Try it out. Write some gcode such as
G28 ;home
G1 Z10 ;raise nozzle for safe movement
G1 X0 Y0 F2000 ; GO TO ORIGIN
G1 X0 Y100 F5000 ; MOVE FAST TO 1ST POSITION
G1 X100 Y100 F5000 ;POSITION 2
G1 X0 Y100 F5000 ;POSITION 3
G1 X0 Y0 F5000 ; BACK TO POSITION 1
By all means substitute 100 for a larger number if your machine is larger, longer moves demonstrate acceleration better.
Copy and paste lines 4-7 about 20-30 times then run it.
You'll see it move around the square over and over.
Now you can send M201 XA, YA (where A is your new acceleration value) and see what happens, using low acceleration will make the acceleration more obvious.
Try a few different settings
Now send M205 XB (where B is your new XY jerk value) and see what happens. M566 on RRF firmware.
Since most firmware's have look ahead planning, which reads the next few (6-10) moves and then decides whether they can flow into one another it will not stop at each corner of the square, I'd expect it to slow to the jerk speed and then change direction then accelerate. It would be interesting to see what happens with a Jerk value of zero, will it decelerate to a momentary dead stop then accelerate away on the next move? It should do.
Jerk is just a way of saving processing time for the small detailed moves, its really asking you how fast are you happy for your nozzle to be flung about on small moves depending on your printer's rigidity/motors etc...
Its also worth saying when you issue a move which has to be coordinated, i.e.
G1 X100 Y100 Z50 E0.45 so you are moving 4 axes at the same time, the axis with the lowest Jerk/Accel values determines the behaviour of the move, there's no point flinging the nozzle rapidly 4 cm in a direction whilst the extruder is still trying to accelerate to extrude sufficient filament.
To answer your last question yes it should change speed without decelerating, if this was a travel move it wouldn't have any consequence, but if you are also simultaneously extruding, given that extruder jerk/accel are usually way lower than X and Y, the lower extruder jerk/accel setting presuming its below 200 in your example, would force a degree of deceleration on the x axis move. Just as well or you'd get an unacceptable variation in extruded width during the transition.
Edited 2 time(s). Last edit at 08/24/2017 05:25AM by DjDemonD.
Simon Khoury
Co-founder of [www.precisionpiezo.co.uk] Accurate, repeatable, versatile Z-Probes
Published:Inventions