[BITList] Fwd: True Position Tolerance

[HUGH]

John,

I've worked for firms in which the principle of stating tolerances on drawings was unknown, or had fallen into disuse, until I put them on my drawings.  In one case there, I had to arrange a bow thruster system which had a drive motor way high up, and a vertical drive shaft with flexible couplings, each of which had very narrow permissible tolerances for misalignment.  I received a complaint from the yard that the tolerances I had given for the motor spigot recess on the seat was an interference fit (so somebody knew his tolerances), to which I replied, "Correct, that's how it's to be made."  In another place, back in the 60s, they had a system that I recall as, eg, 2" full, and 2" bare.  I could never understand that system, but the shipyard did.  As in the later case above, I did it my own way.  My training, from 1949, was in Kincaids, and I grew up with tolerancing as a fact of life.  I still have a copy of the B&W Standard SB, containing Standard EN 4R, Form and Position Tolerances, 37 pages.  This, and related Standards, were the bibles when I was with Kincaid from 1980 onwards.  When the firm moved into general engineering as a sideline in the mid/late 80s, the same Standards were used. Incidentally, we didn't much worry too much about projections, preferring to use the old "View on Arrow A" a lot.  When we did elect to use projection, the view was projected to the right so that one was looking on the LH side ; or to the left, on the RH side ; or below, on the top. But mostly it was component parts lent themselves to that.  All section, elevation and other views of a ship were unambiguously labelled in the engine DO as, eg, Section Looking Aft on Frame 23. It might be of interest that shipyard drawing offices drew the ship with the stern to the right, whereas engine DOs drew it the other way round. This caused no problems that I ever heard of.

Hugh.
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