RE: [CAD_CAM_EDM_DRO] High Speed Machining Look ahead EMC

Hi Byron

I can give you some experiences.

I am using full servo EMC on a large machine that is used mostly
for wood and plastics but is also metal capable.

The typical cutting speed I use of 120 ipm with max 600 ipm rapids
(ballscrew critical speed limited) qualifies for high speed machining I
think. I go much slower on metals due to spindle hp limits. I run at 2000
servo updates/sec, and the first significant mechanical system pole pair
occurs at about omega= 120 or about 20 hz.

The machine is mechanically capable of a little over 1 g acceleration
(400 in/sec^2).

At Bridgeport-type cutting speeds EMC results in utterly smooth motion
as you say.

At higher speeds however many cam programs that are in the price range
of hobbyists and small shops do not do a particularly good job of
things like circular interpolation and generate curves with long series
of G1 commands.

In my case I often get quite jerky motion on paths that really could
be nice smooth curves.

Oh I can certainly turn down the MAX_ACCELERATION parameter, but that
is not really the answer. I need to have rather sharp corners, and the
minimum corner radius in a hard G64 constant contouring 90 degree turn is
velocity^2/ acceleration. So I run at a MAX_ACCELERATION of about 30 which
is probably ten times higher that one might use on a Bridgeport.

Does EMC have look ahead? Well, it's supposed to. It has a cubic
interpolator which should require a 4 position look ahead. Paul
Corner and I had a look at the source code and I can't really say
I know exactly what it is doing. There are many different ways to
interpolate and it may be optimized for relatively low speeds.

Sadly I could not attend the NAMES (recovering from some surgery
right now) but I asked Paul to bring it up, and I think from the
recently posted synopsis it was.

It could be argued that cutting smoothness is a cam program issue
rather than a machine controller one. But given that many cam programs
that we can afford generate fairly crude machine code it would be nice
to have some options in EMC.

Some external control via the .ini file of interpolation control
points and basis functions would be nice. In other words have the
option of controlling the smoothness parameters- and perhaps not smooth
certain hard corners beyond a certain trajectory change angle. It would
be nice.

This I think is a trajectory planning issue, not a servo loop one.
Note that EMC does circular motion G2/G3 commands totally smoothly
at any speed. Trajectory planning can be causal however; since the
machine code is known it can be done independently of servo calcs,
and perhaps is.

For the time being I will watch the machine sometimes stutter around smooth
corners sounding like a machine gun even though it could be perfectly
smooth. The machine can do it. It probably puts unnecessary
wear on some of the components though.

Although accused sometimes of writing c code I am mostly a math guy
and could easily generate some equations for what I have mentioned.
It's not complicated. I'm sure others on the list could do this as well.
I do need to know what's going on in the interpolator source code though.

Les

L M Watts Furniture
http://www.alltel.net/~leswatts/shop.html

-----Original Message-----
From: byron@... [mailto:byron@...]
Sent: Saturday, May 03, 2003 2:23 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] High Speed Machining Look ahead EMC



To All,

Today most conventional CNC controls cannot machine 3D molded and sculpted
surfaces [well] because
of the lack of 'look ahead' which helps produce a uniform contour with a
feedrate approaching what you
have programmed from your CAM software.

Where does EMC stand with regard to High Speed Machining and look ahead
contouring in 3D machining ?

I know EMC does a great job with conventional CNC Milling drilling etc.

Look forward to hearing from your experiences


Byron Damania







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