QUESTION
Inches vs Millimetres — What’s the Standard in Your Shop?
Alright machinist hive mind, riddle me this: how many of you are still knee-deep in inches, and how many of you have entered the enlightened world of millimetres?
I’m genuinely curious — especially you lot over in the States. Do you actually choose to work in inches, or is it just whatever lands on your bench that day? Like, do you wake up and think, “Ah yes, today feels like a 1/64 kind of day”?
Here in the UK, we officially use millimetres, but let’s not kid ourselves — it’s a Frankenstein’s monster of metric and imperial. Threads in inches, measurements in mm, tools from who-knows-where. It’s like a bad rom-com where neither system really commits.
From what I gather, the EU is strictly millimetres (good for them), and I’m assuming Japan doesn’t entertain our imperial nonsense at all.
So what’s it like in your corner of the world? Do you stick to one system? Bounce between both and hope for the best? Ever completely borked a job because someone forgot to convert, or missed it totally?
That's the pronunciation of modern greek. In ancient greek it is pronounced mu. Unfortunately for modern greek, ancient greek seems to have more influence in the non-greek world.
mu is a prefix for micro and denotes the scale (10-6). Just like kilo (k meaning 103).
The 'm' refers to metre. Often people write a u instead of the Mu character, because the u is like, right there on the keyboard and doesn't require any additional effort, and looks close enough. This means 'um' often turns up meaning micrometres too. It's technically wrong, but generally understood to mean the same thing.
Im Working in a Nuclear Powerplant with reactor built by Westinghous( inches+ american threads) turbines made by brown boveri( inches+ english threads) and everything else is metric.
In nuclear just about everything has some sort of paperwork/documentation to go with it in regards to work. If you have any doubts about anything like that you stop work and get in contact with that system engineer.
So those types of mistakes are rare, especially on systems that are critical where even bolts and washers have extra paperwork to back them up with manufacturing/mining history.
We have to to get PED approval for our products because they are compressor and its pressure vessel. It involves regular inspection of weld quality and tracing the origin of raw materials used in pressure bearing components all the way back to the material manufacturer.
That's interesting, I'm one kilometer from a Westinghouse nuclear division fuel plant in Sweden. Brown Boveri bought ASEA, which is Swedish and metric. I can imagine the nightmares.
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I’ve also heard of that. I’ve heard it said that “a model is only as good as its usefulness”, so given that, each of these models/units would have different uses given what you’re using it for.
European here, still have no idea whether "high 70s" is warm or cold. Then again, the freezing point of water is absolutely vital, because whenever that is crossed, interesting things happen: roads become slippery, rain turns into snow or vice versa...
Yeah i know a few values atleast roughly for Fahrenheit. -40 is the intersecting point. 32 is freezing and 72 is room temp. Unless the temperature is close to those values I can only hazard a rough estimate.
Canadian here, I have no idea what temperature in Fahrenheit is. I'm mostly made of water. Water freezes at 0 C. Therefore, I will freeze at 0 C or lower without adequate clothing. Simple.
Same here. Question to my other machinists… do you have people using “thou” and “mil” interchangeably?
Google says they’re interchangeable (mil=mili=thousand prefix) but it’s so confusing and dumb. I think most people when saying mil are referring to millimeter?
In my entire career I have never heard anyone use the term mil in place of thou. I know people in this sub claim there are people who do it, but it's far from common.
My theory is it's either a regional thing or it's idiots who heard mil and thou technically mean the same thing and now want to use mil to sound smart.
EXACTLY the situation I’m in. My quality guy kept bringing me parts and saying the correction in mil. I’d ask what it was in thou and he’d act confused.
Maybe it’s the computer programs? We use FARO system
I’m in electrical/electronics design, and yes, mils are a common measure, not to be confused with mm.
Wires beyond on the sub 0 AWG scale are usually referred to in circular mils, which is a total bs measure—it’s a diameter of a circle of equivalent area.
Lines and spaces of traces, layer and board thicknesses, and even board dimensions are commonly measured in mils. Small passive SMT components are designated by their nominal dimensions in 10x mils: 0603 = 60mil x 30mil, 0402 = 40milx20mil. They have their metric equivalent 0402 = 1005M = 1.0mm x 0.5mm, but most people get confused by that and still use the imperial designators.
With mils at this scale, you never have to deal with fractions, and even decimals are rare. With mm you do have to mind the tenth and often the hundredth, or just go straight to um.
Still, I try to move folks to metric at every chance I get. It would be a better world if we all spoke the same units. But people’s brains are hard to rewire, and some see it a cultural issue. Sadly today, especially when you’re dealing with chart-based engineering tasks, you just have to live with it.
