Re: [CAD_CAM_EDM_DRO] Re: Z-Axis Knee Woes

That brings up the subject of inertia matching....

On very high speed, high acceleration systems the load inertia
and the motor inertia is often matched. For example my large gantry
is inertia matched in the x - axis. Amazingly the 1000+ lb gantry system
has about the same equivalent inertia as the motor armature.
The SEM motors are about 0.18 in oz s^2 and the
gantry / twin ballscrew/ jackshaft equivalent is about 0.2 in oz s^2.

I will forgo the technical subject of inertia and mechanical impedance
matching here as there is ample very detailed technical information
out there for the searching. Also, it is certainly not required on many
systems. For vertical mill knees the primary issue is not inertia ...
it is just plain old raw torque and getting that at an ample speed.
Most of the load is frictional, not inertial (unless you use ballscrews).

Les

Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.alltel.net/~leswatts/wattsfurniturewp.html
engineering page:
http://www.alltel.net/~leswatts/shop.html
Surplus cnc for sale:
http://www.alltel.net/~leswatts/forsale.html

----- Original Message -----
From: <bwjarandall@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Tuesday, January 14, 2003 9:18 PM
Subject: [CAD_CAM_EDM_DRO] 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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> OFF Topic POSTS: General Machining
> If you wish to post on unlimited OT subjects goto:
aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if
you have trouble.
> http://www.metalworking.com/news_servers.html
>
> http://groups.yahoo.com/group/jobshophomeshop I consider this to be a
sister site to the CCED group, as many of the same members are there, for OT
subjects, that are not allowed on the CCED list.
>
> NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM.
DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
> bill
> List Mom
> List Owner
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
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>