Cylindrical gear chamfering and deburring is a rather “unloved” process that adds cost but without delivering readily apparent improvements in gear quality. However, the chamfer process, when performed correctly, provides significant advantages for downstream handling and processing. This is why manufacturers of automotive — and truck-sized — gears are increasingly exploring new technologies to chamfer their gears.
Two major chamfer technologies are used: forming and cutting. While chamfer rolling is a highly proven forming process that has been used for decades mainly in mass production, cutting chamfer technologies are of increasing market interest due to cost reduction and increased quality requirements — especially in dry cutting conditions.
This paper will cover new chamfer cutting processes: fly cutter chamfering (chamfer-contour milling) and Chamfer Hobbing and compare them with the existing chamfer-roll technology.
Chamfer-contour milling uses a universal fly cutter tool with indexable carbide inserts. Chamfer angle and chamfer size depend on programmable machine movements. Therefore, this process provides the highest flexibility for coarse pitch gears — even with different modules, pressure angles, or number of teeth.
Chamfer Hobbing has been developed for modern gear production focusing on low tool cost per part with dry cutting and short cycle times in mass production. As for the left and right gear flank, separate and dedicated chamfer hobs are used to meet most customer specifications in the market.
By comparing the advantages and limits of the aforementioned chamfer processes in gear production for workpieces up to 400 mm diameter and module 8 mm, it is possible to select the right process depending on the specific requirements.
1 Introduction
1.1 Reasons for Chamfering
Chamfering and deburring add cost and effort to the production process, but add no readily apparent value. Therefore, these processes are often under-appreciated and sometimes even ignored. However, there are good reasons for taking this important auxiliary process into consideration. Innovations such as chamfer-contour milling and Chamfer Hobbing or developments in chamfer rolling are now available to suit the needs of the user more precisely. Chamfering and deburring addresses several issues that can occur during gear manufacturing. Without it, opportunities to streamline subsequent gear processes and improve production effectiveness may be missed.
Following common gear generating processes such as hobbing, shaping, or power skiving, a burr at the tool exit position remains (Figure 1). This primary burr is typically removed right after the cutting process using deburring discs, deburring blades, or file discs. With a high percentage of gears, it would be desirable to perform an additional chamfer on the edge between the flank and the gear face by chamfer forming like chamfer rolling or chamfer cutting like Chamfer Hobbing or chamfer-contour milling (Figure 2). Chamfers prevent damages in gearboxes if acute edges or remaining burrs become brittle during super-carburization in the heat-treatment process. Under load, especially with edge load situations, material could break off, and these hardened particles can reduce transmission life and increase the potential for major damage causing premature transmission failure. Improperly chamfered and deburred gears can lead to overloaded edges and unexpected — and undesirable — noise behavior. This is of special interest with gears for electrical vehicle (EV) drive systems where significantly higher RPMs are applied compared to combustion engine transmissions.
When gear faces are used for clamping or locating purposes, burrs must be eliminated. In part handling, chamfers prevent damages such as nicks. And finally, when handling workpieces manually, sharp burrs can injure operators. Therefore, chamfering and deburring is a widespread requirement.
From a gear manufacturer’s perspective, depending on the subsequent finishing process, burrs on the edge between the flank and the chamfer could have a negative impact on cost-per-piece produced. While a burr in the flank is not an issue with, for example, shaving, a hardened burr will reduce the tool life significantly during power honing. Figure 3 shows a flank measurement of an automotive-type gear with a remaining burr of approximately 60 µm on the right flank after hobbing and heat treatment.
After 40 power-honed parts, the right flank shows a plus material of 3 µm on the fine finished flank (Figure 4). This results from an excessive wear of the abrasive honing ring caused by the remaining burr shown in Figure 3. As this plus material would lead to an edge-load situation, additional dressing of the honing ring is necessary, which increases the tool cost per part and decreases the productivity.
Over the years, several processes have been developed to remove burrs and to chamfer gears with an undefined or defined chamfer (see Figure 5). Chamfer rolling (sometimes referred to as press chamfering or rotary chamfering) is a well-known and fast process that is most often applied to smaller cylindrical gears. With increased tendency toward dry gear cutting processes and more hard finishing, the request for efficient and precise chamfer processes grows. New cutting chamfer technologies such as Chamfer Hobbing and chamfer-contour milling have been introduced to the market to respond to these requirements for automotive and truck-sized gears.
