We explore the precise measurement and machining of small distances and their importance on modern industrial society. The history of distance measurements is explained as well as intuitive examples of small distances given. Further, we discuss some of the engineering issues that emerge as we try to machine at smaller tolerances.
In 2010 an unprecedented milestone of the post-industrial world occurred. The total vehicles on earth superseded the 1 billion mark. Even more astounding, in 2018 it is estimated that 2.5-3 billion internal combustion engines exist on earth. This all exists because of small distances.
Let explore how small these distances are and what they look like. This is the science of small mechanical distances.
In order to make things with any precision, we need dimensional units. In our modern world, we look to the meter and the modern derivative of the foot as the basis for all distance measurement. It took over two millennia to get here. The first forms of standardized length measurement were the cubit and the ancient foot. Archeologists believe that the Egyptians, Ancient Indians, and Mesopotamians preferred the cubit while the Romans and the Greeks preferred the foot.
We’re all familiar with tape measures. Most tape measures divide down to a 1/16” or about 1.6mm. With the exception of intricate artisan work and some highly specialized processes, this seems to be the practical limit for hands only manufacturing and construction. Our ability for our hands and eyes to work in tolerances lower than this without the aid of positioning tools, start becoming difficult.
At 1/64” or 0.396875mm we’re at the practical limit of what our eyes can distinguish without tools. At this point, fractional expression of dimensions become decimal based. Machinist may also refer to these distances in “thousandths”, “thou” or “mils”. A unit equal to 1/1,000”. A 1/64” distance would be expressed as 0.015625” or 15.625 thousandths of an inch. At this length hand-guided work is still possible with positional toolings such as jigs and guides or through slow subtractive processes such as sanding or grinding.
At 1/100" we’re now entering the realm of thickness. This envelope is stamped from a thick stock paper, 0.01” or 0.254mm thick. Most thin sheets of material can be found at these sizes. The fine wires that makeup Ethernet cables and audio cables are also here. At this range of small distances, our eyes have a hard time intuitively deducing size. These are ranges more easily recognized by touch. Machine tools such as mills, lathes, and drill presses are a requirement in order to work with these distances.
Working at the 0.01” or 0.254mm size range and tolerances, the concept of engineering fit begins to emerge. Let’s say a hypothetical part pair calls for a 1” cylinder to be fitted in a 1” opening. On paper this makes sense, but in real life, there are other factors at play. To being with, the machine making the parts can never truly be perfect. It has a tolerance for its accuracy.
Engineers solve both of these problems by introducing the concept of a fit clearance. If we take into account the purpose of the part’s fitment, a purposeful gap between components can be designed in, to both accommodate for manufacturing inaccuracies as well as assembly. Fitting parts together can be categorized into three types: clearance fit, location fit, an interference fit.
At 1/1000” or 0.0254mm, the next step on our journey. We are now in the realm of machine only processes. How small is a 1/1000”? This thin razor blade is 4 times thicker than a thousandth. This is the tolerance in which simple machines exist. Firearms, quality hand tools, lower end machine tools, and less critical automotive parts can all be found here. Coincidently, this is also the rough size of the popular standard of small sizes – the thickness a human hair.
Moving an order of magnitude down, we enter the realm of 0.0001” or 0.00254mm. This the practical limit of general machining. At these tolerances not only does part temperature have to be taken into account but even the operating temperature of the equipment used to create the parts needs to be considered. Often grinding a surface slowly is needed to hit tolerances this low.
Directly machining lengths even smaller than 1/10,000” are possible but are reserved for highly specialized machinery usually designed for instrumentation or science. Working at those scales, we begin transitioning into processes such as photolithography, nanotechnology, and other sub-micron processes that are beyond the scope of this video. With current semiconductor manufacturing technology expected to push distances of 5 nanometers by 2020 – at this level of small we are literally dealing with distances 50 atoms wide.
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| Education | Upload TimePublished on 17 Nov 2018 |
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