I believe the USA already has. I have found using both metric and imperial measurements can be helpful for different applications. Sometimes using one or the other give the perfect fit. For example, no imperial/english standard size is close to 0.700", but the metric 18mm is almost perfect. So, I would go with the standard 18mm for parts, sizes, etc.
I like the fact that the USA has both in practice. It give us the ability to choose.
Jon, I agree about the ability to use Phillips and slot head screwdrivers on everything. Those are probably the two tools I use most around the house. I've encountered Robertson screws occasionally, but find it harder to turn them (even using the correct tool!) since the geometry takes more torque.
Jon, while I am travelling in US and UK, most of the time I get confused with these types of metrics. In my country we are following Kilometers, gram, cm and second. When I first landed in US, the taxi driver talked me about distances in miles and weight in pounds, I got confused. This may be true with all travelers, who are following different metric systems in their home land.
I have indeed come across the Robertson scres, but this is the very first time that I have heard thier name. Also, from China we get them made to also accept a Philips driver, and even a straight driver. And when thier forming die wears out they send screws that are hard to use with any type of driver.
The biggest failure of metric unit hardware is that it is not sized in practical proportions, meaning that the strength ratios are strange, and for thread pitches, it seems that they are sort of insane. I recall small metric screws with pitches of 1.2 and 1.5, and then some other pitch as well. That makes much less sense than our more reasonable 4-40, 6-32, 8-32, and 10-32 screw thread sizes. THose sizes have a useful strength ratio, each about twice as strong as the previous size.
Canadian lumber is sold in metric units, but most stores also give the equivalent in the normal inches and feet. So you can buy an 8-foot 2x4 or the same lumber with metric units. Just about everything else is metricized.
I usually end up multiplying by 0.22, Dave, but the number seldom sticks with me. I constantly have to go back and look it up, whereas I don't seem to need to do any mental calculations to evaluate a centimeter or millimeter or a kilogram. I guess it's a matter of teaching an old dog a new trick.
Many of the arguments about why the metric system is more expensive or more difficult are specious. The problems come from dual systems. Sure, for now, metric materials are more expensive to obtain in the U.S. But, that's because we still mostly use imperial. It costs extra to mill and inventory the smaller quantities of "special" sizes. If we were metric, those economics change.
One poster commented that Canada still largely uses imperial units in construction materials. Isn't it possible that happens because the U.S. buys a lot of material from Canada and vice versa? Easier to maintain one inventory. If the U.S. really went metric, I'd bet that Canadian lumber would be in millimeters too.
I recall back in the 70s the push to convert to metric. From what I remember, the main objectors were the automotive and aircraft industries, based on the cost of materials and even more, tooling. Tooling wears out. Most of it has been replaced many times since then. Materials are less expensive if they are all based on one system, especially in a global market. Now, the automotive industry has changed over. In contrast, Boeing has supposedly suffered huge monetary losses and months of delays on the 787 program, in part because of the difficulties in maintaining a global supply chain for an aircraft designed in imperial units.
The bigger problem isn't the arbitrariness of one system or another, it is the existence of dual systems, with dual supply chains, and conversion factors. One of the complaints from many people back in the 70s was that the metric system isn't easier, it's harder...because you have to remember so many conversion factors and multiply everything to figure out how much of something there is or how big it is. That's the cost of not just switching over, as Australia did. Get used to how big a centimeter is. Get used to how much is in a liter. Get used to how much a gram or a kilogram weighs.
I can never remember how many tablespoons in a cup, or how many cups in a quart, or a gallon, even though it's largely binary. On the other hand, it is really easy to figure milliliters to liters. I can easily envision how much a kilogram weighs because I know about how big a 10cm x 10cm x 10cm cube is, and what that amount of water weighs. How much does a gallon of water weigh? How many gallons in a cubic foot? How much does a cubic foot of water weigh? How many foot-pound-seconds in a horsepower?
Units we don't use often might take a little longer to get used to. Even though the definitions are obvious, I've never been able to internalize foot-pounds as work or foot-pound-seconds as energy, unless I stop to think about lifting something a foot in a second. But, I know about what a Watt and a Joule are. They are just the units I'm used to.
I know 30psi or so is about the pressure in a car tire. Twice that is the tire pressure in a trailer tire or what an air wrench runs at, half is about standard sea-level air pressure. But, I also know that sea level is about 100kPa, car tire pressure is about 200kPa, the air wrench runs at about 400kPa, and so on. It is about developing internal references, not how arbitrary or complicated the system is.
The faster we switch make the change, the less it will cost and the faster people will get used to it.
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