Re: Re: High Speed Machining Look ahead EMC

> Date: Tue, 6 May 2003 09:23:49 -0400
> From: "Les Watts" <leswatts@...>
>Subject: Re: High Speed Machining Look ahead EMC
>
>Jon, Keith, Fred, Byron,
>
>I wanted to study this a little before I answered.
>
>I think it really gets back to the old problem of cubic interpolation
>that we have discussed much in the past.
>
>Smarter programs like Fred S. mentioned can help, but I think
>the real issue is the discontinuous acceleration of the cubic.
>

The real problem is that you have a huge massive assembly that
cannot physically make a 90 degree turn at 150 ipm. If you slow
down to 25 ipm just before you get there (JIT-just in time) the
cornering will be more accurate, and the machine will take the
corner smoothly, within the limits of the strength of the frame,
mass of the assy, and acceleration required.

How far in front of the corner is a matter for experimentation, and
will probably vary with the weight of your tool, depth of cut, material
type, and desired max feed rate. In short, it is situational, and not
totally dependent on the controller capabilities. 2 axis cutting will
be different from 3 D.

Aluminum and wood will yield widely varying sets of capabilities.

>EMC is just doing what it's told as many even very high end
>cam programs are going to generate a lot of G1 moves for
>curves. An optimizer (circular interpolator) would help loads, but
>it still will have hard corners- just fewer of them.

The trajectory planning should be independent of circular or linear
interpolation. The only reason to use arc fitting is to reduce program
size for drip feeding into old controllers with limited memory. The
resulting cut from either technique should be identical within the
arc-chordal error tolerances

>Any corner
>at any angle (even shallow ones) will cause a command to
>go to MAX_ ACCELERATION... Paul and I saw this clearly
>enough on the scope. Only the pulse width varies. And that's
>as it should be... the program is just doing what is commanded.
>

The effects are reduced, if your feed rate is less as the corner is approached.

>For me typical speeds one might use to cut metal on a vertical
>mill result in very smooth motion. Here's the problem- the
>v^2/r accel requirements for a corner. At 10 in/min things are
>nice and smooth, but go to 100 in/min with the same corner radius
>and the accel at corners goes up 100 times!!

That is what Vector HSM option does. It also controls a distance
past the corner at which the slower feed rate is maintained to assure
that the controller does not try to reaccelerate too fast on the back side.

Best Regards, Fred Smith- IMService

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