Learning from the Big Boys
Yesterday I took a few hours to go over to the Artisan’s Asylum, a “hackerspace” just outside Boston proper, to get checked out on their 2-axis Sharp CNC mill. If you live anywhere near Somerville, it’s a seriously cool place, and for $75 a month, you get access to a full shop with the aforementioned mill, a 13″ Clausing Colchester lathe, woodworking shop, and welding shop with 220V MIG and TIG machines.
My main reason to be there was that every so often, I have a part I can’t quite fit on my benchtop machines, but I was also interested to get a chance to work with a “real” CNC control, in this case, an Acu-Rite MILLPWR of early 90s vintage. Compared to a VMC, which is designed mainly for production, 2-axis knee mills like this are used to assist the machinist in making one-offs or very short runs, rather than replacing him completely. As such, they’re probably the closest “serious” industrial equivalent of what we do with little “toy” machines like mine.
While the control was organized differently than EMC or Mach, I found it very user-friendly and efficient to work with. After the jump, a few more detailed observations on what I really liked about it…
1. Manual Operation: In manual mode, the Acu-Rite control disables the motors but still reads machine position from linear scales, allowing the operator to do things like edge-finding by hand. After using this, I never want to run a machine with nothing but a keyboard again. If you make parts in manual mode, using a pendant with MPG and axis/scale selector buttons will probably make you at least 25-30% faster.
2. Conversational Part Design: While the control has a floppy drive to load programs, it’s strongly designed to allow an operator to program parts right on the machine. Parts are built up conversationally from a handful of primitives including lines, arcs, rectangles and circles, pockets, and hole patterns. Dimensions are input directly into dialogs rather than as G-code, and as you complete each primitive, the result is displayed on a preview screen. While the wizards used in Mach or EMC’s ngcgui are capable of doing individual primitives very well, neither has a good concept of working with a “part” consisting of multiple primitives. I wrote about this before in this post on the EMC forum.
3. Dry Run Mode: The control had two modes for verifying program operation. In “graphics only” mode, the control would run the program onscreen, without moving the mill, while “dry run” mode would run the program on the mill at rapid speed. The graphics-only mode made it easy to verify the program while programming the part, and the dry run was useful to quickly sanity-check things before making chips.
4. Operator Stupidity Reduction: Compared to Mach or EMC, the Acu-Rite control did a lot more to assist the operator in terms of pausing and prompting to do something at appropriate moments. For instance, if you programmed it to mill two unconnected lines, after finishing the first, it would stop and prompt you to raise the quill, then it would rapid to the start of the next line, and prompt you to lower the quill. With Mach or EMC, the onus is on the programmer to insert the appropriate pauses and moves to clearance planes in the right places. My experience has been that rapid moves like this (whether generated by hand or CAM) are responsible for well north of 50% of crashes, so anything that reduces this would save a lot of scrap.
The big thing I came away from this with was a much stronger sense of the utility of programming parts on the machine itself. While CAD/CAM systems are essential for complex parts and larger projects, the learning curve is steep and the overhead never goes away completely. While I’m still neck-deep in the pendant project right now, as soon as I finish that I’m planning to return to some of my earlier thoughts on building better machining wizards.