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Practical aspects of Metric -v- Imperial measurement systems


old man emu
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Skippydiesel said that, for aviation purposes, having a number of different specs base on the Imperial measurement system for fasteners is confusing when compared to having one spec as for Metric. That maybe so, but for construction purposes, I hold that the Imperial system of measurement is a lot simpler to use in practice.

 

The Imperial and Metric systems indicate the specifications for the materials the fasteners are made from. The names and history of the specifications are:

 

IMPERIAL

 

AN- ARMY-NAVY specification series started in the early 1940s as a means to standardize military items for World War II.  Mostly canceled in the 1950s, a few have survived to only a few years ago, AN3-AN20 bolt is one of the longest lived specs.

 

NAS- National Aerospace Standards, started in 1941 is handled by the Aerospace Industries Association a group of aerospace companies  The NAS series is best known for its state-of-the-art, high strength, precision fasteners. 

 

MS- Military standard started around the 1950s and for the most part replaced the AN hardware series.  However, a few of the AN standards have stayed around.  The MS series was canceled in 1994 by the Secretary of Defense, at the request of contractors in order to save money.

 

NASM- approximately 500 military standards were converted by the NAS group to commercial specifications, but retain the original MS part number.  The spec that defines the part is NASM and then the numerical portion of the MS number.  Example, MS20426 rivet spec went to NASM 20426, but part number stayed MS20426.

 

AS- Aerospace Standards created by SAE International (originally Society of Automotive Engineers) some MS specifications were replaced by AS standards.  Unfortunately, the part number changed to AS then the number of the MS part.  MS21919 cushioned clamps changed to AS21919.

 

METRIC

 

The metric specification system designates strength capabilities via property classes rather than grades. The numbering system is very simple; the number that appears before the decimal, when multiplied by 100, will provide the approximate minimum tensile strength of the bolt. In this example the 10 in 10.9 multiplied by 100 tells the user that this bolt has an approximate minimum tensile strength of 1,000 MPa (Mega Pascals).

Property class example

The number which appears after the decimal, when multiplied by 10, will provide the approximate yield strength percentage in relation to the minimum tensile strength. For the 10.9 bolt, the 9 tells the user that the yield strength of the bolt is approximately 90% of the first number: 1,000 MPa. Thus, the 10.9 bolt has an approximate yield strength of 900 MPa (940 MPa by specification).

 

MEASUREMENT

 

What about their physical dimensions?

 

At first glance, the Metric system, using measurements based on groups of 10 units seems easy to work with. Lengths are expressed in whole numbers, or the whole number divided by factors of two -  0.5, 0.25, 0.125. However as you start to get into small dimensions, it becomes difficult in practice (ie by a tradesperson) to be able to discern the space between portions of a millimetre, which is the usually unit used to describe small dimensions in manufacturing.

 

On the other hand, the Imperial system is based on one inch. Lengths are expressed by whole numbers or the whole number divided by factors of eight - 1/64, 1/32, 1/16, /8, 1/4, 1/2. For finer measurement, the base unit is 1/1000" and dimensions are give in decimal form.

 

Is it easier to make fine measurements accurately in the field using the Imperial system, or the Metric system. Is it easier to use a ruler to measure 1/64" or 0.396875 mm (OK, call it 0.4mm).

 

The biggest confusion in selecting aviation fasteners comes from the size coding for the overall length of the bolt (Grip length + thread length). Let's take a simple example - AN 4-14A, although the measurement system also applies to MS and NAS fasteners

image.jpeg.383f5e28135f5c9e141868271bb75b6d.jpeg

AN:  Army-Navy Specification for materials

4   : Thread diameter = 1/4"

-   : Cadmium plated steel with no holes through the head of the bolt from the faces

14 : The last number, either single or double digit denotes length. If it is a single digit, the length is that number of eighths of one inch. Single numbers have a possible range of 1 to 7 (1/8 to 7/8). If the number is a double digit, then the first number is the number of inches and the second is the number of eighths. So this bolt is 1 and one half inches long.

A   : This letter indicates that there is no hole drilled through the threads of the fastener to accept a split pin.

 

The thing to remember about measurements is that the single, or final digit does not use 8 or 9. And that the length of a bolt is measured from under the head to the end of the thread.

 

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The American imperial fastener system uses dashes on the heads for fastener grade strength, along with a "Grade" rating.

 

So, no dashes is a low-strength mild steel bolt, 3 dashes is Grade 5, a medium carbon steel fastener often referred to a "High Tensile" fastener.

 

But in tensile strength terms, Grade 5 is still only of modest strength at 120,000 psi  (827 MPa) tensile strength. It's equivalent in metric, is Grade 8.8.

