Design for multi-country manufacture - Metric vs. inch stock 14

[BobVo]

We are a US company which recently merged with a European company. How do we design 'common' products, when we can't but metric stock economically here, and they can't buy US gage, or fractional, stock there?

We certainly aren't the first in this situation. What do other companies do? Any suggestions appreciated.

Thanks.

 

[TheTick]

We missed a window of opportunity when the steel industry tanked in the 70s. We should have retooled metric back then.

 

[HDS]

The large multinational companies I have worked for have been all metric but they were in electronics. As many have said it depends what industry you are in and what you need. You are doing the right thing by picking one as a total standard.

There are some things in the US that are sold as inch sizes but made to mm. The best example is plastic sheets and films. Even if custom extruded in the US a 1/4 sheet is really a 6mm sheet. I have also seen this with some Al sheet.

I heard the US military switched to metric for new milspec components. Is this true?

(Ever since the 90s most engineering courses are taught in metric with English units only used where they are tricky like slugs in thermodynamics.)

 

[SincoTC]

However, please note that most so-called "metric" electronic components have pin pitches of 1.27mm or 2.54mm spaced at 7.62mm, 10.16mm and so on at standard conversions of imperial fractions, spindles are 6.35mm and panels have 12.7mm holes and these aren't because they're American, many are Japanese, French, German or Chinese, but it is the "Industry Standard" so we all live with it.

Here in the UK, all the aluminium and brass extrusions we buy are sold in imperial fractional inch dimension for the sections and in multiples of metres for the length! Timber is metric but sheets of ply are so many millimetres thick but still sold in sheets eight by four (feet).

Us old wrinkly's can cope OK, but it's the kids I feel sorry for, they haven't been taught imperial measurments for years now, but all the speed limits are in MPH, road sign distances are in miles with local warnings in yards or fractions of a mile, petrol is sold only in litres, but car mileage is qouted in MPG. Finally, milk and beer is in pints! This is our country's metric policy and I don't see any of this changing much anytime soon!

 

[JStephen]

"(Ever since the 90s most engineering courses are taught in metric with English units only used where they are tricky like slugs in thermodynamics.)"

I never heard of a slug in thermodynamics, it was all in lbm or kg. I've seen that in a few old books, but never used in school. I think it went out of style along with dynes, poises, etc. Unlike kg-f, bars, calories, and other non-SI units which I still see.

 

[gt6racer2]

At my place of work we're moving slowly towards the metric system - one inch at a time.
We're international too, and have had many issues just as described. We make small valve parts from spring steel, and so the difference between 4 thou and 0.1mm is significant in terms of bending stiffness. We've standardized metric as we see this as the long term global direction.

 

[BobVo]

gt6racer2

So you use metric stock..

How do you address dimensioning the parts? Do you dimension them in all metric, and shops in the US live with that? Or dual dimension the prints? Or...

What about punched hole sizes? Do you call for standard metric hole diameters? What do US companies do to punch those hole sizes? (Buy metric punches?) Or...

Bob

 

[eromlignod]

"I never heard of a slug in thermodynamics, it was all in lbm or kg. I've seen that in a few old books, but never used in school. I think it went out of style along with dynes, poises, etc. Unlike kg-f, bars, calories, and other non-SI units which I still see."


"lbm" are strictly bush-league. Try putting lbm in a formula as simple as F = ma and you'll see what I mean. The imperial unit of mass is slugs, force is in pounds.

1 lb = 1 slug-ft/s^2

As long as you use slugs for mass you can use all the same simple formulas that you use in metric.

Don

 

[juergenwt]

After having read the response on metric or inch material and the use of metric vs. imperial I find my point of view confirmed. Confusion, added cost and loss of leadership are only some of the immediate results. My advise to BobVo would be (and I do no know what product he is making and how much material he is using) - stick with metric on all new product. Design and build all tooling for metric product in metric. If not - you will create a big mess for yourself and for your company. It will cost a little more, but you will not sell a product build to imperial standards anywhere but in the US and you will run into big problems when working with your new partner.
The US used to be the leader in machine tools and wrote the book on many standards like tapers, CNC, machine building and design etc. etc.. Unfortunately that is no longer true. Many new Standards being written for ISO seem to have very little US input or am I just plain wrong on this?
It looks to me like most are an exact copy of the German DIN Standard.
Neglecting our manufacturing sector has changed our society to one more resembling some in the Near East or Mediterranean area where the preferred way of making money is not by engineering and making a product but by buying and selling. Nothing against it - it's the way capitalism works.
However, the results of a free wheeling and unregulated capitalistic society are now coming to haunt us. I am by no means promoting socialism but something needs to be in place to assure that critical industries and skills are still available for future generations.
We have to make sure that one thing is kept under control, and that one thing is "Greed".
We do not need politicians who forever are looking to make sure they do not offend anybody. We need leaders in Washington who can lead and make decisions that are good for the future of the country, even if these decisions are sometimes not very popular.
Otherwise we might es well be governed by a bunch of bureaucrats who than make decisions by plebiscite.
One of these decisions should be to make a final and clear cut to convert this country to metric asap.
And "yes" the US Army is metric - has been for a long time. It was not easy to teach soldiers how to direct Artillery fire and call in adjustments by meters when they where all thinking in feet. But that was in the seventies and now we have GPS.

