Cutting stone with a CNC machine is surprisingly simple with the right tools and a few tricks.
I've brought back a nice stone from my last hiking trip in Switzerland and christmas is getting closer and closer - so let's make a tealight holder...
Step 1: Tools
Most stones are hard enough to dull and burn even the toughest carbide tools within seconds.
So we need some help of girls best friends: Diamonds...
I'm using 6mm diamond grinder bits with 3mm shaft.
Things you need to know:
Never ever cut vertical: the fastest way to destroy a diamond grinder bit is to plunge vertically into the stone. You need to use a ramp of just a few degrees to work your way down into the material.
Use lots and lots of water: cutting hard stones creates large amounts of heat. Your grinder bit will glow red hot after just a few seconds if you try to cut without water. I prefer to submerge everything under water.
Step 2: CNC Program
I'm using my CAM software Estlcam to create the CNC program:
(Click here for a larger picture)
First we need to create a new tool with appropriate parameters:
My tool is coated with diamonds at a length of about 9mm (.35") and I'll be going to cut 21mm (.83") deep, so 7mm would be 3 passes and use most of the diamonds.
I'll later only use Estlcams "spiral drill" function where the tool never cuts at its full width - and I'll also use very small step over rates - so this rather fast speed will be OK (but depends on the grit of your tool and also on the stone material).
If you use an operation that requires your tool to cut at its full width much lower feed rates in the range of 60 - 180mm/min would be required.
Reduced speed for cutting downwards because this is much more difficult for the tool than cutting to the sides.
Thats all my spindle can do - go as high as you can.
The most important setting: it makes sure the Z-axis will move downwards at a 1° slope instead of plunging in vertically.
This very small step over allows to use the high feed rates above. It works faster this way than a large step over with slow feed rates.
Creating the toolpaths:
(The actual pockets just require 16mm depth but the stone is not smooth so I set zero for the z-axis 5mm above the stone to prevent collisions)
Lets have a closer look how the pockets are made:
Step 3: Cutting
I've definded 21mm deep pockets but only require 16mm, so Z-axis origin was set 5mm above the material. You'll see in the video that the cut starts well above the stone.