1.2 Chamfer Definition
While gear tolerances are defined in common standards such as ANSI/AGMA 2015-2 or ISO 1328, there are no standard definitions or tolerances for a gear chamfer. The ISO 13715 describes edges of undefined shape and is used by some gear manufacturers as a basis for describing chamfers. But it misses important information to complete the chamfer description. This is why gear drawings show a high variation of chamfer descriptions and tolerances. With the introduction of chamfer rolling in the 1970s, chamfer tool manufacturers regularly use the chamfer angle E and the chamfer width A (see Figure 6) with automotive gear definitions.
With comma-type chamfer forms, measuring is done below the tip of the gear. With parallel chamfer forms, measuring of the chamfer angle and chamfer width could be performed in the middle of the profile height. Measuring of the chamfer takes place in most cases at a line tangentially to the base circle of the gear to be measured.
In any case, new chamfer technologies need to comply with different chamfer definitions and tolerances requested by gear manufacturers.
2 Chamfer Hobbing
Chamfering with hobs has been known for decades [1] but is limited to spur gears only if a defined chamfer on both gear flanks is required. With the development of the “Chamfer Hobbing” process (CHH), chamfer cutting technology has made an important leap for use on modern machine systems. CHH is a cutting chamfer process for medium- and high-volume production of “automotive-sized” gears that fulfills most of the known chamfer tolerances and those expected in the future. Chamfer Hobbing employs one dedicated chamfer hob for each tooth flank (Figure 7). The chamfer on the left and right flank is cut in a generative mode with separate, dedicated chamfer hobs with diameters similar to a gear hob made of high-speed-steel (HSS) materials featuring AlCroNite Pro Coating. The hob profile is specifically designed for the particular chamfer form to be realized.
This approach allows chamfer hob designs for comma or parallel-chamfer forms, with or without root chamfering. Similar chamfer angles such as those commonly produced in the chamfer rolling process are achieved (e.g. 15-30 degree on obtuse edge, 25-45 degree on acute edge).
In the chamfer hob design process, technology software is used to simulate the required chamfer specification and avoid potential collision of the tool with the counter flank and interfering contours above and below the actual gearing. As the chamfer is cut with separate hobs for the left and right edge, it is important to avoid any material accumulation or steps in the root when chamfering the root. The simulation program (Figure 8) and the ability to both alter the chamfer hob macro-geometry and simulate the outcome immediately help to optimize the root area.
By cutting into the gap, burrs are avoided on the face side of the gears, and, therefore, no secondary deburring is necessary. With common chamfer angles up to approximately 40-45°, there are no measurable burrs on the flank that require removal downstream in case of a subsequent gear-honing process (Figure 9b). With unusually large chamfer angles, small burrs could be recognized (Figure 9a).
While two chamfer hobs are sufficient for workpieces with parallel gear faces, up to four chamfer hobs on a spindle are necessary for parts with non-symmetric gear faces such as inclined gear faces or special gears such as beveloid, sprockets, or asymmetric profiles.
When developing a new chamfer process, it is essential to fulfill the gear manufacturer’s tolerance expectations. Typical chamfer width tolerances are between 0.3 to 0.6 mm. Measuring a series of 50 chamfer-hobbed parts, the chamfer width on the acute edge was within a tolerance band of 0.14 mm and on the obtuse edge within 0.18 mm (Figure 10). These tolerance bands include blank face tolerances that were not compensated with this cutting trial. The chamfer angle tolerance band is within 1.5°.
These results prove Chamfer Hobbing’s ability to conform with current and future chamfer quality expectations.
As chamfer hobs use materials and coatings similar to gear-generating hobs, low tool-cost-per-part is expected, especially since tool shifting is possible. Existing sharpening capabilities for gear hobs can be used as well for chamfer hobs. Field experience shows tool-cost-per-workpiece of less than 0.02 euros for standard automotive shift or e-drive gears.
