Making a Gear Block

What should be in it? and Why?

Gear specification is generally not in the curriculum of engineering classes, so here is a breakdown of how to specify gears, what the terms mean, and why the information is necessary for your next gear project. When gears are correctly specified on drawings it makes the quoting and manufacturing process much more efficient and accurate. Some of the gear features below can be interchanged with other gear features, but one must be specified. Some of the features below could also be omitted if the gear is standard form.

*Measurement Over Wires when become Measurement Between Wires if the gear is an internal instead of an external gear.

Terms Defined as defined by the AGMA and Machinist Handbook:

• Number of Teeth: the number of teeth in 360 degrees of the gear circumference. In the case of a sector gear the theoretical number of teeth in 360 degrees should also be shown in addition to the actual number of teeth. 
• Diametral Pitch: Diametral Pitch is the ratio of the number of teeth to the number of inches in the pitch diameter in the plane of rotation, or number of gear teeth to each inch of pitch diameter.
• Pressure Angle: The pressure angle is the angle between a tooth profile and a radial line at its pitch point. In involute teeth specifically it is defined as the angle between the line of action and the tangent line of the pitch circle. The pressure angle gives the direction of the normal to the tooth profile.
• Helix Angle: the angle between any helic and an element of its cylinder. This element would be the standard pitch circle unless otherwise specified.
• Hand: The direction in which the teeth twist as they recede from an observer looking along the axis. Right Hand would twist in a clockwise direction, while the Left Hand would twist in a counterclockwise direction.
• Tooth Form: Tooth form defines the gear tooth profiles, and helps to indicate whether it is standard or modified gear tooth profile.
• Outside Diameter: the diameter of the addendum circle. This dimension not only helps with the manufacture of the gear blank, but also with the gear inspection to insure that the profile of the gear is correctly generated.
• Maximum Root Diameter: The maximum diameter of the circle that contains the roots or bottoms of the tooth spaces.
• Pin Diameter: The pin diameter is the diameter of the pin that is meant to be used when workpiece gears are inspected for Measurement Over Wires, or MOW. Generally pin sizes can be modified if the size specified is not available.
• Measurement Over Wires: Measurement Over Wires is the measurement of the distance over a pin position in a tooth space and a reference surface generally a tooth space directly across the gear. This measurement is used to determine tooth thickness.
• Total Composite Tolerance: The tolerance for Total Composite Variation which is defined as the total change in center distance while the gear is being double flank roll checked.
• Tooth-to-Tooth Composite Tolerance: The tolerance for Tooth-to-Tooth Composite Variation which is defined as the greatest change in center distance while the gear is being double flank roll checked.
• Test Radius: The Test Radius is defined as the arithmetic convention established to simplify the determination of the proper test distance between a master and a workpiece gear for a double flank roll checking (an inspection method). It is used as a measure of the effective size of the gear, which is why when paired with the Measurement Over Wires it is considered a redundancy.
• AGMA Quality:  The AGMA gear quality In most cases, AGMA quality is defined by the Total Composite Tolerance and Tooth-to-Tooth Composite Tolerance which is why including both on a component drawing would be considered a redundancy. The quality number would be defined as the lowest quality number obtained between the two measurements.