On Wed, 7 Feb 2018, Chris Albertson wrote:
Date: Wed, 7 Feb 2018 11:35:03 -0800
From: Chris Albertson <albertson.ch...@gmail.com>
Reply-To: "Enhanced Machine Controller (EMC)"
<emc-users@lists.sourceforge.net>
To: "Enhanced Machine Controller (EMC)" <emc-users@lists.sourceforge.net>
Subject: Re: [Emc-users] CSS and spindle-at-speed during a G33
The G33 says to do a synchronized move. This is physically impossible.
The Z axis has some limit on it's ability to go from zero speed to the
commanded speed. So the controller is reducing the spindle speed so as to
match the ability of the z-axis motor.
ALL multi-axis moves are this way. You see it best on a milling machine
where perhaps the z-axis is also the slowest. Say you are milling a 45
degree ramp that climbs upward to the left. The X motor would have to slow
to match the max speed of the z motor.
When a lathe cuts threads it is in real-life cutting a ramp. The spinach is
turning in x-direction the carnage in z. You can only cut as fast as the
slowest motor can move. And nether of the motors has infinite acceleration.
Spinach and carnage aside,
Actually for G33 moves LinuxCNC does not change the spindle speed but rather
gears the Z axis motion to the spindle rotation, of course at the start of a
thread the Z axis must accelerate to match the spindle rotation.
Heres some information from the LinuxCNC documentation on G33:
"Technical Info
At the beginning of each G33 pass, LinuxCNC uses the spindle speed and the
machine acceleration limits to calculate how long it will take Z to accelerate
after the index pulse, and determines how many degrees the spindle will rotate
during that time. It then adds that angle to the index position and computes
the Z position using the corrected spindle angle. That means that Z will reach
the correct position just as it finishes accelerating to the proper speed, and
can immediately begin cutting a good thread."
Note that the "gearing" from encoder count can result in noise in the Z axis
motion because of position or timing jitter or position extrapolation errors
if the encoder resolution is low.
Peter Wallace
Mesa Electronics
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