The On-Line Magazine of Rideable Model Railroading
  NUMBER 130

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© April 24, 2009   

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Pattern Making

Written by Jerry Braun
photos by
Jerry Braun

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:

  1. Simple patterns
  2. Two part patterns
  3. 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

 

Written by Jerry Braun
photos by
Jerry Braun

 

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©Discover Live Steam. This material may not be published, rewritten, or redistributed without written permission.

 

 

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