Saturday, May 2, 2020

Weekend Shop Update

I'll do a weekend shop update today, since it all transpired earlier in the week. 

When we last talked, I was just about ready to cut the connecting rod after spending too many hours troubleshooting something that I might have remembered if it hadn't been 10 years since I last had to think about it. Once I got the coordinate offset resolved and verified it with a quick cut in air above the part, it was easy to cut.

You'll note some thin flash metal looking like square tabs on both ends that hadn't been cut off on the final pass.  I fastened the rod down with 10-32 cap screws on top of a 3/16 thick piece of scrap steel and the piece deformed under the clamping pressure.  I didn't worry because my plan was to cut a circular arc around each end and I assumed when I turned the part over, those would get cut away. 

The design called for the bulk of the rod to be thinned by 1/32" on both sides with a nice circular arc around the now thicker ends.  I wrote the G-code to cut the circular arcs around both screws (and the path between them) by hand and tested the code with the software I have for that.  The results were as intended. 

Each smaller grid square is 1/4" on a side, and the green lines show the center of the 1/2" diameter cutter, so 1/4" offset from the part.  The red arrows are drawn in to show the direction of the cut from where the cutter starts.  I can count squares and see the dimensions look right.  So I took it to Mach 3 to do an air cut.  This is what Mach3 thought the motion should look like:

Without the grid squares it's hard to know that the small circle on the left is the right size, but the circle starting on the right (end of the blue line) is very obviously wrong.  It's cut in the wrong direction and way too big. 

The code runs correctly in the testing program but not on Mach3?  Aren't they speaking the same language?  Well, yes and no.  As with the CAM program, Mach3 had a default command I hadn't found.  The code it self is simple.
G03  X2.7190 I0.532
G03 tells it cut a circle counterclockwise, the X coordinate (2.7190) is superfluous here and not even needed, the important part is the I 0.532.  My intent, and the way the software I tested it with understood the instruction was that the center of the circle was 0.532" to the right of the X coordinate,  or X = 3.251, and the diameter was twice 0.532 or 1.064" Mach3, however, has an environment variable telling it any coordinate I give it is absolute; in other words, the center wasn't 0.532" farther right of where the cutter was, it was centered at 0.532.  That meant the circle had to be (2.719 - 0.532) or 2.187" radius or 4.374" diameter.  That kinda looks like what those purple circles could be. 

Once I found that in the Mach3 manual, it was easy to see what was going on and change the code so that it cut properly.  The cutter goes to the right side of the left circle on the X axis and cuts it.  When it's done, the cutter is in the middle of the connecting arm again, so I send it to the right circle starting point.  This thins the bar by 1/32" as well.  Finally the right circle is cut.  Take out the retaining screws, flip the piece - this time attaching it directly to the table - reset the Z zero for the 3/16" lower work piece and run the circle cutting program again.

I spent more time sanding and cleaning up the part than it took do the cutting. 

I have a few small pieces to work on next.  I started today and they're going pretty well if I remember to RTFD (D for Drawing).  I cut one the wrong size already. 

There's a few dicey parts that I've never done anything like before but most look like things I know how to do.  There's at least a full page of parts for a DIY carburetor, but Webster (the guy who designed the engine and gave away the plans) has a drawing note that says (in essence) do yourself a favor and don't build this carburetor; just go buy an RC model car carburetor.  I'll probably take his advice.  I'm told the magic words are “Traxxas 4033 carburetor.”


  1. Very nice work. I’m glad to see that you went ahead and made the part look like the drawing.

    Major party points!!

    What CAM program do you plan to use in the future?

    1. That's a deep question. For simple parts, it's not crazy to write the code by hand and for many things I've done, I've just used Mach3 in command line mode. Things like telling it to move the cutter to some X/Y location to drill a hole or cut an edge. On the other hand, even simple things like hogging out a square opening is very tedious when you write it by hand.

      There's a free version of the program I have, DeskProto, following the Fusion 360 model of "free for hobbyists or startups making less than $X per year" but I've never seen a way to get it to code circles. It always makes curves into a series of points connected by straight lines.

      Likewise, the CAD program I'm using, Rhino, updated to a new version over a year ago but I never updated because their emphasis was putting in a language for making computer renderings. That's a complete waste of money to me. I've played with Fusion360 but there are things I just don't like about it. I'm going to play with FreeCAD and see how that compares.

  2. G91.1 in the g-code safety line for the win. G01.1 puts Mach3 in the “Incremental IJ” mode.

    1. You got it. I used G91.1 followed by G90.1 when done. The entire code to cut that part was
      G01 X0.6250 Y0.000 Z0.125 F25
      G01 Z-0.031 F6.5
      G03 X0.6250 I-0.438 F25
      G01 X2.7190 Y0.000
      G03 X2.7190 I0.532 F25

      and those circle cuts could have been simply:
      G03 I-0.438
      G03 I0.532

      I think G01.1 is going to live in my programs from now on.

      The odd part is I cut some circles a few years ago and didn't encounter this. That must mean that I had it in incremental IJ mode and that got lost when I moved Mach3 to new machines.

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  4. Amazon has that carburetor, but you probably already knew that.

    1. Last time I looked (last weekend?) it was out of stock - and $7 cheaper.

  5. I remember when I programmed robots in G-code and was surprised to find the same controller was used for CNC.

  6. There is a MIL spec for drawings... MIL.TP-4-1

    It means "make it like the print for once".

    1. You left out the F.

      I always heard that as MIL-TFP41C Same meaning. You can figure out what the F stands for.

    2. Gotta admit that's a new one to me!