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Mixed Metric Designs

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tdculbert

tdculbert

Mechanical
Jan 8, 2008
4
Our electronics company has long designed our products in metric and the mating tooling & fixturing in imperial to continue making use of the in-house machine shop for tooling fabs.

I've recently began designing and submitting tools designed in metric, much to the shop's anger. The company continues to be split about which dimensioning scheme to use. The main argument being the cost of re-tooling the shop to metric "unnecessarily" - "just because the product is in metric doesn't mean the tooling needs to be" they argue.

Their pushbacks have produced several compromise proposals:
1) Don't convert - it's always worked; don't rock the boat.
2) Dual dimension prints; Since the shop (and our auxillary machining vendors) will be converting back to english anyway.
3) Design dimensions in metric, but use english fastening components - english dowel pins & fasteners/tapping.


My responses -
1 & 2 - there's inherent conversion error introduced in switching from one to the other; additionally, since our english unit precision is governed by decimal places, this complicates conversion of tolerances
3 - I don't have a good response to this; just feels wrong to go half-way.

I'd be curious about advice on this battle and input on these objects from those who've gone this road before?
 
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Well, with regards to inherant conversion error, this can be addressed to some extent by 'adding' a significant figure as appropriate - there have been threads on similar here so maybe take a look.

If you are letting your title block being related to decimal places stop you, frankly that's verging on lazyness. Title block tols are 'unless otherwise stated', i.e. you may have to directly tolerance most dimensions but so be it.

We have a mixture because we use a lot of off the shelf parts that come in a mixture of metric & inch. Recently we've been trying to standardize a bit on metric threaded fasteners, but most drawings are still in inch. If we need to convert we do, if we need all a sig fig we do, if we need to directly tolerance instead of relying on the tol block we do.

There have been quite a few threads about similar, both on this forum and over in I think it was "Mechanical engineering other topics" or maybe something like "Industrial/Mfg engineering other topics". Do a google search of this site using the field at the top.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies: What is Engineering anyway: faq1088-1484
 
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There's no real reason to move to metric from inch.
Measurement systems are arbitary. In the big picture view of the universe, there's no difference between inch or metric. Metric has some advantages in that it easier to think in a metric system (base 10, sizing aligns with mm). But there are some economical advantages to inch in America. There are certainly economical advantages to sticking to one standard, regardless to which one is picked.

Switching is much more complicated that you might be thinking, and the advantage of metric is fairly slight (there are disavantages to a purist metric adoption, as well).

If you are in an Inch environment, just suck it up and do everything in inch, the same if someone who favors inch moves into an metric environment.

I would agree that the swtichover is unnecessary.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group
 
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It's all about the money - if you can put together a convincing proposal for management that shows the bottom line ($) then that's what (should) happen. You could try arguing that conversion leads to mistakes, but do you have proof? Can you find a batch that needed rework because of that particular issue and assign a cost to the issue?

Be prepared to discuss the actual costs of switching, including training and potential new hires. Honestly the print is about design intent - you want to put the little squiggles on there that will best represent what you want your design to be - that's why we (here) use black ink on standard sized white paper, with English notes, in imperial units because we know that our suppliers want to see them.
 
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If your work is exclusively with length/position, then I can agree with fcsuper regarding limited benefits of switching to SI. If your work involves the first, second and third derivatives of length with respect to time, or F = ma, or linking electrical and mechanical systems, then there are more advantages to SI. If your work involves amount of substance, and/or linking to the periodic table, then there are enormous advantages to using SI.
 
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tdculbert,

How bad is this problem, really? Can you easily purchase metric components and drills and taps and stuff?

Your machines are indexed in inches and millimeters are they not?

Is this a hardware problem, or is it a machinist problem?

Critter.gif
JHG
 
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In my experience choose one or the other don’t try and mix and match. Good design is done in either imperial or metric.

At the end of the day features that matters say a location pin need to be a certain size it will probably be an odd size in either metric or imperial, features that don’t matter should wherever possible be standard sizes and put in to “round figures”.

