When you’re a machinist, your stock in trade is precision, with measurements in the thousandths of your preferred unit being common. But when you’re a diemaker, your precision game needs to be even finer, and being able to position tools and material with seemingly impossibly granularity becomes really important.
For [Adam Demuth], aka “Adam the Machinist” on YouTube, the need for ultra-fine resolution machinist’s jacks that wouldn’t break the bank led to a design using off-the-shelf hardware and some 3D printed parts. The design centers around an inch-metric thread adapter that you can pick up from McMaster-Carr. The female thread on the adapter is an M8-1.25, while the male side is a 5/8″-16 thread. The pitches of these threads are very close to each other — only 0.0063″, or 161 microns. To take advantage of this, [Adam] printed a cage with compliant mechanism springs; the cage holds the threaded parts together and provide axial preload to remove backlash, and allows mounting of precision steel balls at each end to make sure the force of the jack is transmitted through a single point at each end. Each full turn of the jack moves the ends by the pitch difference, leading to ultra-fine resolution positioning. Need even more precision? Try an M5 to 10-32 adapter for about 6 microns per revolution!
While we’ve seen different thread pitches used for fine positioning before, [Adam]’s approach needs to machining. And as useful as these jacks are on their own, [Adam] stepped things up by using three of them to make a kinematic base, which is finely adjustable in three axes. It’s not quite a nanopositioning Stewart platform, but you could see how adding three more jacks and some actuators could make that happen.
It may come as a shock to some, but TV used to be a big deal — a very big deal. Sitting down in front of the glowing tube for an evening’s entertainment was pretty much all one had to do after work, and while taking in this content was perhaps not that great for us, it was a goldmine for anyone with the ability to monetize it. And monetize it they did, “they” being the advertisers and marketers who saw the potential of the new medium as it ramped up in early 1950s America.
They faced a bit of a problem, though: proving to their customers exactly how many people they were reaching with their ads. The 1956 film below shows one attempt to answer that question with technology, rather than guesswork. The film features the “Poll-O-Meter System,” a mobile electronic tuning recorder built by the Calbest Electronics Company. Not a lot of technical detail is offered in the film, which appears aimed more at the advertising types, but from a shot of the Poll-O-Meter front panel (at 4:12) and a look at its comically outsized rooftop antenna (12:27), it seems safe to assume that it worked by receiving emissions from the TV set’s local oscillator, which would leak a signal from the TV antenna — perhaps similar to the approach used by the UK’s TV locator vans.
The Poll-O-Meter seems to have supported seven channels; even though there were twelve channels back in the day, licenses were rarely granted for stations on adjacent channels in a given market, so getting a hit on the “2-3” channel would have to be considered in the context of the local market. The Poll-O-Meter had a charming, homebrew look to it, right down to the hand-painted logos and panel lettering. Each channel had an electromechanical totalizing counter, plus a patch panel that looks like it could be used to connect different counters to different channels. There even appears to be a way to subtract counts from a channel, although why that would be necessary is unclear. The whole thing lived in the back of a 1954 VW van, and was driven around neighborhoods turning heads and gathering data about what channels were being watched “without enlisting aid or cooperation of … users.” Or, you know, their consent.
It was a different time, though, which is abundantly clear from watching this film, as well as the bonus ad for Westinghouse TVs at the end. The Poll-O-Meter seems a little silly now, but don’t judge 1956 too hard — after all, our world is regularly prowled by equally intrusive and consent-free Google Street View cars. Still, it’s an interesting glimpse into how one outfit tried to hang a price tag on the eyeballs that were silently taking in the “Vast Wasteland.”
The Mars Climate Orbiter was a spacecraft launched in the closing years of the 1990s, whose job was to have been to study the Martian atmosphere and serve as a communications relay point for a series of other surface missions. It is famous not for its mission achieving these goals, but for the manner of its premature destruction as its orbital insertion brought it too close to the planet’s atmosphere and destroyed it.
The cause of the spacecraft entering the atmosphere rather than orbiting the planet was found in a subsequent investigation to be a very simple one. Simplifying matters to an extent, a private contractor supplied a subsystem which delivered a reading whose units were in the imperial system, to another subsystem expecting units in the SI, or metric system. The resulting huge discrepancy caused the craft to steer towards the surface of the planet rather than the intended orbit, and caused the mission to come to a premature end. Billions of dollars lost, substantially red faces among the engineers responsible.
This unit cock-up gave metric-using engineers the world over a brief chance to feel smug, as well as if they were being honest a chance to reflect on their good fortune at it not having happened on their watch. We will all at some time or another have made an error with respect to our unit calculations, even though in most cases it’s more likely to have involved a simple loss of a factor of ten, and not with respect to a billion dollar piece of space hardware.