 

6 dashes on the head is Grade 8, and a true "High Tensile" fastener, made from low alloy steel, and with a tensile strength of 150,000 psi minimum (1034 MPa). Grade 8 metric equivalent is Grade 10.9.

 

Grade 9 is not often found, but is used in applications where even higher strength is required, such as the bolts on track shoes on dozers, and many other heavy-duty industrial positions.

 

Grade 9 has 7 dashes on the head, and is rated at 180,000 psi (1241 MPa) tensile strength. The metric equivalent is Grade 12.9.

 

Even higher strengths are available in fasteners such as the socket head range of Unbrako fasteners - which are mostly rated at Grade 9 minimum, and up to Grade 12 (190,000 psi)  for some applications.

 

https://www.unbrako.com/images/downloads/engguide.pdf

 

Bolt Depot is the best reference site for fasteners.

 

https://www.boltdepot.com/fastener-information/measuring/notation.aspx

 

I still use a vast amount of inch fasteners and inch hand tools, because the biggest percentage of the equipment I own and work on, is American-built, or built in Australia under American licence.

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I don't see why the AN system is simpler? The metric system is not trying to convert imperial to metric eg 1/64 = 0.396875mm, it is metric so dimensions are in metric not a converted imperial measure. Shank length, thread length, grip, head size, thread pitch etc are all metric specs.

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The metric fastener standardisation system started off O.K. - ISO - but then the likes of the Japs decided they needed their own metric fastener system - JIS.

 

So we end up, that there are basically 4 competing metric standards for fasteners - ANSI, DIN, ISO and JIS. The Jap stuff is the worst, with their additional thread pitch variations.

 

https://grassrootsmotorsports.com/articles/understanding-metric-hardware/

 

Funnily enough, America uses the metric system all through science and industry - but consumers and fasteners are the only areas, where inch measure still prevails there.

Vast amounts of U.S. manufacturing is done in metric, on metric machinery, and then converted.

 

https://www.nist.gov/blogs/taking-measure/busting-myths-about-metric-system

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I really picked a poor way to open this discussion.

 

What I intended to convey was that it is easier to work out what half of a measurement is when using a system based on 8 divisions of 1 larger unit than it is with 10 divisions of a larger unit. Although it is also possible to get graduated straight edges where the inch is divided into 10 units.

 

How do you measure half of a half of a millimetre? My eyes can't discern spaces that small.

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Daffyd Llewellyn wrote a piece on why AN bolts are what he chooses for aircraft work, and Jabiru obviously agree, although those 2 sources are not really independent. Daffyd Llewellyn did Jabiru stuff at the beginning.

Personally, I reckon that using AN stuff is an easy way to keep out of trouble. For example, once our first club Jabiru had an annoying habit of bending a rear u/c bolt. Our maintenance guy of the time decided to try a high tensile auto bolt. He was not trying to save money, he just wanted to fix a problem.

A few weeks later, that bolt failed suddenly when the plane was being wheeled out backwards from its hangar.

Yes it had high tensile strength but it was also more brittle than the AN bolt it replaced. 

All ended well and we  learned a lesson about properties classes. Jabiru issued an AD changing the bolt to a bigger diameter one.

 

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AN fasteners are manufactured to much tighter specifications than standard SAE bolts - and they go through a lot more levels of inspection than standard fasteners.

 

Standard SAE bolts can have quite a substantial level of variability in shank diameter, depending on which company made them and even which batch they came from.

 

In addition, many standard SAE fastener threads are simply cut through the shank metal. Good quality fasteners have rolled threads - the shank is heated and the thread rolled into it, using formers.

 

This ensures the grain of the metal in the shank is not cut, as in cut threads. With rolled threads, the grain follows the thread form.

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AN bolts are quality controlled and batch identified and tested  but that is irrelevant to the Metric/Imperial measurement system comparison. I assume that European metric Aviation hardware is equally quality controlled. I can't imagine Airbus or any other aircraft manufacturer that uses the metric system (practically everywhere in the world except for the US & here), using standard metric high tensile Bunnings bolts that are not quality controlled, tested or unable to be traced.

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17 hours ago, kgwilson said:

I don't see why the AN system is simpler? The metric system is not trying to convert imperial to metric eg 1/64 = 0.396875mm, it is metric so dimensions are in metric not a converted imperial measure. Shank length, thread length, grip, head size, thread pitch etc are all metric specs.

There I go using the wrong example at the fork in the road. The idea I was expressing was that, for practical purposes, it is easier to see with the Mk I eyeball,  the half unit divisions of the inch than it is to see those of the metric system.

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OME, that may be the reason feet and inches are hanging on. Yes we can blame the yanks, but I still think of my height in ft and inches and tyre pressures in psi. Even without any help from a yank.

