Sunday, October 30, 2011

Baggage Car Bronze Striker Plate - Making The Pattern

Now here's a real challenge for you pattern makers.  This is a photo of a bronze striker plate from the end-doors of Bytown Railway Society's ex-Canadian National Railways baggage car.  When first examined, it had a couple of cracks in it, some broken pieces, to say nothing of the fact that it had been whacked into place and out of shape some decades ago when it was first installed - who knows when?  Here's a photo of the "front" of the striker plate. 

It's a fact with bronze and brass that you never know when the bronze will melt.  In fabricating the accelerator for the propane burner for my backyard foundry, I quickly discovered that some brass melts almost exactly at the same temperature as the silver solder I was using.  I've got about 4 of these parts that melted just like a wax candle when I applied the heat.  It all depends on what the brass or bronze has been alloyed with which determines the melting point of the brass.

Here's a photo of the "back" view of the striker plate.  You can see that the striker plate is built up from two surfaces - a flat horizontal surface (the piece with the two holes on each side), and a flat vertical surface (the piece with the two holes closest to you).  Notice the two "wings" on each side.  Without these two wings, it would be relatively simple to make a green sand mold.  However, it's these two wings that will be the challenge for this kind of casting.

To continue with our "tour" of the striker plate, here's an inside side view which gives shows the "latch" piece (the wedge on the left) that the door latch latches onto.  The latch piece on the other door is on the right side.  So our design of the pattern will incorporate a "universal" latch piece for both a right-hand and left-hand door opening.

Here's another shot of the above photo but taken from the right side.  This photo really shows how the part has been bashed and smashed over the decades.

In trying to weld the cracks on this striker plate, the bronze started to vapourize which didn't improve the situation at hand.  So, I was handed the piece and asked to make a pattern for some castings.  Since there are two doors at each end of the baggage car, I wanted to see if I could get some accurate dimensions from the other striker plate at the opposite end of the car.  After removing and examining this second piece, while it was in better shape than its mate, it wasn't much better.  So it was a matter of cut- file-and-fit repeated several times until we got the pieces to fit together.   Here's a photo of some of the bits-and-pieces before we glued them together.

And here's a couple of shots of the bits-and-pieces glued together. That belt sander in the woodworking shop saw double-duty these past few weeks.  Needless to say a few of our bits-and-pieces ended up in the garbage pail when we sanded off a bit too much. 

The material we used for the pattern is 1/8" 3-ply Baltic birch plywood.  At first we were going to use wood glue to stick all the bits and pieces together but then we realized that the pattern probably wouldn't stand up to the heavy-duty ramming required when we made the green-sand mold.  So, while we used wood glue to temporarily stick the pieces together, we used epoxy glue to make sure that everything would hold up to some rough abuse.

In any pattern, we have to consider three things -
  1. Shrinkage of the metal when it transforms from molten metal to a solid, 
  2. Draft (or slope to the sides) so we can extract the wooden pattern from our sand mold, and 
  3. Fillets (or rounded edges where two pieces of wood fit together) to reduce the "tearing" of the metal as it goes from a molten mass to a solid.  
We made our pattern a bit oversize to take care of the first issue, added some "slope" to the sides to take care of the draft, and used epoxy glue to create fillets where two pieces of wood joined together.  The darker parts on the pattern is epoxy glue.  In addition to creating fillets, the epoxy glue has added extra strength to the whole pattern.  Hard to see from this angle but they do create nice rounded fillets. 

Using epoxy glue required gluing the bits-and-pieces together over several days as the old-fashioned 5-minute epoxy has disappeared from the store shelves.  All you can get now is a 24-hour epoxy which takes about 48 hours for the glue to set - sometimes even longer - and cure. 

Here are some "before-and after", or rather, "original-and-pattern" shots taken from various angles.

The difference is quite visible in the above shot, isn't it.  We aren't going to include the holes in our pattern as it will be easier to drill the holes after the part has been cast.  Here's a side view of the two.

And an end view.  You can see how we've added draft (slope) to the pattern.  This will help in extracting the pattern from the green sand when we make the mold. 

We've still got to add the latch part to the pattern but we'll have that done for next week.  We'll then spray on a couple of coats of paint to make sure we don't have any "ripples" between the glued joints, followed by a couple of coats of lacquer.  After that we'll "test" the pattern by making a mold in our green sand.  That will be the real test!

Tony Z from my "basement machine shop" group gave me the name of a person who has a backyard foundry who does castings in brass and bronze.  We'll have to see if he would like to do a casting or two, eh!?

PS - Many thanx to Jack L for his photography expertise.

1 comment:

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