Tooth Tips: Nick Sudzum

This month’s column is the fifth installment of six on bevel gearing, and the conclusion will cover new finished products.


My last column covered the basics of the reverse/inverse engineering process, which is only the tip of the iceberg. Let’s assume that everything went as planned, and the customer has given you the approval to proceed using the drawings that you have submitted. Now is when the planning phase of the actual manufacturing process begins, starting with routing the job properly so that the end result is correct, and the parts that you have produced are as close as possible to a clone of the original samples supplied for duplication.

Figure 1: Good contact patterns will lap in without any offsets.

If questions arise during the manufacturing process it is wise to refer back to the sample and compare it to your drawings, which will usually answer your question. When the situation arises that your answer is not definitive, consult the end user. After all of the general machine work is performed on the raw material, the next process would be generating the teeth, testing the gear with the pinion to develop the proper contact/bearing pattern and mounting distance. The next step in the process would usually be heat treating. In most cases the gear set is then set up in either internal or external grinding machines so that proof datums may be confirmed as concentric and perpendicular with the pitch diameter produced during the cutting process. These areas would be hard green ground to regain the original concentricity, usually still leaving stock for a finish grind operation after the finishing operation is performed on the gear teeth.

Figure 2: Improper contact pattern.

The common process used to finish out the gear tooth area on a commercial replacement set of gears is lapping. During the lapping process an abrasive compound is applied in the area of the contact/bearing pattern. If the contact/bearing pattern did not acquire any distortion during the heat treating process, the lapping compound utilized may be very fine grit. This process lightly wears in the contact/bearing area or seats the gear sat in to insure the gear set will run properly in its respective housing. In all types/styles of generated bevel gears engineers look for a central toe contact/bearing pattern, having what is called V&H (vertical and horizontal) offset criteria to qualify that the contact/bearing pattern is properly positioned. To sum up the lapping process, you are able to make small adjustments to the contact area if the initial manufacturing process was performed properly.

Figure 3: Requires horizontal offsets.

In some applications, especially those involving higher AGMA quality levels, a light lapping process may still apply after tooth grinding, hard cutting, or skiving. This type of lapping is generally performed using an ultra-fine grade liquid compound. The illustrations shown are some basic conditions that may exist in the development of contact patterns during the cutting process.

Figure 4: Improperly cut lame contact, must remanufacture pinion to correct the contact pattern.