[xj-s] British Attitudes Toward Tolerances

["Kirbert" <palmk@mailer.gadcomm.net>]

> My money is that the problem is with the age-old British tradition of
> designing up against the wrong side of tolerances.

When I was working at P&WA on jet engines, we once ended up with a 
contract to make blades for the Rolls Royce Pegasus engine, the 
engine that powers the Harrier vertical takeoff jet.  We got a 
firsthand look at British attitudes toward tolerances.  The first 
thing that got to us was that the Brits had two separate sets of 
drawings with different tolerances -- one for the guys MAKING the 
part and a second (looser) for the guys INSPECTING the part.  This 
causes doubts all around on what the tolerances really are 
(tolerances aren't supposed to be just random numbers, they are 
supposed to reflect the range of dimensions that will actually WORK). 
 It caused the machinists to not really care too much about hitting 
the tolerances since they were aware that the inspectors may accept 
it anyway.

The second lesson was that their tolerances didn't make any sense -- 
and this from P&WA engineers, who are known for nonsensical 
tolerances.  As an example: the blades we were working on were 
something like 3 feet long, and had a "mid-span shroud" -- those 
protrusions from either side of the blade that are supposed to fit 
against the one on the next blade and form a continuous ring.  When 
the blade was held by the root and a dimension to the contact surface 
of the shroud was taken, the tolerance was .002" -- but if you merely 
rolled the blade over and measured it again, the force of gravity 
alone would bend the blade by more than .030".  And trying to get any 
guidance from RR about just how you're supposed to measure this thing 
to get the desired dimension proved difficult or impossible, I dunno 
if they ever did figure it out.

 -- Kirbert      |     If anything is to be accomplished,
                 |     some rules must be broken.
                 |          - Palm's Postulate

---

References:

• Fw: [xj-s] Re: Dropped Seat Mechanism
• From: "Richard Chapman" <richardc@sanderson.co.nz>

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