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Thousandth of an Inch

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Re: Thousandth of an Inch

#21

Post by 0_____0 »

seniorsassycat wrote: post #11
synack wrote: post #10

When doing PCB layout, you're basically forced to think in thousandths, but work in millimeters... You end up memorizing multiples of 2.54mm (0.100in)

Why is that? Common components use thous but tooling moved to mils?

My sophisticated wild-ass guess: Film and sheet thicknesses are often specified in mils, and PCBs are essentially fancy sheets of stuff laminated together.

My unsophisticated wild-ass guess about why they persevere: Like Fahrenheit, it so happens that mils as a scale intuitively correspond to sizes of trace we care about. A 5 mil trace is pretty run of the mill for signals. 20 mils for most IC supply traces. 3 mils is small and expensive to make (last I checked! capabilities have changed rapidly in my decade of experience)

Re: Thousandth of an Inch

#22

Post by klodolph »

taneq wrote: post #17
akgoel wrote: post #6

Like the article says, we machine parts with tolerances in thousandths of an inch, but certain specifications like paint thickness are specified in “mils” which is also thousandths of an inch.

I think it was all over once America started using ounces to measure volume.

Ounce / fluid ounce is roughly the same as gram / milliliter. A fluid ounce of water weighs about one ounce (1.04 oz/fl oz or something), like a milliliter of water weighs about one gram (0.997 g/mL or something).

The US customary units are pretty logical, they're just not powers of 10.

Re: Thousandth of an Inch

#23

Post by russdill »

seniorsassycat wrote: post #11
synack wrote: post #10

When doing PCB layout, you're basically forced to think in thousandths, but work in millimeters... You end up memorizing multiples of 2.54mm (0.100in)

Why is that? Common components use thous but tooling moved to mils?

A lot of tooling and tolerances still uses thousands of an inch as well, complicating the use of a metric grid.

Re: Thousandth of an Inch

#24

Post by nickpinkston »

seniorsassycat wrote: post #18

Curious why we jump from 16ths or 32ths to thous rather than 64ths, 128ths, 256ths, 512ths and 1024ths I guess they are a mouthful without good abbreviations and there was a tech jump between woodworking and machining so in betweens aren't used

I had an old machinist ruler that actually did have 64ths - barely could read it haha - time to get out the calipers!

One reason these whole even numbers are used is because it's easier to inspect with basic tools - ie just fold/layer it a few times and measure the thickness - like using a rule to measure 64 sheets of 1/64" stock and hoping to get 1". The evenness is about doubling/dividing to achieve this effect.

They also had very primitive measures for bore size that worked the same - ie "gauge" for shotguns was the number of how many balls of the barrel's size added up to 1 pound of lead, so you could have a basic scale and infer everything from there.

Re: Thousandth of an Inch

#25

Post by bittercynic »

seniorsassycat wrote: post #18

Curious why we jump from 16ths or 32ths to thous rather than 64ths, 128ths, 256ths, 512ths and 1024ths I guess they are a mouthful without good abbreviations and there was a tech jump between woodworking and machining so in betweens aren't used

I don't think it's usual to make that jump, but to use fractions in some contexts, and thou in others. Around a machine shop the measuring devices go by decimals, and in wood shops they use fractional measures.

Re: Thousandth of an Inch

#26

Post by Cerium »

rootusrootus wrote: post #12
akgoel wrote: post #6

Like the article says, we machine parts with tolerances in thousandths of an inch, but certain specifications like paint thickness are specified in “mils” which is also thousandths of an inch.

Another really common one is plastic sheeting. That's probably the item I most often personally come into contact with that is measured in mils. (that is, things I need to know the thickness of, which excludes the paint all around me)

Disposable gloves are also measured in mils. When I bought a micrometer one of the first things I measured was a disposable glove and I was concerned that the measurement was really wrong. Then I searched online and found that the specified thickness is only on the finger pad area of the glove. Measuring there was dead on.

Re: Thousandth of an Inch

#27

Post by rtkwe »

seniorsassycat wrote: post #18

Curious why we jump from 16ths or 32ths to thous rather than 64ths, 128ths, 256ths, 512ths and 1024ths I guess they are a mouthful without good abbreviations and there was a tech jump between woodworking and machining so in betweens aren't used

It mostly stops around 32nds to at most 64ths at best in woodworking where it gets hard to measure accurately using a tape and the Mark 1 Human Eye. Then in machining there's usually digital or dial readouts for things so it's easier to work in a round number like 1000s.

Re: Thousandth of an Inch

#28

Post by tedunangst »

seniorsassycat wrote: post #18

Curious why we jump from 16ths or 32ths to thous rather than 64ths, 128ths, 256ths, 512ths and 1024ths I guess they are a mouthful without good abbreviations and there was a tech jump between woodworking and machining so in betweens aren't used

"instead of our engineers and machinists thinking in eighths, sixteenths and thirty-seconds of an inch, it is desirable that they should think and speak in tenths, hundredths, and thousandths"

Re: Thousandth of an Inch

#29

Post by rootusrootus »

Cerium wrote: post #26
rootusrootus wrote: post #12

Earlier quoted context omitted.

Another really common one is plastic sheeting. That's probably the item I most often personally come into contact with that is measured in mils. (that is, things I need to know the thickness of, which excludes the paint all around me)

Disposable gloves are also measured in mils. When I bought a micrometer one of the first things I measured was a disposable glove and I was concerned that the measurement was really wrong. Then I searched online and found that the specified thickness is only on the finger pad area of the glove. Measuring there was dead on.

> the specified thickness is only on the finger pad area of the glove

TIL. Thanks for sharing that, this is handy to know.

Re: Thousandth of an Inch

#30

Post by Jedd »

seniorsassycat wrote: post #18

Curious why we jump from 16ths or 32ths to thous rather than 64ths, 128ths, 256ths, 512ths and 1024ths I guess they are a mouthful without good abbreviations and there was a tech jump between woodworking and machining so in betweens aren't used

Yeah, it's a bit of weird disjoint. Almost a tacit admission, in fact.

I've heard that in some crazy parts of the world - say 95% of the planet - it's base-10 all the way down.

Makes you wonder if they're on to something with that kind of whacky scheme. Hopefully it works out for them.

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