Comment on xkcd #3138: Dimensional Lumber Tape Measure

• captain_aggravated@sh.itjust.works ⁨2⁩ ⁨days⁩ ago

  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.

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  • alsimoneau@lemmy.ca ⁨2⁩ ⁨days⁩ ago

    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.

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    • captain_aggravated@sh.itjust.works ⁨2⁩ ⁨days⁩ ago

      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.

      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. The tabletops overhang the edges of the carcass 7/8", and the legs are 1 3/4". So the area between the legs that the doors fill is 3’ 6 3/4". 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 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.

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