This month will be looking at CNC machining. I’m sure a lot of you have heard or seen the term “CNC machined” but what does it actually mean? CNC is actually an acronym for Computer Numerically Controlled. In the early days of semi automated machining, before the computer revolution, there were also NC, or Numerically Controlled machines. These relied on things such as punched tape feeders to input the data by shining light through a series of holes punched in a continuous loop of paper tape. As you can imagine these methods were very slow and unreliable.

With the advent of cheaper and faster computers, CNC machines became much more reliable and easier to use as the computing power increased. These days they have features such as network and internet connections for remote diagnostics and monitoring. Some come with a web cam so that you can show the technician in Japan exactly what your problem is if the machine breaks down. You can even get the machine to send SMS messages to your phone during unmanned operation to let you know if something goes wrong. Advancing technology has also improved the mechanical aspects of today’s machines. Most high speed machines no longer use pulleys or gears to get the power from a motor to the spindle. They actually wrap the motor around the outside of the spindle, so the spindle becomes the shaft of the motor. There are now machines available that move so fast, they use linear motors, like the technology you may have seen for the futuristic trains using magnetic fields to move things around. Spindle speeds on high speed machines exceed 25 000 rpm.

There are various types of CNC machines, all suited to a particular task. The CNC machining centres are very similar to your standard milling machine in construction and operation and are available in both horizontal and vertical configurations. In fact you can buy a CNC controller and retrofit it to the appropriate standard milling machine. The basic CNC lathes are once again very similar to your standard centre lathe. Where today’s CNC lathes become a little different is in the number of axes available. You may have heard the expressions “4 axis” or “5 axis” lathe and wondered what it means. Before we get into multiple axis lathes I will explain the meaning of the axes.

For the CNC machine to do anything it must have a reference in space. Any milling machine must have a minimum of  3 axes, x, y and z, and a lathe must have a minimum of 2 axes, x and z. If you hold a pen vertically on a piece of paper and draw a circle, you would be imitating a machining centre using the x and y axis simultaneously. If you lift the pen off the paper this would the z axis. Similarly, if you spin one of the wheels on your kart and move along the outside diameter this would be the z axis of a lathe. The x axis of a lathe refers to the diameter of the work piece being turned.

So where do the other axes come in on a multiple axis lathe? The latest catch cry in CNC machining is “done in one”. This refers to the ability to finish a component in one setup rather than turn it in a lathe and then put it on a different machine for milling or drilling holes. To satisfy the demand to be able to do this, manufacturers have now produced machines which combine a lathe with a machining centre, and the multi axis lathe, or multi tasking machine tool, is born. These machine can turn, mill, drill, tap, and even cut gears and perform grinding operations all in the one setup. So this creates the c axis, which allows the chuck to be locked and rotated in very small (0.0001 of a degree) angular increments to allow milling and drilling operations to take place. However the c axis only allows milling and drilling operations to take place on the centreline of the chuck. To allow full milling capabilities the y axis is required to move either side of the chuck centreline. Once the b axis is introduced to allow the milling turret to swivel, this allows full milling capacity on any angle and on any surface. The Mazak Integrex series of machines were the first to introduce a fully integrated b axis. These are the machines we use to produce the Sniper in one complete operation. Holden Racing Team also have a couple of Integrex’s.

Surely such complex machines must be a nightmare to operate? This is probably the main area where faster computers have made operating CNC’s so much easier. There are two main types of control systems that are used. The first is called ISO and the second is called Conversational. ISO consists of many M and G codes that are used to move the machine around and perform different functions. In an ISO program you physically have to tell the machine every step of every move it is going to make. These programs can be many hundreds of lines long. In the past on an ISO controller you had to input all of this data directly into the machine one digit at a time. You can now buy CAM (Computer Aided Machining) software which allows you to perform all of the machining on the screen of your PC in a virtual environment, and it will then generate the program to feed straight into the machine via a cable. A conversational control is so named because it will prompt you with questions about what you are trying to do, and you simply answer the question and the control generates all of the code in the background. For example, if you wanted to drill a hole, the CNC will ask you for the type of drilling cycle to be used (straight drilling, peck drilling, reaming, etc), the position of the hole and how deep you want to go. The CNC will then calculate all of the necessary movements it has to make to perform this operation. One line of this conversational program can replace 10 or 20 lines of an old style ISO program. The CNC will then generate a solid model on the screen and perform the whole machining program using virtual tools before a single piece of swarf is produced. This allows the operator to be sure the work piece will be exactly what he expects before machining begins.

Machine tools are not the only area where technology has had a big impact. The cutting tool technology has also made massive leaps forward. This has been driven by manufacturers such as ourselves always striving for more efficient and faster cycle times. If you can run all your tools twice as fast, then obviously you can manufacture twice as many parts in the same amount of time. There has also been the necessity to develop specially balanced tooling and tool holders to withstand the sustained high RPM of today’s super fast milling spindles.

During the nineteenth century, various iron and steel processes were developed to produce tool steels such as high-carbon steel and alloy carbon steel. Tool life was extremely short and unreliable as heat treatment and metallurgy were undeveloped sciences. The first big development was in 1900, when high speed steel (HSS) was demonstrated. This cutting material could endure a red hot cutting edge while maintaining sharpness. HSS is still used as an efficient and cheap material today in drills, reamers, taps and dies. Today’s HSS is alloyed with cobalt and can be coated with all kinds of hard materials by vapour deposition to produce a super hard cutting edge.

The next major development for cutting tools occurred in the 1930’s when cemented carbides, or tungsten carbides, were developed. These are produced by powder metallurgy, where various carbide powders and binders are pressed into a mould under extreme pressure and temperature, to produce an extremely hard and tough cutting insert. Virtually all CNC’s use carbide tooling now due to cutting speeds up to 30 times that of HSS. The same metallurgy technology has allowed production of man made diamond tooling which will cut virtually anything.

CNC is not limited to lathes and machining centres, however. Other machinery available with CNC controls include grinding machines, gear cutters, laser cutting, bandsaws and routers for metal and woodwork. There are also wirecut machines which cut metal of almost any hardness using electricity passed through a wire as thin as 0.07 of a millimetre. The work piece and cutting wire are submerged in a fluid bath and the material is sparked away along the length of the wire to produce a very thin cut line. Almost any shape imaginable can be cut using this technology.

We will finish with something that was science fiction only a few years ago. You can now buy a laser prototyping machine which produces a part in front of your eyes, from what appears to be thin air. Nylon powder is laid down in layers 0.1mm at a time on a work table. A laser then traces a shape on the powder which causes the nylon to bond together into a solid medium. The powder keeps building up 0.1mm at a time and the laser keeps bonding layers together. At the end you have what appears to be a solid block of nylon powder, until you blow off all the powder and reveal the three dimensional prototype underneath. As long as there is a passage to remove the nylon powder you can even create full assemblies. In theory you could watch a nylon Rotax appear out of thin air, fully assembled with all moving parts intact!