Friday, April 9, 2010

1.1 - Woodings Motor Car Sweep Bracket

1.1.1  Purpose of the Woodings Motor Car Sweep Bracket
Sweep brackets are fastened on all 4 corners (front and back) of the Woodings motor car.  Rubber sweeps are bolted to the brackets and hang below the motor car slightly touching the rails.  As the car moves along the track, the sweeps remove rock and other obstructions that might be on the rails which could derail the motor car and injure the occupants.   The right front and left front brackets are mirror images of each other as are the right rear and left rear brackets.  Or, restated, the right front and left rear brackets are similar as are the left front and right rear brackets.  We will need 4 aluminum castings - the right front and left rear being the same and the left front and right rear also being simlar.  This means we will have to make 2 patterns, each being a mirror image of the other.  As precision is not important for this casting, shrinkage of molten aluminum will be ignored.  We will take the measurements from the existing casting when making the patterns. 

1.1.2 A Tour Of The Sweep Bracket
An examination of the existing casting indicates that it is made up of 4 unique elements. 

Firstly is the BODY on which the casting is built.  This is shown in the photo below.  While this consists of an irregular shape, it is built on a rectangular "footprint" measuring 5 3/8"x 11 1/2".  This fact will make the cutting of this part of the pattern relatively easy. 

Secondly, the casting has an elevated WING mounted to the body at an angle of 20 degrees to the BODY.  Rubber sweeps are bolted to the WING via 2 slots in the WING. 

Examining the underside of the casting in the photo below, we have two L-shaped RIBS that provide strength to the casting.  In order to provide clearance for attaching the rubber sweep via bolts, the RIB on the left side is indented about 1/8" into the BODY of the casting.  The right side of the BODY has also been recessed on an angle of 20 degrees to provide clearance for the bolts.  At the bottom-right corner of the casting is a BOSS that is 3/4" thick.  The casting is fastened to the motor car via a hole in the BOSS.  To simplify the pattern and making of the mold, this hole will not be incorporated into the pattern as it is much easier to drill a hole after the parts have been cast. 
The photo below shows the BODY incorporating the RIBS, the angle of the WING, and the hole in the BOSS on the left side of the photo.  Note that the end of the BOSS is rounded which allows the casting to be swung away from the tracks when the motor car is being removed from the rails. We won't incorporate the top part of the rounding in our pattern as this would create an UNDERCUT in our mold.  We will simply round this with the band saw after we have cast the part.  We also get a good view of where we can establish the PARTING LINE when we make the pattern for the casting. 
1.1.3  Problems To Resolve With The WING
The casting is unique in that it is cast along two planes.  The BODY, RIBS and BOSS of the casting covers a horizontal plane (the XZ-plane).  The WING of the casting is sloped 20 degrees to the XZ plane.

If we rotate the above photo 180 degrees along the XZ-plane, we can see where we will have some problems to resolve in making the pattern as the WING of the casting will be embedded in the sand in both the COPE and DRAG. We will ruin our mold if we attempt to pull the embedded PATTERN from the sand. 

We can easily resolve our problem with the part of the WING embedded in the DRAG by simply having the parting line follow this part of the wing as shown in the photo below.  

In order to preserve the mold, we will have to fill in part of the PATTERN between the BODY and the WING so that we don't destroy the MOLD when we remove this revised PATTERN.  These fillers will be the CORE PRINTS.  We can subsequently fill in the hollow created by the filled part of the PATTERN with a specially prepared sand CORE which will give us our final outline of the casting we want to make. 

SAND CORES are made in a specially prepared MOLD which is filled with a sand and glue mixture, baked to form a solid mass, and then inserted into the larger MOLD.

The photo below has been rotated 90 degrees in a counter-clockwise direction along the XZ-plane. We can clearly see the mounting hole in the BOSS and the 20 degree elevation of the wing.  The RIBS have been seamlessly incorporated into the BODY of the casting. 

Our first step will be to fabricate the BODY of the pattern.  We will next fabricate and incorporate the RIBS and BOSS into the BODY.  Lastly, we will fabricate and incorporate the WING and CORE PRINTS into the PATTERN, remembering that we will have to also fabricate CORE BOXES in which to cast the sand CORES.

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