Manufacturing a high-quality skateboard truck is a hands-on process – literally. It involves as many as 27 different craftsmen from different industries working in tight coordination to produce something that will hold up to the kind of abuse we call a good time. At the end of the “Diary of a Truck Part 1” we left off at the completion of the masters and making the sand casting tooling called the pattern plate. In Part 2 we’ll pick up at the beginning of the casting stage and follow our parts all the way to a finished truck.
Casting 101
There are several ways to cast a skateboard truck. The OG method, dating back to Mesopotamia circa 3,300 B.C., is called sand casting. It’s done by making an impression in a pair of damp sand blocks with a pattern plate and pouring molten metal into the cavities left by the plate’s impressions. Another, more recent method is casting into steel or iron molds.
Much confusion surrounds this method, as it is referred to variously as die casting, injection casting, pressure casting and permanent molding. Some overseas manufacturers have long claimed the results of this process are just as good; but for one skilled in the art of casting, nothing could be further from the truth. Die casting does produce a very clean finish and has been widely used to produce the smoothest, best-looking truck parts being imported from overseas today, but there are serious problems with this method. While a die casting is extremely clean, it requires special alloys, which are combinations of two or more metals that together have enhanced properties not present in either of the individual elements. For example, stainless steel is an alloy of iron with small amounts of carbon and chromium, and it’s harder, nonmagnetic and does not rust, which is quite different from the properties of the iron it’s primarily made from. It’s the same with aluminum alloys.
The aluminum alloys used in die casting need to contain higher concentrations of iron so they don’t solder or stick to the steel of the die mold. This unfortunately reduces the alloy’s ductility, or amount the aluminum can bend before breaking, in effect producing a hard but brittle part. These alloys, typically classified as 360 or 380, are in the same family as 356 but are much cheaper and not able to be heat-treated. By contrast, the sand-casting alloy 356 contains higher amounts of magnesium for strength, which provides for the ability to heat-treat the aluminum. It also contains silicon, which helps it to flow a full 200 degrees cooler than for die casting, which prevents burning off the magnesium that is so crucial to heat-treat hardening.
Heat treating is the process whereby the aluminum parts are heated in huge ovens to 950° F for a precise amount of time (nearly nine hours), and then quenched in water to chill them quickly. This process will change the crystalline microstructure of the metal’s grain, grouping it in a specific rearrangement called allotropy, which will increase the strength and flexibility of the aluminum. After quenching it’s reheated to 350° F for another five hours in a process called aging, which takes the metal to its final T6 Rockwell hardness rating. This produces what we consider the ideal combination of hardness and flexibility to guarantee a strong, long-lasting truck.
Permanent molding is like sand casting in that it’s a gravity-fed pour, where the molten aluminum is literally hand poured into the mold, but the mold is made of steel instead of sand. The finish is not as smooth as die casting and it’s not as quick in production, but it’s smoother than sand casting, and can also be used with 356. It’s more costly and fickle than sand casting, but once it’s worked out, it requires less finishing costs to make the part smooth.
The other day we were at a local foundry and the owner was telling us how numerous truck brands he worked with were unhappy with their trucks from overseas, from the brittle metal to the rising prices, and were trying to bring production back to the U.S. He said these brands had spent a lot of money shipping their tooling back home, expecting that they could re-use it to cast with here. What was arriving, though, was not the permanent-mold tooling they had been promised, but unusable die-cast tooling specially designed to fit foreign injection-molding machines.
The lesson that has been learned over and over regarding overseas production: Beware the old “bait and switch” trick, where specific materials and processes are promised and cheaper substitutes are delivered. Now many companies are scrambling to remake their tooling, and learning that not all truck castings are alike.
