grand plate flanges...

[Joel Rappaport]

Jim,
Thanks for the great answer.  To summarize: the angle is for a clean
release of the _wood pattern_, not the finished plate.  (We learned that
the wood pattern is made 2% larger to allow for cast iron shrinkage when
the actual plate cools).  And the sand is used for only one plate at a
time.  OK, thanks again, Jim
----Joel
Jim wrote:
>
> Joel and Priscilla:
>
> I tho't I'd jump in and answer this one.
>
> When a plate has been designed, a wooden pattern (often solid mahogony) is
> prepared using a shrink rule (1/8 inch per foot larger than normal).
> This pattern is mounted to a follow board (heavy platform). A rectangular
> flask (like a heavy formed steel band) is placed on top of the board.  This
> flask is about 12 inches high all the way around. There is a steel
> reinforcing grid across the top of the flask which holds the sides rigidly.
> A powdery releasing compound is lightly dusted over the entire follow
> board and the pattern.
>
> Sand is poured down thru the grid to completely cover the pattern.  Workers
> standing on top of the grid begin ramming the sand tightly around the pattern
> and solid upon the follow board. Jack hammers and ramming rods are used.
>
> This sand is a special mixture of sand, bentonite, molasses and perhaps other
> ingredients which make the sand stick together when compacted.
>
> There a flask which holds the molded sand for the shape of the top side of
> the plate pattern and then another flask which holds the molded sand for the
> shape of the bottom side of the plate pattern (like the horn, bosses, etc.).
>
> These two flasks are called the cope and the drag.  When they are lifted
> off of the pattern, the draft angle is important to avoid any sand being
> accidently dislodged and falling down from the flask which holds
> the reverse shape of the pattern.  The cope side has little sprue holes
> carved into the sand to allow pouring in the white hot metal mix when
> the cope and drag are mated together.  When they are mated together, there
> is a piano plate shaped space between the cope and the drag.  The
> sprue hole are placed so that all parts of the cavity between the cope
> and drag are completely filled and running over.  After the metal cools
> sufficiently, the flasks are removed, or the sand is shaken out.  The sand
> usually goes onto conveyor belts which takes it up to be renewed and
> processed for use in some future casting.  The plate is then shot peened,
> sent for curing, then ground down at rough edges, and then drilled.  If any
> sand had fallen into the cavity before pouring, it would have ruined the
> plate.  Hence, the need for good draft angle.  the foundry would
> really like 4 to 7 degrees draft angle to help avoid cave-ins.
>
> Jim Coleman, Sr.
>
> On Wed, 27 Nov 1996, Joel Rappaport wrote:
>
> > Danny,
> > You are the perfect one to ask a question that occurred to me while
> > reading the various responses in this thread.  How many times is a sand
> > mold used to cast a plate?  Is the draft angle necessary for a clean
> > release so as not to disturb the sand of the molding so it can be used
> > again, or for a clean casting?
> >
> > ----Joel
> >
> > Danny Dover, RPT
> > Dartmouth College, Hanover, NH wrote:
> >
> > > Patternmaking has its own set of rules, just as we have, to design
> > > pieces that will release cleanly from the sand mold.  One of those rules is,
> > > the deeper the impression in the sand (that is, the higher the vertical wall of
> > > the piece coming out of the sand), the more draft angle required to get a clean
> > > release.  So if the height of the flange changes, you can expect the
> > > patternmaker to have changed the angle as well, to make sure they pour a clean
> > > plate every time.
> >