Same idea, but we have a little different coordinate system to work with on a turning center or CNC lathe. It’s really hard to know a safe intermediate point in absolute mode, whereas it’s pretty easy to use the relative mode with X0 Y0 Zn to make a safe move to an intermediate position that’s straight up from the current tool position. In absolute mode, the intermediate point is relative to part zero. You’ll notice the examples started with a G91 to put the machine in relative mode. This is much less likely to hit some errant fixture or part of the workpiece than just commanding a move straight to the reference position. This variation is telling G28 that the intermediate position is 4″ above the current tool position, hence the machine moves up 4″ before heading straight from there to the reference position. Move straight a distance, then on to the Reference position Since it thinks it is already at the intermediate position, this version essentially causes the intermediate position to be ignored and we get coordinated motion straight to the reference position. In this case, we’re telling the machine the incremental position is at 0, 0, 0 relative to the tool’s position. Let’s try some examples: Move Straight to the Reference Position
In relative mode, the coordinates are relative to the tool’s current position. If the machine is in absolute mode, those position coordinates are relative to program zero (part zero). The intermediate position is specified using one or more position words (X, Y, or Z) after the G28. Specifying the G28 Intermediate Position on Mills On many machines, if you have Single Block on, you push the Cycle Start twice–once to go to the intermediate position and once to finish at the reference position. The movement to the reference position is done at rapids (G0) speed,and the intermediate position is used to ensure there are no collisions along the way. Typically, G28 allows the movement to be done via an intermediate position. That zero return position is where most progams begin, most machines will go to this position when you manually home or reference the machine, and it is the reference or zero position for calculating fixture offsets for mills and geometry offsets for lathes. It’s function is to return to the machine’s reference position, sometimes called the zero position. G28 is one of those odd g-codes that you don’t use very often, but when you need it, it’s pretty darned handy. G28 G-Code: CNC Return to Reference CNCCookbook’s G-Code Tutorial What does G28 Return to Reference Position Do?
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