Gear Cutting: A Machining Process
Gear Materials and Manufacturing Methods
Gears can be made from the following materials: plain and alloy steels, brass, bronze, stainless steels, aluminum, zinc, cast iron, and many plastics. The type of material utilized is often dictated by the environment that the gear is to be utilized in. In turn, the choice of materials will often dictate the method of machining or other manufacturing process utilized. Material choice and method of manufacture is important because gears are utilized in everything from your wristwatch to the engine of your automobile.
Here are a few illustrations of how varied the gear making process can be and how material choices may dictate the method of manufacturing. Sometimes smaller size gears that are under low stress will be produced ready for use by producing the part by powder metallurgy or die casting. Plastic gears of any size can often be made in a similar fashion utilizing the injection molding process. Small or medium-sized gears made of brass or bronze may be extruded in bars of 12’ lengths and then saw cut to the desired thickness. Larger gears for slow-moving equipment may be produced from sand-cast iron or flame-cut from steel sheets.
Most gears are produced from blanks which are cast, forged or machined to the approximate width and diameter of the finished part. When blanks are utilized, a machinist will typically locate on one dimension and then machine the outside diameter and face the ends of the blank off so that the gear is prepared for finish machining.
Hobbing is the most often used method of making gears because it has a quick setup, enabling economical short and long production run. A gear hob is a cylindrical cutting tool with rings of teeth that are the involuted shape of the desired gear tooth on it. It is important to note that the gear hob is positioned at a right angle to the gear blank being cut. In this manner, the gear blank rotates with the hob feeding across the cutting plane. Each successive tooth on the hob cuts a little deeper until it has generated the precise gear tooth profile desired.
Gear shaping is a machining process that generates the desired tooth characteristics much the same as gear hobbing except that it moves on the same axis as the gear blank. Gear shaping is often used to produce spur and herringbone gears. This process can also be used to produce ratchets, splines, and sprockets of almost any pitch or diameter. One of the advantages is that gear shapers can work much closer to the shoulder of the gear blank.
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