Drilling / Milling Stone - Large Diameter Ball End Mill
Does anyone out there know if it is possible to get a 3" inch diameter drill bit or ball end mill for stone? I am trying to drill a half-sphere into the top of a stone (so the hole would be ~3 inches wide and 1.5 inches deep.
I don't know if this is possible, what procedure to use, or where to find equipment.
Yes you can get a 3" ball end mill but you will probably have to have it made. We have our bits made at Cold Springs Granite co. The problem I have found when trying to plunge a ball end mill into stone (Granite, Marble, Limestone, ect..) is chatter, and if you use a plated diamond bit you may burn off the diamonds. I think if you use a sintered diamond drill bit (.5" dia. or something of that size) and drill a hole to 1.5" depth and then use a 3" diamond ball end mill and plunge you will be able to do it.
I need to cut a bunch of 1/4" deep x 12" diameter pockets in granite countertops. The end result needs to be a cosmetically polished "bottom" of the pocket but I don't mind a couple of steps to get it there.
I've never milled granite, and have NO CLUE what tool(s) would be best suited for this. I have a 50 taper machine, but I also want to consider making a portable machine to allow doing this on a jobsite.
What tool should I use on my CNC to rough out the pocket, and what tool should I use to finish it.
Granite is about the most difficult material to work with on the planet. If all you've done is metal work, and you think it can't be much harder - think again.
If you don't have a machine specifically built for granite work, you really don't want to attempt this. It will destroy the screws and slides in your machine, and you'll find it very difficult to put that polish on the bottoms of those pockets. Granite is a mixture of various minerals, each of which has different hardnesses. Achieving a smooth overall polish is difficult in the best of circumstances, flats are harder than rounded surfaces and doing flats at the bottom of a hole is a lot harder than on the surface. You need a lot of time, specalized tooling, and many successive grits of diamond to get to polish. Of course, everything has to be done wet - if the stone heats up, flakes start popping off the surface.
If it is really necessary to do this, you'd be much better off using a big hole saw to cut all the way through the granite, and gluing a pre-polished round slab in it 1/4" low. Stone guys use a polyester-based glue called "Akemi" for this sort of thing.