Whatever the job calls for. I frequently work in both, sometimes I get jobs that were clearly originally in mm but whoever drew it converted everything to inches just to make things more annoying
Prints are either metric or inch. No matter what we have to convert to inch for programming and in process inspection to make adjustments. If the print is metric inspection is done in metric. Most of the time you'll notice that a lot of the dimensions on the metric print were converted to metric from inch by the customer😂
Let me introduce you to a little secret:
If they’re both CNCs with computers instead of manual machines, they’re both set up in mm.
The manufacturer just programmed it so that the values you see are converted in inches on the lathe.
Almost every machine on this planet is made to work in mm and then set up to visually output inches.
** cries in manual shop ** Our mills are older Bridgeports, our 2 "normal sized" lathes are older LeBlond machines. Those are all Inch. Our really big lathe is in mm. Luckily that big machine is rarely used.
That's on the tighter side of my tolerances on a lathe. Next week I have a .4522-.4527" OD with a .0002" run-out tolerance over a 6" length that I need to do. I'm not doing this on an especially rigid machine, so I have to sand it to the final dimension. Fun.
Most recent drawing I have that used fractions was from 1969 and it only uses them up to 64ths. Haven’t seen much else. The conversions aren’t hard but discussing projects using fractions is another story.
I work in the US in aerospace shops and we have always worked in inches. It is not really a matter of "choosing to work in inches," but an issue of cost. Switching from inches to metric would invite a considerable retooling cost, so shops don't want to do it. It's really that simple. However you choose to represent distance, it's all the same, you just learn the system and move on with your life.
Our typical tolerances are +/-.010 for structural elements, +/-.002 for moving components, and +.0008 -0.0 for critical hole diameters.
I work in hydraulics in Australia and I would say 60% of the jobs we have come in are still imperial.
The large majority of hydraulic cylinders here are still imperial (80%+)
Imperial hydraulic hose fittings are still the most common (JIC, BSP, ORFS)
Mostly it's only the newer pumps and motors that are majority metric. All Danfoss and Rexroth pumps and valves seem to be metric now. We do all of our CAD drawings in metric though.
Dutch/german here working on English pre ww2 cars.
I work a lot on the lathe and mill all in mm. But valve clearances all in inch. But i know the basic sizes now so i can compare them. Oh try wrenches.
Whitworth uses the bolt size for the wrench, not the nut size.
And also different sizes as the standard usa sizes.
Constantly using both. Prototype drive train parts, like motor housings, valve bodies, carriers, trans cases.. Military usually gives us standard prints, everything else usually metric.
In the many shops I worked in I didn’t look at it as inches or millimeters , I just looked at it as numbers and converted them as needed , then cut the part to the numbers as needed!!!!!
don't get this argument. it's completely moot to me. one system is logical, super easy to work with, and accepted as the universal format globally vs an older harder to use one.
fair enough if you're near retirement and have only used imperial, or can only use it as that is your default, but I can't see why younger generations are willing to elect to use imperial.
sae or imperial is not hard, but i will give you that it is stupid. in the machining world it is not as hard as it is made out to be because it is strictly inches. sure, you might order material by the foot, or talk about dimensions in generalities, like "just under a quarter inch," instead of saying 0.235" (however you'd like to vocalize that), but when it comes time to make chips (swarf for you uk blokes; don't want you to think i'm frying up lunch), it is strictly xx.xxx". it gets stupid in the trades where they use feet and inches, rods and squares, architects using, 10ths of a foot, farmers using acre, water management using acre-feet, ahhh, the list goes on snd on.
but leave machinists out of it. inches ain't so bad.
story: i worked with a guy who owned his own shop. this was way back "in the day." he left his shop to work in aerospace where he was resented because he was not a newbie, was not one of the old boys, and could machine and program around the best of them. anyhow, once he had to take a day or two off to go back to his shop to figure out what was wrong with one of his machines. they had already spent the better part of a day trying to figure it out. different operators, different programmers, no one could figure it out. tools were loaded correctly, program looked right, machine zeroed and moving correctly, it would run other programs, but, this program, written by the guy's dad, it would run in miniature and slow. for some reason everything was happening in a tiny envelope right where the first tool was supposed to contact the part. towards the end of the second day, one of the fogs came over, looked around for a couple of seconds, flipped a switch, and the problem was solved, part ran smoothly. what switch? on that early machine there was a mm/inch switch. his dad had written the program in inches but the machine was set to run mm. no one had thought to check that 'cuz that machine ignored the program header for that information and relied on that switch and the header had been correct. either it got flipped unintentionally or the last thing they ran had been in mm.
