John: ISO 2768 Part 1 specifies the usage of General Tolerances (Allgemeintoleranzen) for linear and angular dimensions in technical drawings. Part 2 deals with form and position. There are 4 classes: f (fine, fein) m (medim, mittel) c (coarse, grob) v (very coarse, sehr grob) Then, each class assigns a certain tolerance depending on the value of the nominal dimension. For example a dimension between 6 and 30 (mm of course) would get a general tolerance in class m of +- 0.2. A dimension between 2000 and 4000 would get in the same class a tolerance of +-2 General or self-tolerancing as you call it *has* to be dependent on the nominal dimension. The style 1; 1.0; 1.00 as you describe them cannot be a standard as it does not take in account the nominal value. This system I have seen it employed in the English design school. Drawings from UK, Canada and of course US use this system. They often specify at the bottom what tolerances are given for each style. I am a strong opposer to this system which is very limited and not conforming to ISO. I see it as ridiculous (nothing personal here) as the specifications: "Make Good" that I often see on US technical drawings. I wonder if the Space Shuttle drawings have some "make good"s in them. With the ISO all you have to write is: Tolerance class medium or ISO 2768-m or General tolerances ISO 2768-m Another example. In construction all dimensions are in mm. So if one uses the rule you mentioned the tolerance between two walls of a warehouse would be +- 0.5?? As you see it has to be dependent on the nominal value. If you wish I will be more than glad to fax you a copy of ISO 2768 (in German). A> -----Original Message----- From: "kilopascal" <[EMAIL PROTECTED]> Sent: Tue, 19 Jun 2001 00:06:34 -0400 To: "U.S. Metric Association" <[EMAIL PROTECTED]> Subject: [USMA:13894] Re: Edison Film 2001-06-18 I can't speak for inches, but what I was taught concerning metric numbers in millimetres, is that the way a number is written determines its tolerance. The term self-tolerancing comes to mind, even though it might not be the correct term. Another term is implied tolerancing, but that might not be the correct term either. I'm sure someone will help me with the correct term. (Bill????) Unless otherwise stated in a drawing, the self-tolerancing works as follows: 1 mm = 1 �0.5 mm 1.0 mm = 1 � 0.05 mm 1.00 mm = 1 � 0.005 mm etc. It works well with millimetres because of the size. I can't say if the same method works well with inches. In fact, I've never seen it with inch numbers. And yes, rational was not the correct word to use, but it was the one that came to mind. There must be a term for fractional number to show that the fraction can have a range of meaning based on practical usage. 0.375 is not a practical representation of 3/8 as the number of digits implies a greater than need precision. There are many engineers out there who will show this on a drawing because that is the way the calculator displayed it not knowing that they pay a premium to have something machined to 3 place accuracy. In most cases 2 place accuracy in inches is good enough and one place in millimetres (with the decimal digit either a 1 or a 5) is sufficient. John Keiner ist hoffnungsloser versklavt als derjenige, der irrt�mlich glaubt frei zu sein. There are none more hopelessly enslaved then those who falsely believe they are free! Johann Wolfgang von Goethe (1749-1832) ----- Original Message ----- From: "Joseph B. Reid" <[EMAIL PROTECTED]> To: "U.S. Metric Association" <[EMAIL PROTECTED]> Sent: Monday, 2001-06-18 21:05 Subject: [USMA:13893] Re: Edison Film > Kilopascal wrote in USMA 13889: > > > Remember 1-3/8 > >is an irrational number and as a measured value can mean anywhere from 1.37 > >to 1.38. It just goes to show you how ambiguous 1-3/8 can be and how > >inexact that makes it. > > Any fractional number can be converted to a decimal fraction. Fractions > whose denominator contains factors other than 2 or 5 convert into > continuing decimnal fractions. Then there are so-called irrational numbers > such as pi, e, and the square roots of prime numbers. 1-3/8 does not > convert into a continuing decimal nor is it an irrational number. 1-3/8" is > simply 34,925 mm. > > To say that 1-3/8 can mean anything between 1.37 and 1.38 is a matter of > manufacturing tolerance. The target dimension implies 1,375 000 > 000...There is no way this can mean 1,37, but that is where 34,8 mm came > from, as you so rightly pointed out. But following your reasoning a > dimension of 1" would imply something between 0.5" and 1.5". > > Joseph B. Reid > 17 Glebe Road West > Toronto M5P 1C8 Tel. 416 486-6071 -- _______________________________________________ FREE Personalized E-mail at Mail.com http://www.mail.com/?sr=signup FREE PC-to-Phone calls with Net2Phone http://www.net2phone.com/cgi-bin/link.cgi?121
