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          Technology of Machine Tools
                                                   6th Edition

                                         Krar • Gill • Smid


                            Gear Cutting
                                       Unit 70


Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
                                             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
                                                 70-3



            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
                                                   70-4



      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
                                                   70-5


                     Comparative Gear-Tooth
                        Sizes: 4 to 16 DP




Copyright © The McGraw-Hill Companies, Inc.
Permission required for reproduction or display.
                                                             70-6



               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)
                                                       70-7



                       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)
                                                        70-8



         Involute Gear Cutters
Cutter                         Cutter
Number       Range             Number       Range
 1       135 teeth to a rack   5      21 to 25 teeth
 11⁄2     80 to 134 teeth      51⁄2   19 and 20 teeth
 2       55 to 134 teeth       6      17 to 20 teeth
 21⁄2    42 to 54 teeth        61⁄2   15 and 16 teeth
 3       35 to 54 teeth        7      14 to 16 teeth
 31⁄2    30 to 34 teeth        71⁄2   13 teeth
 4       26 to 34 teeth        8      12 and 13 teeth
 41⁄2    23 to 25 teeth
                                                       70-9


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.
                                                     70-10


   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 …
                                                      70-11



d. Indexing (using Cincinnati standard plate)




2. Turn gear blank to proper outside diameter
   (6.750 in.)
3. Press gear blank firmly onto mandrel
4. Mount index head and footstock, and check
   the
   alignment of the index centers
5. Set dividing head so index pin fits into hole on
   39-hole circle and sector arms set for 30 holes
                                                        70-12



6. Mount mandrel (and workpiece), with
   large end toward indexing head, between
   index centers
7. Move table close to column to keep setup
   as rigid as possible
8. 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
                                                             70-13


9. 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
10. LOCK THE CROSS-SLIDE
11. Start milling cutter and run work under
    cutter
                                                     70-14



12. Raise table until cutter just touches work
13. Set graduated feed collar on the vertical
    feed to zero
14. 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
15. Slightly notch all gear teeth on end of
    work to check for correct indexing
                                                  70-15



16. Rough-out first tooth and set automatic
    feed trip dog after cutter is clear of work
17. Return table to starting position
   •   Clear end of work with cutter
18. Cut remaining teeth and return table to
    starting position
19. Loosen knee clamp, raise table to full
    depth of .270 in., and lock knee clamp
20. Finish-cut all teeth
                                                   70-16



              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
                                                70-17



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
                                                   70-18



Module
            Milling Cutter Numbers
                                             Metric
Size (mm)   Cutter No.   For Cutting         Module
0.5   3.5
0.75 3.75     1          12 to 13 teeth      Gear
1     4       2          14 to 16 teeth      Cutters
1.25 4.5      3          17 to 20 teeth
1.5   5       4          21 to 25 teeth
1.75 5.5      5          26 to 34 teeth
2     6       6          35 to 54 teeth
2.25 6.5      7          55 to 134 teeth
2.5   7       8          135 teeth to rack
2.75 8
3     9
3.25 10
                                              70-19



        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
            70-20


Solution:
                                            70-21



 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
            70-22


Solution:
                                                  70-23



   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
                                                       70-24


    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 1.728 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 1.728 multiplied by module of gear
                                                      70-25


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
30.4374 in. divided by DP.

				
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