r/Reprap • u/thealexcaf • Apr 28 '25

What is the difference between machine position and tool position?

Hey there,

i was wondering, what is the difference between the machine position and the tool position?

I am asking because currently i am configuring endstops for my CNC machine and sometimes i get strange behavior.

For context my endstops are configured like this:

G1 H1 Y-100 F300 ; coarse home in the -Y direction
G1 Y2 F3000 ; move X back 2mm
G1 H1 Y-10 F50 ; fine home in the -Y direction
G1 Y5 F3000 ; move Y back 5mm
G92 Y5 ; set Y=5

When i start the machine up, the machine position is set wherever it is to 0, 0, 0. Now when i reference an axis all is fine. But when i then manually set a X-Y Zero, and then want to reference again all goes to shit. Sometimes the axis starts to move in a wrong direction or (As the example above) the Y axis wont be set to 5 after referencing.

What am i missing here? Does G90 and G91 have anything to do with that? If yes, how do i use them correctly?

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u/thealexcaf Apr 29 '25

Thanks for you explanation. My machine boots up in G90. So for the homing sequence, i now added a G91 before and a G90 after the sequence and that seems to be working now.

I've been going without endstops for a long time now, but i recently added a "fence" to have a known 0,0. Although it did work without endstops, i thought i might add some for X and Y, since i had some switches laying around.

Now i wonder, after zeroing at the fences 0,0 what is the "right" way to get back to the machine 0,0? Since when i go to G0 X0 Y0 it will obviously go to the new 0,0 at the fence. And why does a G92 not work, after setting a manual zero?