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Casting Foundry & Lathe
Patterns

Ok, so you might not agree with me but the heart of metal casting are the patterns. Without some sort of a pattern all that other stuff like the furnace, crucibles, even greensand, can do little more than cast ingots. A pattern is an exact representation (replica) of the item you wish to cast. Patterns can be made of many things wood, plastic, fiberglass, metal, foam, or for that matter an actual item you wish to reproduce. The only requirement is that it is able to withstand being rammed into greensand without being broken or deformed, and that it has sufficient draft and lack of complexity (?) to allow it to be drawn from the greensand mold, without damaging the sand mold. However very complex shapes can be cast using lost wax or foam casting, in which the pattern remains in the mold and is burned away by the molten metal.

patterns.jpg

The photo above shows some examples of various wood patterns that go into the gingery lathe. From left to right you have the split step pulley, carriage box slide, leadscrew bearings, cross slide/apron sitting on top, compound swivel base, and the compound slide. These patterns when cast and assymbled make up the lathe's carriage with leadscrew.

Take special note of the step-pulley pattern, that is the one that is in two halfs. The reason for this is because it is very difficult to properly mold and to remove if it was in just one piece. This way when the mold (flask) is opened one half of the pattern remains in each part if the flask, where it can then be easily removed.

Also take note that some of the patterns have eyescrews in them. These are screwed into the pattern to remove the pattern once the flask it is rammed up, and opened to remove the pattern. It does not need to be eyescrews, and wood screw will work. Also I use the eyescrews to rap on to loosen the pattern.

Although it is difficult to see from the photo all patterns have what is called draft. Also all the patterns have what is called a parting line. This simi imaginary line is the point on the pattern that is in the same plain where the two half of the flask mold come together. In some cases as in the carriage apron and the core plates the parting line is the bottom or top surface of the pattern. In others such as the split step-pulley pattern it is in the middle of the pattern. This means that part of the pattern is above and part is below the parting line.

Draft is one of the most important parts of a usable pattern. Draft is what allows the pattern to be removed from the mold without damaging the delicate cavity formed by the pattern. In the simplest terms I can think of draft is a slight taper from the parting line inward towards the part(s) of the pattern imbeded in the sand. The widest part of the pattern is at the parting line and the narrowest part is furthest from the parting line. This tapper is only a few degrees. Often two or three dregrees of taper is all that is needed, but the more you have the easier it can be to remove. I often have around 5-7 degrees, although excessive it has saved me a lot of problems.

A Match-plate is a way of casting many patterns all at once. I have not tried this, but I can see a lot of advantages, especially if you are casting a lot of little parts. Basicly it is a board (sheet of 3/4 ply-wood) that fits between the cope and the drag. Depending on the shapes, etc., some patterns are only on the cope side and some are on the drag side. But others may be split patterns with one half of the pattern on each side. Patterns, runners, gates, vents, etc., are all attached permenently to the match-plate.

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