Thursday, Aug. 25, 2011
If you know Greg Falk, OG founder and president of Carver, you know that the first thing he does when you visit the factory is show you his machines. Banks of rebuilt vintage American Walker-Turner drill presses are lined up in rows in the shop, each drill head dedicated to a specific operation. Today a load of parts arrives from the foundry; our machinists take the parts one by one and load them into the new jigs that hold the parts at the correct angles while the holes for the kingpin, pivot cup and mounting hardware are drilled. The jigs are custom-made from machined and welded steel plates, tested weeks ago in preparation for this day. One of these jigs is for a new process designed to “chase” or remove the parting line around the pivot pin of the hanger. Because this parting line runs along the pivot pin on every cast RKP hanger, nearly all pivot pins tend to be slightly out of round from sanding, or have a raised seam that prevents a precise fit inside the pivot cup. This leads to either binding or slop, neither of which is good for performance. Greg has developed a method to chase the pivot pin so it’s perfectly round and fits the pivot cup exactly. It’s great to see this new technique in operation today. Finally a cast truck with a pivot pin as precise as a costly CNC truck!
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Friday, Aug. 19, 2011
Today we’re at the foundry, a cavernous building where everything has been blackened by the oily dust that hangs in the air. The furnaces roar constantly, and steam rises from mountains of cooling black-sand molds. It’s like Dante’s Inferno at the dawn of the Industrial Age. At the squeezer, the pneumatic stand that compresses the sand tightly into the molds, a foundryman takes the pattern plate with the shapes of the truck parts on it, and an open-ended pair of boxes called flasks, and packs a special mix of damp sand and clay onto the plate, forming an impression of the trucks into each of the two sand-filled boxes. After removing the pattern plate from between the two flasks and reassembling the sand blocks together again, another craftsman pours 1,380° F molten aluminum into the cavities, filling them until the metal begins to spill out of the sprues (the opening through which the metal is poured). After they have cooled, they are fished out of the steaming sand blocks using steel hooks, and the individual parts are sawn off the trees (so called because the parts look rather like branches). Then the stubs, called gates, are sanded flush, and the seams, called parting lines, are ground smooth by hand.
Thursday, Sept. 1, 2011
The first batch of parts has been machined, and today they’ll travel to the heat treating ovens in downtown Los Angeles. This is a crucial step in the process, as the special 356 alloy is quite soft when it’s cast, and needs to be heated to temperatures of 950° F over many hours and then quenched in water in order to harden the alloy. The beauty of 356 is that it can be precisely tempered to the ideal hardness, which is hard enough to withstand the beating the truck will soon take, but not so hard that it’s brittle. Brittleness is the biggest drawback to the cheap die-cast alloys used in trucks made overseas.
Friday, Sept. 9, 2011
The heat-treated parts are now taken to the finisher, where they will be loaded into huge rubber-lined vats filled with water and ceramic stones. These massive “VibraHone” vats are mounted to motors that shake them vigorously; as they shake, the stones vibrate against the truck parts like pebbles on the beach, wearing down the parts’ sand-cast texture and giving them a dull, smooth finish. It’s so loud you leave with a ringing in your ears! Once the parts have been smoothed, a process that can take as many as five hours, they are removed from the vats, sorted from the stones and loaded back into barrels to be taken to the powder-coating facility nearby.
Tuesday, Sept. 13, 2011
Once the parts arrive at the powder coater, the axles are masked off and all the parts are hung on hooks traveling along a suspended conveyor, while a pair of skilled painters spray a magnetically charged black powder onto each side of the parts as they pass by. Then the parts are again put in an oven, where the powder is melted onto the surface at 400° F.
The powder-coating process is completely free of volatile gases, unlike traditional paint, producing an environmentally sensitive finish that is much more durable than regular sprayed-on paint. Even the ovens run on clean-burning natural gas. It’s so clean that the EPA doesn’t even need to regulate the process. Once the parts have cooled, they are individually wrapped in paper like candy, boxed and brought back to our factory.
Friday, Sept. 16, 2011
Once the finished parts arrive, the bases are unpacked first, and one at a time the Grade 8 kingpin is pressed in with a six-ton Denison hydraulic multipress, another of Greg’s classic American machines. From here the bases are loaded onto carts and brought to the assembly table, where another craftsman individually presses in the pivot cup, slides a cup washer and a brightly colored urethane bottom bushing onto the kingpin, then fits the hanger onto the base and traps it with the top bushing, flat washer and nut. The finished trucks are then loaded back onto the carts and either assembled onto boards or packed into boxes, either to be shipped to your local shop or purchased online and mailed directly to your house. It’s all downhill from there.
