Rule
No. 10:
"The Tool Rule"
Datums and pick-up points need to be agreed between the pattern
or tool maker, the casting engineer, and the machinist to
avoid unnecessary scrap after the casting has been made, and
the inevitable delays in supply.
This agreement has to be put in place at the time of the
placing of the order for the parts. Thus it is essential that
the buyer spends time with the people who machine his parts
in order to understand their needs in this area.
Once the pick-up points have been agreed, the patternmaker
or tool maker can check the tooling, the foundry can check
the casting, and the machinist can set up the part for machining,
all parties working from the same working datum points. Only
then will a trauma-free, integrated, supply of castings be
achieved, all accurately within dimensional specification.
Strongly recommended are the robust, reliable and low cost
"passive" location systems
(1) A rectilinear job can use a classical "6 point" system.
(Click here to view diagram.)
The incorporation of lugs that simultaneously provide clamping
points is helpful. This is an absolutely accurate system.
(5 points is not enough, since the part is not uniquely
located; 7 points is too many, and will have some points
in conflict).
(2) The traditional "cone, groove and flat" system is another
absolute location technique useful for some geometries of
casting.
(3) A part with some cylindrical form that needs to be
held in a 3-jaw chuck needs the 3 locations at 120 degrees,
plus end stop, plus a "clock" stop (to define its angle
of rotation). This is usually less easy to work as an absolute
system, but has sufficient accuracy for most applications.
Active systems are those that require several measurements
to be taken from a casting when it is presented to the machine
tool, and an average is calculated (usually by built-in computer).
This approach may be necessary for flexible, open box type
products (such as an oil pan with flat, unstrengthened walls).
However, this route is, of course, more expensive and the
machine tools to deal with such castings not so widely available.
Also, of course, the method does not have the benefit of providing
the integration of the supply chain that is so valuable for
trouble-free supply.
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