Wednesday, April 14, 2010

Patterns Finished!!

Got the patterns finished today.  The original casting on the right and one of the patterns on the left. The extra blocks of wood on each side of the wings and the two protrusions on the top are for the sand cores. 

Top view

Bottom view.  The blocks on each side are core prints.  These create voids in the mold that will hold the sand cores.  This allows us to draw the pattern out of the sand without destroying the mold.  The pink in the middle is body putty that we used to create fillets between the ribs and the base plate. 

The wing patterns.  There's an extra piece of plywood on the side of each wing.  This helps us to line the pattern with the base plate.  Once we have holes drilled for our alignment dowels, these pieces will come off. 

Drilling holes for the dowel pins to keep the top half aligned with the bottom half.  You can see that extra piece of plywood that we use for aligning the wing with the base plate on the left side of the wing.  This piece will be removed once we have the holes for the dowel pins drilled. 

Once we have the alignment holes drilled, we glue 1/4" wooden dowels in the holes in the wing part of the pattern so that they protrude about 3/8".  You can see them sticking out on the left side of the photo below.  We next cut the dowels protruding on the top part (right side) of the wing pattern, then sand them so that they are flush with the top surface of the pattern.  
Details of how we did it in the next couple of installments.  Core boxes to be done next.

Friday, April 9, 2010

Step 1 - Fabricating the Base Plate Pattern.

The BASE PLATE is an irregular shape that is based on a rectangular footprint measuring 5 3/8"x 11 1/2" and 1/2" thick.  On top of the base plate is the angular pad to which is fastened the rubber sweep.  On the bottom of the pad are two rows of reinforcing ribs and a 3/4" mounting block which fastens the casting to the motor car.

The wooden pattern for the base plate is made from 1/2" Baltic birch.  Using the table saw, cut a rectangular piece of 1/2" Baltic Birch measuring 5 3/8"x 11 1/2". 

Using the existing aluminum casting, trace the pattern of the base plate onto a piece of stiff card stock.  Retrace the lines on the card stock using a straight edge to make sure that all straight lines are parallel.  Make a copy of the drawing and add the key measurements to the copy.

An additional pad will have to be included in the base plate underneath the upper part of the wing.  This pad will serve as a platform to mount the core prints. The base plate pattern should look like this. 

Cut out the copy of the card stock, leaving all lines in.

The base pattern is cut out of the birch plywood using the band saw.  Use the band saw fence to make sure that straight lines are cut.  Free-hand cut the inside curves.  DO NOT CUT THE OUTSIDE CURVES!  These will be cut after the ribs and mounting block have been glued and fastened to the base plate.

Sand the inside curves to produce a smooth finish.

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Parts of the Speeder Sweep

If we carefully examine the speeder sweep, we can see that it is comprised of a series of distinct pieces and shapes. 

The sweep is built on a base that is 1/2" thick.  To stiffen the base we have two L-shaped ribs.  To fasten the sweep to the speeder, we have a 3/4" plate with a hole.   The wing is another 1/2" thick piece with two slots in it.  It is mounted on the base via a pedestal that is 20 degrees to the base.  So our task will be to fabricate all of these bits and pieces, glue and nail them together. 

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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Colour Coding Of Patterns

In order to identify the different part of the pattern (pattern, core prints, surfaces to be milled), the American Foundryman's Society has adopted the following colour codes for painting patterns. 

Pattern - yellow
Coreprint - black
Machined areas - red
Loose piece mating areas - yellow with red strips
Gating for mounted patterns - yellow

Pattern - black
Coreprint - yellow
Machined areas - red
Loose piece mating areas - yellow
Gating for mounted patterns - yellow

There are other colours for other types of metals.  

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Thursday, April 8, 2010

Sand Core Basics - Part II

In our previous blog on sand core basics, we were dealing with a solid pattern.  Wherever possible, patterns with an irregular shape are made in two halves called a SPLIT PATTERN.  With a SPLIT PATTERN, we place the bottom part of the pattern face-down in the drag.  We then ram green sand around the pattern, flip the drag, pattern, and rammed sand over.  We then place the top-half of the pattern on top of the bottom-half, ram green sand around the pattern, insert some risers and sprues.  We then lift the cope off of the drag, remove the patterns and complete the rest of the mold (SPRUES, GATES, and RISERS) so that it is ready for pouring.

