <mark@c...> wrote:
> > I have an old mill (1965 index)in great shape it is 9 x 40 and

its

> > lead screws are 1.125 dia. .200 lead i am planning to change the
> lead
> > screws to ball and cnc of some sort. I have a pc for the job now
> im
> > looking for best or should i say reasonable way to go, servo or
> > stepper ive been on ebay and looked at many sites for home cnc. I
> > want to have an accurate and reasonable to program cnc setup
> > any help would be appreciated. thanks --mark
>
> I'm building now a CNC machine with servo motors. I've had also the
> same question, and also did the same question. There are two

aspects

> to consider: 1 - the price and 2 - the performance.
>
> Servo motors can have more torque at greatest speeds, while

steppers

> don't (check the graphics of speed vs torque). Also, steppers are
> told to do a step, for example, and without feedback, if it was not
> done (for several reasons), then you lost him - servos are used in

a

> closed loop control, with enconders. But if the steppers are good,
> and if enough torque is avaiable for what you are moving then it
> seems ok to go for steppers.
>
> Servos are more expensive, but I think that they compensate the

price.

> If you have questions, then look to some industrial machines and

you

> will see that the serious ones use servo motors. This was one of

the

> main influences for me.
>
> I'm still learning - so don't take for sure my info. You can still
> ask again to someone else.
>
> If you are living in Europe, I'm looking for a couple of persons to
> buy servos from the USA, for less expenses.
>
> Filipe Tomaz