Kb58's granite CNC router build

[TorontoBuilder]

With great trepidation, "conversational G-code" was created to drill one hole, and the process stepped through slowly. It's very intimidating watching the spindle moving fast straight down toward the granite, worrying that it'll keep going due to some programming goof and at best, destroying the drill bit.

The awkward thing about drilling granite with a CNC machine is the material's hardness. No matter how slowly it's fed, each time the Z axis advances into the stone, the diamond drill can't instantly drill into it like a normal drill in steel or aluminum would. With each advance, the Z assembly and gantry deform slightly, until the weight drives the drill bit in to relieve the pressure. That was the primary reason that the feed rate was set to a very slow 1.2 mm / minute. Could it have gone faster? Probably, but I didn't want the risk.

This was something of a worst-case first customer for the spindle, and it was a little disappointing to feel the spindle vibrating during the drilling. I don't know if it's a bent drill bit, or flexing in the entire assembly. For the next hole, the drill bit will be recessed much further into the collet, as there was way too much of it hanging out and could only be contributing to the vibration.

So all that worry aside, it actually worked - slow, but it worked. A few settings in the drill sequence will be adjusted to save some time, then I'll try setting it up to do an entire column - carefully.

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Okay I was just informed of your thread here, because I'm building a cnc router with a granite surface plate after seeing Piotr Fox Wysocki's build.

I've chosen to make my sides and gantry beam from epoxy granite over a metal structural assembly so as to aide a few key areas like truing surfaces and having pre-drilled holes for rails and mounts etc.

I've been wondering the best method of drilling the surface plate itself, and it appears you have the best arrangement I shall seek to copy.

 

[TorontoBuilder]

Well this kicked my butt. Remember how I said how a diamond core drill can drift due to not having a point? Well it sure does, quite a bit as it turns out. The inner rows and columns aren't an issue because the spindle forces it to stay put. The outside rows and columns... not so much. They were drilled by hand, and just how messed up they were didn't become apparent until the fixture plate was put into position. Some of the holes were off by a little, and some by a lot, up to as much as 5 mm! Having no choice, out came the file, and hours later, it's "good enough." Not pretty, but functional. Partially because of this, the important holes will only be placed where the router can reach, which will leave open areas around the outside, but oh well. That area will be reserved for large 1/2-13 threaded holes for hold-down clamps, and they don't have to be precisely placed, so it works out.

so this is one of my fears... and I'd thought of perhaps renting a drill with a suction base plate. That perhaps is now a certainty