In the graphic below, we have the SPLIT PATTERN for a hollow pipe with flanges on each end.  The pattern can be split in half along a "PARTING LINE".  The two halves of the pattern are kept aligned by holes in the bottom half of the pattern and wooden dowels that protrude from the top half.  The wooden dowels align with the holes in the bottom half of the pattern.  Here's what our pattern would look like.
However, our pattern doesn't account for the hollow centre of the pipe.  To create this hollow centre, we make a cylindrical SAND CORE that we insert into the mold to create the hollow centre.  In order to ensure that the SAND CORE is properly placed in the mold and is kept there, we add CORE PRINTS to each end of our SPLIT PATTERN.

Now that we have added CORE PRINTS to the PATTERN, we place the bottom half of the pattern face-down in the drag, and ram the drag with green sand.
 We then flip the drag and bottom half pattern over on the mold board, place the cope on top of the drag and set the top-half of the pattern on the bottom half, using the alignment pins and holes that we have built into the SPLIT PATTERN.  We then ram the cope with green sand.
We lift the cope off the drag and remove the patterns from the mold.  We now have the external shape of the casting we want to make, including two round voids at each end that will hold our SAND CORE in place.
We now insert our cylindrical SAND CORE which will give us a hollow centre when we pour our molten metal into the sand mold.  Notice how the SAND CORE fills the voids made by the CORE PRINTS in the green sand.
Wherever possible, when we are making patterns, we will use a split pattern and sand cores to create the holes that are required to complete the casting.

Mold Board Basics - The Follow Board

In some cases, the parting line of the pattern will not be a surface that is even and parallel across the cope and the drag but may protrude above a "normal" flat-surface parting line.  And we may also not be able to fabricate a "split pattern" in the usual two halves.
This presents a problem when we place the pattern face-down on the drag.  In order to overcome this problem, we build a special mold board for this pattern that conforms to the outline of the pattern so that most of the parting line will rest flush with the mold board.  This special mold board with a cavity or socket in it and which conforms to the form of the pattern and defines the parting surface of the drag is called a "FOLLOW BOARD".  The pattern rests on the FOLLOW BOARD while making up the drag half of the mold and, in so doing, establishes the correct parting line.

When the drag (or in this case, the FOLLOW BOARD) is flipped over to make the cope part of the mold, the FOLLOW BOARD is removed exposing that part of the pattern that will be incorporated into the cope.  The green sand can then be formed about this protrusion so as to make a clear parting line between the cope and the drag.

Sand Core Basics

3.0  SAND CORE Basics
Making a solid rectangular cube casting is pretty straight forward.  We simply place the solid cube face down on the mold board, shake on some parting dust, riddle some sand over the rectangular pattern, and ram the sand.  Next, turn over the drag, attach the cope to the drag and repeat the process.  Removing the cope, we lightly tap the pattern and lift it out of the drag.  Add some sprue and vent holes and we're ready to pour our casting.
However, when we add holes, slots, and protrusions to our rectangular cube, things get a bit difficult when we try to make a mold for our cube.  Holes, hollows, and undercuts in castings present unusual challenges in that the mold will be destroyed if we attempt to remove the pattern.

In order to overcome this problem, we have to modify the pattern.
Remember that molten metal will flow into all voids in the mold.  Whenever we want to create holes, slots, etc, we have to fill those voids in the mold.  We do this through the use of SAND CORES.  As defined by the American Foundryman's Association, a SAND CORE is simply "...a bonded sand insert that is placed in the mold to form an undercut or hollow section in the casting which cannot be shaped by the pattern.  A core is frequently used to create openings and various shaped cavities in the casting. The shaped body of sand which forms interior of casting and also selected external features".