But these days I think of timber sizes in mm and m. Fuel in liters and of course electricity in volts and kilowatts.

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9 minutes ago, Bruce Tuncks said:

But these days I think of timber sizes in mm and m

If the metric system is the duck's guts, why is it that when I go to a hardware store, sheet material is sold in 1/8ths of 2400mm? Why is it when I go to by lumber, it too is sold in 1/4ths of 2400 mm? The same applies to buying lengths of metal.

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1 hour ago, old man emu said:

If the metric system is the duck's guts, why is it that when I go to a hardware store, sheet material is sold in 1/8ths of 2400mm? Why is it when I go to by lumber, it too is sold in 1/4ths of 2400 mm? The same applies to buying lengths of metal.

ome, I'm not quite sure I follow you there. Could you elaborate.

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Cos the hardware shop guy cuts it in half then half again( quarters) and even once more ( eighths)

I don't see why metric guys can't do this...  there is no imperial copyright on quarters for example. If you grew up with 3 sisters like I did then you got good at cutting things into quarters.

but here's a question  for you guys...  why are there seven days in a week and not some other number?

 

 

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1 hour ago, willedoo said:

Could you elaborate.

Ye Metric system is a system based on 10. So you would expect that products we buy, such a "full" sheet of plywood, or a full sheet of gyprock would be in a multiple of 10. So you would expect a sheet of plywood to be 2500 mm long x 1500mm wide. But it is sold in 2400 x 1200 mm. The same goes for gyprock, MDF and melamine. Sheet aluminium and sheet aluminium come in those dimensions. If the material is supplied in smaller pieces, the sizes are based on 300 mm increments, and 300 is 1/8th of 2400.

 

Likewise, if you want to buy lumber or tubing the most common lengths vary between 2.7 m and 5.4 m in steps of 300 mm. Once again, 300 mm is 1/8th of 2400. 

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I know that a standard room height is 2400mm or 2700mm. So sheets of plasterboard come in 1200 or 1350mm widths.

I have built rooms at 2550 mm height by using one sheet of each.  Maybe people pre-dated the metric system? I wonder now for the first time if standard room heights are different overseas.

 

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The recurring cycle of seven days comes from the fact that the appearance of the moon follows a roughly 28 day cycle. The average length of the synodic month is 29.530587981 days (29 days, 12 hours, 44 minutes, and 2.8 seconds). The synodic month is the average period of the Moon's orbit with respect to the line joining the Sun and Earth. This is the period of the lunar phases. The earliest uses of the Moon as a time-measuring device can be traced back to 28,000-30,000 years ago.

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AND !.

If you find your self on the proverbial desert island, without a measure.

YOU WILL find the Imperial system suddenly makes sense.

Your foot is the foot length. The first joint of your thumb  is an inch. 

And it goes on .

Don,t forget there is Twelve  segments on your four fingers. 

spacesailor

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23 minutes ago, old man emu said:

Ye Metric system is a system based on 10. So you would expect that products we buy, such a "full" sheet of plywood, or a full sheet of gyprock would be in a multiple of 10. So you would expect a sheet of plywood to be 2500 mm long x 1500mm wide. But it is sold in 2400 x 1200 mm. The same goes for gyprock, MDF and melamine. Sheet aluminium and sheet aluminium come in those dimensions. If the material is supplied in smaller pieces, the sizes are based on 300 mm increments, and 300 is 1/8th of 2400.

 

Likewise, if you want to buy lumber or tubing the most common lengths vary between 2.7 m and 5.4 m in steps of 300 mm. Once again, 300 mm is 1/8th of 2400. 

ome, the reason sheets are 2400 is as Bruce says, the standard wall height was 8 foot which they have now converted to metric measurements. there was simply no reason to change standard wall heights to 8'4"  to make the measurement divisible in metrics. To save mass disruption in the building and manufacturing industry, old imperial measurements were converted to their nearest metric equivalent. Most wall heights in modern times were 8,9, or 10 foot in height, hence 2400, 2700, and 3000 sheets.

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Conversely, the conversion to metrics worked out ok with bricklaying. Formerly, a standard sized bed joint of mortar was 3/8" and is now 10mm..

The introduction of metrics made bricklaying about 100 times easier when measuring and estimating gauge. Same could be said for the building industry in total. I could never figure out why schools taught kids to think of centimeters as the primary measurement. When they leave school, they find out everyone works in meters and millimeters, so have to learn all over again.

 

The good thing about metrics is that you can measure a wall 4800mm. long and divide easily by 4 to get 1200. It's a lot easier than dividing 15'9". If anyone suggested the building game goes back to Imperial, they'd get eggs thrown at them.

Edited by willedoo
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