In the May Tooth Tips column, we covered design — the third major element that dictates the ultimate performance of the required gearing. In this column, we conclude the series by taking a look at the final component: the manufacturing process.
Defining the Manufacturing Process
When you reach the manufacturing stage in the gear design process, the majority of the related variables such as gear type, number, size, and material have been finalized. The manufacturing process, much like the design process, commands a thorough understanding of the many methodologies available to the industry as well as what is available to you. An understanding of the machines that will be used and their capabilities, the effect of any post-cutting operations, and the fixturing and tooling of each will be critical. Every operation has an effect on the gear’s ultimate performance and will directly impact cost, longevity, and quality. Let’s take a look at the stages of a typical routing sequence that’s common to steel gearing.
Blanks: Blanks can start as castings, forgings, or cutoffs from bar stock. In lower-stressed designs, powdered metal, stampings, or molded blanks may be an option depending upon the application. If the blank has a simple cross-section, turning it down on a lathe is most likely the better option. If the gear has a number of features or size changes such as hubs or a web cross-section, then a casting or forging (near or net) may be less costly. Tooling costs and annual volumes need to be considered and weighed against machining cost offsets.
In general, whenever you remove material, you are likely inducing or relieving stresses. This can result in unexpected or excessive distortion, especially during heat treatment. If distortion is a concern and subsequent processes cannot accommodate it, stress relieving (annealing and/or normalizing) the blanks should be considered following any operation that has removed significant material.
Post-Blank Operations: After blanking, gearing frequently will require a number of operations prior to a gear tooth generating operation. Features such as internal splines, keyways, lift holes, weight reduction, or datum surfaces may be needed and can be accomplished in any number of ways. Broaching offers a cost-effective and quick way to generate internal features. Shaping equipment may also be used, but this may increase cycle time. Milling, whether horizontal or vertical, is typically driven by part size. In many cases, the number of operations, machines used, and setups can be greatly reduced with the more capable multi-axis CNC mills and multi-turret live tooled lathes.
Cutting the Gear Teeth: In producing gear teeth, the most common form is the involute — from which numerous variations have been developed. When creating the tooth form, there are many ways to cut the part. Milling, broaching, shaping, hobbing, and bevel generation are most common. The appropriate method is often dictated by whether the tooth is external or internal, helical or bevel.
After tooth roughing, gears may require finishing to meet specified quality or life requirements. Finishing operations include shaving, honing, lapping, or grinding. These operations can sometimes “persuade” a part slightly out of tolerance back into acceptable ranges.
Proper fixturing, cutting fluid, and cutter type are influential factors that should be considered when defining the cutting process.
Heat Treating: When heat treating is required to produce the desired mechanical properties, the appropriate method must be determined. Frequently used applications of heat treating include flame hardening, induction hardening, carburizing, and nitriding. Heat treating can be the most critical and intricate operation. In some cases, a special recipe may need to be designed to minimize distortion without compromising specified tolerances. It is common for different parts with similar heat treating procedures and material to be treated in batches. How the parts are stacked, handled, or hung and where they sit in the basket can influence their outcome. The temperatures and time at which they are held and quenched are critical. Special racking, loading, fixturing, and process controls may need to be implemented to ensure consistency of expected results and to minimize distortion. Stress relief operations incorporated earlier in the process can dramatically reduce distortion encountered at this stage.
Finishing: After heat treating, a finishing grind on bores, bearing surfaces, and tooth form may be necessary to meet final size. During this process, care is required to prevent burn, which is generally induced by localized overheating of the surface due to an aggressive removal rate or inadequate cooling. The side benefit of grinding at this stage is the ability to remove any remaining distortion if there is enough remaining material. Other finishing operations such as shot peening, isotropic superfinishing, or coatings can enhance ultimate performance.
Packing: Handling and packing — an often-overlooked cause of many rejections — should be carefully considered throughout the process. Damage from handling, movement in shipment, or exposure to moisture and chemicals are the root of endless warranty claims.
Final Inspection: A solid quality plan throughout the operation is critical for any efficient manufacturing process. Process controls and metrics need to be in place, communicated, and constantly audited at every operation and at final inspection.