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Re: Level of Detailing - round 2

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On Aug 20, 2006, at 3:15 PM, Daryl Richardson wrote:

I'm sure that any of the older engineers on the list will be able to confirm that the origin of the practice of not scaling drawings originated from the fact that the liquid based printing processes of the 1950s and 1960s stretched the paper when printing.
The practice of not scaling drawings comes from the fact that complete drawings have all necessary dimensions stated explicitly (and incomplete drawings are just cartoons…). You don't want the fabricator using scaled dimensions and ignoring the necessary design features: he may not scale the dimension accurately or by using a scaled dimension he may be allowing a vital feature to float. Bad practice and one of the hidden pitfalls of CAD.

The engineer's responsibility is twofold
--to specify only those dimensions and tolerances required to make the design work --to specify dimensions in such a way that they can be verified by measurement.

For example if you need an overall dimension to maintain a clearance, it's stupid to specify the individual components and assume that the overall will be the sum of the components. Nor do you specify dimensions from a datum like a centerline that exists only as an abstract. Good drafters and good designers know this and checkers used to beat on the hacks until they learned it too. Isn't that quaint? Aren't we glad we have CAD so we can dimension everything to three decimal places?

I'm doing a project now from sketches abstracted from a CAD model. The designers figure they'll machine everything perfectly from the CAD model so everything will come out perfectly. They only have the vaguest notion of what dimensions must be held and which dimensions should float, because SolidWorks makes the geometry exact. But in fact, the design is going to be made up of several machined weldments and the geometry isn't going to be as perfect as they think. When I point this out I may as well be trying to explain weldment design to my granddaughter.

Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)

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