Hobbing is a machining process for gear cutting , cutting splines , and cutting sprockets on a hobbing machine , which is a special type of milling machine. The teeth or splines of the gear are progressively cut into the material a flat, cylindrical piece of metal by a series of cuts made by a cutting tool called a hob. Compared to other gear forming processes it is relatively inexpensive but still quite accurate, thus it is used for a broad range of parts and quantities. It is the most widely used gear cutting process for creating spur and helical gears  and more gears are cut by hobbing than any other process as it is relatively quick and inexpensive. A type of skiving that is analogous to the hobbing of external gears can be applied to the cutting of internal gears, which are skived with a rotary cutter rather than shaped or broached.
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Upon completing this chapter, you should be able to do the following:. Describe the materials used to manufacture gears. Explain the manufacture of gears, splines, and sprockets. Explain the process used to set up gear trains. This chapter covers the manufacture of spur gears, helical gears, bevel gears, stub tooth gears, worms, worm gears, splines, and sprockets.
Gears have always been a highly essential element in machinery used aboard ships and at naval shore facilities. This requires great skill and precision. This chapter will cover gear cutting practices on a standard milling machine. As with any shop equipment you must observe all posted safety precautions.
Review your equipment operators manual for safety precautions. The choice of material for a particular gear is usually based on the function of the gear. This involves factors such as the speed of operation, the type of stress, the importance of quiet operation, and the need for resistance to corrosion. The easiest way to determine what material to use for a replacement gear is to find out what material was used for the gear you must replace. In most cases, you will have the original gear to go by.
If not, you may have to find the specifications or blueprints for the original gear. Do this to be sure the material you are using will hold up under the stresses the gear will encounter. Gears are made from ferrous, nonferrous, and nonmetallic materials. Steel, for example, is used whenever great strength and toughness are required. Nonferrous metals such as bronze and brass are often used aboard naval ships for gears that must resist saltwater corrosion.
Monel and aluminum may be used for gears, where corrosion resistance is of primary importance. Non- metallic gearing is frequently used where quiet operation is important.
Nonmetallic gears are most effective at high-speeds. However, they do not always hold up against the wide fluctuations of load and the high shock loads encountered at low speeds. Gears made of nonmetallic materials have a lower tensile strength than those constructed of metallic materials, but their greater resiliency gives them approximately the same power-transmitting capacity as cast iron.
A gear is made by cutting a series of equally spaced, specially shaped grooves on the periphery of a wheel see fig. To calculate the dimensions of a spur gear, you must know the parts of the gear. You also must know the formulas for finding the dimensions of the parts. To cut the gear you must know what cutter to use and how.
Figure The following terms see fig. The symbols in parentheses are standard gear nomenclature symbols used and taught at MR schools. In parallel shaft gears, you can determine the pitch diameter directly from the center-to-center distance and the number of teeth. The symbols the American Gear Manufacturers Association uses to describe gears and gear teeth are different from those the Navy uses.
The following list will familiarize you with both sets of symbols:. American Gear. Spur Gear. Pitch circle. Pitch diameter. Working depth. Whole depth. Root circle. Outside diameter OD. Circular thickness CT. Circular pitch CP. Number of teeth. Root diameter RD. Chordal thickness tc. The diametral pitch system was devised to simplify gear calculations and measurements.
It is based on the diameter of the pitch circle rather than on the circumference. Since the circumference of a circle is 3. When you use this system, there is no need to calculate circular pitch. Indexing devices based on the diametral pitch system will accurately space the teeth, and the formed cutter associated with the indexing device will form the teeth within the necessary accuracy. This system simplifies all calculations such as center distance between gears and working depth of teeth.
Many formulas are used to calculate the dimensions of gears and gear teeth, but we will only use those needed in this discussion. Appendix III of this manual contains a more complete list of such formulas.
Appendix IV contains explanations of how you determine the formulas to calculate the dimensions of gear teeth. Usually, you can get the outside diameter OD of a gear and the number of teeth NT from a blueprint or a sample gear. You may then use these two known factors to calculate the necessary data. For example, use the following procedure to make a gear 3.
Find the pitch diameter PD by using the formula:. Find the diametral pitch DP by using the. Find the whole depth of tooth WD by using. You can select the cutter to machine the gear teeth as soon as you compute the diametral pitch. Formed gear cutters are made with eight different forms numbered from 1 to 8 for each diametral pitch. The number of the cutter depends upon the number of teeth the gear will have. The following chart shows which cutter to use to cut various numbers of teeth on a gear.
If, for example, you need a cutter for a gear that has 24 teeth, use a No. Range of teeth. Number of cutter. Most cutters are stamped to show the number of the cutter, the diametral pitch, the range for the number of the cutter, and the depth. Involute gear cutters usually. To check the dimensional accuracy of gear teeth, use a gear tooth vernier caliper see fig.
The vertical scale is adjusted to the chordal addendum a c and the horizontal scale is used to find the chordal thickness t c. Before you calculate the chordal addendum, you must determine the addendum ADD and circular thickness C t. To determine the addendum, use the formula:. Using the values from the preceding example,.
To determine the circular thickness, use the formula:. The formula used to find the chordal addendum is. The formula to find the chordal tooth thickness is. For example,. Now set the vertical scale of the gear tooth vernier caliper to 0. Adjust the caliper so the jaws touch each side of the tooth as shown in figure If the reading on the horizontal scale is 0.
Sometimes you cannot determine the outside diameter of a gear or the number of teeth from available information. However, if you can find a gear dimension and a tooth dimension, you can put these dimensions into one or more of the formulas in Appendix II and calculate the required dimensions.
Use the following procedures to make a gear with the dimensions given in the preceding example:. Select and cut a piece of stock to make the blank. Mount the stock in a chuck on a lathe. At the center of the blank, face an area slightly larger than the diameter of the required bore. Drill and bore to the required size within tolerance. Remove the blank from the lathe and press it on a mandrel. Set up the mandrel on the milling machine between the centers of the index head and the footstock.
Dial in within tolerance. Select a No. Set the index head to index 24 divisions. Start the milling machine spindle and move the table up until the cutter just touches the gear blank. Set the micrometer collar on the vertical feed handwheel to zero, then hand feed the table up toward the cutter slightly less than the whole depth of the tooth. Cut one tooth groove. Then index the work- piece for one division and take another cut.
gear cutting formulae
If you see errors or omissions please contact us at. GearSolutions , a technical publication, has many gear topics of interest. Example: hobbing throated worm gears Fly Cutting and Tapered Hobs on machines with tangential feed, as well as Radial infeed. EngineersEdge is a website of specs, formulas,etc.