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Lynn Javoroski CSI CCS LEEDŽ AP SCIP Affiliate Senior Member Username: lynn_javoroski
Post Number: 931 Registered: 07-2002
| Posted on Friday, September 25, 2009 - 02:13 pm: | |
The other day, I sent the following to ASTM: Re: Table 2 Dimensional Tolerance for Rectangular Shapes of Type 1 Transparent, Flat Glass. For SI designation, 6mm, 1/4 inch, the thickness range in inches has as its minimum 0.219 and as its maximum 0.244. The maximum is therefore less than 0.25, which is equal to 1/4 inch (at least in the math and number system I learned in school). I understand the need for a range of thickness, and the tolerances to govern that range. I don't understand how the maximum can be less than the traditional designation nominal measurement. (even allowing for the differences between 6mm and 1/4 inch) Can someone please explain this? Thank you for your attention to this question. Here's the response I received: Sorry to say there is no good answer; this has been this way for many years. The fractions are called Traditional or nominal dimensions (i.e., not true valuates). It is similar to why a 2" x 4" lumbar piece is not really 2" by 4"; it is less. To which I responded: For every other thickness of glass, the minimum is below (or less than) the nominal measurement and the maximum is above (or greater than) the nominal. 1/4 inch is the ONLY one where the maximum is LESS THAN the nominal. [Therefore, were I to put 4 panes of 1/4 inch glass together, using the maximum allowable thickness, I would get a piece of glass with a thickness of 0.976, which is dangerously close to the minimum for 1 inch glass (0.969). You'd think that it ought to come out closer to the nominal.] To use your analogy, it's like saying that the maximum width of a 2x4 is 3...when we all know that the nominal width is 3-1/2. If this has been this way for many years, perhaps it has been wrong for many years? And the answer: A 2 x 4 piece of wood is less than 2" thick and less than 4" wide, but it is called a two by four. A 1/4" piece of float glass is less than 1/4", but it is called one quarter inch.. To me, that analogy makes sense. But WE are not going to change this industry accepted nomenclature by talking about it. And I honestly do not think the industry will ever change, but I could be wrong. You may want to join ASTM C14.08 and submit your comments on the next ballot of C1036. Typos not withstanding, anybody have additional insight into this? |
John Bunzick, CCS, CCCA, LEED AP Senior Member Username: bunzick
Post Number: 1112 Registered: 03-2002
| Posted on Friday, September 25, 2009 - 04:16 pm: | |
I always specify glass by metric dimensions. If they call in 1/4 inch in the field, so be it. |
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