Machine tools have come a long way since their Bronze Age forerunners. Milling machines in particular, which were first used in the 1800s, have evolved to the point where they’re capable of easily holding tolerances that were simply unheard of a century ago. You may not realize it, or may take it for granted, but the levels of precision found in products we use every day are incredibly impressive. The sort of precision found in common products and the parts that make milling machines run accurately is built on a long history of brilliant minds, fascinating tools, and processes that have evolved over years of trial and error.
PRECISION FROM NOTHING
Although self-replicating machines have become popular in recent years with the advent of affordable 3D printing, particularly through the RepRap project (short for REPlicating RAPid prototyper), the idea of partial self-replication is not new. Henry Maudslay, a prominent British inventor of the 1800s, is credited with the invention of the screw-cutting lathe, which was used to incrementally improve itself through the precise turning of its own lead screws. As the lathe became capable of creating more and more precise lead screws, its own accuracy improved with each iteration. With the newfound ability to cut threads with ease, Maudslay’s lathe made the now-common screw and nut available for all - an invention that has had a profound impact on our lives today. Prior to his invention, screw threads were cut by hand, often with a chisel or file.
A method of building up a competent machine or shop is available to all through the famous series of books, Build Your Own Metal Working Shop from Scrap, by inventor and machinist David Gingery. The series starts with instruction on how to build a simple metal foundry, which is then used to pour the castings for a lathe, which can be used to build improved parts for itself. From there, more complicated machinery such as a shaper, mill, and drill press can be built.
While iterative self-improvement provides increased precision to a degree, at some point you need to measure those improvements to make more. A precise machine must be built with precise components, and those components must be measured with precise tools. Metrology, the study of measurement, is necessary for achieving precision. Attempting to create a precise machine or product without taking measurements is like attempting to cook a great meal without tasting it as you go.
A good general rule to follow is to ensure the measuring tool you use is at least one order of magnitude more precise than the degree to which you’re attempting to measure. For example, reliably measuring to the thousandth of an inch requires a tool that’s capable of measuring to a ten-thousandth of an inch.
One of the key inventions that kicked off the creation of precision machine tools is the handheld micrometer, invented by Jean Laurent Palmer in 1848. Still available in many analog and digital forms today, the micrometer granted machinists the ability to measure features that they hadn’t been able to before. Shortly after, in 1851, the first vernier caliper capable of measuring to one thousandth of an inch became available. As tools such as these were introduced, and at a price that most could afford, the ability to measure with precision grew, therefore allowing machinists to further scrutinize their work and aim for more accurate parts.
A key contribution to precision machine tools was the understanding of the importance of flatness, and the development of processes that are used to make surfaces very flat. Machine tool carriages require reliably flat surfaces to slide on, called “ways”. As you can imagine, ways that have poor surface finishes or are not flat will yield poor results in the milling machine.
Thanks to his work developing machine tools, Henry Maudslay understood the necessity of flatness and contributed to the invention of the now-common surface plate, which serves as the backbone of metrology today. Typically made of granite, surface plates act as a datum, or the basis upon which precise measurements and movements can be made. They can be finished to a variety of grades of flatness, based on their intended use, and should be treated with care. When dealing with such flat surfaces, tiny imperfections or gradual wear can have drastic effects: using a measuring tool over the same spot on a surface plate or leaving it in a space where the temperature varies by more than a few degrees can negatively affect their flatness.
The neat thing about surface plates is that they do not require precision tools to create. By using the “three plate method”, developed by Joseph Whitworth, flat surfaces can be created by using gravity and a simple hand-scraping tool, or by lapping the plates against each other. By starting with three plates of relative flatness, rubbing the plates against each other in alternating pairs to remove the high spots can yield fantastic results. Machinist Tom Lipton goes into great detail about how to do this at home with only basic hand tools in his fantastic video series, found here.
The process can be completed in six simple steps, and then repeated until the desired level of flatness is achieved. Note that, in this visual explanation, the surface finishes of the three plates is exaggerated. Before beginning this process, the three plates (Red, Green and Blue) should be machined to as flat a surface as possible, to remove all unnecessary lapping work. In addition, a fine abrasive compound is often used between plates to assist in material removal.
- To begin, the Red and Green plates are lapped against each other in an alternating manner. That is, one plate remains stationary while the other is lapped against it, and then the opposite is performed:
At the completion of this step, the Red and Green plates “agree with” each other, but that is all.Next, the Red plate acts as the control (it remains stationary) while the Blue plate is lapped against it:
At the end of this step, both the Green and Blue plates have picked up the error from the Red plate. They do not agree with each other, however.
- Next, the Green and Blue plates are lapped against each other in an alternating manner:
Since both of these plates picked up the error of the Red plate from the first two steps, lapping them together removes some of the error from the Red plate, bringing them closer to flat. At this point, the Green and Blue plates are more flat than the Red plate:
- Next, the Green plate acts as the control and the Red plate is lapped against it:
At the completion of this step, all three plates are of roughly equal flatness, but one (Green) is convex and the other two are concave:
- Next, move back to an alternating pattern and lap the two concave plates against each other:
This results in two fairly flat plates. At this point, only the Green plates needs to be brought to the same level of flatness as the other two.
- Finally, the single convex plate is lapped against the Blue control plate:
And the result is three plates that are in agreement with each other! It’s worth noting that these plates are by no means perfectly flat - this process should be repeated a number of times until the desired level of flatness is achieved.
Unlike the method described above, hand-scraping is more commonly used for flattening the ways in machine tools. Since no large machinery or matching plates are needed, it’s perfect for touching up surfaces that are too large or too heavy to be moved easily. A practiced hand-scraper can even scrape in oil-retaining textures that are ideal for extending ways’ longevity.
We’ve come a long way since the 1800’s, but despite how difficult it is the achieve extreme precision, it’s clear that fairly precise components can be made with only simple tools and a bit of ingenuity. And from there, incremental improvements can be made until the desirable level of precision is achieved. The accuracy with which we can measure and machine today exists entirely thanks to the brilliant inventions of those who came before us.