3 Chamfer Contour Milling (fly cutter chamfering)
With larger “truck-size” gears, requirements are changing. Smaller batches, larger variety of parts, and longer cycle times are typical. Therefore, chamfer-rolling and Chamfer Hobbing with dedicated tools may not be the best fit, since tool investment could be significant and flexibility is limited.
The continuous “fly-cutting” process, chamfer contour milling — more commonly known as auto path chamfering (APC) in the bevel world or fly-cutter chamfering — offers this flexibility. While fly cutter chamfering has been long employed on bevel gear cutting machines [2], it has just recently been adapted as a viable chamfering process for cylindrical gears.
This process generates a chamfer along the gear-edge contour by synchronizing fly cutter and workpiece rotation such that the fly cutter contour mills the chamfer with the desired characteristics by a generation motion. The tool is generally a star-shaped body with four standard, replaceable indexable carbide inserts configured so the straight cutting edge is used as much as possible for tool-life reasons (Figure 11).
The chamfer-contour milling process is continuously indexing and creates a cutter path that envelopes the tooth geometry from the outside of the part in or vice versa. The result is a faceted chamfer finish (Figure 12) similar to the finish after a hobbing process. Since each edge of the tooth is done separately and the chamfer size and angle depend on machine movements and not on the tool design, the process is universal, and the tools used are flexible. During the data input of workpiece and tools into the machine operating software, chamfer size and chamfer angle could be defined and do not depend on the cutting tool.
The standard indexable carbide inserts are mounted on a four-star body (Figure 13). One set of tools could be used to cut the chamfer of a large variety of workpieces with different modules, pressure angles, and number of teeth. The cutter geometry defines the minimum size of the gap of the gear to be chamfered. Cutting insert wedge angle and the blade tip radius are the limiting factors. With standard commercial inserts, a minimum module of approximately 2-2.5 mm can be chamfered. In small batch production, avoiding tool changes improves the overall productivity.
Chamfer width and angle can be set in the machine software but may be influenced by the gear-data like module, pressure angle, and by the workpiece geometry. The existence of a hub, flange, adjacent gear zone, or tooling in close proximity to the root may limit the tool path required to complete the chamfer in the root. When chamfering the workpiece on a separate chamfer spindle and not on the hobbing spindle, a slim fixture design is preferred to avoid interferences.
Since chamfer-contour milling is a cutting process, secondary burrs can build up. When cutting with a slot-enter method (cutting from the gear face into the gap) and targeting chamfer angles smaller than 40 degrees, burrs on the edge between chamfer and flank are imperceptible. No burrs are generated on the gear face. Alternatively, the slot exit method (cutting direction from inside the gap to the face side) can be applied with no burrs remaining in the flank and any potential burrs on the face side removed with a secondary burr reduction disc mounted on the chamfer cutter head.
Considering first production results and experiences with chamfering of bevel gears, tool cost-per-part could be less than 0.03 euros for a truck-sized gear with standard chamfer sizes. With a cycle time of roughly 1 minute when chamfering all four edges in parallel to the hobbing process, the target market is the manufacturing of medium-size gears with generally longer cycle times and where achieving greater flexibility is desirable.
4 Comparison of New Cutting Solutions with Chamfer Rolling
4.1 Chamfer Rolling
Chamfer-rolling, is an extremely fast, versatile process that is most often applied to smaller cylindrical gears up to module 5 mm to remove burrs formed by a preceding soft gear cutting operation. Chamfer rolling is a forming process that creates chamfers along the tooth edges with gear-shaped tools that mesh with the workpiece (Figure 14).
Excess material flows mainly to the face side of the gear where it is cut away by simple single blades, deburring discs, or filing discs, depending on the shape of the gear. Some material may bulge out into the flank at the edge between the chamfer and the flank. Depending on the subsequent finishing process, this burr may be a factor. While there is no issue with shaving and in most cases with threaded wheel grinding either, the secondary burr could limit tool life if a power-honing process follows. In the latter case, a second (hobbing) cut is applied or chamfer tools with an integrated burnishing function are used.
