suntravel wrote: ↑Fri Oct 27, 2023 12:59 pm
No Risk No Fun
2000rpm 10mm/min 1.5mm peck drill should be ok to start. Depends on the quality of the drill bit and on good flushing
No MDF, directly mount a fixture plate 20mm flat milled AW5083 or cast iron
If you use a vise, directly mount on granite
Uwe
If he's running a router, he'll be stuck at a very high speed, probably 12k+. I have a 4hp spindle and the lowest that will go is 6k.
Another consideration for a brittle material like granite is that it's tough and scratch resistant but it can shatter with uneven loads or sharp blows. And once it's cracked, it's cracked forever. No fixing it.
One possibility is to flood it with epoxy about 1/4" thick. Epoxy would provide a decent, albeit shallow, spoil board and be repairable easily, and it would be perfectly flat and level.
One advantage of an MDF spoilboard is that it can work with a vacuum hold down system. I love using a full table hold down system for sheets of material, be they ultra thin plastic or metal, or warped sheet goods like an old piece of plywood.
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While the overall machine is indeed a "CNC router", a 3.2KW spindle is mounted, capable of 24,000 rpm, hence my concern about its low-end torque for this considered operation.
Last edited by kb58 on Fri Oct 27, 2023 5:34 pm, edited 1 time in total.
Every time I think I'm out of the rabbit hole, another presents itself - this time it's CAM. I'm using Alibre CAD, which doesn't have CAM abilities. Yes, there are a million third-party CAM apps, most very expensive, and I'll have to pick one eventually. This is mentioned because without the ability to program the entire drill sequence, it means working with the free version of CNC12 and its 20-line CAM limit. I hope that I can at least program it to drill one column or row, then "shift and repeat." Heady times...
Yes that's what I'm hoping to use to fit it all into 20 lines. The plan is to create steps to successfully drill one hole, then transfer those values into one of the batch-drill routines to do either a column - or all columns if it can fit.
The awkward thing about drilling granite with a CNC machine seems - to me - to be the material's hardness. No matter how slowly it's fed, each time the Z axis advances, 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 collet, as there was way too much 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.
