Grbl: Proofing precision and accuracy
After building the DIY Sherline CNC, I had asked myself the question: “How do I proof the CNC/grbl to make sure that everything machines correctly and to spec?” I posed this question to my friend Machinist Mike, and he quickly responded: “Machine a diamond-circle-square test block!” and awesomely generated a g-code program for me to immediately use.
The diamond-circle-square test block is an old school method to machine a set of known shapes and dimensions to gauge the accuracy and precision of a CNC mill. The diamond gauges how well and straight the CNC tracks diagonal cuts; the circle gauges the circularity of the cuts; and the square gauges the primary axes and perpendicularity. This test also will show the effects from backlash, if your machine is not square, smooth feeds from the surface finishes, among other things. Today there are new and better methods to gauge the accuracy and precision of a CNC machine, as the link shows, but for DIY’ers, the diamond-circle-square test is a plenty robust enough test.
The exact sizes and depths of the shapes do not particularly matter, just as long as the user knows what they are. I would recommend to create a large enough test block that is about as large as the parts you intend to machine. A large shape will tell you if you have problems in a certain area of travel. For me, the Sherline has about a 5″ Y-axis travel and the test block was sized to be about 2″x2″ with shape depths of 0.1″.
Also, Mike had noted that when machining with a mill without rigid anti-backlash ball screws, one should always conventional cut, opposed to climb cut (unless doing a finishing pass.) This is because the cutting forces with a conventional cut always push against the leadscrews in the direction of cutting travel. Where as, with climb cutting, the cutting forces push away from the leadscrews in the direction of cutting, causing the axes motion to rattle between the length of the backlash. This can effect precision and surface finish, as well as prematurely wear your lead screws.
Anyhow, a couple of weekends ago, Machinist Mike and I ran the g-code program with the new changes to my grbl fork. We ensured the Sherline anti-backlash nuts were nice and tight with a backlash of less than 0.001″. (The higher torque of my DIY CNC build allowed me to tighten up the anti-backlash nuts more than I would have with the OEM CNC.) We ran the program without any problems from start to finish.
I’m very happy to report that the surface finishes were excellent, the circularity and perpendicularily were also excellent, and all measured dimensions are within <= 0.001″ of the design dimensions. Meaning that grbl and the DIY Sherline CNC are good to go!