But it also touches on one of those fundamental divides in the world between the metric and imperial systems. It’s a divide that brings together threads of age politics, geography, nationalism, and personal choice, and though it may be somewhere angels fear to tread (we’ve seen it get quite heated before to the tune of 885+ comments), it provides a fascinating subject for anyone with an interest in engineering culture.
As a Hackaday writer, you can never predict where the comments of your posts will go. Some posts seem to be ignored, while others have a good steady stream of useful feedback. But sometimes the comment threads just explode, heading off into seemingly uncharted territory only tangentially related to the original post.
Such was the case with [Steven Dufresne]’s recent post about decimal time, where the comments quickly became a heated debate about the relative merits of metric and imperial units. As I read the thread, I recalled any of the numerous and similarly tangential comments on various reddit threads bashing the imperial system, and decided that enough was enough. I find the hate for the imperial system largely unfounded, and so I want to rise to its defense.
Measuring length is a pain, and it’s all the fault of Imperial measurements. Certain industries have standardized around either Imperial or metric, which means that working on projects across multiple industries generally leads to at least one conversion. For everyone outside the last bastion of Imperial units, here’s a primer on how we do it in crazy-land.
The basic unit of length measurement in Imperial units is the inch. twelve inches make up one foot, three feet make up one yard, and 5,280 feet (or 1,760 yards) make up a mile. Easy to remember, right?
Ironically, an inch is defined in metric as 25.4 millimeters. You can do the rest of the math for exact lengths, but in general, three feet is just shy of a meter, and a mile is about a kilometer and a half. Generally in Imperial you’ll see lots of mixed units, like a person’s height is 6’2″ (that’s shorthand for six feet, two inches.) But it’s not consistent, it’s English; the only consistency is that it’s always breaking its own rules. You wouldn’t say three yards, two feet, and six inches; you’d say 11 1/2 feet. If it was three yards, one foot, and six inches, though, you’d say 3 1/2 yards. There’s no good rule for this other than try to use nice fractions as often as you can.
Users of Imperial units love fractions, especially when it comes to parts of an inch or mile. You’ll frequently find drill bits in fractions of an inch, which can be extremely frustrating when you are trying to do math in your head and figure out if a 17/64″ bit is bigger than a 1/4″ bit (hint, yes, it’s 1/64″ bigger).
If it wasn’t hard enough already, there came the thousandth of an inch. As the machine age was getting better and better, and parts were getting smaller and more precise, there came a need for more accurate measurements than 1/64 inch. Development of appropriate tools for measuring such fine resolution was critical as well. You can call a 1/8″ bit a .125″ bit, and that means 125 thousandths of an inch. People didn’t like to wrap their mouths around that whole word, though, so it was reduced to “thou.” Others used the latin root for thousand, “mil.” To summarize, a mil is the equivalent of a thou, which is one thousandth of an inch. It should not be confused with a millimeter. It takes about 40 mils to make 1 millimeter. Also, the plural of mil is mils, and the plural of thou is thou.
Measuring length is done with a variety of tools, from GPS for long distances, to tape measures for feet/meters, and rulers for inches/centimeters. When it comes to very small measurements, the caliper is the tool of choice. This is the kind of tool that should be in everyone’s toolbox. Initially it started with the inside caliper and outside caliper, which were separate tools used to measure lengths. The Vernier caliper combined the two, added a depth meter and a couple other handy features, and gave machinists an all-around useful tool for measuring. Just like the slide rule, though, as soon as digital options became available, they took over. The digital caliper can usually switch modes between decimal inches, fractional inches, and metric.
Every industry has picked a different convention. Plastic sheets are usually measured in mils for thin stuff and millimeters or fractions of an inch for anything greater than 1/32″. Circuit boards combine units in every way imaginable, sometimes combining mils for trace width and metric for board dimensions, with the thickness of the copper expressed in ounces. (That’s not even a unit of length! It represents the amount of copper in one square foot of area and 1 oz is equivalent to 1.4mil.) Most of the time products designed outside of the U.S. are in metric units, while U.S. products are designed in either. When combining different industries, though, the difference in standards gets really annoying. For example, order 1/8″ plexiglass, and you may get 3mm plexiglass instead. Sure the difference is only .175mm (7 thou), but that difference can cause big problems for pieces that are press fit or when making finger joints on boxes, so it’s important that when sourcing components, you not only verify the unit, but if it’s a normal unit for that industry and it’s not just being rounded.
Often you can tell with what primary unit a product is designed with only a few measurements of a caliper. Find a dimension and see if it’s a nice round number in metric. If it’s not, switch it to imperial, and watch how quickly it snaps to a nice number.
Use metric if you can. The vast majority of the world does it. When you are sending designs overseas for production they will convert to metric (though they are used to working in both). It does take time to get used to it (especially when you are dealing with thou/mils), but your temporary discomfort will turn to relief when your design doesn’t crash into the Mars (or more realistically when you don’t have to pull out the Dremel and blade to get your parts to fit together).