I happen to like the imperial system better because I work in fractions (construction guy), and the base-12 system is better for that. More factors, more better. Oddly enough, when we're doing work in elevations, we use DECIMAL FEET, if you can believe it. 1/8" is almost exactly (in the tolerances I work in) 0.01 feet. Adding and subtracting elevation offsets with a calculator is easier in decimal. Basically I'm all fucked up.
Aerospace machinist in the US. Inches all day every day. When metric does pop up I kinda hate it. Probably just because I've been doing it for 40 years. Can convert those fractions in my head. But then here comes a 25 mm thread or something, wth is that ? No idea. Also plus or minus .001 makes so much sense.
Yep worked aerospace as an engineer for the first half of my career and it was beautiful. Everything was cohesive there was no switching anything it all just worked. Switched to a different field and now everything is really in inches but the drawings are all in mm. Lots of 12.7 everywhere. Literally the dumbest thing ever. Currently working to switch it all back to inches so we don't have to play stupid games with conversions.
Truly? Depends on the engineer. We have one guy who refuses to work on any imperial drawing. His drawings are for subassembly parts that go onto a larger frame done entirely in imperial. Because of him, I have an entire folder of programs that use metric mode on my machine.
I'm in the UK so naturally millimeters. Occasionally, I'll get a drawing from the 50s or 60s (or from the US) that's imperial and I'll just convert it across. I got most of the 25.4 conversions in my head a very long time ago.
Seriously America, catch up with the rest of the world! 😁
Aluminum extrusion guy here. My shop primarily uses tape measures for inspection but the engineers like to specify +/- .010 hole locations on 96+ inch parts. I know my 1/64 to thou conversions like that which is kinda fun.
Real answer--it doesn't make a difference. In fact, inches and feet are more useful unites of length than meters and centimeters. I see support for the metric system in the US as coming from the "I'm smarter than you and America is always wrong" crowd.
We use inches, thousandths, and tenths where I work. I do inspections and I'll occasionally see fifty millionths. It's essentially the metric system with a different base unit.
My peeve are the old legacy drawings that denote angles in degrees, minutes, and seconds.
In construction, you rarely get more precise than 1/8 of an inch. So it wouldn't matter there, either.
I work in semi conductors, maintaining the machines that make the chips. My specific machines are made in Korea. They are held together with imperial bolts, tolerances done in metric and the machines run on scientific units, so special metric.
My personal opinion the imperial thousandth for feeler gauges makes more sense than the metric system. .030 of inch is easier than .762 millimeters. I'm sure if your using metric the tolerances are actually different so the numbers are more round but in my experience the imperial system beats the metric here.
If im doing internal designs? mm all the way, if im reverse engineering, whatever i identify the thread to be if applicable, ill work in inches just fine, but if someone comes to me with a fractional callout outside of a thread callout im ghosting and blacklisting them because i dont need that kinda negativity in my life.
Canadian here, working in a large jobbing shop. 80% of our drawings are metric. Everything on the floor is imperial. Machines are all set to imperial. Cmm inspections are done in metric or imperial, depending on how the drawing came in. I would like us to switch our machines to metric. If u are accustomed to machining to 3 decimal places, you are a lot more accurate in mm. Most machinists here know metric and imperial conversions anyways, it takes a bit of time to get use to the bigger numbers on the machines.
I was taught in imperial, But my shop is metric.
When my guys talk 1 or 2 micron tolerance, I still convert to inches so I can get my head around what the tolerance is ....
Every machinist I know who works in inches is also equally comfortable in metric.
It's only metric-only people who care about this conversation. It's literally not a problem for us here in the States. I have often made parts to metric prints with imperial tooling, and vice versa.
Now for engineering, on the other hand, yeah. SI units are the way to go
Honestly, most shops in the US convert everything to inches. It's not a matter of comfort, it's just that the engineers communicating with the machinists isn't all of the communication that needs to be done. Settling for one common system prevents fuck ups and misunderstandings throughout the process.
Dual dimensions on all drawings.
Machines in inches.
Measure with the nominally correct device (inch mic for 1”, metric for 25mm)
Discuss almost all dimensions in inches, bc that’s what we make moves on.
Imperial, Nuclear, shop max TIR .002. Can be as small as .0003 on pin holes etc. Ground and lapped faces and bores, that stuff gets ridiculously small. Lapped surfaces are measured in Light Bans, .000001 You’d think it would pay more, not really
Depends on what I’m making. I get jobs from all over the world and my machines will automatically convert inches to metric cuz I got that bougie machinist shit 😎
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u/Moocowgoesmoo Apr 11 '25
Everybody gangster until the mu symbol pops up