1) With the exception of the #12 tap, the major diameter of number size taps, change in increments of .013". So that if we start with the major diameter of a #5 tap @ .125", the major diameter of a #8 tap would be .164" ie: .125"+(3x.013")=.164". A #2 tap=.086", a #0 tap=.060" etc. (Jim Davis)

You Don't Say #2) Most machinists give little thought to the origins of our milling machine when we turn on the spindle. We probably give even less thought to how the system was developed that allows our finished product to be assembled with other components.

Both of these factors can be traced back to their origins in Eli Whitney who was a prolific inventor. He was issued his first patent on the cotton gin in 1793. This machine created a commodity for export and was a financial blessing for our struggling, young Republic.

Whitney also invented the first, true metal cutting mill in 1818*. Over the last 185 years, this machine tool has developed into a wide variety of equipment that has helped make our lives what they are today. But the real story is in the manufacturing process that fostered this magnificent machine.

After we defeated the British in our war for Indendence, the arsenal of our newly formed nation was in a dreadful condition. Whitney knew this and in 1796, met with Thomas Jefferson and made a bold proposal to manufacture fire arms for the federal government. In May of 1797, he nailed down a contract in which he promised delivery of 4,000 muskets to the federal armory by September of 1799 and 6,000 more by September of 1800. Whitney's proposal would prove to be a huge undertaking. Up until this point, all muskets and pretty much everything else mechanical was crafted by hand, making individual parts that were unique to each assembly.

The shop Whitney used to manufacture his cotton gin was modestly equipped and lacked a water wheel for power. He would have to find a location with suitable water fall for the new machine shop, construct a plant, design and build new machine tools, train a work force and most importantly create a manufacturing process that would facilitate production and guarantee his first delivery of goods in less than 28 months. This man had some serious cajones underneath those silk stockings and knee breeches.

The project was not without its setbacks and delays. Whitney made his first delivery of 500 muskets in September of 1801 (right month, wrong year). The fact that other manufacturers were late on their deliveries played a minor role in his salvation. But what really saved the day for Whitney was the interchangeable parts of his muskets. Any number of muskets could be disassembled, put in a pile and reassembled by picking components at random. Parts could be substituted from one musket to another. In its day this process was unheard of!

Whitney had inadvertantly created a system of uniformity based on interchangeable parts that sparked the industrial revolution. Today, this system is the foundation of how we design, manufacture and assemble virtually every product made in machine shops today. So think of Eli Whitney the next time your parts pass inspection. We have him to thank for the tolerances we hold today.

* Up until this point, mills were classified as metal filing machines.

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