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  1. #1
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    Dumb newbie needs some help with a few Cad/Cam terms on CNC milling. This is very basic, but after searching the web I still can't find a definition that will sink thru my thick skull....

    Work Offset: the work piece offset from what? why?

    Z axis cordinate of clearance point: Have no idea what this would refer to, other than the Z axis goes up and down.

    Thanks KJ

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    KJ,

    A modern cnc machine has one machine coordinate system that is established shortly after startup. The machine travels to various limit switches to teach itself 'where it is'.

    Now, once the table has arrived at this 'home position', then the coordinate system is typically set to all zeros. Or, the machine manufacturer may also assign certain coordinates to that home position. The short of it is, that the machine coordinate system (called G53) cannot be altered after the machine is homed.

    Now, you throw a vise on the table and load a piece of stock to machine. You will typically be thinking, "ok, the datum or reference point on this part, I choose to be the top, back right corner of this stock. This is my X0Y0Z0 on this part."

    Now, what is that location in the machine coordinate system? It could be something like X12.4563 Y4.5872 Z-11.6859 Handy, eh? This is not convenient to program from, for the sake of knowing where your tool is at any given time while cutting this part.

    So, they invented work offsets, which is a new imaginary coordinate system based on a shift of position from the machine's home position, to this new datum on your part. So in simple example, you would use
    X12.4563 Y4.5872 Z-11.6859
    as the values in one of your work offsets, lets say in the G54 offset table. There are several other imaginary systems as well: G55 up to G59 and often even more than those, which you may want to use if machining several parts, at various locations on the machine table.

    The particular work offset system which holds the coordinate system shift for a given part is called in the program by name,eg.,
    G00 G54 X0 Y0
    This causes the machine to move to my example:
    X12.4563 Y4.5872 Z-11.6859
    in the G53 system (which is the real underlying system), and then reset the axis counters to zero to establish a new origin at that point, from then on. This shift stays in effect (it is modal) until such time as you call a different work offset, or, change the values in the current offset table.

    After that, all your programmed moves based on certain absolute distances from the datum on your stock become valid moves.

    You can always make a movement in the machine coordinate system by specifying G53 with the axis movements.

    There is a bit more to this topic involving how Z axis tool length offsets work within a work offset, but I won't confuse you with more details than you asked about.

    Z axis clearance is a safe distance above the part, for the tool to move around in X and Y at rapid speed, without fear of collision with the part or fixture.

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    Work Offset: from your defined zero at work(piece) to your tool.
    It is common to use the point of your work wich is most near your tools as Zero most of the time.

    Z axis cordinate of clearance point: the point where you let your tool stop after a rapid, you wont rapid to Zero, you set a clearance coordinate(point).

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    Wow Huflung you should go in politics

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    Yup, who wants to hear a short answer?

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    Hu, that was a very well written explanation.

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    HuFlungDung--

    Although I use work offsets and Z axis reference/clearence points every day, I have never quite seen it explained like that. Good Job. I'm going to print that out for some of the newbies around here to read.

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    HuFlungDung,

    Thanks a lot. That is the first explanation that made sense to me. Thanks again KJ


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