August 2007 Edition

CONTROLLING INTEREST

Muskets and Machining

How the evolution of interchangeable parts created the American System of Manufacture and the development of CNC

New students in machine shop classes are given exercises to measure a variety of parts with micrometers. Later, the students’ first actual machining exercise is usually to turn a workpiece to a specified diameter within a tolerance. Tolerance is so central to machining that it is hard to believe that from the Bronze Age to relatively-recent history, metalworkers did not work to tolerances.

It was roughly 200 years ago when the challenge arose to produce muskets in large quantities with interchangeable parts. The prevailing method of manufacture back then was based on craftsmanship. Craftsmen produced the lock, stock, and barrel of a musket using forges, hammers, anvils, and files. The craftsman fitted them together with additional filing, shaving, and hammering. Each musket was unique.

Tolerance in Machining

The craftsman’s mind probably would reel if he had to imagine how to manufacture to tolerance, but inventive people of the 19th Century developed what came to be called the American System of Manufacture.

The American System of Manufacture is the name of the system that supplants craftsmanship with machinery. This machinery was put in motion by differently-skilled workers to manufacture metal parts to specified tolerances for assembly into muskets from interchangeable parts. This system was developed in the United States in the first half of the 1800s at the federal armories in Springfield, MA, and Harpers Ferry, VA.

Manufacturing to tolerance changed metalworking from shaping and fitting to stock removal. This spurred the invention and production of machinery to exploit the cutting-tool process of stock removal, usually with a spindle – not always – using some kinematics arrangement of mechanical linkages to drive the hard cutting material into the softer workpiece.

The high pressure produced on the edge of contact forces the softer material into a plastic state, where it fractures to expose new material for the driving edge to remove stock in a continuous process. Machinery of this kind became known as machine tools or cutting tools, although the latter also implies the harder material – think of a twist drill – shaped in a way for use with a machine tool.

If the cutting material is harder than the workpiece, and is sharp – not sharp like a knife, but like an edge at the intersection of two almost normal planes – the line of contact between the two materials concentrates the pressure in a very small area and cutting can be accurate on the microscopic scale.

The Birth of CNC

In the 1950s the quest for more accurate machining inspired John Parsons of the Parsons Works, Traverse City, MI, to propose a by-the-numbers scheme for machining to close tolerances. His idea was taken up by the Servo Mechanisms Laboratory of the Massachusetts Institute of Technology, where it was redefined as numerically-directed interpolation of a cutting tool in the work envelope of a machine tool. It was dubbed Numerical Control – NC.

Numerical control machine tools use position servos with digital signal processing. If we think of the workpiece as the memory of the interpolation then the digital processing and servo systems have to be good enough to achieve the necessary tolerances. This has been no small accomplishment.

Traditionally, with numerical control, the information that defines the path is specified as a sequence of rectangular coordinates. The sequence, as a whole, is the essence of a workpiece program. The execution of the program produces the workpiece of the design.

Nurbs

Numerical control spurred the development of computer-aided [geometric] design – CAD – employing spline algorithms based on polynomial methods. By the 1990s CAD had converged on a common geometry of non-uniform rational b-splines – nurb-splines, also called nurbs. Siemens followed this development with the Sinumerik 840D CNC. The 840D was the first CNC designed with a nurbs algorithm and a polynomial interpolator. The 840D advanced the American System of Manufacture from manufacture-to-tolerance to manufacture-to-design. Of course, manufacture-to-design folds into itself the idea of manufacture-to-tolerance.

The American System of Manufacture has become so universal and ubiquitous that it is no longer strictly an American phenomenon and what it has spawned is no longer talked about with proper nouns. Still, it is important for us to know our origins so we know how to think about our future.

Norman Bleier manages Siemens machine tool applications engineering in support of U.S. OEMs. His special interest is technology on the shop floor.

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