Published: March 12th 2010March 12th 2010
This is basically a way of making oddly shaped metal objects that would be difficult or impossible to make on shop machines. The method of making these objects is basically by filling a box with sand, putting a pattern in the sand, pouring more sand over top and then removing the pattern to create a hole (or a cast) of the object. Then molten metal is poured into the cast and when it has cooled down sufficiently it can be removed and filed.
So, to create the cast, the first box called the Drag is placed on the table and the inside is painted with water so that the sand will adhere to the sides. The drag is simply a box with four sides. Both ends are open for ease of removing the cast once it had cooled. Then sand (a special composite of sand, clay and water) is piled high in the drag, almost three times as tall as the height of the drag. Then it is compressed down by pounding it with various implements. The first simply looks like an inch thick metal rod that has been bent 180 degrees at the end. This ensures that the sand is packed evenly all around the drag. Then it is stamped with a tool that is basically a circular base with a handle. This just even out the rest of the sand and compresses it even more. Finally it is smoothed with a spade. This process leaves you with a box packed with as much sand as you can fit into it. The sand should be compressed as much as possible to prevent the cast from deforming at any point in this procedure.
Next, the pattern is placed in the sand. The pattern is usually made of wood, if it is an original creation, or it can be the piece you want to reproduce. Either way, it must be placed around center of the drag and then hammered into the sand, about halfway. If the object is complicated with many curves and angles, you have to make sure that you will be able to remove it once it is encased in sand. This is one of the limitations of casting. I won't get into all the various types, but let me assure you, there are more ways to cast objects than you probably want to know about.
Then Parting Sand is applied to the top of this layer of sand. It is also called silicon sand. It is not nearly as cohesive as the packing sand and works quite well to keep the two layers apart.
After this the Cope, which is identical to the drag, is stacked on top of the Drag. Again, water is applied to the inside and sand is poured in. The same procedure is applied to the top until the sand is again even. Then, a small tool similar to an awl is used to make several holes through the sand to the cast. This is so the gases that are trapped inside the cast when the metal is poured can escape. Then a funnel is made on top of a hole directly down through the cope. This acts as a passageway for the molten metal to be poured.
Lastly the Cope is taken off the drag (carefully) and is flipped upside down. The pattern is taken out of the Drag and the cast is inspected for accuracy. Sometimes we had to repeat the cast three times to get it right. This got very frustrating. Finally an alleyway between the cast and the passage for the metal is made. Otherwise, there is no way for the metal to actually reach your cast.
The Cope is then replaced on top of the Drag and then molten metal is poured into the funnel. If everything was done right, the metal flows right into the cast and can be taken out to be cooled after a few minutes. If it is not, as we found out, the metal flows into the cast, out through accidental holes in the sand and starts pouring onto the floor of the workshop, lighting the cope and drag on fire on the way. The metal that we used was aluminum and it was heated to 660 degrees celsius for pouring. It hardens at 640 degrees, so the instructors stressed that hardened was not the same as cooled.
The shaper machine is probably one of the more boring machines in the workshop, so I will not spend too much time talking about it. It is a machine that has one arm that moves horizontally back and forth. On the end of the arm there is a single blade. This blade cuts straight lines (or occasionally angled ones) as it moves accross the part. So, for example, if you had a metal cube that was 1mm too large on some of its sides, you would use a shaping machine to remove the excess material.
The turning machines are the other part of machining that we get to do and it is much more entertaining than shaping. A turning machine, as its name probably gives away, spins a vice in a circle. Whatever is fixed into the vice spins with it. Then a stationary blade is placed at varying spots to remove material and change the diameter of the object being turned.
So, to turn an object, first it must be fixed and centered in the clamp, which is much harder than it sounds. The vice on this machine has four teeth that need to be tightened the same amounts to ensure the object is spinning on a central axis. To do this, you start off tightening the teeth around the workpiece (called the work) and turning on the machine. The work is rotating (off the center axis most likely), so chalk is applied to the work to see which side is jutting out. The teeth are then tightened on the side with the chalk markings and loosened on the opposite side. This procedure is repeated until the work is center, which can take quite a long time.
After the work has been centered sufficiently (or the instructor takes pity on you and does it himself) the blade can be positioned to take off material at specific points along the length of the work. In this way you can make a single cylinder with varying diameters along its length. Turning is very useful and can make things like screws, simple hammers gears etc.
By far the most fun of any skill we are learning. Welding. Okay, I think most people know the basics of it. You use a current to melt metal to fuse other pieces of metal together. I am currently learning how to do this properly, so I don't know all the different types yet, but I can give you a basic idea of what we are doing.
First, there is machine to supply DC power. It is hooked up at one end to the work table and at the other to a clamp with an electrode (a metal rod that is melted onto the work as you weld). Basically, when you touch the electrode to the work table, the circuit is completed and you get an arc. For Arc welding (what I have learned currently) this is how you weld the pieces together. The skill itself is fairly simple when you are doing straight, flat, horizontal welding. All you need to do is keep the electrode 3mm from the work and move it straight, steadily and slowly accross. Once you get into different joints and doing inclined, vertical or overhead welding, well, let's say that things get interesting. And there is no shortage of sparks flying all over the place.