Chapter Three - Variations in Materials and Design

INVESTMENT CAST LUGS

Investment cast lugs are sometimes referred to as being made by the lost wax process. With investment cast lugs, an actual full sized copy of the lug is made of wax copy. (The wax copy is not actually full sized; it must be made with allowances for shrinkage during the casting process. This shrinkage is only a very small percent. If this shrinkage is not planned for, the poor builder must file or grind out the inside diameters of the lugs for large amounts of time.) This wax copy (it is not a mold) has all of the characteristics built into it (e.g. tolerances, finish, shape). The wax copy is then attached to a "tree" with other wax copies. Through a lengthy process, the tree is "invested" in a ceramic case. By putting the whole assembly into an oven, the wax is melted away, leaving only hollow cavities in the shape of the desired parts. The porosity of the ceramic mold allows air to escape while molten steel is being poured into it. After the castings are cool, the ceramic mold is broken away to leave the cast pieces on the tree. The individual parts are cut away and then heat-treated.

FIGURE 3-59: The gate at the back of the seat post binder on a Henry James seat lug

FIGURE 3-60: The gate on the head tube lug where the facing tool will clean it off later

Some castings are better than others. Some castings are very porous or grainy and lack sufficient strength. Other castings may have excessive seams or have unsightly gates, which make more work for the builder and tend to show up as blemishes after the frame is painted. The "gate" of the casting (an example can be seen in FIGURE 3-61) is where the metal enters the mold. The gate often shows up as a hump or irregularity on one side of the casting. For that reason the casting company will carefully design the mold so the gate will either diminish or disappear after final preparations are done. For instance, the gates on head lugs are usually located where the head tube-facing tool removes metal. The gates on bottom brackets are usually located on the faces. Sometimes the gate cannot be hidden very well and the builder must file it off during the initial preparation. Some castings may be more porous than others. Porosity in the metal can substantially weaken the piece. FIGURE 3-61 shows a stylized version of investment casting.

FIGURE 3-61: Investment casting process

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