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 😳
Neat!
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’.
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.
My sawyer showed me a measuring tape graduated in tenths of a foot. Someone manufactured that for some reason.
Oh yeah, we use these “decimal feet” rulers at my job, for surveying. Nobody bothered to tell me ahead of time, so my first few days out in the field were pretty much a complete waste. I still maintain they are an abomination, the worst of both systems.
If it’s used for surveying, aren’t those survey feet? (a survey foot is 1.000002 US customary feet). Dividing a survey foot by 12 would not be technically an inch, even though it’s only a couple millionths off.
…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.
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
Frankly, using a base 12 measurement system solves more problems for a woodworker than a decimal system does. It works very well for the task of woodworking. I’m familiar with and use the metric system for other things but I’m never building furniture in centimeters.
Genuinely curious. Mind giving an example?
Okay, a simple mortise and tenon joint. If I cut my board to 3/4" wide, if I want a tenon that is half the width of the board, it is 3/8" with 3/16" on either side. All my tools have these markings, I have router bits and such that are these sizes, easy. If I want a tenon that is 1/3 the width of the board, that’s 1/4" with 1/4" on each side. Also quite easy to find tools for.
In metric land, they often mill wood, or manufacture plywood, to 19mm. Because that’s quite close to 3/4". Show me a half, or a third, of 19mm on a metric tape measure.
You’ve got a 4 foot cabinet with 3 doors in it. How wide is each door? 1 foot, 4 inches. You’ve got a 400cm wide cabinet with three doors, how wide is each door? 133.3333cm.
Now do it with 5 doors.
You can always make up examples where one is easier. The truth is the easiest one is the one you’re used to.
48" / 5 = 9 3/5" = 9 9/15" ~= 9 9/16" or 9 5/8". Dividing by five gets a little messy, but I divide by 2, 3 and 4 a lot more often than I divide by five. Thing is, that works out to be some pretty narrow doors, like, middle school locker narrow. You can indeed contrive scenarios where the math is ugly, but inevitably the cabinet you’d make would also be ugly. In actual scenarios you face in the real world it has a way of working out.
I’ll give you a real world example. I recently built this dining room cupboard and hutch. The absolute overall width of the cabinet is 4 feet at the tabletops. The tabletops overhang the edges of the carcass 7/8", and the legs are 1 3/4" thick. So the area between the legs that the doors fill is 3’ 6 3/4" (4’ minus a total of 5 1/4"). The upper doors are 1’ 2 1/4" and the lower doors are 1’ 9 3/8". In reality each is 1/16" narrower than that to allow for some space for the doors to swing open and closed. The drawers have a 3/4" thick bulkhead between them, so each opening is 1’ 9", and the drawers are 1/8" narrower than that to allow a 1/16" gap on either side so each drawer is 1’ 8 7/8".
The leg dimension was chosen so I could have two layers of 3/4" boards, one for internal structure one for the outer rails, doors etc. and still have the legs stand 1/4" proud to make the legs look like legs (which they are; they’re genuine posts) and to hide any impreciseness in fitment or milling of the rails, doors, drawers etc. The top overhang on each side is half of the leg’s thickness, and then every dimension after that comes from the plan of the cabinet.
Tell me that wouldn’t have been a pain in the ass to do in metric.
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.
That will go nicely with a tape measure that uses the Chinese inch (cùn), which is equal to 1.312 imperial inches
Not Chinese imperial inches?
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.
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.
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.
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
The carpenter or woodworker cares about how big the board is. The sawyer cares about how much of the tree it took to make that board. The lumber yard has to make those two ends meet at a price point.
If you sell me a 2" by 4" board, you cut it 2"+the width of the sawblade, just like the metric boards are sold.
Also is a saw really 1/2" wide? That’s like 1.27cm, that’s wild.
If you really really cannot sell a true 2"x4" at the price of a 1.5" x3.5", just keep the same price and call it what it is.
These boards are factory planed, not sawn.
I feel this was the true decline of America.
ITT: people arguing about how to measure their wood
A tale as old as manhood
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.
And that’s what men the world over use to measure their appendage!
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…
As if they did not have enough issues with their backwater imperial units…
Its 2x4 before drying and planing.
The tree was 2x4, I’ve no idea what they’ve done with it.
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!
Eh, not a problem at all. You can’t make it look exactly like my other three and I make sure to use the same tape measure throughout a project, really after the first couple cuts I’m not even using it I’m using the cut pieces to measure against.
I don’t care what the number actually is, just that I can mark in the same spot consistently.
This is how you become a character in House of Leaves.
