Dual Drawings vs. Dual Dimensions? 3

If either is truely acceptable & interchangeable, could the tolerances just be relaxed to accomodate both the metric and nearest inch equivalent?

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies: What is Engineering anyway: faq1088-1484

I don’t really see how it helps much. If for example there is a 12mm plate with a +/- 1 mm limit then a ½” plate would do, if the limit is say +/- 0.25 you would have to machine it anyway, so there would be no cost saving.

If you start re-dimensioning everything to take into account the fact it will be made from imperial stock you are opening up a whole can of worms.

I wish it were that easy. For example if you have 2 20mm plates seperated by a 50mm long tube the overall lenght is 20+20+50=90mm. Now the closest imperial plate to 20mm is 7/8" which is 22.225mm. Using the same 50mm long tube I would now have a finished part at 22.225+22.225+50=94.45mm.
The tolerance cannot be relaxed to that extent.

So in that case the inch part and the metric part aren't equivalent/interchangeable then.

Which means having the same part number for each is a bad idea.

I suppose you could still have a common drawing, with different "-" numbers for the metric or inch parts but this might be messy.

You would have to have different BOMs as you say for metric or inch.

At a previous employer we'd sometimes re-design American designed equipment to use metric standard sizes etc. It was never as simple as it sounds.

Unless you have fairly high run-rates it may cost more to create the inch equivalent than the extra cost of the metric material.

In some instances dimensioning at the assy level and just calling up stock can accomodate this but that has its own issues.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies: What is Engineering anyway: faq1088-1484

2 x 7/8 + 1-3/4 spacer = 3.5" = 88.9 mm

MintJulep, that is exactly what I would propose for the 2nd drawing page which would also have it's unique BOM. But since the 50mm was cut to length I could substitue a pipe cut at 45,55mm so the overall dimension will match the metric drawing page 1. Same form-fit-function only with local materials.

For simple things that will work, but needs to be separate part numbers & probably drawings up to the point that interchangeability with metric is reached.

How much money will it cost to do these extra drawings, is it worth it?

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies: What is Engineering anyway: faq1088-1484

Nordern,

I think design change rules apply here. Any change to form, fit or function or any combination of the above, requires a new part number. Any two parts with the same part number must function identically in the same application.

If you can open up tolerances enough to allow inch or metric material, go ahead. If you need to re-design your assembly to use inch components, they must have separate part numbers.

It is possible that at the assembly level, the inch system is interchangable with the metric one. Your maintainance people still need to know what components were used. You could tabulate the assembly drawing.

Maintaining two versions of an assembly at the same time is more complicated than revising the assembly, changing metric components to inch components. The assembly drawing and/or parts list revision letters allow you to distinguish between inch and metric, assuming that the metric design is not manufactured any more.

Is there any chance you can contain the inch modifications to a functionally identical sub-assembly?

JHG

Thank you all for the advise and suggestions. I will take all of it into account and discuss our next steps with my engineering group. This sure is a great website to get some very good insite!

Soft conversions to metric or imperial units often confuse contractors and fabricators. Give them one or the other. If they are metric savvy, then it shouldn't affect the price. If they are not, then you'll get a price that reflects their learning curve or perceived learning curve.

We put both dimensions on the drawings.

HOWEVER - BE CAREFULL. We have seen some drawings over the years shrink or grow as people converted back and forth between Imperial and Metric. The conversion is NEVER exact.

Here is what we experienced. Our company transferred most of our German product line to the US. As a result we had to duplicate or revise a large number of tools including dies. All tool prints in metric to DIN standard and in German. All prints filed by DIN size,cataloged and stored in DIN print cabinets.Quantity - more than 1300 part prints and over 3000 tool prints. Decision made by top management and chief engineer on how to do this.
First mistake: Change all DIN print size to nearest print size used by the company - they have their own sizes.
Result: Once the company print sizes were put on the company computer file (only the symbol for the company print size was listed and filed) the system mixed and listed our existing US prints with the DIN prints since all had the same company size listed.
End result: Nobody could find a print unless you knew whether it was a German or US part. Now that the DIN sizes were all filed in DIN cabinets by DIN size (A0, A1, A2, A3 etc) you had to guess what US size was closest to the DIN size. You always needed to look in separate file cabinets.

Than they employed a draftsman to re-draw all German part prints in inches. Total disaster. Not only did the lack of know how and understanding of both systems (Tolerances in DIN or ISO) turn out to be a big hurdle, but after retiring the original German print and issuing new prints among others to "receiving inspection" another problem occurred.
We had hundreds of vendors in Europe and all of these had our original print on file (and parts in stock). Now when we received parts they were checked against the new English prints and often returned to the vendor because of a difference in tolerances or material spec's. What a mess!
Part two.
Each part had a manufacturing route in German.
First mistake: 1300 routes, average 20 operations ea. Decision was made by top management to use computer translations from a reputable translating company. The asked whether we wanted a straight computer translation or a manual translation. It was decided to use a straight computer translation. Cost - over $10000.00 vs. a price into the millions for a manual translation (the translating co. would have to send a team to Germany and the to the US company to learn all the terminology specific to the product line).
End result: Nothing but garbage.
Part three.
Building tools and dies to German prints.
Since I was the foreman of the tool room I was able to influence that part (after some flack).
I decided to build all the tooling and gages as they were drawn in metric and to DIN. From our company in Germany I ordered a computer printout listing all tooling and DIN print sizes. Had the DIN tool print cabinets separated from the mixed up part prints.
As to actually building the tools.
At first we tried to save money by keeping the material sizes listed in metric to the nearest inch size. The idea was to cut down on cost for milling inch sizes down to metric dim's. Sounded OK at first but we soon found out different. Many of these tools were very complicated multi-station CNC-tools on top of comlicated milling-, drilling-. grinding and other fixtures and gages. When the tool & die makers started working with metric print specifications and inch material they soon found themselves in deep trouble. It proved very difficult to work in both systems and finally we decided to build the tool as drawn in metric and start cutting our inch material down to the correct metric size (in time we bought more and more metric material)
The lesson learned here: DO NOT MIX THE TWO SYSTEMS!.
I had our German company send me some DIN - books from "Beuth" so we could look up all the DIN standards.
The problem with translating manufacturing routes was finally solved (after suggestions of using someones German speaking grandmother). I insisted (finally finding an open ear) on a line for line manual translation. We found a very intelligent bilingual young lady who would spend hours with me and keeping detailed notes. After some time she got better an better and all I needed to do was proof read her translations. It took a few years, but the job got done right and no parts were scrapped because of errors in translating.
Final analysis: Don't ever mix the two systems. Make your prints either completely in metric or in inches.
If you get a metric print, leave it. If you try to change part of it you are already asking for trouble.
Now that all NEW parts are in metric (we are still fighting this) we decided that all tooling for metric parts would be in metric. This is very important.
P.s.: I can see some "very simple" parts or tooling being made using solid inch material to save cost. Nothing complicated.
Nordern - I hope this helps you in you decision.

A star for you juergenwt, that covers the “whole can of worms” I mentioned earlier but in far more detail and in a more concise way than I could have done.

Thanks for the great reply! I am sure to put some of your knowledge to good use.