CNC Stone Carving

In a form of architecture so ancient, CNC machines may seem a little out of place.  However, when one considers that the medieval builders were always on the forefront of the technology of their day, it no longer seems so strange.  To build Gothic today, it must become a reality, not a romantic idea locked up forever in one’s head.

(The Carmelite Monks spend their allotted work time on construction. See their Horarium here.)

CNC machines and robots have unlocked the ability to relatively quickly carve the intense details of a Gothic church.  Ornate pieces that used to take months for a skilled carver, now can be accomplished in a matter of days.  Instead of cutting out the beauty, using the excuse that it takes too long, thus doesn’t fit into the budget, modern technology can be used to make true Gothic in all its beauty a reality again today. 

The use of modern technologies in stone carving unlock the potential of modern youth.  Though they may be untrained in the use of a hammer and chisel, young men and women have grown up in a world of computers.  The skills they possess can now be channeled towards a higher end, the building of a church for the glory of God.

What is a CNC machine

CNC stands for “computer numerically controlled”, where a computer, through a series of XYZ coordinates, controls the movements of a machine.  The first CNC was invented for the manufacturing of airplane parts.  Stone cutting CNC machines are a relatively new invention.

Stone carving CNC machines typically use diamond tipped blades and milling bits, not water or laser, to carve the stone.  The diamonds, harder than the stone, grind away at the stone and wear it away.  The water is simply to keep the diamond tool from heating up from the friction and to wash away the stone sludge created by the carving process.

Normally the stone carving CNC machines used for sculptures and foliage have 5 axes, being able to move left and right, up and down, front and back, and rotate their heads in two different ways. The machines interpolate these multiple axes in order to carve the stone.  “Interpolate” means to work together, and an axis is one direction of motion.  So, the machines can move left, and down, and rotate their heads all at the same time to reach into a small area or leave a particular finish on the stone.    Sometimes they will even have a 6th axis, typically a lathe or a turntable, for carving tall stones such as pinnacles or statues.  The lathe interpolates with the other axes, carefully positioning itself to allow the CNC machine to reach each area of the stone.   CNC machines can also carve stone unattended, sometimes for long periods at a time and overnight.  This means 40 hours, a typical week of work, is less than 2 days for a CNC, a significant time savings!

Prussiani CNC machines

The Carmelite Monks use CNC machines from Prussiani Engineering, a stone CNC manufacturer based out of Bergamo, Italy.  Italians have always been at the forefront of stone carving, and it is no different with modern technology.  With Prussiani’s machines the monks can carve levels of detail previously reserved to the hand carving of highly skilled craftsmen.  With the mindset “where there’s a will, there’s a way” and careful programming the machines’ capabilities are practically limitless and have the ability to carve unprecedented 3D reliefs.

The Prussiani machines also have a reputation of being indestructible, a reputation proven in the battlefield of stone carving.  Stone is an incalcitrant material, and it takes robust machines to subdue it; no small task!  Here’s a little story to illustrate.  For the first time, a few months after we started carving stone, we left our first CNC machine running overnight on several 7 foot long window sills.  We woke to a shocking surprise the following morning.  The huge stones, weighing several hundred pounds each, had been thrown about as if a tornado had ripped through the machine.  Stones were snapped and shattered, the fragments scattered about the inside of the CNC milling area.  After the shock settled, we proceeded to analyze what had happened.  It turns out there was a mistake in the particular batch of code the machine had read and followed that night.  At the very end of each window sill, after it had completely finished carving, the code told the CNC machine to repeatedly pound down into the top of the sill, almost as if it was a giant fist.  The stones didn’t stand a chance, but that same machine is still carving stones to this day, almost a decade later.

The Carving Process

The process of carving stone with a CNC is a digital workflow.  The ornate pieces covered in foliage, the statues, the gargoyles, and many other distinct Gothic elements begin with the artist, the head carver.  He sculpts the various flowers and leaves using a 3d modeling program in a process much similar to molding a ball of clay.  It still requires a great deal of artistic talent, but there is a major advantage to this work in a computer, the Undo button!  The artist can experiment with an almost infinite number of designs to find the one most suited to the space.  And with the ability to visualize the pieces in a full model of the church, there is no longer any guesswork as to what the finished building will look like. 

Once the artist has finished a particular sculpture it is passed on to the actual stone carvers, the CNC programmers.  The programmers take the 3d model into a CAM program, which stands for computer aided machining, and create the toolpaths that the CNC machine will use to carve the actual stones.  Utilizing a large arsenal of tooling and methods they gradually carve the stone down to the finished piece.  They do their best to transform the stone block they start with to as close a resemblance of the artist’s model as possible, similar to the way stone carvers have historically used a pointing machine to transfer measurements from a plaster model into stone.  At the same time they have to juggle a wide array of concerns, from which tools to use depending on speed and finish, to how to keep the stone from moving, to avoiding collisions between the cnc and the stone, or even other stones they might be carving at the same time.  This is a lot to keep track of, but if done well, it will quickly bear fruit.  It is extremely rewarding to see the keystrokes and mouse clicks of a computer turn into not only a finished product, but an ornately carved part of a Gothic church, a place God will be loved for centuries to come.

Finally, once all of that Gcode (the programming language of a CNC machine) is ready, it is handed over to the sawyers, who operate the CNC machines themselves, as well as the various other saws that prepare the stone blocks for carving.  The sawyers are the “boots on the ground” guys and keep a clear line of communication open with the programmers to ensure that everything runs smoothly.  They carefully follow the layouts and tooling lists the programmers prepare for each piece as they set up the stones for carving, set reference points so the machines can locate each stone, and monitor it as it carves.  They are constantly keeping a watchful eye to ensure that everything goes smoothly.  There is a symbiotic relationship between the sawyers and the programmers as the work of each is essential to the successful completion of each stone. 

The sawyers also operate the wire saws and bridge saws, cutting each stone to either be carved into something more ornate, or to go straight into the wall.  The wire saw, a simpler type of 2 axis cnc itself, can cut simple profiles, including any straight piece, such as columns and the straight parts of window surrounds.  The sawyer, using the wire saw, will take a large slab of stone and sometimes cut even 20 or 30 different smaller stones out of it; looking similar to pieces from a cookie cutter.  This frees up the 5 axis CNC machines to focus on the more ornate pieces, and is also significantly faster.

There are many different facets to the art and science of stone carving today and only a few have been touched on here.  Pneumatic chisels and hand carving certainly have a valuable role to play, as well as a multitude of other methods not discussed here.  The learning curve to dive into such a work is indeed daunting, the workload intimidating, but the fruit of it will stand for centuries to come.