In our last post, we discussed where CNC machines came from and how they can make things move around. Now, we need to decide what we are going to move around. In our case, it will be a table, a gantry, or perhaps a rotary device. What is important for this next part is what it travels on. We are talking about motion control, so it’s going to be something in motion, and for it to be in motion it will be traveling on some kind of bearing. There are many kinds of bearing, each with its own application—there are radial bearings, axial or thrust bearings, linear bearings, and combinations of each type. For our first step, we need to define our application. Let’s say we are going to move a table or gantry. For that, we need a linear-type bearing.
Computerized Numerical Control, or CNC as we all now it, came about shortly after WWII as a result of the aircraft industry’s desire to produce more accurate and complex parts. Below is a partial reprint of an article from the August, 1996 issue of American Machinist that explains the history of CNC much better than I ever could:
“Numerical control as a concept developed in the mind of John Parsons as a way to produce integrally stiffened skins for aircraft, and this led to a series of Air Force research projects at the Massachusetts Institute of Technology, beginning in 1949.
“The initial planning-and-study phase was followed by the construction of an experimental milling machine at the Servomechanisms Laboratory at MIT. Prof J.F. Reintjes, director of the lab, James O. McDonough, Richard W. Lawrie, A.K. Susskind, and H.P. Grossimon were the people involved in the research.
“A 28-in. Cincinnati Hydro-Tel verticle-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.
“By 1951, the system had been assembled, and application studies were begun. By 1953, enough data had been assembled to indicate practical possibilities that could be developed. A detailed 24-page report on the process that appeared in American Machinist on Oct 25, 1954, started a flurry of further development. […] But it was the initially more awkward, less accurate prototype at MIT, which employed a Flexowriter and its eight-column paper tape, a tape reader, and a vacuum-tube electronic control system that was to become the prototype for the developments that followed.”
The article included a photograph of the prototype machine with the heading “Pioneering setup at Servomechanisms Lab at MIT had control system that surrounded modified milling machine it controlled.” In the photo, you can see the control systems, which consisted of metal cabinets the size of school lockers, extends 12-15 feet (my guess) and is filled with electronics. The controls were larger than the mill itself! A sign on top of these “lockers” is also visible, reading “PILOT SERVO OUTPUT UNITS.” Underneath there are the signs “Table,” “Head,” and “Slide,” each corresponding to a “locker” that the respective sign was above.
The article continues: