Designing Mechanical Components - English vs. Metric

I suppose there is a list somewhere but I haven't bothered to check.

AutoCAD has a feature where you can do your drawings in English units and then convert it to metric automatically. However, I suspect your question has something to do with items or materials that are sold in metric or English standards like:

• a 2 inch vs a 50mm flange
• a 1/2 inch vs a 12mm thick steel plate
• a 10 foot vs a 3 meter length of pipe

Whether it's more expensive to go metric or stay in English depends on several factors. Do the suppliers in your area stock English or metric (do they have to import if you need to buy one standard)? Is the size of the item larger or heavier in one standard or the other (a 1/2 inch plate is a little thicker than a 12mm plate)?

If you ask me, it's better to buy and design with what's available locally. In my place, English standards are available locally. If I go metric, the supplier has to import. This is a problem because my company is actually European-based so our standards are in metric units. We made a compromise. If I buy, say, a flowmeter, I'd specify the process connection should be in English units. The instrument, however, measures in metric units.

I guess the answer to your question is, it's up to you.

It very much depends on what system your country used in WWII. Here in Aus for example we technically converted to metric in 1976 however all that meant was the parts were measured in metric they were and still are, produced to Imperial sizes. so your 2" steel bar is still 2", bright bar is still 3 imperial thou less than 2" etc; It will be sold as 50MM bar though. Of course some things did change. Like the superfoot went up slightly in the 1990s sometime.

Pipes are also another mess. as they can be measured by ID or OD, metric or Imperial.

also we still use a lot of Whitworth thread here as it runs better and doesn't sieze like metric. sometimes this means metric pitch size whitworth thread design, metric hex....

For over 5,000 line items of stock in metric size steel in a variety of shapes sizes, and grades, that ship most items same day to Canada, USA and Mexico go to www.MetricMetal.com

It is not simply the unit system: the whole calculation strategy is sometimes different. (especially for vessels and piping)

Be sure that you know upfront which documentation the customer wants and to which standard the whole system will be reviewed. It is better to start working in this standard.

Also check your supplier: it might be that they don't know what a mm or an inch is. There have been serious mistakes in the past, costing lots of money through conversion errors.

Matching to the closest metric component after design verification: quite stupid as you will need to redo the whole work.

it might be that they don't know what a mm or an inch is

That is so right! Soon after moving to my present location (my wife and I didn't have any furniture whatsoever), I commissioned a guy to build me a small dinner table and a set of chairs. I did everything in millimeters. He looked at my drawing, scratched his chin and said,

"That's a rather large table, isn't it?"

"What do you mean? I used standard dimensions for the table and chairs. They should be fine."

"Well, I've never seen a 914.4 inch x 914.4 inch square table."

I had to redo the drawings.

This may be a little tenuous as I am not sure of the exact facts, but didn't a NASA satellite fail because they used the wrong units?

Sure did, Forgot to convert units. I think it was for retro burn and either over or under shot the target. Couple million burned up.

You would probally be better off using the base system to start. If your designing things for the US, you could actually use both, or be crazy and use SI/metric. It would be like IKEA.

What is wrong with IKEA?

The puzzle game for grown up men. I love it, solving the mess another man has made.

IKEA is Swedish, Sweden is a part of Europe, since Napoleon we tend to use the SI system.

The intentional SI system integration in the US (the SI system is the official US system) is going very slow. But NASA decided to swap over completely and to prohibit the imperial system. This might speed up the integration.

Gwen

Hi Gwen,

Absolutely nothing wrong with IKEA, except the weight of even the smallest coffee table, and we have had to re-inforce our floors to accept a wardrobe!

Regarding the English v metric system, I was brought up on English and (at my age) find that metric equivalents just do not relate e.g driving at 100 mph; a persons height of 6 feet; weight of 10 stones or 1lb; a gallon of any volume. Converting any of these into metric does not work.........you Europeans really missed out!

Quite funny,

100 MPH must be 160 km/h

6 foot man : that really is to difficult for me to translate. I can't imagine how tall he is. 1.76 is the average.

Stones are to be thrown to the police. and a pound is half a kilo (a bit more)

the metric system is logical and not build related to something old.

there is completely no logic in the conversion factors.

A calorie is the amount of energy needed to heat up a gram of water with one degree centigrade. We calculate a joule, which is the energy needed to accelerate a mass of one kilogram 1m/s². Funny thing is that the conversion of a calorie to a joule is approx, not specific. (depends on which definition of a calory and a kilo of water)

Everything in the SI system can be found back in nature and checked by everyone (if one wishes): a meter is related to the speed of light, a second to the period of a certain isotope, ...

It is education: if you would have grown up on mainland Europe, the meter would have been the most logical system. growing up in GB/US makes the meter a funny rare dimension, nearly a yard but not completely.

Gwen

I am not able to respond with the complexities or otherwise of the history of the English units, except that 1 foot (12 inches) is probably the length of .......a foot?

I hope that another reader will respond with further answers to enlighten us all!

Fortunately for me and my school mates, we were taught both Imperial and Metric all through K-12. The US was to convert to the metric system, never happened.

So, I know both, know them very well, just can't convert I have memorized some stuff, like 2.54 cm is an inch, 39 and something inches is a meter.

When I go to Canada, my excuse for speeding is "I thought that was MPH"

But I cannot see quite the same thrill in doing 160, and the Monty Python song from The Meaning of Life "you don't have to be six footer....." ain't the same converted to "you don't have to be a one thousand eight hundred and twenty eight point eight millimeter...er"

....and do I detect a personal history with "Stones are to be thrown to the police"? Scary!

My wife loves IKEA, it gives me fits. Those crazy little allen wrench things.

AUGH

Well

Well,

Gentlemen and Ladies,

This is fun. I appreciate all of your comments. Especially those that shed some light on the specific design practices in various geographical regions.

To better clarify, I have an assignment to figure out if changing my company's design practices based on geographical plant location would be worth while. Intuitively, the gut tells me "yes". However, I need some data to support the hypothesis. So, I am interested in knowing a rule of thumb design practices in various location, common component sizing's, cost differences, and such......

I have been searching the internet but so far I have not been able to locate a good source of data... this is how I ended up on this web site....

Great discussion guys , thanks

Zuzanna,

We normally keep it simple and respond to the question. but sometimes it floats a bit to all directions.

You should follow the discussions on perpetual motion and free energy systems, real fun.

You should always keep in mind that dimensions used on a plan must be logical for those who need to read them. It is the engineer who has to do the conversion and check the results, not the one in the warehouse.

If you plan to build a house in the US you will use the imperial system, the same house in France will be build in meter. The same applies to plant design: everyone on a chemical plant knows what an eight inch pipe is, don't expect that they know in Houston that a DN200 is exactly the same pipe. That is the tricky part: we call them DN200 but in fact it stays and 8" pipe. Only some dims like DN125 are really metric.

For non-steel piping and food you will find completely different dimensions.

Only for bolts and nuts we work completely metric. (The Germans left if after the war)

Gwen

Oh yea, there really was a question for this. I work in the imperial system mostly. If a device or a system uses the metric system, I use that.

So, I use what ever system the end-user will utilize.

I think using one set of units internally is most beneficial throughout a project. That way there is less chance of potentially costly errors in conversion.

The benefit of using local units is that everyone on site will understand what is going on and there is less chance of error due to confusion about interpretation of specs.

Drawings sent to Corporate can be converted if necessary or just left in the units they are designed in. Some conversions are not precise since they result in repeating decimals (a weakness of the metric system.)

In summary--Corporate should be most concerned that the project is built right, on schedule, at the predicted cost. The rest is gravy!

The company I work for is based in Switzerland and they use the metric system for all their measurements and designs. When we do a new project, they send over drawings in metric. Sometimes, they send over equipment and that would be metric.

After some time, those equipment will need to be replaced or repaired. If we order metric, it's more expensive since it has to be imported (not all but a lot of them, we do have some suppliers with metric equipment). Furthermore, since it's imported, it will take longer to arrive. This can be solved with proper planning but it's still expensive.

A 2" flange is not the same as a DN50 flange. One of our mechanical engineers once got his head chewed off because he ordered 2 inch flanges for our DN50 level transmitters. I was working with a team of instrumentmen and, nothing we tried worked, we couldn't get them to fit.

Today, our factory is a happy world where metric and imperial live together. We even have tools for each type.

This may be off at a tangent , but recently my wife sent our son and his cousin to get some shopping. She wrote "loo rolls" on their list (this is toilet paper/tissue if you don't have that term). A short while later they phoned from the shop saying they didn't have enough money for 100 (bread) rolls. At least they phoned and didn't get c.o.d.