xkcd #3138: Dimensional Lumber Tape Measure
Title text:
A person with two watches is never sure what time it is, especially if I got them one of the watches.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3138/
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Interestingly enough, this concept was used in pattern making for casting machine parts back before modern machining and parts manufacturing.
They were colloquially called shrink rulers, and looked like a standard ruler, but were actually longer to account for the shrinkage of the material being cast.
For example, say you’re casting a part from iron, which shrinks 1% as it cools, which amounts to 1/8 inch per foot.
An iron shrink rule would look standard, but actually measure a foot as 1 foot 1/8 inches to account for the shrinkage (this is an example and not meant to be actually accurate).
Source: am historian that interviewed pattern makers that used shrink rulers in their work.
Edit: spelling
It shrinks? Like a frightened turtle!
That’s what happens when things cool down… It is the cold, that why it looks small 😳
I need to get a shrink tape ruler like this. I own a skoolie (used school bus converted to a motorhome) which is 35’ 4" long from bumper-to-bumper. A lot of campsites have rules where RVs can’t be longer than 35’. My thought was to get a tape measure with feet just slightly longer than normal and use it to make my bus appear to be shorter than 35’.
Just when I thought I managed to contain my metric arrogance…
harrumph…
Those nonsensical rules exercise the brain more. Helps stave off mental deterioration.
Carpenters in the USA have a higher mental acuity at advanced ages than scientists
Maybe they just measure their age wrong.
I don’t think so, they are all 6 doggers old or more
It’s from a time when you bought undried and planned wood rather than dried and planned like we typically do now.
It’s less a quirk of the imperial system and more a quirk of the lumber retail system, which is older than the metric system.
The biggest difference is that in places that use dimensional lumber and the metric system the pattern is to sell by actual dimension, rather than nominal. So a wall stud might be 45mmX145mm, or 63mmX75mm for a rafter, depending on your country.Most north American hardware stores also sell by finished sizes now.
At one time a 2 x 4 really was 2" x 4". Very old houses will have these in the walls, not planed and quite rough and splintery. I think I still have splinters from the 1913 bungalow I renovated more than 30 years ago.
I’ve worked on a lot of pre-1900 houses (I even grew up in one) and the 2x4s from back then really were 2" x 4" instead of the modern 1.5" x 3.5". Two years ago I bought a house built in 1942 and I demolished one interior wall and re-used the studs from it to build some new walls. I kept building these walls 1/2" too tall even though I measured and re-measured the spaces I was putting them into very carefully. I eventually realized that these 1942 studs were not in fact 1.5" x 3.5" like I had been assuming, but were actually 1.75" x 3.75" (the extra 1/4" in width of the top and bottom plates of my walls is where the phantom extra 1/2" was coming from). So apparently there was a transitional period between the real 2" x 4" 2x4s and the 1.5" x 3.5" ones.
I discovered another weird transitional thing in this house. The old houses I worked on all had lath-and-plaster walls, with strips of rough wood lath covered with a thick rough plaster layer which was in turn covered with a thin smooth plaster layer. Modern houses of course use sheetrock, but my 1942 house covered the bare studs with 16" x 16" pre-formed interlocking blocks of 1" thick rough plaster, and then smooth plaster was laid over these blocks. I first encountered these when tearing down the ceiling in my kitchen, and these things were unbelievably fucking heavy. They basically weighed as much as solid stone of these dimensions, and I can’t imagine what it must have been like to install them initially. It surely must have been a two-man job.
Edit: another fun experience I had was renovating an Atlanta house that had been built in 1843. When we tore down the original lath-and-plaster walls, we found embedded in every single wall and ceiling a single dead, flattened rat. That house must have stunk to high fucking hell when they first moved into it. I like to imagine that it had been built with slave labor and this was some well-deserved payback.
Interesting. We found some 3/8" drywall in the 1913 house, dating from some renovations that appeared to have been done in the 1950s or 60s. We also found a mummified sandwich.
This hurts so bad to think about
Which part? The ruler that can’t exist or the part where finished lumber is smaller than the listed size?
The thought of reaching into my toolbox and pulling out a measuring tape that’s labeled wrong without knowing it. He did a good job with this comic. That thought sucks so bad.
…the ruler exists, it’s a metric ruler. This is a metric joke comic. The 2x4 sizes are in metric…
You are welcome.
No, this comic is about nominal vs actual lumber size. Both tapes are imperial.
I want whatever crack you’re smoking that made this make sense
Overconfidence and bad eyesight I guess.
Huh, the explain link says the dimensional sizes originated from the wood being cut at the listed size while green, then shrinking as it dried. I was told that it was done for construction purposes, where the wood would likely be covered by plywood or drywall that would bring the dimension up to size. I never questioned it before; that always seemed plausible enough.