It probably doesn’t matter a jot if a support plate is 12mm or ½” or if bolt centres are at 25mm or 1” but it soon becomes a complete mess when you mix them up and costs unnecessary money if you start machining ½” plate down to 12mm, or causes huge problems if you just choose to ignore the odd 0.7mm.
 
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Look at previous threads above. Take some advise: If the part is designed in metric insist the tooling and manufacturing be in metric. You will regret it if you start mixing the two systems.Just think what some people are asking you to do. Twenty years down the road you would still be working with two systems - just so you can keep a view old machines around.
The people advocating staying with a mixed system will all be retired but your company will still be fighting with all types of conversions - if they are still in business.
 
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There's a tendency for metric fans to work as though metric is the default. It's not. I get a kick out of the old "The US is the only 1st world country not using metric." argument. Hello! the US has the largest economy on the planet by almost 2 times. It doesn't really matter what rest of world does because the US is so big. It's like saying that the 800lb gorilla in the room should wear jeans just because all the chimps in the room are wearing them. Maybe the gorilla is happy with his corduroys instead.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group
 
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Only if you exclude the European Union which was bigger in 2008 by GDP and the US is not all imperial and that it was in 2008 only 18/60 of the world GDP.
 
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I agree that an entire assembly should be designed in one format or the other. It's probably not a good idea to mix and match in the design phase BUT if the part is designed in metric but manufactured on machines that are in inches, it doesn't matter. I do it all the time. I worked at a company that created all their drawings in metric and when a component had to be machined, I would scale it in Mastercam and program it in inches. The part would go up on a machine that would cut it in inches, the machinist would convert his dimensions to inches (this is certainly a point where errors could be made...no argument here) and he would check his work in inches. When all is complete, the part would go to QC and get checked in metric on the CMM. The only place there was ever any issue was when the machinist would convert incorrectly or a programming error unrelated to the metric/inches conversion. To insist that the machines be switched over to mm is unreasonable.

Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
 
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"Only if you exclude the European Union which was bigger in 2008 by GDP and the US is not all imperial and that it was in 2008 only 18/60 of the world GDP."

And the US President is just some person with an oddly shaped office. :)

I think many Americans might be surprized by the number of countries that ARE NOT 100% metric, many of which are in Europe. :)



Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group
 
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Unless they've come on leaps and bounds in the last 5 years the UK is not all metric.

Some areas are more metric than others but it's not across the board. All our new design was metric but interfacing with US equipment or working on older designs we frequently still used inch.

If I understand the OP correctly, I think they may be better sticking with inch for the fixturing drawings. That way the conversion only gets done once and can hopefully be verified. Allowing the default title block tolerance to impede this is ridiculous as far as I can tell.

The end product may be metric but the fixturing doesn't have to be.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies: What is Engineering anyway: faq1088-1484
 
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I worked in France for a couple of years. Had to put a pipe thread on a fixture so I looked up the metric tapered pipe threads & put one in the design. The French manufacturing engineer came back to me and asked "What the #$%& is this?" Turns out they don't use metric pipe threads in France, they use good old 'merican NPT.
 
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powerhound said:
I agree that an entire assembly should be designed in one format or the other...
Click to expand...

Try selecting ball bearings sometime. I have been in a couple of situations where I wanted a bearing slightly smaller than my shaft, and switching units solved the problem.

Here in Canada, we are officially metric, but lots of people do English drawings. When I send out panels for silkscreening, I have to use inches on the drawings, even if everything else is metric.


Critter.gif
JHG
 
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I am now designing tooling/fixtures.
The final product comes from Canada (metric), my tooling (USA) is in imperial. In order for things to work, my drawings use dual dims, metric secondary. The machinist complained at first, but I told him to stop b*tching...both dims are on the dwgs! There I stand.

Chris
SolidWorks 09 SP4.1
ctopher's home
SolidWorks Legion
 
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