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Soft Metric Drawings

Soft Metric Drawings

Soft Metric Drawings

I'm presently the drafting supervisor for a US company that was purchased by a European company. Because the company is based in Europe, the company's global standard is ISO. The corporate office has not mandated that we convert to the ISO system however; my manager has tasked me with being proactive in looking at converting to the company's global standard of ISO or an adaptation of it because we will be looking to expand the US business to Europe and abroad. Because the US part of the company that was purchased does most of its business and manufacturing in the US, it would not be cost effective to totally adopt the ISO standard. So we are considering a "soft metric" system which states that the design is done using ANSI shapes and dual dimensions.

Here are my questions and would appreciate any feedback:

1. What symbols would better to use with a "Soft Metric" system?
2. Will international fabricators accept drawings with ANSI standards?

RE: Soft Metric Drawings

Many ASME standards are already metric; and tables with the former British units in an appendix.  Some metric units such as bar for pressure are not the SI unit of choice.

Many electrical standards include metric units.  Coordination exists such as ISA and NEMA with IEC.  This may apply to IEEE standards too.

RE: Soft Metric Drawings

from my own experience: try to create all new design in "hard" metric. If you go "soft", the dimensions on your drawings will look very strange for the people working strictly metric. Millimeters are tiny units (something what the inch users do not realize when they look at the drawings/dimensions). Do not use the decimal fractions of millimeters unless really necessary. The "soft" approach will lead you to dimensioning in hundredths or thousandths of the millimeter. Something what is really not necessary and not easy to measure.
There are also "series of preferred sizes" in metric design (standardized in both ISO and ANSI) - in new design use those if you can, the tooling/measuring (metric) will be easier to select.
Good luck, plilly and Merry Christmas to all who recognize and celebrate it.  

RE: Soft Metric Drawings

Plilly, is your company currently adopting GD&T?

RE: Soft Metric Drawings


There are also "series of preferred sizes" in metric design (standardized in both ISO and ANSI)

I first ran into this concept when writing a screw thread calculation program for ISO threads.  ISO 965 provides formulas for calculating the various diameters, but then states:

"The values for pitch and crest diameter tolerances and for fundamental deviations have been calculated from the formulae and then rounded off to the nearest value in the R40 series of preferred numbers.  However, when decimals appear, the value has been further rounded off to the nearest whole number"

It gets worse.

"In order to reproduce a smooth progression, these rules of rounding have not always been used."

ISO 965-1:1998(E), section 13

Software For Metalworking

RE: Soft Metric Drawings

There is not too much to calculate in ISO or any other threads. Use the existing standardized threads dimensions from the appropriate standard(s). Similar situation is with thread tolerances.


RE: Soft Metric Drawings

ISO 965 has tables of tolerances and allowances only for some combinations of major diameter and pitch.  Combinations not listed, and there are many in use, must rely on formulas and, presumably, rounding using the R40 series of preferred numbers.

This is also true for metric trapezoidal and metric buttress threads.


Software For Metalworking

RE: Soft Metric Drawings

I do not see any reason to look for metric thread which is not in that standard (or in the Machinery's Handbook and other books). If there is a necessity to do it, then you are right, and then you have to "design" both internal and external threads and fully dimension it on the drawing. It also means that you are losing the primary advantage of the standardization. I never needed to do it.

RE: Soft Metric Drawings


What is the main aspect of your concern, going metric or using iso specs?  The newer versions of Y14.5 and several related specs already are (or at least allow for) metric.  There are implications in slightly different definitions for some of the GD& T between the 2 though.

We have fabricators in Asia that work to our inch drawings with ASME/ANSI standards invoked.  Whether they actually fully understand some of the implications is another matter.


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?

RE: Soft Metric Drawings

Plilly - Do yourself and your company a great favor - Go hard metric and stick with ISO or DIN. If you don't, it will come to haunt you forever and you will come to regret it. Make the change now and complete. Bin there-done that.

RE: Soft Metric Drawings

Thank you Kenat for some very valuable information. I think plilly is looking for directions of which way to go in regards to his company working together with the European parent Co.. I think most European companies are more or less with ISO. If it is a German Co. they will be DIN which in most cases is identical to ISO. Thank you for bringing up some very interesting points.

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