Re: Z-Axis Knee Woes

If you want the definitive answer look at
<ahref="http://www.motionvillage.com/welcome_center/articles/Inertia.P
DF"> The bottom line is that Gear Ratio = Sqrt(Load Inertia / Motor
Inertia). Using the 75 lb/in load that Les quoted and 0.012 lb/ in
from a larger SEM motor, peak torque 1408 in oz, that works out at a
ratio of 80:1 if I got all my units and sums right.

I do know that using one of the 600 oz in motors on a 9x42" mill with
a 4:1 reduction, Z is slooowwww. The problem is the motor just can't
get up to the speed required to develop sufficient torque, i.e. half
rated speed equals max power.

There have been a couple of people on the list who have used servos
successfully on the knee, but from all the responses there are a lot
more who haven't, unless they are using ballscrews, gas struts, or
some form of counter balance.

IMO Mariss has it right, use a big enough stepper on the knee and you
don't have to worry about big gear ratios and a lot of other problems.

Bernard

sorry for the repost - something went wrong somewhere.

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Bernard R > --- In
CAD_CAM_EDM_DRO@yahoogroups.com, "Dan Mauch" <dmauch@s...>

> wrote:
> > I have one of those 600 oz in motors on my 8X36 knee mill. I am
> using a
> > 2.5/1 ratio timing pulley. I would recommend a 3/1 ratio. I can
> raise
> > and lower my knee with a #105 mill vice on it all day long

without

> the
> > motor getting warm and at good speed. The G320's never fault

except

> when
> > I run the knee into the stops. (AAAARRRGGGHH I hate it when I do
> that)
> > The trick is make sure you have a 3/1 and I also put a lower

encoder

> > count onto that motor so that I would not have a larger count on
> the Z
> > axis. I ended up using a 60X037 pulley on the handshaft and a
> 24XL037
> > pulley on the motor. These are not that expensive! But I would
> recommend
> > a full 3/1 ratio and possibly a 4/1.
> > Dan
> >
> >
> >
> > -----Original Message-----
> > From: jbordens <jake@a...> [mailto:jake@a...]
> > Sent: Monday, January 13, 2003 4:59 PM
> > To: CAD_CAM_EDM_DRO@yahoogroups.com
> > Subject: [CAD_CAM_EDM_DRO] Z-Axis Knee Woes
> >
> >
> > 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
> >
> >
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