Re: [CAD_CAM_EDM_DRO] Oops wrong details - Rotor inertia.

Oooops, gave you the wrong specs.

The motor is as follows.

7.8nm cont
23.3 peak
Inertia is 13.21kgcm^2

----- Original Message -----
From: Jason Cox <sinergy@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Friday, October 31, 2003 2:15 AM
Subject: Re: [CAD_CAM_EDM_DRO] Rotor inertia.


> Les,
>
> I was under the impression that the power of the motor wuld over come the
> rotor inertia. Or is the inertial rating given taken this into account
> already.
>
> The mill im working on is similar to a Haas mini mill but about 30%
larger,
> it runs on linear bearings. Uses 32mm 5mm lead ballscrews, the x table
> weight is around 100kg and y axis moves around 220kg.
>
> The motors I have selected are as follows:
>
> 12nm cont. stall
> 35.25nm peak
> Rotor inertia is 43.61kgcm^2
>
> They will be directly coupled, and I would like to run at 600-700ipm.
>
> Am I on the right track, or are these motors overkill and with too much
> inertia?
>
> Thanks again, I appreciate your help.
>
> Regards,
>
> Jason Cox.
>
>
> ----- Original Message -----
> From: Leslie M. Watts <leswatts@...>
> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> Sent: Friday, October 31, 2003 1:31 PM
> Subject: RE: [CAD_CAM_EDM_DRO] Rotor inertia.
>
>
> > Hi again Jason
> >
> > Yes you need to consider it some... otherwise you might pay too
> > much or suffer poor performance.
> >
> > Acceleration is important because the minimum corner radius
> > = velocity^2/acceleration.
> >
> > What kind of machine are you considering?
> >
> > A bridgeport x axis works well with about a 0.2 in oz sec^2
> > polar moment of inertia motor for example. That is considered
> > a medium inertia motor.
> >
> > Les
> >
> > Leslie M.Watts
> > L M Watts Furniture
> > Tiger Georgia USA
> > (706) 212-0242
> > http://www.lmwatts.com
> > Engineering page:
> > http://www.lmwatts.com/shop.html
> > CNC surplus for sale:
> > http://www.lmwatts.com/forsale.html
> > CNC carved signs:
> > http://www.lmwatts.com/signwp.html
> >
> >
> >
> >
> > -----Original Message-----
> > From: Jason Cox [mailto:sinergy@...]
> > Sent: Thursday, October 30, 2003 9:38 AM
> > To: CAD_CAM_EDM_DRO@yahoogroups.com
> > Subject: Re: [CAD_CAM_EDM_DRO] Rotor inertia.
> >
> >
> > Hi Les,
> >
> > Thanks to you and Mariss for such detailed replies.
> >
> > That does help me to understand things much better... But do I really
need
> > to take this into account when selecting servo's for a milling machine?
I
> > mean a motor with a higer moment of inertia will not affect accuracy
just
> > acceleration and deceleration times? Is that right...
> >
> > Surely matching the inertias must be difficult, what happens when you
add
> a
> > large load to your table?
> >
> > Thankyou again,
> >
> > Jason Cox.
> >
> >
> > ----- Original Message -----
> > From: Leslie M. Watts <leswatts@...>
> > To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> > Sent: Friday, October 31, 2003 12:32 PM
> > Subject: RE: [CAD_CAM_EDM_DRO] Rotor inertia.
> >
> >
> > > Hi Jason
> > >
> > > I'll give a stab at an answer for you.
> > >
> > > Rotor inertia is simply a measure of the "flywheel
> > > effect" of a spinning mass. In a cnc this is usually
> > > transformed into linear motion and then becomes the
> > > effect of a moving mass tending to stay in motion
> > > unless acted on by an external force.
> > >
> > > With rotating things inertia is actually measured
> > > as a moment of inertia so you will see that term a lot.
> > >
> > > Now classically it was desirable to match rotor moment
> > > of inertia with the load equivalent moment of inertia.
> > > This came from transmission line theory.
> > > You will find it often called impedance matching,
> > > but the term refers to mechanical impedance- not electrical.
> > > Inertia is a portion of that term.
> > >
> > >
> > > When impedances are matched maximum power can be delivered.
> > > A certain length and weight of baseball bat can transfer the maximum
> > > energy to a ball... but the regulation range bats have a mismatched
> higher
> > > mechanical impedance than the ball. That is one reason some would like
> to
> > > hollow out and "cork" the bat.
> > >
> > > Nowdays with modern servos and amps it is a little less critical
though.
> > >
> > > I will give an example of my latest gantry machine. It happens to
> > > be inertia matched. The rotor inertia of the NEMA 56 size servo
> > > is about the same as the load- about 500 kg of mass and some
ballscrews.
> > > The equivalent moment of inertia of that is transformed considering
> > > things like ballscrew pitch etc.
> > >
> > > But precise matching is not so important as I mentioned. If the rotor
> > > inertia predominates the motor will determine how fast the system
> > > will accelerate. If the load predominates _it_ will... by loading down
> the
> > > motor.
> > >
> > > Effective inertia can be changed by gear ratios and the like.
> > >
> > > High torque to moment of inertia ratio motors generally cost
> > > more for a given rpm... it's kind of like 0 to 60 mph time in a car.
> Light
> > > but powerful.
> > >
> > > In rotating things torque = moment of inertia x angular acceleration
> > >
> > > In sraight line moving things force = mass x acceleration
> > >
> > > A good basic engineering text will have a wealth of information about
> > this
> > > as will countless internet sites.
> > >
> > > Did this help some?
> > >
> > > Les
> > >
> > > Leslie M.Watts
> > > L M Watts Furniture
> > > Tiger Georgia USA
> > > (706) 212-0242
> > > http://www.lmwatts.com
> > > Engineering page:
> > > http://www.lmwatts.com/shop.html
> > > CNC surplus for sale:
> > > http://www.lmwatts.com/forsale.html
> > > CNC carved signs:
> > > http://www.lmwatts.com/signwp.html
> > >
> > >
> > >
> > >
> > > -----Original Message-----
> > > From: Jason Cox [mailto:sinergy@...]
> > > Sent: Thursday, October 30, 2003 7:26 AM
> > > To: CAD_CAM_EDM_DRO@yahoogroups.com
> > > Subject: [CAD_CAM_EDM_DRO] Rotor inertia.
> > >
> > >
> > > Hello All,
> > >
> > > Just wondering when selecting servo motors what impact does rotor
> inertia
> > > have on the final function of a cnc machine?
> > >
> > > From my research motors with a far lower inertia are much cheaper, but
> > does
> > > this really matter. Surely the redution in inertia of the rotor would
be
> > > minimal say compared to the inertia of the machine table.
> > >
> > > Thanks for any thoughts.
> > >
> > > Regards,
> > >
> > > Jason Cox.
> > >
> > >
> > >
> > >
> > >
> > >
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OT
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> OFF Topic POSTS: General Machining
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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
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> 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........
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> List Mom
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