Re: Z-Axis Knee Woes

Bernard,

A toolbox that only had screwdrivers in it wouldn't be much of a
toolbox.

By all means use the right tool for the right application, in this
case servos where they fit best and steppers where they fit best.

I assume the X and Y axies are much lighter and need to move much
faster than the Z axis. For those, servos make sense. A servo could
also be used on the Z axis but the large reduction ratio may make
that impractical.

If so, a stepper here makes more sense. It has way more torque at low
speeds, needs little or no reduction and is unaffected by an
overdriving load that would make a servo suffer.

Mariss


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Bernard R
<bwjarandall@c...>" <bwjarandall@c...> wrote:

> Hi Mariss,
>
> I suggested using a mix of servos and steppers in the Mach1 group a
> couple of weeks ago for exactly this problem and one person brought
> it to CCED shortly after. At the time the proposal was not well
> received, but looking at the charactaristics of both type of motors
> it seemed the logical step. Is there any practical reason not to

do

> so? the servos in question need a power supply of about 42 Volts

and

> if chosen carefully the stepper could live with this voltage. Any
> further guidance would be appreciated.
>
> Thanks
>
> Bernard
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis
> <mariss92705@y...>" <mariss92705@y...> wrote:
> > Jake,
> >
> > This may be a situation where a step motor is a better match.
> >
> > Servos develop their maximum rated power at about 80 to 85% of no-
> > load speed. The correct reduction will be the ratio of that RPM

to

> > the maximum RPM you expect to have on the jackscrew. That may be

a

> > rather large number.
> >
> > More than likely the 600 in-oz is the motor's stall torque. That
> > means the motor's continuous rated torque will be between 60 to

120

> > in-oz. The motor will overheat if you apply more than the
> continuous
> > rated torque to the motor for more than a few dozen seconds.
> >
> > The problem is not the motor's power but rather the gearing. If

the

> > motor is rated at 200W, then it would be capable of lifting a 500
> lb
> > weight at over 212 IPM vertically.
> >
> > The situation is similar to being in 5th gear instead of 1st when

a

> > red light turns green. The engine will lug and possibly stall; in
> any
> > case the getaway will not be nearly as spectacular as it would be
> in
> > 1st gear.
> >
> > On the other hand, steppers have the torque you need at low

speeds

> > with little or no reduction.
> >
> > Mariss
> >
> > --- In CAD_CAM_EDM_DRO@yahoogroups.com, "jbordens <jake@a...>"
> > <jake@a...> wrote:
> > > Hi folks. I'm retrofitting a G3102 Grizzly Knee Mill, a

smaller

> > type
> > > mill somewhere bigger than a drill mill, but smaller than a full
> > > bridgeport.
> > >
> > > I just finished my Z-axis, after success with X and Y. I

decided

> to
> > > motorize the knee for various reasons. I'm using Camtronics

600-

> in-
> > oz
> > > motors and a 2:1 ratio on the pulleys.
> > >
> > > The knee is driven by a shaft that is at 90-degrees to a
> jackscrew.
> > > The shaft is connected to the jackscrew via a set of helical

spur

> > gears.
> > >
> > > Here are my problems:
> > >
> > > (1) the motors aren't powerful enough, apparently, to raise the
> > knee.
> > > They stall and eventually cause my geckos to fault. I'm

pretty

> > sure
> > > I have the "limit" set to Max on the geckos.
> > >
> > > (2) when lowering the knee, the motor turns and every now again
> > > there's a 'thump' from the mill, as if the knee is not lowering
> > > smoothly, but in little increments.
> > >
> > >
> > > I guess I have two options:
> > >
> > > (a) go to a higher ratio, but 2:1 was the most practical. 3:1

is

> > > possible but expensive, and anything more is really not

possible

> due
> > > to pulley size constraints.
> > >
> > > (b) counterweight the knee. I think this is complicated and
> > probably
> > > not a good idea.
> > >
> > > In either case I'm not sure it would solve problem #2. I'm
> > currently
> > > at a loss on how to proceed. Any suggestions are greatly
> > apprecaited.
> > >
> > > Thanks,
> > > Jake