Computer Aided Manufacturing of Natural Stone

Computer Aided Manufacturing of Natural Stone

Micro-Project

Science and Engineering of Glass and Natural Stone in Construction

Lukas Neukom

ETH Zürich

December 2015

Summary

This short report discusses new possibilities of computer aided manufacturing of natural stone. It illustrates them with the example of the MLK Jr. Park Stone Vault which will be built in Austin, TX, USA.

There, a computer numerically controlled machine will cut the the voussoirs with a circular saw blade only in a highly optimized process.

Introduction

Natural stone has lost much of its importance as a load-bearing building material to concrete and steel. Today it is used mostly for flooring tiles and facade cladding. As it would be a sustainable building material, the interest in natural stone has been renewed by the new possibilities of computer aided manufacturing and design.

The MLK Jr. Park Stone Vault in Austin, TX, USA which will be built in 2016 illustrates in an impressive manner, how historic masonry structures can be reinvented and reinterpreted. It will be built as an unreinforced, dry-set cut-stone masonry shell.

MLK Jr. Park Stone Vault

The structural form of the MLK Jr. Park Stone Vault is designed to take advantage of the high compressive strength of natural stone. The vault sustains itself by compressive forces only. Therefore the stone shell can stay unreinforced.

The vault consist of several hundred individually shaped voussoirs. Each of them is curved on the top and the bottom and planar on the other sides. The flat faces of each voussoir are oriented roughly parallel and orthogonal to the force flow in order to reduce shear forces between them. The thickness of the blocks varies depending on the required compressive strength.

To find the optimal geometry of the voussoirs, several aspects have to be considered. One has to keep in mind how they will be machined and which limitations that puts on the possible shapes. A lot depends also on the tesselation of the shell. It should consist of staggered bonds to ensure that the individual voussoirs are interlocked as much as possible.

Model of the MLK Jr. Park Stone Vault

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Interlocking voussoirs show the force flow

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Machining of natural stone

The fabrication of natural stones is a subtractive process, which is unusual for building materials. Because of this, the amount of wasted material is generally higher and should be kept at a minimum with appropriate design decisions. The cutting time and tool degradation be reduced through that as well.

For cutting stone with multi-axes computer numerically controlled (CNC) machines, there are three techniques available:

• Abrasive water jets

• Diamond wire cutters

• Milling / circular saw blade machines

Each of them has its advantages and limitations. Diamond wire cutters have relatively high tolerances and can only cut ruled surfaces, but they are efficient for pre-cutting of large pieces. Abrasive water jets can cut only about 50-150 mm deep and it is very difficult to control the depth of the cut. They are very precise and do not generate heat during the cutting process though. Milling and circular saw blade machines are versatile but slower. The milling heads can mill the most complex shapes, whereas the circular saw blades are used for planar cuts.

Milling and circular saw blade machines are suited best for the machining of voussoirs like the ones that will be used for building the MLK Jr. Park Stone Vault. They are very accurate and can handle large enough stone pieces. The cutting process needs to be optimized though, because it would be not efficient to mill all stones.

Optimizing the fabrication process

Without any optimization of the fabrication process, the voussoirs for the MLK Jr. Park Stone Vault would probably be milled from larger stone blocks. But since it is far more efficient to use a circular saw blade as the main tool, the geometry of the voussoirs was adjusted and new cutting techniques were developed to use the circular saw blade exclusively. This resulted in a process where no milling head is used but some manual labour is needed instead.

Since the force flow is not linear, the contact faces would not be planar if they were orthogonal or parallel to the forces in each point. They are simplified to planes in order to cut them with a circular blade. The best approximation is iteratively searched.

The curved surfaces at the top and the bottom of each voussoirs can be approximated with parallel cuts. This is possible as long as the radius of the curvature is larger than the radius of the circular saw blade. Afterwards, the leftover material between the cuts needs to be removed manually. But since the tolerances for the curved surfaces are not as small as for the contact faces, the resulting roughness of the surface is not a problem. With this technique, it is possible to reduce the amount of stone dust that would be produced by milling and sawing the whole surface.

Parallel cuts to approximate the curved surface

Re-referencing

After cutting the top surface, each voussoir has to be turned over. Since the tolerances are small, it is important to align the stones exactly before processing the bottom surface. With five precise cuts on some leftover parts, that problem is solved in an elegant manner. These cuts intersect in six well defined points. A steel rack with six pins, each referring to one intersection, is used to hold the turned over voussoir. Because that rack itself can also be placed exactly in the CNC machine and the location of its pins are known, the voussoirs can be re-referenced from the other side.

Further optimizations

The processing time can be further reduced if the CNC machine works at its full capacity. This is achieved by adjusting the feed rate dynamically, according to the depth of the cut. The feed rate represents the speed at which the circular saw blade moves forward and the sawing capacity of the machine could be measured in volume per time.

An other possible optimization is to balance the distance between the parallel grooves that are cut. Increasing the distance results in fewer cuts and therefore less processing time, but comes at the cost of a rougher surface and more manual labour to remove the leftover parts.

With all these optimizations the research group estimates a processing time of 2 to 2.5 hours per voussoir.

Possible applications and open questions

This building process is expensive and difficult. It will probably be reserved for only a few, representative buildings. But because of the interesting shapes and the sustainability of such structures, it might spark innovation in «computer aided masonry».

A question regarding the stability in rare events such as earthquakes has to be asked though. Since every single voussoir is needed to support the structure, partial damage will collapse the whole vault. If the foundations of the cornerstones move relatively to each other, the structure would most certainly fall apart.

Collapse mechanism of the vault

References

Matthias Rippmann, Philippe Block, «Rethinking structural masonry: unreinforced, stone-cut shells», 2012 ICE Publishing

Matthias Rippmann, John Curry, David Escobedo, Philippe Block, «Optimising Stone-Cutting Strategies for Freeform Masonry Vaults», 2013

Matthias Rippmann, Philippe Block, «Innovating structural masonry - MLK Jr. Park Stone Vault»