CNC Machines: Development and Differences
Computerized Numerical Control (CNC) machines automatically cut metal according to a design specification given by the CNC technician.
CNC machines are commonly employed to manufacture parts for various industrial uses.
Types of CNC machines
There are many types of CNC machines that perform different functions. Here are some common ones:
● Milling machines make precision cuts using a cutting spindle. In the mill, the cutting spindle remains stationary as the part moves around it on X, Y, and Z axes according to the design provided by the technician. Cutting with a milling machine is predictably known as “milling”.
● Lathes make highly precise subtractions from parts using a spinning arm. The part is mounted onto the spinning arm, which rotates it against a cutting tool. For that reason, cutting with a lathe is known as “turning”. Because of their cutting mechanism, lathes are best suited to pieces that have symmetry about an axis that can be attached to the spinning arm.
● Grinders, well, grind. Grinders trade the precision of a milling machine or a lathe for ease of programming, so they are mainly used for projects where less precision is needed.
● Other common machine types include drills, plasma cutters, EDMs, and laser cutters.
History of CNC
CNC was first conceptualized post WWII by the aircraft industry as a way to manufacture uniform parts for airplanes. The August, 1996 issue of American Machinist Magazine describes the very first NC machine, assembled in 1951:
“A 28-in. Cincinnati Hydro-Tel vertical-spindle contour milling machine was the starting point. It was extensively modified: all of the table, cross-slide, and head drives and controls were removed, and three variable-speed hydraulic transmissions were installed and connected to leadscrews. Each transmission would produce, through gearing and leadscrew, a 0.0005-in. motion of the table, head, or cross-slide for each electrical pulse received from the director. A feedback system was provided to make sure the machine was doing what it was told. A synchronous motor geared to each motion generated a voltage response to movement; this was sent back to the director and compared with the original command voltage.”
Those first machines took commands via punched tape. NC was slow to catch on, despite offering immediate cost savings, partially because a universal CNC machine code had not yet been developed. But that code was eventually developed, at MIT around 1958, and given the name g-code. G-code was standardized in the early 1960s by the Electronic Industry Alliance.
Meanwhile, the field of computer-aided design, more commonly known as CAD, was gaining popularity, and computing technology was advancing, paving the way for the commercial CNC machines of the 70s.
CNC Machine Code
These days, almost all CNC machines, as well as 3D printers, are controlled with some variant of g-code. G-code gives the machine instructions about how and where to move the workpiece to perform the necessary cuts. The accuracy of g-code depends on many precise calculations made by the coder.
Any errors could cause the various parts to become out of sync, potentially leading to a machine “crash”. A crash occurs when the parts of the machine is out of sync, causing the various moving parts to collide, damaging the workpiece, or worse, the machine.