If you start with off the shelf material you save a pile of money with no
tooling costs and a material universally supplied by most mills so not tied
to any vendor. You can specify very loose tolerances again saving money
there will be distortion at various step of finishing and their tolerances
as well so can specify them initially pretty loose all with the final
grinding op the only one with tight tolerances. You save a fortune with
loose tolerances through all the initial machining and heat treating steps
and the initial material spec as well. It is all about cost of manufacture
and cost of material. The science is there is no science just money. It also
allows for only 2 QC checks one for hardness and one for final diameter with
no intermediate checks needed which save yet another big pile of money in
time and equipment. Good engineering builds in manufacturing costs along
with strength. If you start the engineering with the premise the machinery
is worn to some degree and the operators semi skilled tolerances are your
biggest cost as it requires new machines and skilled operators. The biggest
challenge for a cutting tool is to make a very light cut rather than a heavy
one. The light one will allow backlash and bearing / gib clearance be the
determining factor for straight and round a heavy cut will load everything
down to a predictable location every time so contrary to initial naive
thinking about machine work.
----- Original Message -----
From: "Kirkwood" <saltfever@comcast.net>
To: <land-speed@autox.team.net>
Sent: Friday, November 25, 2011 6:40 PM
Subject: [Land-speed] Dumb Question # 4857 - Wrist Pin Diameters
> These were designed in the days long before FEA. The accepted method was
> to
> start oversize, as Mark has said, and then continue reducing size along
> with
> tensile testing until the breaking strength is at an acceptable level. The
> other thing to consider is the OEM's subcontractor's existing tooling. As
> Ed
> mentioned cost was, and still is, the driver. If you are making 40 million
> pins a year you must consider the total cost to make small changes to a
> multi-million dollar transfer line or process. The size was probably the
> most economical result of changes to existing tooling. 40 million pins
> (3.75" length) is about 2,400 miles of bar stock. At that volume the
> factory
> can get whatever nominal size it wants directly from the mill without cost
> penalty. However, material costs savings still result with smaller
> diameters. As Ed indicated, one penny less in material cost is about
> $400,000.
>
> From: Larry Mayfield <drmayf@mayfco.com>
> (snip . . . ) fords is 0.912 inches and GM's is 0.927. Why the odd
> dimension used?
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