Gears Cut Method

Polytechnic University of Puerto Rico 
Mechanical Engineering Department
ME-5250-09 
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Homework: # 10
Gears Cutting Methods
 

Francisco J Muñoz                                                                                                      02/04/13            #47973                                                                                                     Prof. Antoni Skrypinzky 
 

Gears cut method
Broaching

where:
* P = pitch
* RPT = rise per tooth
* nr = number of roughing teeth
* ns = number of semi-finishing teeth
* nf = number of finishing teeth
* tr = RPT for the roughing teeth
* ts = RPT for the semi-finishing teeth
* tf = RPT for the finishing teeth

* Ls = Shank length
* LRP = Rear pilot length
* D1 = Diameter of thetooth tip
* D2 = Diameter of the tooth root
* D = Depth of a tooth (0.4P)
* L = Land (behind the cutting edge) (0.25P)
* R = Radius of the gullet (0.25P)
* α = Hook angle or rake angle
* γ = Back-off angle or clearance angle
* Lw = Length of the workpiece (not shown)

Broaching is a machining process that uses a toothed tool, called a broach, to remove material. Thereare two main types of broaching: linear and rotary. In linear broaching, which is the more common process, the broach is run linearly against a surface of the workpiece to effect the cut. Linear broaches are used in a broaching machine, which is also sometimes shortened to broach. In rotary broaching, the broach is rotated and pressed into the workpiece to cut an axis symmetric shape. A rotarybroach is used in a lathe or screw machine. In both processes the cut is performed in one pass of the broach, which makes it very efficient.
Broaching is used when precision machining is required, especially for odd shapes. Commonly machined surfaces include circular and non-circular holes, splines, keyways, and flat surfaces. Typical workpieces include small to medium sized castings, forgings,screw machine parts, and stampings. Even though broaches can be expensive, broaching is usually favored over other processes when used for high-quantity production runs.
Broaches are shaped similar to a saw, except the teeth height increases over the length of the tool. Moreover, the broach contains three distinct sections: one for roughing, another for semi-finishing, and the final one forfinishing. Broaching is an unusual machining process because it has the feed built into the tool. The profile of the machined surface is always the inverse of the profile of the broach.
The rise per tooth (RPT), also known as the step or feed per tooth, determines the amount of material removed and the size of the chip. The broach can be moved relative to the workpiece or vice-versa. Because all of thefeatures are built into the broach no complex motion or skilled labor is required to use it. A broach is effectively a collection of single-point cutting tools arrayed in sequence, cutting one after the other; its cut is analogous to multiple passes of a shaper.

Hobbing

Hobbing is a machining process for making gears, splines, and sprockets on a hobbing machine, which is a special typeof milling machine. The teeth or splines are progressively cut into the workpiece 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 byhobbing than any other process since it is relatively quick and inexpensive.
Machining

Spur may be cut or ground on a milling machine or 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. To...