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UK back to "imperial" measurements ?

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Benta:

--- Quote from: TimFox on May 30, 2022, 08:53:55 pm ---It brags about measurement precision down to low millionths of an inch.

--- End quote ---

Yes. Those would be the reference Jo Blocks kept in a temperature-controlled safe.
The calibration blocks would be good to 1/1000 mm, the production blocks at around 5/1000 mm.
Normal practice.

TimFox:
A cool thing about high-quality "Jo Blocks" (and competitors' such as Starrett and Mitutoyo) is how they "wring" when you place the broad faces against each other, due to the flatness and other properties of the blocks beyond their calibration accuracy.  It can be hard to pull them apart if mated properly.
The wikipedia article  https://en.wikipedia.org/wiki/Gauge_block  discusses how to wring them properly.

nctnico:

--- Quote from: Benta on May 30, 2022, 08:11:57 pm ---@TimFox:
which proves my point. If there's no common reference, you're deep in manure. Henry Ford and US industry were smarter. Imagine specifying a precision-cut 1/4" milled slot when the supplier doesn't have the same inch size as you.

--- End quote ---
And that isn't far fetched. Gas piping thread sizes in inches are referenced to 1 inch being about 33.7mm. Try measuring that with a caliper that has 1" = 25.4mm.

TimFox:

--- Quote from: nctnico on May 30, 2022, 10:45:33 pm ---
--- Quote from: Benta on May 30, 2022, 08:11:57 pm ---@TimFox:
which proves my point. If there's no common reference, you're deep in manure. Henry Ford and US industry were smarter. Imagine specifying a precision-cut 1/4" milled slot when the supplier doesn't have the same inch size as you.

--- End quote ---
And that isn't far fetched. Gas piping thread sizes in inches are referenced to 1 inch being about 33.7mm. Try measuring that with a caliper that has 1" = 25.4mm.

--- End quote ---

Pipe sizes are complicated:  smaller pipes for gas, etc., are approximately the inner diameter of the pipe, and with changes to wall thickness over the decades the actual inside diameter has changed somewhat.
"One inch" NPS1 is 33.4 mm = 1.315 inch OD.
The inner diameter is roughly 1 inch, but varies depending on the "schedule", meaning code for wall thickness.
Flexible tubing, however, is specified by its outer diameter.

IanB:

--- Quote from: TimFox on May 30, 2022, 10:54:47 pm ---Pipe sizes are complicated:  smaller pipes for gas, etc., are approximately the inner diameter of the pipe, and with changes to wall thickness over the decades the actual inside diameter has changed somewhat.
"One inch" NPS1 is 33.4 mm = 1.315 inch OD.
The inner diameter is roughly 1 inch, but varies depending on the "schedule", meaning code for wall thickness.
Flexible tubing, however, is specified by its outer diameter.

--- End quote ---

Pipe sizes are, however, highly standardized (even if there are many competing standards). If you know which table of pipe sizes and wall thickness schedules you are working with, then the pipe dimensions will be consistent and predictable.

One thing you can't do, is to assume a "one inch" pipe measures exactly one inch, either inside or outside diameter.

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