Today, it seems like every machine that once operated without the benefit of CNC machining has now been retrofitted with servo motors and some kind of programmable controller or computer. So how did parts get made before CNC (computer numerically controlled) machinery arrived on site?
Well, once upon a time, or shall we say 50 years ago or more, transistors did not exist. Then, in the early 60’s, transistor radios became more and more must-have gadgets. The more little devices were made, the more comfortable all the parties involved: chip makers, assemblers, designers, etc. became with the new technology. This laid the background for the development of the PC, whose smaller than mainframe size was needed in order to wed it to a machine.
Before discussing CNC machining equipment, it is important to understand how things got made prior to its existence. Before CNC machining, machines ran mechanically. There were relays, capacitors, and other somewhat helpful timing aids in use, as well as a limited use of fluidics. However, most motion patterns were created with the use of cams, linkages, escarpments, gears, and other mechanical elements. The standard clutch shift transmission is a good example of such a machine. It is basically a gear train with a control (the shift knob) and a manual gear change activator (the clutch). Yet, while primitive compared to a modern automatic transmission, it made it possible for a vehicle to move.
Cams were widely used for control of automatic lathes dating from around 1880 to the early 1970’s. They are irregularly cut plates or drums cut with winding paths cut into them. Each cam has a follower which imparts movement and timing to the tool or axis above according to the irregular scallops and paths on the cam. Programs and algorithms were developed to coordinate the various actions so as to eliminate conflicts and collisions. Tool makers were necessary to fabricate any special tooling which might be needed to make a particular part. Often, production lathes were sold already tooled up to make only one or two specific parts. For example, for years screwdriver handles were completely fabricated on Brown and Sharpe machinery. The machinery was also specially adapted to insert the screwdriver shaft and blade.
Don’t think that these older machines were inaccurate or incapable. The better non-CNC machines could still hold well under a thousandth of an inch. They were also considerably faster than all but the most expensive CNC machines made today. In fact, some large automatic lathes, such as the Acme Gridley, are still being used by reputable companies today. They are rebuilt, and at least partially fitted with modern, CNC slides and other components. The CNC version of this machine is still built, although not by Acme-Gridley. It is also a multi-spindle machine. This means that it is essentially a Gatling gun which feeds material through tubes, instead of bullets. Every time the machine rotates, the part being made encounters a different tool or tools which perform another operation on it. The last station is, of course, usually the cutoff tool. The material then advances forward and the whole process begins again. Needless to say, the set-up time required to use these machines, and the general complexity, means that only long runs are usually put on either the manual or the CNC version. Check out a video of one of these machines:
Despite the fact that electromechanical systems continued to advance, with the first NC machines (using a punch tape, not a true computer controlled unit) utilizing vacuum tubes which, in order to handle the voltages were sometimes enormous works of art in themselves. For example, tubes 24” long were not uncommon. Given the various difficulties of using vacuum tubes of extreme construction, it was not surprising that engineers seized upon the use of transistors and integrated circuits when they became available. Then, as controllers, such as the primitive Allen Bradleys then being offered, came on line, the slow change from analog to digital began. For the first time pretty much anything that could fit within a certain sized mill, for example, could be completely machined without many inaccuracy-introducing set-ups. This meant that such simple things as 45 degree cuts could be made without specialized fixturing. It also meant that better controls demanded better drives and motors, as well as ball screws and bearings capable of great speed and accuracy. Thus, the development of CNC machinery went hand in hand with the development of more powerful, accurate servo motors, stepper motors encoders, and other CNC components. The development of these components, in turn, led to faster, more accurate machinery. Today, for example, a decent CNC mill should be able to hold one to two ten-thousandths of an inch all day long.
So, finally, what is a CNC machine? Obviously, given how ubiquitous CNC machinery is today, there is no single answer. But most CNC mills, lathes, and the like have some common traits. First, they are given their instructions by a computer, which may either have dedicated architecture, or, more recently, may have a standard computer at its heart. The motors which provide motive power are largely, if not completely, servo controlled motors of one sort or another. There is a (are) encoder(s) which translate and communicate rpms and linear motion back to the controller. There are at least two or three simultaneously controlled axes, making it possible, with the use of software which is capable of generating a tool path from a CAD model, to produce almost any shape desired. Four and Five axis machines can, of course, produce even more complicated shapes, such as turbine blades. Modern, CNC machine tools also have slots for many extra tools, so that program changes can be made without always having to change every tool. The controller also stores the tool offsets for all of these, so once a tool is touched off, its offset numbers are permanently stored until they need to be changed. Today this technology has become so inexpensive and widespread that there are people making home-made CNC machinery, which may not have the accuracy and power of the store-bought variety, but which are perfectly adequate for cutting out sign material or other light tasks. Incidentally, it is this popularization of CNC components which also has been fueling the huge increase in the development and use of Robotics, both in industry, and as a hobby.
So, in conclusion, generally speaking CNC machinery is much more versatile than cam-operated machinery. However, there are specialty products which are still better and more cheaply made with non-CNC equipment.
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