Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: mikrokontrola on April 24, 2008, 06:21:14 AM

Hi,
After updating to 3.039 version (the previous was 2.63) and running below program, some feed rates are different then programmed. Before updating was OK.
My max feedrate (G0 moves) is 1800 mm/min. FRO is 100%.
Here is my program:
G17 (XY plane select)
G21 (mm unit)
G40 (Cancel cutter radius compensation)
G49 (Cancel tool lenght offset)
G90 (Absolute distance mode)
G94 (Feed per minute mode)
G0 Z50
G0 X0 Y0
G0 Y50
F200
G0 Z10
G1 Z0
F1500
G2 I50 (max feed 848, sholud be 1500)
G0 Z50
G0 X150 Y100 (max feed 1897, sholud be 1800)
G0 Z10
F200
G1 Z0
F1500
G1 X200 Y0
G1 X100
G1 x150 Y100
G0 Z50
G0 X0 y0 (max feed 2163, should be 1800)
M30
I've tried it on 2 PCs (633MHz and 2.4 GHz), the same behaviour.
Rgds

The figures your are quoting for the speeds are axis speeds  not straight line distance speeds. Your X axis travels at 180 and your Y axis travels at 180.
If you look at your quoted figures, your cutter travels from pos X0Y50 to X150Y100 (X moves 150, Y moves50). Your compound speed  if you travel on both axis at 180 units per minute simultaneously is 1897.736659610 plus another 10 digits or so, which is what you were quoting. Actually, the time for the move from one position to the other would be just the same as the straight line move from X0 to X150.
This will be true for your other G0 move.
As for the G2 move, the F command will apply to the centre of the cutter  not the circumference  so therefore PI will will come into it somewhere.

jimpinder,
Thank you very much for your fast answer and explanation.
Now, if I understood, if I travel with G0 on both axis then the 'longer' axes travels on max speed, and the 'shorter' travels on max speed mutilplied by shorter/longer ratio.
So the DRO's showing the actual speed always will show something more than max speed, if i move with G0 on both axes. In my previous version (2.63) the actual speed for G0 moves was always equal max speed, even for both axes travels. But I think that now is better.
Regarding circles there is something strange. If the LookAhead is set to 1 line, then the DRO with actual speed show 1500 (after acceleration period). For LookAhead 2 and more it shows 848. Is it normal ?
Regarding

Yes  you are quite right. On double axis moves, the axis with the longer travel will travel at the maximum set in motor tuning. The other axis will travel at whatever speed is necessary so the two finish simultaneously. It is the max speed times the tangent of the angle the cutter is describing.
I must admit I have no idea why the difference occurs on circles  with look ahead. One possible explanation is that once it has calculated the line, it stops calculating until that line is executed. With look ahead, with Constant Velocity on, the system is always calculating so that it can accelerate or decelerate the axis so the transition to the next line is smooth.
As a check try doing the circle cut again, once with the machine set to CV and again with it set to Absolute Stop and see if there is any difference.