| This article pertains mainly to
aluminum castings. Aluminum shrinks approximately ¼ inch to the foot upon
cooling depending upon the alloy and temperature at pouring time. Since I do
not make replacement parts I am not concerned with the shrinkage factor but it
is easily incorporated in pattern making by making a shrink ruler for your
dimensioning. Take a piece of wood, paper or metal 12 ¼ inches in length and
divide it into 12 equal spaces, each representing one inch and drawing or
scribing them into the wood, paper or metal. One of the inch spaces may divided
into 1/16ths giving you all the dimensioning you should need. If you are
going to make aluminum castings you must have a pattern to pack sand around and
be able to remove it before closing the flask and pouring.
The first step is to make a scale drawing of the part you want to duplicate.
Some pattern makers use nothing but hard woods for their patterns. Since I
will not be making hundreds of castings from one pattern, I rarely use hard
wood. Pine and fir are my favorites and they are easier to cut and sand to form
the shapes I need.
You will now need to decide what type of pattern to make. I classify
patterns in three ways:
- Simple patterns
- Two part patterns
- Pattern boards
You will find out how to make all three in this article as well as special
ramming procedures needed..
Simple Pattern
If you are going to make just a few castings from your pattern you will just
make a simple pattern. I’ll use the hood of a 43 ton GE switcher I’m planning
on making all from castings as an example.
The side of the hood is made of ¼ inch plywood and is cut to the proper
dimensions. (In this case the pattern is cut about 1/16 to 1/32nd
inch larger) The reason for this is that one has to incorporate "draft" into
the pattern to make it removable from the sand without damage to the mold (also
spelled mould). The finished casting is then filed or milled square before
final assembly. The top of the pattern is 1/32 of an inch (approximately 3 to 4
degrees) smaller than the bottom so when removing the pattern the bottom of the
sand mold (top of pattern) is smaller and doesn’t rub against the sand upon
removal.
The holes in the plywood ends were filled with wood filler and sanded and a
couple of coats of sanding sealer applied and sanded again. I then glued some
aluminum doors with louvers to the wood pattern. The louvers had to be filled
with Plaster of Paris and wood filler to create draft for easy removal. If you
look closely you might be able to see the draft on the louver bottoms, (Fig. A)
also linoleum nails were placed along the outside of the pattern to simulate
rivets.
Fig. A
They have built in draft. I use a glossy spray paint as a final coating as
it shows any faults in the pattern.
Two Part Patterns
The radiator is made of ¾ in. pine cut out and a draft sanded into it. The
side facing you (top) is always smaller than the bottom creating the needed
draft as it will face the bottom of the sand mold for easy removal, remember?
The pattern is made from 4 pieces of wood glued together. (I use an all purpose
wood glue.) A problem is now encountered because the back of the hood has to
have tabs on which to mount the hood sides. If you were to glue the tabs to the
back of the radiator you could not remove it from the sand because it would be
locked in. (This could be remedied by use of a special bottom board described
later.) We will make a two piece pattern. (Fig. B) The second piece of the
pattern is made of re-sawn wood and draft sanded into it. This time the draft
is reversed and the sides of the pieces that come together are the largest.
Holes are drilled into the tabs which have corresponding holes marked and
drilled into the back side of the radiator. Small wood dowels are glued into
the tab and extend about 1/8 inch. These fit into the corresponding holes in
the back of the radiator and are made for a loose fit.
Fig. B
To ram this pattern it is placed inside the drag (bottom of flask) on a
bottom board with the front facing up. Ram up with sand, strike off and place
bottom board on top and turn the drag over. Remove the board and you will see
the back of the radiator with the holes for the tabs facing you. Apply parting
dust (to keep the cope and drag halves from sticking together) and place the
tabs into their respective holes. You can now ram up the cope (top half of the
flask) and when you open the flask to remove the pattern, the tabs will be in
the upper part of the mold and the radiator in the bottom of the flask. The
pattern is then rapped and removed from each part of the flask. The pouring
spout, sprue and runners are cut out according to the instructions given in the
casting article by Steve Hoerner and the mold is closed and is ready to pour.
Simple Pattern with special bottom board
The hood top is a little more difficult to build. The ends are cut out and
drafted (draft sanded into them). Since the casting will be ¼ inch thick, the
same piece of plywood used for the sides was used again. This time ½ inch wide
strips were cut out and trimmed to the length needed. Start in the middle top
of the hood and glue the strips to the end pieces making sure they are 90
degrees to each other for a square casting. All the pieces are glued to the
ends until you come to the last piece on each side. This piece may have to have
the width adjusted to make them come out even. The gaps left between the glued
pieces are now filled with wood filler, allowed to dry and sanded smooth. A
little draft is sanded into the ends of the hood top. Don’t forget to sand the
inside of the pattern and make sure you again have draft for easy removal.
We now have to make tabs for mounting the sides to the hood top. We could
make this a two part pattern and drill and dowel the holes but this is too
tedious and if all the tabs are not made the same you could ruin the casting if
any of the tabs were off center, etc. Cut all of the tabs out of wood (pine,
fir or plywood) I used ¼ inch pine. File draft into each tab and glue them to
their proper positions.
