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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> OFF Topic POSTS: General Machining
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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.
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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........
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> List Mom
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