Section 13. qxd

Table 13.4.11
No. of cutter
1
2
3
4
5
6
7
8
No. of teeth
135–
55–134
35–54
27–34
21–26
17–20
14–16
12 and 13
For more accurate gears, 15 cutters are available
No. of cutter
1
1
2
2
3
3
4
4
No. of teeth
135–
80–134
55–79
42–54
35–41
30–34
26–29
23–25
No. of cutter
5
5
6
6
7
7
8
No. of teeth
21 and 22
19 and 20
17 and 18
15 and 16
14
13
12
1
⁄
2
1
⁄
2
1
⁄
2
1
⁄
2
1
⁄
2
1
⁄
2
1
⁄
2
Fig. 13.4.17
A gear-cutting hob.
Hob
Hob
Gear 
blank
Gear blank
Side views
Top view
(a) 
(b) 
Section_13.qxd 10/05/06 10:32 Page 13-64

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cutter is different. The spiral cutter is basically a disk that has a number
of straight-sided cutting blades protruding from its periphery on one
side to form the rim of a cup. The machines have means for indexing,
retracting, and producing a generating roll; by disconnecting the roll
gears, spiral bevel gears can be form cut.
Gear Shaving
For improving the surface finish and profile accuracy
of cut spur and helical gears (internal and external), gear shaving, a
free-cutting gear finishing operation that removes small amounts of
metal from the working surfaces of the teeth, is employed. The teeth on
the shaving cutter, which may be in the form of a pinion (spur or heli-
cal) or a rack, have a series of sharp-edged rectangular grooves running
from tip to root. The intersection of the grooves with the tooth profiles
creates cutting edges; when the cutter and the workpiece, in tight mesh,
are caused to move relative to one another along the teeth, the cutting
edges remove metal from the teeth of the work gear. Usually the cutter
drives the workpiece, which is free to rotate and is traversed past the
cutter parallel to the workpiece axis. Shaving requires less time than
grinding, but ordinarily it cannot be used on gears harder than approxi-
mately 400 HB (42 HRC).