Sand smoothly, filling in any irregularities, and apply two coats of sanding
sealer, sanding between coats. Apply the linoleum nails and use glossy spray
paint to finish the pattern.
You will need to make a special bottom board to set the pattern on prior to
ramming with sand. Slots are cut into the board corresponding to the tabs which
fit into the slots so the bottom of the hood top sets against the bottom board.
Ram up the cope (top of the flask) add another bottom board and turn cope
over and carefully lift off the board that the pattern was sitting on. You will
now see the tabs clearly. Since they are properly drafted they will release
from the drag (bottom of flask) easily. Apply parting dust and ram up the cope.
You will notice that upon removing the pattern from the sand that a mound
representing the inside of the hood is in the drag (bottom). It is best to plan
for it to be in the drag otherwise the mound would be "hanging from the
ceiling" if rammed in the cope (top) and could fall into the casting if the
mold is bumped. Cut the pouring spout, runners and sprue and close mold. Note
that when a large part of the casting will be in the cope (top) of the flask
that weight must be applied to the top of the mold to keep it from separating
while pouring.
Pattern Boards
I have found that making a pattern board (pattern mounted to a board that
fits between the cope and drag of your flask) really speeds up your sand box
time. One-half of the pattern is on each side of the board with the runners and
sprue already in place so all you have to do is place the board in your flask
(between cope and drag) and ram it with sand, rap and remove the pattern board
and close the flask and it’s ready.
For this example I will use the skate pattern.
This casting is used in ride-on railroads of the 1-1/2 in to 2.5 in. scale
size to hold uncoupled cars on a grade. The skates are ½ inch thick and have
two tabs (approximately 3/32nds inch thick) on each side to set over the
railhead.
After drawing the skate the dimensions were transferred to ¼ inch plywood
and each half cut out. Before finishing they are placed back to back and a 1/8
inch hole drilled through both pieces in two places spaced on each end of the
pattern. It is best to use a drill press so the holes are drilled at a 90°
angle. I then place a 1/8 in dowel through both to hold them together while
sanding the draft in them. Remember that the draft is now from the inside
(where both sides meet) of the pattern to the outside. You will now cut out a
pattern board. Plywood ½ to ¾ in. thick is best for this as plywood is less
likely to warp than dimension lumber. This board must fit between the cope and
drag of your flask and extend outside it both width and length. Holes were
drilled in the board so the alignment pins in your flask will go through the
board with plenty of room to spare. See picture. Fig 1.
Place the pattern board in between the cope and drag of your flask and
position one half of the
pattern on the board leaving room for the sprue, runner and pouring spout as
seen in picture of the finished pattern board. (Fig 2) Outline the half pattern
on the board with a pencil. Remove the board and half pattern and using a drill
press drill through the holes in the pattern through the pattern board. Place
longer 1/8 in dowels through the half pattern, pattern board and the other half
pattern on the other side. Cut the dowels short enough to be slightly indented
on both patterns. When satisfied with the fit, glue the patterns to the board
keeping the dowels in place and filling indentions with wood filler. This
insures that both side of the pattern will align with each other once the mold
is closed.
Fig. 1
Cut out the tabs and glue them in position as shown in the picture.(Fig 1 &
2) These tabs could be made as a two part pattern and allowed to be removed
later when the mold is separated or a filler piece the same size of the tab
glued underneath it making it solid through and through. I used the latter
procedure and just mill the slot in them with my milling machine after the
casting is made. This way I can control the width of the slot for different
sized railheads.
Fig. 2
Cut out the sprue and runners and glue them to the bottom half of the board
(the side that will go into the drag). Fig. 2. In the middle of the runner
where the pour spout will join it, drill a hole from the bottom through it and
the pattern board. Fill the bottom of the pour spout drill hole of the runner
with wood filler and smooth off. This leaves a centered hole in the other side
of the board to put a pour spout with a dowel the size of the drilled hole .
See picture Fig. 1 & 3. Use wood filler to fill all gaps between runner and
sprue and make sure you give them plenty of draft for easy removal. This seems
a little tedious but you will see the rewards later. Sand and seal with 2 coats
sanding sealer and use glossy paint to finish both sides of the pattern.
Fig. 3
Fig.4 Pour spout with centered dowel
Place the cope (top half) of the flask upside down and put the pattern board
over it with the hole for the pouring spout pointing into the cope (top). Add
the drag and start ramming the drag (bottom half) of the flask, place bottom
board on and turn flask over.
Insert a ½ - 1 inch dowel (Fig. 4) with a smaller centered dowel the same
size of the hole into the bottom board and ram it up and scrape off top. Jiggle
the pour spout dowel and remove it and use a spoon to form a small funnel to
aid in pouring the aluminum.
Rap the pattern by hitting the pattern board on each end and sides before
removing the cope and drag.
Notice how the runners and sprue are preformed and require little if any
slicking, etc. Close and pour.
I know many questions will arise about this article and I will try to answer
them as quickly and thoroughly as I can as far as my ability permits My e-mail
is jerrybraun1@comcast.net
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