Newbie- Routing Slate

[Cormac]

Greetings!

I am a newcomer to CNC Zone, and this is my first post.

Having recently retired, I am indulging a long-time hobby of mine, and wish to make sundials. To that end, I have invested in a router-engraver with 1.2 kW spindle. I have chosen 20mm honed slate for the sundial plate.

I've had reasonable success engraving the plate, using CNC, but in cutting out the plate (about 300mm diameter), I'm having trouble. I have used 4-tooth carbide cutter, 8 mm diameter, at spindle speed of 12,000 rpm (I'm limited to 12,000, 18,000 or 24,000 rpm). I use water as coolant. The cut is clean, but I set a very slow feed and shallow (0.1mm) depth of cut, so the job took ages. One operation ruined the cutter, and despite the spray, the tip sometimes glowed red.

Can anyone advise me as to best tool, speed, feed, plunge rate, depth of cut and coolant for slate?

Many thanks, and best regards,

Cormac Lalor
Cork, Ireland.

[ger21]

Not sure about rpm and feedrate, but you need diamond tooling for stone, and lots of water.

[Cormac]

Thanks, Gerry!
You're absolutely right about plenty of water. As to diamond, it's very expensive, so I've tried the following, with some success (very slight tool wear, but I'll get many jobs out of it, I hope).

At bottom of 20mm slate slab, I fixed a 3mm hardboard sheet (sacrificial, that will take breakthrough, and prevent leaking). On the top I sealed a 12.7mm thick "bottomless bath" of MDF around the perimeter to be cut. Filled the bath with water and away. The tool was a 6.35mm grinding burr, which gave a surprisingly clean cut. Having no flutes, I had little or no splashing. Depth per pass: 0.3mm; speed 12,000; feed and plunge each at 0.3 m/min.

I wonder if I’m being a bit timid with these settings…

Best regards, Cormac.

PS Thanks also to JD for similar advice.

[z28tt]

Look into a local shop to waterjet the slab. It shouldn't be too expensive if you get a few done at once.

Andris