Mixed Metric Designs

[tdculbert]

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?

 

[juergenwt]

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.

 

[Yagonyonok]

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

 

[fcsuper]

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

 

[NomLaser]

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.

 

[ctopher]

GM who??

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

 

[drawoh]

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

 

[drawoh]

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

 

[KENAT]

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?

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[drawoh]

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

 

[Yagonyonok]

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

 

[drawoh]

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

 

[KENAT]

I thought Whitworth was a 55° thread, although there are several threads that get called Whitworth and one does have 60 or something, I think the bicycle version.

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[Yagonyonok]

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

 

[KENAT]

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.

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[juergenwt]

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.

 

[fcsuper]

Juergenwt, exactly. BTW, it's a Martian probe. The moon probe was intentionally slammed into the moon.

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

 

[juergenwt]

Thanks fcsuper.

 

[PeterStock]

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

http://www.infotechpr.net

 

[MechNorth]

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[juergenwt]

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.