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
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-----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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