Although it appears that CNC manufacturing popped up yesterday, in fact it’s been over 60 years in the making. If you work with stone or other similar materials, CNC processing can be an extremely versatile method of producing benchtops in granite and marble but also for a range of other products such as cladding, tiling, shower bases and fire-place surrounds.
CNC machines are still a large investment for most companies with $350,000 to $500,000 being a ball-park figure. They do, however, offer the potential for significant improvements in output and quality that far outweigh the initial cost.
NC control came about as a result of the American military aircraft industry. At the end of the Second World War it was apparent that faster jet aircraft required more complex and demanding parts. Conventional means of manufacturing were falling short of the required tolerances and a faster, more accurate method of parts manufacture was needed. The movie “The right stuff” chronicles the race to break the sound barrier and conquer space and it is within this time frame that the history of CNC is set.
During the late 1940s John Parsons was working on a system where punched cards containing position data controlled a machine tool. The idea was to machine flat templates to check the contour of helicopter blades. Parsons submitted his idea to the US Air Force in 1948 and was awarded a development contract with the University of Massachusetts (MIT) Servo Mechanisms laboratory. During the next three years a conventional milling machine was fitted with positioning servomotors for three axes (or movements) known as X, Y and Z and the first NC routing machine was born.
The machine demonstrated in 1952 looked very similar to a modern-day single spindle CNC router. Different were the banks of computer consoles needed to drive the servomotors, almost equal in area to the machine itself. All this was driven from a punched paper tape, and became known as numerical control, or NC. Numerical control is where machine codes are used to “drive” the machine. There are codes to start and stop motors; move the cutter-head left, right, up and down; and other codes for feed speeds and spindle RPM, etcetera.
Development in the woodworking industries began in the mid 1960s when the Ekstrom Carlson Company offered the first NC router in the USA. The Japanese Heian company developed its first NC router in 1968 and in 1969 Shoda claimed the first circular-cutting NC router. Early machines of the NC type relied solely on a punched tape. The paper tape had holes punched into it that the machine controller read as code. No calculations or modifications were possible on the computer and a new tape would be made even if only one code was changed.
Computer numerical control (CNC) was not to follow until IBM developed the personal computer.
The American Thermwood Company claims the first CNC machine, based on the Intel 8080 chip. Unlike NC, a CNC machine is able to perform some calculations such as acceleration and deceleration of the axes, and offer modification of programs at the machine control. These early CNC routers used G-code as a programming language, and this may still be used today on all CNC machines processing metal, wood, stone and even glass. In the stone industry, the first CNC machine is claimed by the Italian CMS brand in 1985.
In the early 1980s the first point-to-point machines appeared. These were primarily drilling machines and were developed from engineering machines of the same type where the workhead rapidly moved from point to point to insert pre-programmed drilling patterns. A sawblade was often added but there was no means of simultaneous movement of the three axes and therefore no possibility of a shaping option. From the late 1980s, profiling machines for wood and stone have been rapidly accepted in Australian industry where we use more CNC machines per capita than most other countries.
These machines originally used a proprietary controller and software but now all CNC machines accept code from CAD software programs. The CAD drawing is converted into code with a post-processor written just for that machine.
The most common stone cutting machine in Australia is still 3-axis with a few 4-axis machines for specialist manufacturers. The main suppliers of CNC stone equipment are CDK, Breton and Biesse (Intermac). Recently, cheaper machines have been developed and software is getting much easier to use so 5-axis may be just around the corner.
Your new CNC machine will have a flat table, usually of aluminium, and on which are placed ‘pods’ or supports to hold your stone workpiece. The pod raises the workpiece up to 100mm above the table to allow your cutting and shaping tools to cut through the part, as in the case of a kitchen top, or shape the outside edge with shaping (routing) tools. The pods may be positioned with the help of a laser that points to where the stops and pods need to be placed. The laser may also be able to show exactly where the cuts are to be made and this usually requires a laser mounted above the machine.
The cutting tools will be located in a tool-holder and this may be placed at the back or side of the machine table. You may also be offered a tool changer that travels with the main spindle and this saves a great deal of time. Every time a new profile is needed the machine will drop the existing tool and replace it with a new profile.
The more you spend, the more tool change positions you will get. Depending on the type of work you do, you may need tools for drilling, engraving, milling, grinding, carving, polishing, sawing and profiling.
Stone is cut with a large diameter diamond tipped blade and the edge finished with diamond and abrasive polish tooling. Feed speeds for stone are in the order of three metres a minute using a bridge saw and up to five metres a minute for edge profiling. Diamond tools can be expensive but they last a long time, especially in homogenous materials.
Today, the software is provided on a personal computer and is very advanced but easy to use. You will need to draw the shapes to be produced and then indicate the direction of the tool travel and on which side of the drawn line the tool should be. The drawing is then converted to the code the machine understands.
You can import drawings from other software programs if the code is in a generic format. Generic formats include DXF for 2D shapes and STL for 3D shapes. A feature of modern CNC machines is the ability for “parametric” programs. A parametric program could have a sink cut-out in the centre of a bench top and if the bench top length is changed, the sink remains in the centre of the top.
With CNC, anything is possible right up to and including Michelangelo’s statue of David. CNC technology gives you the ability to provide accurate, repeatable products and custom, niche market goods.
If you want to remain a serious player in the stone industry, CNC could be the way to go.
Article by Philip Ashley, Machinery Editor, Discovering Stone Magazine.