Gear Finishing & Testing

Gear Finishing

For an effective and noiseless operation at high speed, teeth profile is accurate, smooth and without irregularities. Finishing operation is done to:

1. Eliminate after effect of heat treatment. 
2. Correct error of profile and pitch.
3. Ensure proper concentricity of pitch circle and center hole.

Types of Gear Finishing Process

Following are the types of gear finishing process:

1. Gear Shaving

It is defined as, "a finishing process in which gear tooth is running at high rpm in mesh with a gear shaving tool".

• Serrations  ↠  serves as cutting edges (scrapping operation).
• It involves simultaneous rotation of workpiece and cutter as a pair of gears with crossed axes.
• Soft materials  ↠  Aluminum alloy, brass, bronze, cast iron and unhardened steels are finished by shaving process.

Limitations

• The lack of chance to remove the distortion caused by heat treatment.

2. Gear Rolling / Roll Finishing / Gear Burnishing

It involves revolving of gear between two hardened rolling dies containing very accurate tooth profile of gear to be finished. 

• Surface irregularity of gear teeth is squeezed by hard die through plastic deformation.
• Cold working  ↠  improves surface finish, induce compressive residual stresses.

3. Gear Grinding

It is defined as, "abrasive grinding wheel of a particular shape and geometry are used for finishing of gear teeth". There are two methods used for gear grinding:

1. Form Grinding (Non-Generating)
2. Generation Grinding

Form Grinding  ↠  It is similar to machining gear teeth by a single disc type form milling cutter. Gear to be finished is mounted and reciprocated under the grinding wheel (which is rotating about its axis).

• Need of indexing (teeth are finished one by one)  ↠  makes the process slow and less accurate.
• Grinding Wheel  ↠  changes with module, pressure angle, number of teeth.

4. Gear Honing 

It is carried out with steel tools having abrasive or cemented carbide particles embedded in their surface. 

• For super finishing of heat treated gears.
• Used in automotive, aerospace, truck industry.

Benefits of Gear Honing

1. Corrects dimensional errors.
2. Corrects distortions caused by heat treatment.
3. Improves surface finishing.

5. Gear Lapping

It is done generally on gears having hardness more than 45 RC to remove burns, abrasions from the surface and to remove small errors caused by heat treatment.

• Gear to be lapped is run under load in mesh with a gear shaped lapping tool.
• Abrasive paste is introduced between the teeth under pressure.

Benefits of Gear Lapping

1. Improves wear properties.
2. Corrects minute errors in involute profile, helix angle, tooth spacing and concentricity.

6. Blast Finishing 

It uses the high-velocity impact of particulate media to clean and finishing a surface. It uses blasting media as Al2O3, SiC, Nylon beads and crushed nut shells.

• The media is propelled at the target surface y pressurized air or centrifugal force.

Applications of Blast Finishing

1. For cleaning, smoothening, roughing of metal casts and forged parts.
2. Paint and rust removal.
3. Etching of shiny surface.

7. Shot Peening 

It is defined as, "a high-velocity stream of small steel pellets is directed at a metallic surface with the effect of cold working and induce compressive stresses into the surface layers (to improve fatigue strength)".

• Increase fatigue life up to 100%.
• Cleaning is accomplished as by-product.
• Use to avoid further propagation of microcracks in surface.

8. Phosphate Coating

It is used on steel parts for corrosion resistance, better adherence of lubrication, lubricity as a foundation for subsequent coatings or painting.

Gear Testing

Gear testing method is used to compare the performance or characteristics introduced in the gear to a baseline. Following are the test method for gears.

1. Gear Testing-Parkinson Testing

Principle

A master gear is mounted on a fixed vertical spindle and the gear to be tested on another similar spindle.

• These gears are maintained in mesh by spring pressure.

Working

• At first dial gauge is set to zero and both gears are mounted on spindles.
• Variation in dial gauge reading are any irregularities in gear under test.
• A recorder is fitted in the form of waved circular chart.

Limitation

• Friction in the floating carriage reduces sensitivity.
• Not suitable for < 300 mm gear diameter.
• Measurements are directly depend upon master gear.
• Error in pitch, helix and tooth thickness are not clearly identified.

References:

• Material from Class Lectures + Book named Fundamentals of Modern Manufacturing Materials, Processes and Systems by Groover + My knowledge. 
• Photoshoped pics are developed. 
• Some pics and GIF from Google.  
• Videos from YouTube (Engineering Sights).