The SAND CORE is made in a specially prepared CORE MOLD that resembles the shape of the hole slot, etc that we want to create.  The CORE MOLD is filled with a sand and glue mixture, baked to form a solid mass, and then inserted into the larger MOLD.
In order to hold the CORE in place in the middle of the hollow mold, we have to make the SAND CORE longer than the hole we want to create and make depressions called CORE PRINTS in the casting sand.  To make these CORE PRINT depressions, the PATTERN is modified so that the CORE PRINT is an integral part of the PATTERN and is created when we make the mold. In this case, we add two pieces of dowel to the solid cube of our pattern to accommodate the longer length of the sand core (the second dowel piece is hidden in the back of our pattern).
The sand core will be lowered into the voids created by the two dowels. However, if we try to pull our pattern out of the sand, we will destroy the mold.  We have to further modify our pattern to account for the two dowels that are attached to our basic solid core pattern as shown below (the back part of the modification is hidden).
However, when we remove the pattern, we will have two "V-shaped" voids because we modified our pattern so that we could remove the dowel part of the pattern whereas we want a cylindrical void.  In order to create this cylindrical void and fill in the V-shape, we have to make a V-shaped SAND CORE with a circular bottom.  This will fit over our cylindrical SAND CORE and fill the V-shaped void in the mold.
 If we place our two sand cores together - the cylindrical sand core and the V-shaped sand core - we thus create a hole through the middle of our rectangular casting.
In creating any pattern, we always have to be aware of those places on the pattern where SAND CORES will be required and design our pattern accordingly.

Mold Making 101 - Very Basic Instructions

1.1 Parts of the Mold.
Castings are made in a 2-piece wooden box called a FLASK.  The bottom part is called the DRAG.  The top piece is called the COPE.  The DRAG does not have a bottom.  Instead, it is placed on a MOLD BOARD which is an oversized piece of 3/4" plywood on two skids.  LOCKING PINS allow the two sections to be separated and then later re-joined in perfect alignment.  The joint and surface separating the COPE from the DRAG is called the PARTING LINE. 

To recap.
  • FLASK – a 2-piece wooden box. 
  • DRAG – bottom half of the FLASK
  • COPE – top half of the FLASK
  • MOLD BOARD – oversized thick plywood on skids that the DRAG rests on. 
  • PARTING LINE – joint and surface which separates the COPE from the DRAG.
1.2  Making A MOLD
A CASTING is made from a MOLD.  The MOLD is made in the FLASK from a PATTERN using GREEN SAND to create a hollow cavity that is in the shape of the PATTERN.  To make a mold:

The pattern is placed top-down in the DRAG on a MOLD BOARD.  The DRAG is filled up and tightly packed or RAMMED to the top of the DRAG with GREEN SAND.  The excess sand is struck off with a straight edge which  produces a flat surface on the DRAG.

A second MOLD BOARD is placed on top of the leveled sand.  The DRAG, along with both MOLD BOARDS, is flipped over.  The top MOLD BOARD is removed so that the top surface of the pattern is now exposed.  This surface now forms the PARTING LINE.  

The PARTING LINE is next dusted with TALC so as to allow the COPE to be separated from the DRAG when it comes time to remove the PATTERN. 

The COPE is then fitted to the DRAG using the LOCKING PINS. 

Tapered SPRUE and VENT PINS are then pressed about 1"-2"into the green sand of the DRAG.  The SPRUE and VENT PINS will form openings through the sand in which to pour the hot metal and to allow the hot gasses to escape.
 The COPE is filled and rammed to the top with green sand.  The excess sand is struck off with a straight edge.

With the COPE and DRAG tightly packed with green sand, the PATTERN next has to be removed or DRAWN from the MOLD.  The SPRUE and VENT PINS are removed from the COPE, the LOCKING PINS removed, and the COPE lifted off the DRAG.  The PATTERN is lightly RAPPED so that it can be loosened from the green sand and lifted out. 

The green sand in the DRAG between the SPRUE and VENT holes and the hollow impression left by the PATTERN is removed using a spoon-like instrument called a SLICK to form channels called GATES which will allow the molten metal to flow into the MOLD and the hot gasses to escape.
The COPE is then re-placed on top of the DRAG using the LOCKING PINS. 

Hot molten metal is then poured into the SPRUE hole until the metal returns to the surface by the VENT hole. 

When the metal has solidified, the FLASK is upturned off the MOLD BOARD.  The green sand is then knocked out of the FLASK to expose the CASTING.  The SPRUE and VENT holes, which are now filled with metal, are next cut off of the CASTING.