Metric To Ansi Conversion

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Marybelle Bailey

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Aug 3, 2024, 4:12:35 PM8/3/24

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Two systems of weights and measures are derived from English Units: the US Customary System and the British Imperial System. Neither is dominant worldwide, but they do retain significant usage. For commercial and everyday use, the US Customary System is used in the United States, and the Imperial System still applies to volume and vehicle speed in the United Kingdom.

Therefore, it is important to understand US Customary and Imperial units and their conversions to metric. In fact, there have been some substantial errors in the past associated with unit-based misunderstandings. Most notably, in 1999, due to a mishap with one engineering team using English units while another used SI units, a $125 million Mars orbiter was lost to the great expanse of space.

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L-3 Communications RE: English to Metric Tolerance Issues MadMango (Mechanical)16 May 06 14:17Shouldn't one or the other be the controlling dimension that inspection inspects, with the other listed as reference? "Art without engineering is dreaming; Engineering without art is calculating."
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Have you read FAQ731-376 to make the best use of Eng-Tips Forums? RE: English to Metric Tolerance Issues TRRMan (Mechanical)(OP)16 May 06 14:39Yes in a perfect world it should be reference but in the real world I have China building my parts in mm and doing QC in mm because thats the tools they have and I have those parts being checked by my companys QC in English because those are the tools we have. I didn't make the mess I'm just trying to work with in it. We are checking to see if our QC can check everything in metric and at that point the issue goes away. I was just wondering if anyone else has had to deal with this and came up with a clean solution. I could loosen the tolerance but that could open up a whole host of other issues. I could make all the english dims 3 digits but then I would need to specify the tolerance everytime to make sure QC used the correct one and to do that on every drawing would become a problem. Thanks for any help. RE: English to Metric Tolerance Issues metalonis (Mechanical)17 May 06 08:49The problem appears to be in the conversion of the tolerances from inches to mm. Looks like the mm values were rounded instead of truncated. It's necessary to truncate the converted value so they end up being smaller than the original inch tolerance. That wy the millimeter dimensions will always be within the orignal inch dimensions.

For example, a 2.00 in dimension would have a size range of 1.99 to 2.01. The way you've converted the tolerance (0.01 in = 0.254 mm rounded to 0.3 mm) the equivalent range is 50.5 to 51.1 mm = 1.98 to 2.01 in. By truncating the converted tolerance to 0.2 mm you end up with 50.6 to 51.0 mm = 1.99 to 2.01 in. Everybody's happy. RE: English to Metric Tolerance Issues rconner (Civil/Environmental)17 May 06 10:23While I don't know what if any standards specifically address this issue, I think in principal I like metalonis's approach of converting the original tolerance to in effect get effectively the same allowable "range" of actual dimensions (in either unit). RE: English to Metric Tolerance Issues rconner (Civil/Environmental)17 May 06 13:48I have noticed ASTM E380, Standard for Metric Practice may in fact discuss "Rounding Tolerances Inches to Millimetres" (you may wish to obtain a copy of this standard). I think this may be allowed per this standard by a couple different explicitly defined methods, one of which "rounds to values nearest each limit" (meaning that even in the most "unfavorable cases" the two original limits will be changed by no more than a small percentage), and another that assures that the converted tolerances are never larger than the original tolerances (the latter of course necessary if "original gages" are used for inspection). googletag.cmd.push(function() googletag.display('div-gpt-ad-1406030293255-2'); ); Red Flag This PostPlease let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.
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The UK has switched gradually, with the latest units changed around the year 2000, I think? Today in the UK the old imperial units are only allowed for "drinking and driving", as one Englishman said on another forum. (=for road signs and beer bottle labels). But correct me if this info is incorrect. :-)

The USA has officially adopted the metric system in 1875 (yes, 18..., not 19...), although most politicians, government officials, and press people are not aware of this. So they keep converting these metric units back to imperail units for "convenience", although it is very inconvenient for them, and it slows down conversion in everyday life. This causes big headaches for US-technicians, engineers, industry, and all other internationally oriented businesses.

Although in daily life people tend to keep using their old habits of course, just like we still use horse-power for cars instead of kilowatt. I guess because the number in horse-power is 1.3x higher than in kilowatt, so it sounds more impressive. :-)

When using a format like 3MF, there is no need for a conversion, since the format itself simply defines the unit (which is used by Cura to convert into mm). Some formats (like STL) don't have this, but the most commonly used unit for STL's is metric (so Cura assumes this). If it is in inches, it's also rather trivial to convert it, as it's just scaling it by 25.4.

Also allowing for settings to be in multiple systems would be a lot more difficult, and add an maintenance burden on us. This is why we chose not to support it and keep all settings in the scientific (and thus metric) system.

I also don't think it matters that much. Settings might say things like 50 mm/s, but usually if people change it, they change it based on what's already there (eg; It needs to be a bit faster, so let's make it 60). For these relative changes the unit doesn't have that much impact.

Disappointed that Cura does not provide a unit selection. The statement that it is trivial to convert and then to state it is to difficult to maintain is not consistent. There are heritage designs in inches that will not just disappear. When trying to confirm that parts fit before building the real thing not having a simple conversion built into the tool has the potential to create problems...See Mars mission catastrophic failure due to incorrect trivial conversion! Agree with other commenter: What do you expect when it is free. I will try the conversion plug in soon. Glad to now understand the problem. FYI: Competitors 3D printer has the option for inches and metric and it works very well.

Makerbot....The real world requires all to be experts at conversion in many areas of science. Just wanting a no conversion world will not change reality. Units have been created by experts for hundreds..thousands of years and to make new discoveries typically require standing on the shoulders of Giants using their units. The market will decide when one product allows easy conversion for all they will have a larger market. Telling a customer what they need is not a great market strategy.

Then when I'm modeling for 3D prints, I just model in metric, most objects if you are copying something will be able to be measured by a caliper, wall thicknesses, hole spacings, etc. all can be done in metric with a caliper, if you REALLY don't even want to look at metric numbers, then get a multi feature caliper with inches, fraction and MM and you can measure it all out in inches, then push the button and see it in MM and keep going.

Well, it has been, because we (Ultimaker) have been using the metric system for ages. We get maybe 2 requests a year (if at that) about the conversion. Most of them simply see it as a minor (in)convenience.

This really could not be further from the truth. US mechanical engineers design in ANSI units of measurements. If you go into a engineering lab in the US you find 4-40 you don't find m3. Certain industries are almost exclusively ANSI units. Aerospace for example, a majority of systems are in ANSI units. Boeing, Collins, Honeywell, Raytheon, Lockheed, etc, etc, etc all ANSI. Airbus who integrates many of these systems takes them in ANSI. The joke is there's two type of units in engineering, the ones that have sent people to the moon and everyone else.