How is a rectangular hole cut in a rock?

Water-jet? Laser? The ends look rounded to me....but my eyesight is as old as I am!

A Bridgeport mill could probably do that. (with the right end mill bit)

Hubby made holes in steel that way. They're called slots. The piece is clamped to a moving table that moves under a massive drill. The drill uses special bits called end mills. The bits are held in the mill by something called collets.

Wouldn't be too hard to do on that Petoskey with one of those.

I imagine they used a diamond bit. Probably something similar to this:


Or one of these. Advertised as a diamond end mill bit:

If it can mill granite, Petoskey is a piece of cake.
You can buy a nice used Bridgeport mill for probably anywhere from around $3,500 to $5,000. They only weigh about 1500 lbs.

What do you do to get the core out and leave a smooth bottom without danger of cracking the stone? Stone rarely, if ever does exactly what you want with leverage. To remove the core you would need to break something somewhere.

I can almost guarantee that particular petoskey stone was not slotted the way that you suggest. That is a milled slot if I've ever seen one.

Just use mill cutting fluid. The most mills can recycle their coolant. That's what they use for steel. Its made for them. Won't rust. If you look closely at the bits in the top picture I posted, you can see holes and slots in the cutting area of the bit and a large hole in the end to let coolant flow through when cutting rock. The slurry from the process is forced out through the slots in the bit by the pressure of the coolant flow. Notice there is even a hole in the end for when it is doing the initial plunge into the slot and the groove in it supplies coolant to the outside edge of the bottom of the bit. That is where the most wear and tear would occur with no coolant.

Besides, I've already provided proof that milling machines are used to do this in my pictures of sintered diamond end mills designed to mill granite.

If the slot has a smooth bottom, it was milled.

Like most things in life, there is a really simple workaround solution for the dilemma of water and mills. The answer is: Put the stone in a plastic container filled with water.......Wow....who would have thought of that.  Clamp the entire assembly down. 
I'm not going to waste my time explaining this anymore so just go to this link and read all about it (the page is written in three steps (pages) and the answer is not on the first step.) BTW, the author of this article actually uses a CNC milling router and not a bridgeport. I think you could swing this simple slot on a bridgeport though. A good machinist (like my hubby) would have no issues at all.


Since I have never personally used a milling machine to put a slot on a rock, I could only guess how they cooled the bit without wasting the table and ways. Well, that is why there is google. I googled "milling in rock" and voila, just like that I had the answer to the coolant question. Google is your friend. 

I did know, before I opened my mouth to answer juddlerguys question however, that milling machines are being used for stuff like that.

OH yeah, the bits used are like sintered diamond tool steel rods, much like the bottom picture that i posted. With something like a milling machine in your arsenal, you would be stupid to try and do it any other way.

They also make (really costly) purpose built CNC stone routers,

where the stone is clamped down to a table and the milling/routing head moves instead of the table. It is on fancy machines like these that the fancy diamond bits with built in coolant holes are used.

The one pictured above looks like it might use flat linear motors for all the x and z axis moves (also known as Mag Drives.) Please, don't ask me to explain what that is....(although I could because hubby used to install them almost everyday.) That might also be a simple rack driven x/z axis but he can't really tell from the photo. He says the small steel rail next to the main linear bearing rail looks like what might be a rack and the one up top on the moving bridge also looks like a gear drive rack.

These are basically the same kind of machines that my hubby used to design and build for a company called Glassline Corp. but the ones he made were used for cutting/edging glass, not rocks (working with glass requires tools with machining done to extremely small tolerances, capable of working with one of the more brittle and shatter prone substances that are worked this way.)

Go one step beyond the simple CNC mill/router and you have the multiple axis stone cutting mill/router. It can carve statues and busts for crying out loud.


I don't think the Petoskey slot was done on one of those. That would be overkill for a simple 3" long slot. But it could have been.
Oh, and the coolant on this machine??? Looks like simple H2O. Probably not. The water presents a rust issue inside the mill and the stuff being used is most likely filtered and recirculated.

Based on all the facts, I would say that the most likely way that slot was put in that Petoskey was with a regular mill with a diamond bit with the rock submerged in a tank of water. I chose the regular mill based on the fact that Petoskey stones are not a mass manufactured kind of thing. It was likely done as a one off or maybe he does them more than that. Petoskeys don't come made to order so the chances of having enough stones of that size to make a bunch of them are slim to few.
Probably a product likely made by a lone lapidary probably somewhere in Michigan using a re-purposed vertical mill using diamond bits with the rock in a tank is the most plausible answer.

How could it plausibly be done with no mill? Once again, first submerge the piece in a plastic tub filled with water  Very carefully drill a series of adjacent holes to the exact same depth as close together as possible. Break or saw the space between the holes. Use large diamond drill bits or a diamond oscillating  blade and smooth down the ridges in the hole until it looks like a smooth slot (which would take a long time I think.)

A mill would be way easier.

Unless you plan to mass produce products, stop over thinking stuff!! Use existing tools.

I agree, use the tools you have(if you can) and don't over think.
Don't try and reinvent the wheel.

Well, the funny thing is they do have a square bit for wood.
Rocks are a different story.
For metal they brooch hole. (Spelling)

If you mounted a Dremel in a Dremel drill press you could put the petosky stone on a sliding table and push the rock into the bit. Or possibly slide the rock against a fence clamped down.

A diamond bit will eat thru petosky like butter. Drill down first, slowly push rock linearly into bit keeping hole wet. A basic one direction milling machine.

It would not work well on agate, but petosky being so soft.

Using these Dremel diamond bits:



Or a Dremel router attachment. You would have to clamp the rock below a small flat support table. Same concept, just more complicated :




This Dremel rig would do it also, but would have to be turned upside down to keep water out of Dremel



I would go with the Dremel drill press(for petosky or limestone). Factory fence installed. Make a better fence. It would be a poor man's end mill for soft rock.



Dremel diamond chain saw sharpening bits 1/8-5/32-3/16- something like those sizes

I have cut limestone with a diamond blade on a skilsaw. Cuts it like butter. I would think you petosky's should be similar.
Apparently a Dremel is strong enough for side loading since they use it as a router.