Hill Cox 2017-06-27 03:20:26
WE OWE THEM WE IN THE PRECISION WORLD SHOULD NEVER FORGET THESE CONTRIBUTIONS. In an age where, if it doesn’t have a digital display it’s not modern, we tend to forget how the levels of precision we measure to came about in the first place. This is a brief look at one of the people we are indebted to for their discoveries and inventions from many years ago. I’ll look at others in future columns but for now, one man, known the world over for one of his many contributions to the field of mechanical engineering made equally—or more—important discoveries that became the bedrock of dimensional metrology and machine tool design. His name was Joseph Whitworth and he created the Whitworth screw thread system that is still used around the world, but that was later in his career. Whitworth (1803 -1887) was born in Stockport, England. His father made loom frames for the textile industry and when his mother died in 1814 his father decided to become a minister and put eleven year old Joseph and his two siblings into foster care. Three years later he was indentured as an apprentice for a cotton spinning mill owned by his uncle, and in 1821 his interest in machinery resulted in him starting work as a mechanic in Manchester, England. He realized that measurements and machine tool functions could be significantly improved if better flatness or ‘true planes’ could be developed. Having identified a need, while in his late twenties, he set about filling it. The popular method of producing a precision surface at that time was to ‘polish’ one surface against the other—a process we today call ‘lapping.’ However, he correctly realized all that did was make one match the other without being particularly flat. In pondering this dilemma he realized that if three surfaces were finished against each other, flatness to a higher level of precision could be realized but that posed another problem: how to work those surfaces to do so. He developed a process called ‘scraping’ where small amounts of metal representing ‘high’ spots on one plate were shaved off manually so it better matched the plate it was compared to. This process was used extensively to produce surface plates and angle plates prior to granite taking over as the base material used for them. Its unique feather-like pattern was seen on the ways of machine tools for a long time and had the added advantage of oil retention to keep those surfaces well lubricated. In 1833 he opened his own shop in Manchester, and set about his next project which gave birth to plain plug gages to better evaluate the size of a hole effectively when a machinist of that time couldn’t do much better than 1/32 of an inch. But the difference in size between the two pins used for such tests had to be determined to fine limits and this ‘need’ gave birth to a measuring machine by Whitworth. It comprised two flat parallel surfaces, one of which was fixed while the other could be moved relative to it by means of a lead screw with a twenty pitch thread which included a graduated scale. Full marks if you realized this device had all the features of an outside micrometer. In 1841 at the tender age of 38, Whitworth formally introduced the screw thread system that bears his name—a subject that would require several columns alone to briefly outline what was involved. The British government contacted him in 1854 with a request for an estimate of the cost of machinery to make rifled muskets at factory at Enfield. He carried this out and introduced rifling and a caliber which was contested at the time. But tests in 1857 demonstrated that his concepts and manufacturing skills produced a weapon that excelled in accuracy, penetration and range far exceeding what was then available. For the benefit of my younger readers, no, he didn’t have an app for any of it. Space does not permit listing many other contributions Whitworth made to his industry, his country and the world, the accolades he deservedly received and his bequests to education in engineering. We in the precision world should never forget this brilliant man and his contributions to what we know and do. We owe him big time.
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