Gear cutting
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Gear cutting is any number of methods used to manufacture precision gears.
Gear hobbing is a method by which a special hobbing anto cutter and gear blank are rotated at the same time to transfer the profile of the hob onto the gear blank.
Spur and other straight gears may be cut or ground on a milling machine/jig grinder utilizing a numbered gear cutter, and any indexing head or rotary table. The number of the gear cutter is determined by the tooth count of the gear to be cut. Any straight gear can be produced in this way.
To machine a helical gear or twist drill on a manual machine, a true indexing fixture must be used. Indexing fixtures can disengage the drive worm, and be attached via an external gear train to the machine table's handle (like a power feed). It then operates similarly to a carriage on a lathe. As the table moves on the X axis, the fixture will rotate in a fixed ratio with the table. The indexing fixture itself receives its name from the original purpose of the tool: moving the table in precise, fixed increments. If the indexing worm is not disengaged from the table, one can move the table in a highly controlled fashion via the indexing plate to produce linear movement of great precision (such as a vernier scale). An enterprising hobbyist who has need of many helical gears would be well served by a quick-change gearbox.
For very large gears or splines, a vertical broach is used. It consists of a vertical rail that carries a single tooth cutter formed to create the tooth shape. A rotary table and a Y axis are the customary axes available. Some machines will cut to a depth on the Y axis and index the rotary table automatically. The largest gears are produced on these machines.
The old method of gear cutting is mounting a gear blank in a shaper and using a tool shaped in the profile of the tooth to be cut. This method also works for cutting internal splines.
Gear cutting is the process of machining gears into shape. There are several different ways to make gears. These include: drawing, forging, extrusion, casting, powder metallurgy, and thread rolling. Gears are not exclusively made of metal and can also be made of plastic or wood. The use of gears ranges from small everyday operations to extremely important operations on a grand scale. This can be anything from making your watch tick to raising a draw bridge for a large carrier.
When making a gear it is important that the fit between the gears is proper and that the teeth are of good quality. If this is not done then it will result in inefficient energy transfer and will ultimately wear and break down much quicker. A popular way to build gears is by form cutting. This is done by taking a blank gear and rotating a cutter, with the desired tooth pattern, around its periphery. This ensures that the gear will fit when the operation is finished.
Other operations such as broaching work particularly well for cutting teeth on the inside. The downside to this is that it is expensive and different broaches are required to make different sized gears. Therefore it is mostly used in very high production runs.
There are a few different types of cutters used when creating gears. One is a rack shaper. These are straight and move in a direction tangent to the gear, while the gear is fixed. They have six to twelve teeth and eventually have to be moved back to the starting point to begin another cut.
Another is a pinion-shaped cutter that is used in a gear shaper machine. It is basically when a cutter that looks similar to a gear cuts a gear blank. The cutter and the blank must have a rotating axis parallel to each other. This process works well for low and high production runs.
Lastly, there is a cutter called a hob. This is like a worm that turns and cuts the gear. The angle must be set up at minus 90 between the hob and the gear blank, but then the lead angle of the hob threads must be accounted for. The hob must make one revolution to create each tooth of the gear. Used very often for all sizes of production runs, but works best for medium to high.
After being cut the gear can be finished by shaving, burnishing, grinding, honing or lapping. (Kalpakjian, 749-755)
See also
Further reading
- Woodbury, Robert S. (1972), History of the Gear-Cutting Machine. In Studies in the History of Machine Tools, Cambridge, Massachusetts, USA, and London, England: MIT Press, ISBN 978-0-262-73033-4, LCCN 72-0. First published alone as a monograph in 1958.