None of the above is true, or at least isn’t the full answer for why today a “2x4” is missing an entire half an inch all the way around. The shrinkage due to drying is around 5% (and the real math there is more complicated, as wood shrinks different amounts in different directions relative to the grain), which would only account for 1/10" of difference in the thickness of a 2x4. With some species of pine it’s as low as 2%.
No, the lumber industry has consistently shaved boards in order to fit more into rail cars for transport and make more money and spend less per plank on transportation costs. Various lumber consortiums determined via internal testing that the smaller board sizes are still “sufficient” for their intended purpose vis-a-vis structural integrity of stick framed residential buildings.
Of course the answer is “capitalism”. It always is.
that’s one of the common excuses that the mills quote. It’s bullshit of course, but it sounds plausible so they continue to get away with it.
Another bullshit excuse is that they’re providing an additional service by milling and planing the lumber for you, and that the nominal measurement is before that process.
It’s all just greed. If they could get away with selling a 2x4 that was half an inch thick, they would. At least it’s all standardized now.
I don’t think this is true. There was a transitional period around the 1940s where 2x4s were 1.75" x 3.75", and that wasn’t because wood shrunk half as much as it does today.
There is no sensible reason to sell something as a dimension before it’s ready to sell, a 2"x4" should be so when sold, not when curing, it takes nothing to cut oversize to accomodate for shrinkage, or to cure and cut later at the right dimension
Modern lumber is planed, so some of that difference is because of losses from that. If you open up the walls of a house built 100+ years ago, you see these thick rough wall studs that never went through a planer. Even with shrinking, it’s close to being actual 2" x 4".
That will go nicely with a tape measure that uses the Chinese inch (cùn), which is equal to 1.312 imperial inches
I had a client who wound up with one of those not realizing what it was, which caused him no end of problems until I ultimately figured it out confiscated it from him. He got a regular US inch one in exchange. I had to look it up at the time, too, because the notion of there being a Chinese knockoff inch that’s subtly inaccurate is one of those things that just seems so ridiculous on its face that it simply can’t be true, right? Except it totally is.
An Inch meant something different for most countries not too long ago. If the Chinese inch is a knockoff, then so is the US inch. Only the UK inch is the one truly inch!
Except it’s not. It’s simply a completely different and unrelated unit of measurement, which was dubbed colloquially in the west “Chinese inch”. Calling it a “Cinese knockoff inch” is like calling the yard a “US knockoff meter”.
I mean, that’s a valid assessment of the yard as well if you’re looking at it from a comedy perspective.
Yes, calling the yard a knockoff of the meter would be kind of funny, especially since it predates it by about a thousand years. And calling the cun a knockoff of the inch is similar, since it predates that by another thousand years.
Just like how Hydrox are just knockoff Oreos!
Not Chinese imperial inches?
ITT: people arguing about how to measure their wood
A tale as old as manhood
I feel this was the true decline of America.
Its 2x4 before drying and planing.
Maybe I’m being daft (I probably am).
After wood is planed and dried, I thought it would have gotten smaller.
In the image, 9CM lumber measurement is smaller than 9CM metric. Meaning when 9CM lumber shrinks it’ll be even smaller than the 9CM metric.
Have I got this backwards?
The joke is when you purchase a 2x4, it measures 1.5x3.5 and not the advertised 2x4.
lol, learn something new every day!
The tree was 2x4, I’ve no idea what they’ve done with it.
And that’s what men the world over use to measure their appendage!
As if they did not have enough issues with their backwater imperial units…
This is how you become a character in House of Leaves.
You don’t even need a custom one. Readily available is the universal tape measure where the relationship is marked as 2.54 units, equals 1 inch.
It means that a 2 x 4 is actually 5.08 x 10.16, which leaves you with a margin for surfacing and sanding.
american lumber doesn’t use standard imperial units. an “inch” isn’t the standard you’d expect on a so-called “nominally” 2 inch by 4 inch board. just attempting to use metric without accounting for the extra wackiness added by lumber ‘measurement’ on top of standard imperial silliness, you’d end up with a bunch of errors.
tl;dr: a 2" x 4" board actually measures something like 3.81cm x 7.62cm.
I understand.
I was making a joke about secretly introducing a metric tape measure.
I realise that my sense of humour is not universal, like the use of SAE … or Fahrenheit 😇
Can still work as long as you use the same ruler and just use referenced measurements.
That 8” mark though 😂
I was sort of expecting a more in-your-face callout of the 6" being at the 5.5" mark, considering most men…
