re:Re: emc really needs (3 axis vs. 5 axis)
Posted by
ballendo@y...
on 2000-11-20 15:44:00 UTC
Rab writes:
Output of 3 axis cartesian coordinates is trivial, since so much can
be assumed. (90 degrees between planes, Linear motion, independant
axis motion)
Adding a 4th axis can also be trivial, IF it is a linear axis
parallel to one of the original three. Adding a 4th rotary is simple,
if not trivial.
When you add the 5th axis you now have motions "compounded" onto
other axis/axes motion. It is no longer trivial! Imagine cutting a
spherical surface with a FLAT bottom endmill (the end mill axis will
ALWAYS point to the center of the sphere). This means the "tilting"
of the cutter, in two directions, will have to be "compensated for"
by the movements of the x, Y, and Z. And this compensation will be
nearly continuously varying! And this spherical surface is "easy"
compared to some compound-curved mold surfaces!
This is a LOT more "math" than simply moving x,y, and z to the points
of the spherical surface, perhaps offset by some amount.(using a ball-
end cutter)
You also have the requirement(s) of defining the motion(s) of the
additional axes (linear or rotary?, radius of movement?, is the axis
radius fixed or variable?) Also called kinematics. (kinematics is the
mathematical relationship of the axes to each other and the world in
which they move)
It CAN be done and IS done for the "hexapod" version of EMC. Also
most industrial robots.
I should also mention that it is easier to DEFINE the shape needed
(as CAD pkgs do) than it is to DERIVE THE MOTIONS needed to DESCRIBE
(cut) the shape! (which is why the "good" CAM s/w costs so much)
For your second question, Yes. Additional axes can often use standard
Gcode. Standard Gcode supports up to 12 axes:
1st linear set: X,Y,Z
1st rotary set: A,B,C
2nd linear set: U,V,W
3rd linear set: P,Q,R
In practice, the letters P,Q, and R (words, in G code) are used for
other things, leaving 9 axis capability. M codes are sometimes used
to "activate" an additional axis. The non-standard use of some
arbitrary G code may also be used.
NIST EMC and as such, ASSUMES that EMC WILL be "driving" the machine.
Which eliminates the need for several "posts" (post-processors).
Hope this helps.
Ballendo
>Also, as a matter of interest, how much more difficult is it toSimple answer, A LOT!
>output 4 and 5-axis toolpaths than 3, and why?
>Can G code be used regardless of number of axis ?
Output of 3 axis cartesian coordinates is trivial, since so much can
be assumed. (90 degrees between planes, Linear motion, independant
axis motion)
Adding a 4th axis can also be trivial, IF it is a linear axis
parallel to one of the original three. Adding a 4th rotary is simple,
if not trivial.
When you add the 5th axis you now have motions "compounded" onto
other axis/axes motion. It is no longer trivial! Imagine cutting a
spherical surface with a FLAT bottom endmill (the end mill axis will
ALWAYS point to the center of the sphere). This means the "tilting"
of the cutter, in two directions, will have to be "compensated for"
by the movements of the x, Y, and Z. And this compensation will be
nearly continuously varying! And this spherical surface is "easy"
compared to some compound-curved mold surfaces!
This is a LOT more "math" than simply moving x,y, and z to the points
of the spherical surface, perhaps offset by some amount.(using a ball-
end cutter)
You also have the requirement(s) of defining the motion(s) of the
additional axes (linear or rotary?, radius of movement?, is the axis
radius fixed or variable?) Also called kinematics. (kinematics is the
mathematical relationship of the axes to each other and the world in
which they move)
It CAN be done and IS done for the "hexapod" version of EMC. Also
most industrial robots.
I should also mention that it is easier to DEFINE the shape needed
(as CAD pkgs do) than it is to DERIVE THE MOTIONS needed to DESCRIBE
(cut) the shape! (which is why the "good" CAM s/w costs so much)
For your second question, Yes. Additional axes can often use standard
Gcode. Standard Gcode supports up to 12 axes:
1st linear set: X,Y,Z
1st rotary set: A,B,C
2nd linear set: U,V,W
3rd linear set: P,Q,R
In practice, the letters P,Q, and R (words, in G code) are used for
other things, leaving 9 axis capability. M codes are sometimes used
to "activate" an additional axis. The non-standard use of some
arbitrary G code may also be used.
>Would post-processors have to be written for each machine to be usedI believe that Terry's original idea for this program is BASED on the
>with the program ? >Rab
NIST EMC and as such, ASSUMES that EMC WILL be "driving" the machine.
Which eliminates the need for several "posts" (post-processors).
Hope this helps.
Ballendo