A ‘generated’ surface, whether it is curved or straight, is produced by continuous motion of a point, a line or a surface. The direction in which the generating point travels determines the shape produced. The simplest example is the generation of a cylinder on a lathe. The cutting point of the tool generates a circle when the workpiece is rotated. As the tool is led axially, the cylindrical component is produced.
Types Of Gear Shaping :
Gear shaping is one of the gear generating methods. In this process gear tooth are accurately sized and shaped by cutting them by a multipoint cutting tool.
Various gear shaping processes are listed and then described below :
(a) Gear cutting by gear shaper.
(b) Rack planning process.
(c) Hobbing process.
The geometrical accuracy of the circular component depends on:
(a) The trueness of the workpiece rotation.
(b) Parallelism of the tool movement with the work- spindle axis.
(c) The effect of tool wear.
It does not depend on the form of a tool profile. Besides, the process is continuous.
Gear Shaper machine
In gear shapers, the above generating principle is applied in the following way
The cutter is hardened disc-shaped and slightly dished at the bottom (ground with top rake and clearance) to facilitate cutting. The teeth have the desired tooth profile and pressure angle.
The cutter is reciprocated with the required cutting speed for stock removal along the face of the workpiece and is gradually fed radially to plunge if for correct teeth depth utilizing a cam. A relieving mechanism enables the cutter to clear the work on non-cutting or return stroke.
The continuous generation motion is obtained by feeding cutter to full depth and rotating the cutter and the workpiece slowly in the exact ratio of their respective number of teeth by means of a chain of gears including change gears. The cutter is considered as the driving gear and the workpiece as the driven member, exact transmission of motion being affected by an external link of the gear train.
The workpiece gear is fully generated when the cutter has plunged to correct tooth depth (set previously) into the workpiece and the teeth are cut fully and uniformly over the entire periphery of the workpiece gear. The machine comes to rest automatically after the workpiece gear is fully cut.
The cutting action of a pinion type of cutter to generate a gear is shown clearly in Fig. The principle of cutting gear by the generating method has the advantage that with a particular module of DP cutter it is possible to cut accurately gears having identical modules or DP, but different numbers of teeth. This makes the generating process functionally superior and economical for the manufacture of gears.
Thus in brief, it could be seen that since an involute gear will roll with any other involute gear of the same normal base pitch and hence this principle has been utilized in the gear shaper (a method of gear manufacture). In this cutter is made in the form of pinion provided with relieved cutting edges, both cutter and work are given rotary motions about their respective axes to simulate the conditions obtaining, had both members been complete gears rolling together at the correct center distance.
This is achieved by gearing the cutter to the work in the same ratio as the numbers of teeth in the cutter and the finished work. Cutting is effected by imparting axial reciprocating motion to the cutter coincident with its rolling action, the length of stroke of the ram being slightly more than the width of the blank.
Pinion cutter gear shaping process.
In this method instead of rack cutter, a pinion cutter having formed similar to gear to be produced is used
Gear shaping is used for cutting spur and herringbone gears etc.
Principle:- Teeth are produced with the help of reciprocating and rotating cutter with the rotating motion of the workpiece
Gear shaping cutter receives the reciprocating motion which is the principal movement. It reciprocates at a rate of 50 to 450 strokes per minute. Both cutter and workpiece rotate with the same speed. The radial movement is given to cutter when it is to be fed into the depth of cut.
1) Cutter is fed into full depth with cutter reciprocating and blank stationery
2) Both cutter and blank slowly rotate about their axis at high speed
3) At same time cutter is feed to the workpiece
Gear Shaping by Rack Shaped Cutter
In this method, gear cutting is done by a rack shaped cutter called rack type cutter. The principle is illustrated in Figure. The working is similar to the shaping process done by gear type cutter.
The process involves rotation (low rpm) of the gear blank as the rack type cutter reciprocates along a vertical line. Cutting is done only in the downward stroke, the upward stroke is only a return movement. The main difference of this method with the previous one is that once the full length of the rack is utilized the gear cutting of operation is stopped to bring the gear blank to its starting position so that another pass of gear cutting can be started. So this operation is intermittent for cutting larger gears having a large number of teeth over their periphery.
Advantages Of Gear Shaping :
Main advantages of the gear shaping process are described below :
(a) Shorter product cycle time and suitable for making medium and large-sized gears in mass production.
(b) Different types of gears can be made except worm and worm wheels.
(c) Close tolerance in gear cutting can be maintained.
(d) Accuracy and repeatability of the gear tooth profile can be maintained comfortably.
(e) For the same value of gear tooth module a single type of cutter can be used irrespective of the number of teeth in the gear.
Disadvantages of gear shaping:-
- Not Suitable for internal gears
- Owing to the reciprocating action of the cutter, there is no cutting on the return stroke in the gear shaper.
- Worms & Worm wheels cannot be generated on gear shapers.
- The rate of production is lower than the gear hobbing process due to periodical indexing hence
- More machining time.
Applications of gear shaping:-
1) Cutting spur gears
2) Cutting herringbone gears
3) Rachets gears
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