Gear Cutting Unit 70

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

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

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

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

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)

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)

Involute Gear Cutters Cutter Number Range 1 135 teeth to a rack 11⁄2 80 to 134 teeth 2 55 to 134 teeth 21⁄2 42 to 54 teeth 3 35 to 54 teeth 31⁄2 30 to 34 teeth 4 26 to 34 teeth 41⁄2 23 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

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.

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 …

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

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

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

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

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

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

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

Metric Module Gear Cutters Milling Cutter Numbers Module Size (mm) Cutter No. For Cutting 0.5 3.5 0.75 3.75 1 12 to 13 teeth 1 4 2 14 to 16 teeth 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

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

Solution:

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

Solution:

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

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

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.