1. Gear Cutting
Unit 70

2. Objectives Select the proper cutter for any gear to be cut
Calculate gear-tooth dimensions for inch gears
Calculate gear-tooth dimensions for metric gears
Set up and cut a spur gear

3. Gear Cutting
Usually cut to repair or replace gear broken or lost or no longer carried in inventory
Industry mass-produces gears on special machines designed for this purpose
Gear-shaping machines
Gear-hobbing machines

4. Involute Gear Cutters Sharpened on face
Ensures exact duplication of shape of teeth
Regardless of how far back face of tooth has been ground
Available in many sizes
Range from 1 to 48 diametral pitch (DP)
Special cutters for teeth smaller than 48 DP

5. Comparative Gear-Tooth Sizes: 4 to 16 DP
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

6. Gear Cutting
Cutter must be chosen to suit both DP and number of teeth (N)
Tooth space for small pinion cannot be same shape as tooth space for large mating gear
Teeth on smaller gears must be more curved to prevent binding of meshing gear teeth
Sets of gear cutters
Made in series of slightly different shapes to permit cutting of any desired number of teeth with assurance that teeth will mesh with another gear(same DP)

7. Cutters Generally made in sets of eight
Numbered from 1 to 8 (also in half sizes)
Gradual change in shape (sides)
#1 cutter with straight to #8 with curved sides
Number of teeth
#1 will cut teeth in gear 135 teeth to a rack
#8 will cut only 12 and 13 teeth
Cutter number permits a more accurate meshing of teeth (for gears to mesh must have same DP)

8. Involute Gear Cutters Cutter Number Range 1 135 teeth to a rack
11⁄ to 134 teeth
to 134 teeth
21⁄ to 54 teeth
to 54 teeth
31⁄ to 34 teeth
to 34 teeth
41⁄ to 25 teeth
Cutter Number Range
5 21 to 25 teeth
51⁄2 19 and 20 teeth
6 17 to 20 teeth
61⁄2 15 and 16 teeth
7 14 to 16 teeth
71⁄2 13 teeth
8 12 and 13 teeth

9. Example: Selecting a Cutter
A 10-DP gear and a pinion in mesh have 100 teeth and 24 teeth, respectively. What cutters should be used to cut these gears?
Cutter Selection
Since the gears are in mesh, both must be cut with a 10-DP cutter.
A #2 cutter should be used to cut the teeth on the gear, since it will cut from 55 to 134 teeth.
A #5 cutter should be used to cut the pinion, since it will cut from 21 to 25 teeth.

10. Procedure For Machining a Spur Gear
Example:
A 52-tooth gear with an 8 DP is required
1. Calculate all necessary gear data (See previous table)
Cutter number = 3 (35 to 54 teeth)
Continue …

11. d. Indexing (using Cincinnati standard plate)
Turn gear blank to proper outside diameter (6.750 in.)
Press gear blank firmly onto mandrel
Mount index head and footstock, and check the alignment of the index centers
Set dividing head so index pin fits into hole on 39-hole circle and sector arms set for 30 holes

12. Move table close to column to keep setup as rigid as possible
Mount mandrel (and workpiece), with large end toward indexing head, between index centers
Move table close to column to keep setup as rigid as possible
Mount an 8 DP#3 cutter on milling machine arbor over approximate center of the gear
Be sure to have cutter rotating in direction of indexing head

13. Center gear blank with cutter by either of the following methods:
Place square against outside diameter of gear
With pair of inside calipers or rule, check distance between square and side of cutter
Adjust table until distances from both sides of gear blank to sides of cutter are the same
More accurate method of centralizing cutter is to use gage blocks instead of inside calipers
LOCK THE CROSS-SLIDE
Start milling cutter and run work under cutter

14. Raise table until cutter just touches work
Set graduated feed collar on the vertical feed to zero
Move work clear of cutter by means of the longitudinal feed handle
Raise table to 2/3 depth of tooth (.180 in.)
Lock knee clamp
Slightly notch all gear teeth on end of work to check for correct indexing

15. Return table to starting position
Rough-out first tooth and set automatic feed trip dog after cutter is clear of work
Return table to starting position
Clear end of work with cutter
Cut remaining teeth and return table to starting position
Loosen knee clamp, raise table to full depth of .270 in., and lock knee clamp
Finish-cut all teeth

16. Table 70.2 in text gives necessary rules and formulas for metric
Metric Gears
Countries using metric system usually use module system of gearing
Module (M) of a gear equal pitch diameter (PD) divided by number of teeth (N)
M is actual dimension
Table 70.2 in text gives necessary rules and formulas for metric
spur gears

17. Metric Module Gear Cutters
Most common available in modules ranging from 0.5 to 10 mm
Available in set of eight cutters numbered from #1 to #8
Range of each cutter reverse of DP cutter
Table on next slide shows cutters available and rang of each cutter in set

18. Metric Module Gear Cutters
Milling Cutter Numbers
Module Size (mm) Cutter No. For Cutting
to 13 teeth
to 16 teeth
to 20 teeth
to 25 teeth
to 34 teeth
to 54 teeth
to 134 teeth
teeth to rack
2.75 8
3 9

19. Metric Example: A spur gear has a PD of 60 mm and 20 teeth. Calculate:
Module
Circular pitch
Addendum
Outside diameter
Dedendum
Whole depth
Cutter number

20. Solution:

21. Another Metric Example:
Two identical gears in mesh have a CCD of 120 mm. Each gear has 24 teeth. Calculate:
Pitch diameter
Module
Outside diameter
Whole depth
Circular pitch
Chordal thickness

22. Solution:

23. Gear-Tooth Measurement
Measure with gear tooth vernier caliper
Set to corrected addendum
May also be checked by measuring over wires or pins of a specific diameter placed in two diametrically opposite tooth spaces of the gear
Tables in most handbooks give measurement over wires having given number of teeth and specific pressure angle

24. Gear-Tooth Measurement
Accurate measurement of inch gears need diametral pitch and number of teeth of gear
For metric gears, the module must be known
Wire or pin size to use determined by
For external inch spur gears, wire or pin size equal to divided by DP of gear
For internal inch spur gears, wire size equal to 1.44 divided by DP of gear
Metric module gears measured using wire size equal to multiplied by module of gear

25. Example (Inch):
Determine wire size and measurement over the wires for a 10-DP external gear having 28 teeth and a 14.50º pressure angle.
In handbook tables, size over wires for gear having 28 teeth and 14.50º pressure angle should be in. divided by DP.