 

[katmar]

Don, you can use all the standard formulas with lbm as well, as long as you measure force in poundals. Where it gets messy is when you want to use lbm for mass and lbf for force - then you need to introduce the dreaded g_c. This is (was?) very popular in US chemical engineering texts and has lead to a lot of confusion.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[SomptingGuy]

When I was learning physics as a kid, we were taught along the lines that force is proportional to mass times acceleration; pressure is proportional to temperature , current is proportional to voltage (no more examples required).

Then they gave us a consistent unit set. Brilliant, absolutely brilliant. I was so thrilled that power and energy from electrical systems was interchangeable with power and energy from mechanical systems.

- Steve

 

[drawoh]

eromlignod,

m=w/g

F=ma=wa/g

In the English system, you can ignore slugs and just substitute w/g. Now, you can convert units between inches, feet and millimters to your heart's content, as long as you do not screw up your arithmetic. Since the Newton is a unit derived from meters and seconds, you have to be disciplined about using meters and seconds as your units in SI calculations.

Don't forget that the SI system is mks (meters, kilograms, seconds). Lots of metric types still use cgs (centimeters, grams, seconds).

In the English system, slugs are the derived unit. This is an excellent reason not to use slugs.

JHG

 

[KENAT]

That and they leave a sticky trail.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[vpl]

Apologies in advance...

Knowing that BobVo's question has been answered, and that this thread has gone off on a tangent about units and a further tangent about the word "slugs"...

I have to add, that beer is a good way to get rid of slugs. Whether you drink it, or they do.



Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[KENAT]

BobVo, some of our parts get dimensioned in mm and I dont' think we have any real problems with vendors not being able to punch the holes etc.

While perhaps not quite as common as inch series, common mm size tools are I believe fairly readily available.



KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[SnTMan]

vpl, Coors for the slugs, Newcastle for me!

Regards,

Mike

 

[eromlignod]

The imperial system is based on feet, slugs and seconds. Force is a derived unit, just like in metric, and is in pounds.

Using pounds-mass is as stupid as using kilogram-force (which you do see sometimes). As long as you use slugs for mass you can use all the same, simple mechanics formulas as for metric.

How much force does it take to accelerate a 3-slug mass at 15 ft/s/s? Simple:

F = ma = (3 slugs)(15 ft/s^2) = 45 lbs

What torque results from 20 lbs exerted on a 3-ft lever arm?

T = Fr = (20 lbs)(3 ft) = 60 lb-ft

How much work is exerted when you push on a block with 5 lbs over a distance of 10 ft?

W = Fs = (5 lbs)(10 ft) = 50 ft-lbs

Force is in pounds, torque is in pound-feet, energy is in foot-pounds. In all of these units pounds are force, not mass. This is how these quantities are expressed in the engineering world. When you look at spec. sheets and standard tables, this is what you find, not "pounds and poundals" (how confusing). Pounds are force.

Don

 

[drawoh]

eromlignod,

Pounds and Kilograms are traditional units, frequently, however improperly, used for mass and force respectively, as you noted. Slugs and Newtons are derived from force equations involving the traditional units. If you write an equation with slugs or Newtons, you have to convert all your units into whatever units were used to derive the slugs and Newtons. If your equations use pounds and kilograms, you can use whatever units you want, although you have to be consistent.

I have never seen a mass scale calibrated in slugs. On a good day, force scales are not calibrated in kilograms and grams.

In English units, I almost always use inches as my length units. I am pretty certain this rules out the use of slugs, which I believe are derived using feet. I cannot be bothered to look this up.

I must admit that when I am doing calculations in metric, methodically converting all units to the SI MKS is trivial.

JHG

 

[BobVo]

vpl - With all due respect, my question never did get many useful, on topic' replies... (Thanks to a few participants for the limited 'on topic' replies.)


We quoted metricmetal.com on some sheet stock.. but found they want $247 for an aluminum sheet similar to a .060 sheet our vendor pays $55 for. And $690 for a metric sheet similar to a 16 gage sheet our vendor pays $138 for. At these prices 130 sheets per week of metric sheet metal would obviously drive our costs through the roof. :-(

I'll discuss metic punches with our vendors... (but fear they might be forced to raise part prices to cover the cost of the additional metric tooling.)

Also, as some of our low priced suppliers don't have state of the art presses which can store many more punches than needed, press setup times will increase, as the 'normal' tools in the press will never be the ones we need. :-(

BobVo

 

[SomptingGuy]

eromlignod,

So how do you unite the mechanical and electrical systems. What are the equivalent imperial units for current and voltage that allow seamless working between the two domains?

- Steve

 

[SomptingGuy]

Oops, I sound like I'm goading. I'm not. I'm just interested. As I mentioned before, the idea that Volts*Amperes gives the same (W) as N*m/s thrilled me as a kid.

- Steve