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Mixed Metric Designs

Mixed Metric Designs

Mixed Metric Designs

(OP)
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?

RE: Mixed Metric Designs

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: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Mixed Metric Designs

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

RE: Mixed Metric Designs

If your in-house shop is setup for inches, stay with it. Use metric for customer interface, if required.

Chris
SolidWorks 08, CATIA V5
ctopher's home (updated Aug 5, 2008)
ctopher's blog
SolidWorks Legion

RE: Mixed Metric Designs

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.   

RE: Mixed Metric Designs

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.

RE: Mixed Metric Designs

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?

               JHG

RE: Mixed Metric Designs

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.
 

RE: Mixed Metric Designs

http://www.eng-tips.com/viewthread.cfm?qid=242338
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.

RE: Mixed Metric Designs

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

RE: Mixed Metric Designs

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.  

RE: Mixed Metric Designs

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

RE: Mixed Metric Designs

"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

RE: Mixed Metric Designs

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: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Mixed Metric Designs

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.   

RE: Mixed Metric Designs

Quote (powerhound):

I agree that an entire assembly should be designed in one format or the other...

   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.  
 

               JHG

RE: Mixed Metric Designs

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

RE: Mixed Metric Designs

tdculbert - read my thread listed above: http://www.eng-tips.com/viewthread.cfm?qid=242338
Your company is making a very big mistake and the people who would like to return to the steam engine are making the same goofy arguments for mixed drawings etc.
Stick to your guns. I know it's like fighting windmills but if your company does not change their way of thinking - find a new job. The will not survive in a worldwide economy.

RE: Mixed Metric Designs

There are a few products out there that mix both systems.  They Lieca Thread Mount camera lenses, for example, have an M39x26TPI thread on them.  
That was a fun one to try and explain to the machine shop.

Engineering is the art of modelling materials we do not wholly understand, into shapes we cannot precisely analyse so as to withstand forces we cannot properly assess, in such a way that the public has no reason to suspect the extent of our ignorance.
-A R Dykes
 

RE: Mixed Metric Designs

juergenwt, I would challenge your thinking by only asking you to please read the above posted messages.  Some are in industries where it is important, others are not.  I guarentee Chinese vendors are not telling American customers to work in metric.  This is the reverse effect of your same arguement.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

RE: Mixed Metric Designs

Wish someone would tell GM not to mix metric and imperial fasteners.  I never know which size my truck needs until I go to fit the socket or wrench.

RE: Mixed Metric Designs

NomLaser,

   All the metric bolts I have observed in cars have their grade marked on them.  Any bolt with a code like 8.8, 10.9 or 12.9 on top is metric.  I have not paid attention to older cars, but I assume that English bolts are marked too.  You just have to read up on it.

               JHG

RE: Mixed Metric Designs

Yagonyonok,

   If it were me checking the drawings, I would not accept  M39x26TPI as a thread specification.

   How about 1.5354-26UN?  or 1.5354(39mm)-26UN

   The UN thread with weird diameter is not ideal practise, but it is acceptable.  It is not really any weirder than something like 1.350-20UN, which I am pretty certain I have seen around.  26TPI looks weird too, although I just found some 27TPI threads in my Machinery's Handbook.

   The M39 part indicates a metric thread, which is a different profile.  I do not think this should be mixed with an English pitch.  M39X0.977 would be evil.  

   Is there any reason they just did not do M39X1?

               JHG

RE: Mixed Metric Designs

As drawoh says, not wonder it was tricky to explain.  The spec makes no sense.  Either come up with the unified equivalent (UNS I'd expect, not UN) or do it properly in metric, don't mix it like that - what spec do you reference seeing as none I know of spec it like that.  On the thread form, I thought the nominal form was the same, it's the tolerances and obviously way diamter & pitch (and their values) that are different?

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Mixed Metric Designs

KENAT,

   I have not checked carefully.  The specifications are mostly the same, with the 60° angle.  I am sure there are niggling details that do not agree.   

