CNC Machining

Douglas Betts

CNC Machine Tool

Background

CNC or "computer numerical controlled" machines are sophisticated metalworking tools that can create complicated parts required by modern technology. Growing rapidly with the advances in computers, CNCs can be found performing work as lathes, milling machines, laser cutters, abrasive jet cutters, punch presses, press brakes, and other industrial tools. The CNC term refers to a large group of these machines that utilize computer logic to control movements and perform the metalworking. This article will discuss the most common types: lathes and milling machines.

History

Although wood-working lathes have been in use since Biblical times, the first practical metalworking lathe was invented in 1800 by Henry Maudslay. It was simply a machine tool that held the piece of material being worked, or workpiece, in a clamp, or spindle, and rotated it so a cutting tool could machine the surface to the desired contour. The cutting tool was manipulated by the operator through the use of cranks and handwheels. Dimensional accuracy was controlled by the operator who observed the graduated dials on the handwheels and moved the cutting tool the appropriate amount. Each part that was produced required the operator to repeat the movements in the same sequence and to the same dimensions.

The first milling machine was operated in much the same manner, except the cutting tool was placed in the rotating spindle. The workpiece was mounted to the machine bed or worktable and was moved about under the cutting tool, again through the use of handwheels, to machine the workpiece contour. This early milling machine was invented by Eli Whitney in 1818.

The motions that are used in machine tools are called "axis," and are referred to as "X" (usually left to right), "Y" (usually front to back), and "Z" (up and down). The work-table may also be rotated in the horizontal or vertical plane, creating a fourth axis of motion. Some machines have a fifth axis, which allows the spindle to pivot at an angle.

One of the problems with these early machines was that they required the operator to manipulate the handwheels to make each part. Besides being monotonous and physically exhausting work, the ability of the operator to make identical parts was limited. Slight differences in operation resulted in variation of the axis dimensions, which, in turn, created poorly fitting or unusable parts. Scrap levels for the operations were high, wasting raw materials and labor time. As production quantities increased, the number of usable parts produced per operator per day were no longer economical. What was needed was a means to operate the motions of the machine automatically. Early attempts to "automate" these operations used a series of cams that moved the tools or worktable through linkages. As the cam rotated, a link followed the surface of the cam face, moving the cutting tool or the workpiece through a series of motions. The cam face was shaped to control the amount of linkage movement, and the rate at which the cam turned controlled the feedrate of the tool. These early machines were difficult to set correctly, but once set, they offered excellent repeatability for their day. Some have survived to this day and are called "Swiss" machines, a name synonymous with precision machining.

Early Design to Present Day Operation

The modern CNC machine design grew out of the work of John T. Parsons during the late 1940s and early 1950s. After World War II, Parsons was involved in the manufacture of helicopter rotor blades, which required precise machining of complex shapes. Parsons soon found that by using an early IBM computer, he was able to make much more accurate contour guides than were possible using manual calculations and layouts. Based on this experience, he won an Air Force contract to develop an "automatic contour cutting machine" to produce large wing section pieces for aircraft. Utilizing a computer card reader and precise servomotor controls, the resulting machine was huge, complicated, and expensive. It worked automatically, though, and produced pieces with the high degree of accuracy required by the aircraft industry.

By the 1960s, the price and complexity of automated machines had been reduced to the point where they found applications in other industries. These machines used direct current electric drive motors to manipulate the handwheels and operate the tools. The motors took electrical instructions from a tape reader, which read a paper tape approximately 1 in (2.5 cm) in width that was punched with a select series of holes. The position and sequence of the holes allowed the reader to produce the necessary electrical impulses to turn the motors at just the precise time and rate, which in effect operated the machine just like the human operator. The impulses were managed by a simple computer that had no "memory" capability at the time. These were often called "NC," or Numerical Controlled machines. A programmer produced the tape on a typewriter-like machine, much like the old "punch cards" used in early computers, which served as the "program." The size of the program was determined by the feet of tape needed to be read to produce a specific part.