Gears and screw thread , Machine tools , Production , GATE & ESE

 Gears can be manufactured by following methods:

1.Sand casting: 

• Large size gears 
• Applications : lifts , cement mixers , hoist gear box , farm machines

2.Die casting:

• Small size gears
• Applications : cameras , toys , lawn movers

3.Injection moulding :

• Light gears
• Application: speedometer , washing machine , zerox machine , wind shield wipers

Note: circular blanks use for gear manufacturing is produced by semi centrifugal casting.

4.Extrusion:

• Small and light gears
• Application: wall clocks , type writers

5.Sheet metal operations:

• Thin metallic gears
• Application: wrist watches , electric meters

Machining:

• Gear shaping : gear generating process

1. Reciprocating cutter > rack type cutter > external gears
2. Rotating cutter > pinion type cutter > accurate internal gears

• Gear hobbing: hob - circular rotating cutter

1. Accurate external gears can be produced because of continuous indexing mechanism.
2. Number of teeth cut on the circular blank corresponds to number of starts the hob cutter carries.

Question: number of teeth is 60 , double start hobbing , find the ratio of revolutions of hob to job?

Answer : 1 hob rotation = 2 teeth cut

              Nhob/Njob = 30/1

• Angle of tilt of the hob cutter is equal to the sum of hob helix and job helix. eg for spur gear to be cut angle of tilt is hob helix + zero

Question: which of the following is a gear cutting operation?

Answer : milling

• Gear cutting : form cutting application eg milling , broaching
• Gear generating : cutter looks like a gear eg shaping and hobbing
• Gear forming : no material removal

Screw thread manufacturing:

• External threads by grinding 
• Internal threads by tapping (tap drill)

Open die threading:

• Only external threads

Thread rolling:

• Thread formation by deforming material
• Cold rolling method strongest threads are produced
• forming process
• Strongest external threads
• Strain hardening occurs
• Ductile material is used for thread rolling process
• Cold rolling is restricted to ductile materials only.
• After thread rolling -- ductility of material decrease and strength & hardness increases

Thread chasing:

• High quality thread
• Arrangement is very similar to turning process.
• High quality external thread produced

Helical milling :

• Drill flutes can be produced

Thread milling:

• Form mill cutter
• External and internal threads can be produced