               JHG

RE: Mixed Metric Designs

http://en.wikipedia.org/wiki/M39_lens_mount

It is actually a Whitworth thread profile, although we have never had it called out as such.  When I started with the company it was called out as an M39x1, and was working fine.  We never had any problems, but I have read online about some Russian lenses that were done with the 1mm pitch that had problems with the 26TPI mount.

The error was pointed out to me, so I tried to give it the correct callout.  Everywhere I found it online it was always referred to as an M39x26TPI.  I should probably change it to 1.5354-26 as suggested, although the rest of the drawing is metric so that would be mixing the two standards again.

I mostly wanted to point it out as it seemed to be a prime example of what the OP is talking about, mixing metric and imperial.
 

Engineering is the art of modelling materials we do not wholly understand, into shapes we cannot precisely analyse so as to withstand forces we cannot properly assess, in such a way that the public has no reason to suspect the extent of our ignorance.
-A R Dykes
 

RE: Mixed Metric Designs

Yagonyonok,

   On a metric drawing, a 1/4-20UNC thread is called up as 1/4-20UNC.  UN is dimensioned in inches.  Unified National is a different specification from metric, and from Whitworth.  I remember seeing a camera with the tripod thread specified as 1/4-20BSW.  I asked around to see if it was possible to insert a 1/4-20UNC screw into a 1/4-20BSW hole.  The Whitworth thread definitely has a different profile from UN and from metric.

   You are able to cross-thread this stuff because you are only engaging one or two threads.   

               JHG

RE: Mixed Metric Designs

Kenat,
You are right, the Whitworth is a 55° thread.  I have never seen them anywhere before this part so I don't know if there is a 60° version as well (Machinery's Handbook doesn't list any).

Drawoh,
Thanks for clarifying that for me.  Our company has never really cared for the quality of the drawings (managements attitude seems to be that if we can built a prototype in ths shop it is ready to be sold, I had to fight to get a copy of Y14.5).

In our case the lens we use only has three threads (revolutions of thread) to engage, so the minor differences between a 1mm pitch and a 26 TPI have not affected us, yet.  As for the difference between the 55° and the 60° thread profile, is this something that I should worry about?

Sorry for hijacking the thread.

Engineering is the art of modelling materials we do not wholly understand, into shapes we cannot precisely analyse so as to withstand forces we cannot properly assess, in such a way that the public has no reason to suspect the extent of our ignorance.
-A R Dykes
 

RE: Mixed Metric Designs

http://en.wikipedia.org/wiki/British_Standard_Whitworth gives a little info.

If I'm thinking about it correct, one combination of 55 & 60 may work, the other way round wouldn't.  I think a 55 screw might go into a 60 nut but I'd have to draw it out to convince my self.  The other way would wouldn't work due to difference in the pitch & minor diameters.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Mixed Metric Designs

tdculbert - just read all the posts of people promoting mixed dim's and see how everybody has it's own way of dealing with it. Do this and do that, convert this but not that, compensate here but not there, UK not metric(or may be metric in manufacturing but not in the pubs), France still using inch pipes etc. etc. . If that is the route you want your company to take - good luck. As a tool room foreman I have first hand experience with manufacturing a part drawn in metric and providing imperial tooling and gaging. The first time you will regret going with mixed system is when something goes wrong and you are trying to find out exactly where the mistake was made. You will have to check all tooling and gaging made to inches against a part drawing made in metric. The lost NASA moon probe comes to mind.

RE: Mixed Metric Designs

Thanks fcsuper.

RE: Mixed Metric Designs

Not to mention the 767 that had to make an emergency landing in Gander up in Canada. Mistake in calculating fuel load in pounds for a system gaged in metric. The wrong conversion factor was used, not enough fuel was loaded. No one was injured thanks to very good piloting and luck that the car show taking place on the abandonded runway was at the far end (or someting like that).

