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Using a CNC Machine With a CAD File

  1. Aug 10, 2014 #1
    I'm working on a project that requires me to use a CNC metal cutter. I'm using LibreCAD to design the thing I need to have cut (it's a radio chassis). The wiki doesn't really offer much besides a general how-to-use, and after playing around on the program I'm wondering two things. First, switching between millimeters and inches doesn't yeild different coordinate points, rather, the point 3.25 says 3.25 whether in inches or millimeters, and I'm not sure if size is kept ambiguous until fabrication time or if it's a bug in the program (or me not knowing what I'm doing). Secondly, when the machine goes to cut the metal, it's probably a good idea to have the machine cut out the smaller holes in the middle than cut out the chassis; is there a way to specify in the program which parts the machine is supposed to cut first; or is that determined manually during fabrication? I've never used a CAD program before, or a CNC machine (in fact I've never worked with metal before), so any information from people who have done this would be great :)

  2. jcsd
  3. Aug 11, 2014 #2
    "First, switching between millimeters and inches doesn't yeild different coordinate points"
    If it's going to a CNC you shouldn't have no worry if it's in metric or standard. As long as your file is in a format your shop can read, they'll figure out whatever system they want to work in .

    "is there a way to specify in the program which parts the machine is supposed to cut first"
    The programmer/machinist at the shop will determine the most efficient sequence to cut the part. If you have a concern about the sequence just ask to talk to the programmer or someone in charge of the shop. Any shop in busniuss should be able to figure out how they want to cut and stay within your set tolerance though, it is their job to cut metal.
  4. Aug 12, 2014 #3


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    I am assuming you are running this on your own CNC? Is that correct?

    If so, can you edit the CNC file?

    If you can edit the CNC file put a G20 code in the header for inches or G21 for metric. This doesn't change the numbers in the file, it tells the machine whether the numbers in the file are in inches or mm.

    G20 and G21 codes are like a switch. The controller will stick with that setting for the rest of the file and possibly until it sees another G20/G21 code.
  5. Aug 12, 2014 #4
    Okay, thanks for the information guys. It isn't my own CNC machine, it's at a metal shop in my town. I took the file I had to the guy and who basically said the same thing, that the file is put in the machine and then scaled based on a known dimension of some part of the cutout.
  6. Aug 12, 2014 #5


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    Your CAD file is (presumably) just the geometry of the finished part, maybe with some information on tolerances etc.

    Figuring out the operations needed to cut the part on a machine tool is a separate process. Some of it will be specific to the actual make and model of CNC machine that is being used. As well as the geometry, it needs to specify the cutting tools used, and the exact sequence of movement of the tools. For example you might want to make a rough cut to remove material quickly, and then a finer cut to get a good surface finish.

    The machine shop should look after all those details, but they might need to ask you a few questions about exactly what you want to produce.
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