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net

RE: Mixed Metric Designs

You CAN survive in a mixed-dimensional system; I know because I've lived it here in Canada.  Officially, we're a metric system here, but because much of our business is US based, we also do a lot of inch-base as well.  It is better to stick to one unit where possible unless your internal processes are rock solid because it can lead to costly mistakes at some point.

I get a good chuckle when people get all worked up about metric vs inch ... it's an "US (pun-intended) vs Them" argument that's absurd.  We made the switch about 30-plus years ago and we still work in both units and have full availability of components in both dimensional units.  If you look realistically at the US industial base now, you'll see that the driving industries are metric-based now, and have been for what ... 15-20 years now?  Automotive ... do you need a 1/4" socket or a 6mm socket on current products?  Aerospace and defense systems are dominantly metric because of the international market and defence-alliance agreements.  And if you want a real reality kick, look at what unit system is actually codified in the USA; it isn't the "US Customary" system.

I know from experience that it is often easier to find metric cutters, tools, taps, stock and whatever else you may need for manufacturing, in the US than it is in Canada ... we buy much of it from the States.  As for the machining centres themselves, I've seen 60-year-old machines with encoders that were converted to metric.  As long as you're using decimal fractions, there's no functional difference for the user; a number is a number is a number.  And no, shops don't usually dump all their old tooling during a metric migration; they use the closest inch cutter (within tolerance) until it is consumed, then replace it with metric cutters.  I've known many inch-trained machinists, millwrights and master mold makers who made the migration and a few that haven't; it's a resistance to change that is at issue, not the unit or the technology.

Consider the economics of the global situation now.  The US sources a significant portion of its goods from Asia, EU and Canada, and reciprocally sells a lot of their goods to us.  Personally, I try to do business in whatever unit is most convenient for my customer, and that's what US suppliers have been doing, and will do even more in order to survive.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: Mixed Metric Designs

MechNorth- Words of truth. Here is another odd thing I have noticed. Many import tools with American names like "Chicago Tool"
or "Black & Decker" etc. etc. are apparently designed in the US and the prints are than sent to China. Now you can find screws which are in inches (like 1/4-20) but with a metric hex or wrench size. Looks like someone here insisted on inch thread. That is what they gave you! Only, with a metric wrench size. You wonder how many of the parts on these US designed and China made tools are part inch and part metric?

The only thing that is holding back a fast and complete change to metric in the US, is the political will and if I had to compete with the US today I would hire lobbyists and pay off our representatives to keep our schools on the customary US system. Not so sure this is not the case already. What a cheap and easy way to cut down your competition.

We must start with our schools so our children will grow up thinking metric. Our media must stop dumbing down the American people by converting everything to inches, gallons and deg. Fahrenheit. Science moved to metric a long time ago and I am sure we do not want to become a country that is relying on agriculture only. Bushels anybody?
Tdculbert - sorry if I drifted away from your original posting.

RE: Mixed Metric Designs

Would that be a US Bushel 2,150.42 cubic inches or a British Bushel 2,219.36 cubic inches?

Pah, who needs metric when the other option is so great?
 

RE: Mixed Metric Designs

Shoot, anybody can use a simple base 10 system.  Training our students to use PSI, inch-pounds, cubic yards, teaspoons, drams, pecks & BTUs gives them much better mathematical training.  

My favorite unit is the miners inch, which varies by local statute:

    * 1/60 ft³/s (472 mL/s) New Zealand
    * 1/50 ft³/s (566 mL/s) southern California, Idaho, Kansas, Nebraska, New Mexico, North Dakota, South Dakota, Utah, Washington
    * 1/40 ft³/s (708 mL/s) Arizona, northern California, Montana, Nevada, Oregon
    * 1/38 ft³/s (745 mL/s) Colorado
    * 1/36 ft³/s (787 mL/s) British Columbia
 

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