With chamfer rolling, the technology know-how is built into the tools. Most current design improvements ultimately lead to a new generation of chamfer-rolling tools, and this could reduce the tool cost-per-part to 0.04 euros for an automotive shift or e-drive gear using a dry process.
4.2 Applicable Workpieces
The request for defined chamfers is found mainly in automotive- and truck-sized gears. Larger gears do have the requirement for deburring but rarely defined chamfers are expected.
While chamfer-rolling is applicable for parts with module up to 5 mm, chamfer-contour milling needs a minimum module of approximately 2-2.5 mm as described earlier. Upper limits for the cutting processes shown in Table 1 are not technology dependent but instead depend on the machine capability where these processes are currently applied.
The geometry of the workpieces influences the choice of the chamfer method (Table 2). While disc-type parts do not have limitations, shaft-type parts might. Two critical geometries need to be looked at: contours parallel to the gear face and shaft diameters close to the root of the chamfered gear. Chamfer-rolling can handle contours adjacent to the gear face with a minimum distance of approximately 3 mm. Chamfer Hobbing and chamfer-contour milling need larger clearance, depending on root-chamfer requirements and tool diameter. While chamfer-rolling could create a comma-form chamfer even with a shaft diameter larger than the root diameter of the gear (Figure 15), the cutting methods face some restrictions. Depending on the request for a chamfer in the root, the root-chamfer angle and the diameter of the tool may need a certain clearance below the root diameter. The detailed analysis could be performed upfront through use of analysis and simulation tools.
Today, chamfer processes are typically integrated into gear machines that either generate the gear, e. g. hobbing or power-skiving machines, or into soft finishing systems such as shaving machines. While chamfer-rolling is applied either in parallel or sequentially to the main process, chamfer-hobbing and contour-milling are arranged parallel to the main process. With parallel chamfering, it is expected that the chamfer time will be shorter than the main cutting time. Otherwise the machine would have to wait for the chamfer process, thus limiting the overall effectiveness of the machine system.
The right chamfer process therefore depends on the expected overall cycle time of the line or cell as well.
4.3 Chamfer forms and quality
Together with the chamfer angle and chamfer width as described in Section 1.2, the chamfer needs additional definitions, including: with or without root chamfer, parallel or comma form chamfer (Figure 16), and chamfer on obtuse and acute edge or only on the acute one.
The most common chamfer form with chamfer rolling is a comma-form chamfer without root chamfering. Root chamfering with this forming process is more challenging than with other cutting chamfer processes since the material flow is limited. Parallel chamfer form, including root chamfering, is the preferred solution for cutting technologies such as CHH and CCM. There might be limits when chamfer cutting a shaft with a close proximity of a shaft diameter or contour (Table 3).]
4.4 Efficiency and Cost
As described earlier, chamfering does not improve the quality of the gear directly. Therefore, it is even more important to perform the process as efficiently as possible. The efficiency of a process is influenced by the machine execution and cycle time as well as the tool-cost-per-part (Table 4). The machine execution to perform chamfer-rolling is the simplest one, as the basic version of a hydraulic tool infeed, plus a driven tool would be sufficient. In case a chamfer-rolling machine system includes NC-axes, these would be mostly used to improve the usability or user friendliness. To run Chamfer Hobbing or chamfer-contour milling, a 6-NC-axis configuration is necessary, similar to hobbing. From a machine-investment point of view, cutting chamfer executions for CHH and CCM are more expensive than machine executions for chamfer rolling.
The tool-cost-per-part depends on the machining condition (dry, wet), on the gear (module, number of teeth), and the chamfer requirement (size and length of the chamfer). With CHR, tool cost could be as low as 0.01 euros per workpiece with wet chamfering, but with dry process, the tool life decreases significantly and could remain with only 10 to 20 percent of a similar wet operation. The latest developments with chamfer-rolling tools were able to double tool life with dry chamfering to reach about 0.04 euros cost per part for a standard automotive part, comma-form chamfer but without root chamfering. Considering parallel chamfers including root with CCM and CHH, dry machining tool-cost-per-part for CHH (automotive) and CCM (truck) is expected to be below 0.03 euros per part (Table 5).
Part of the tool cost calculation is the reconditioning of the tool. With standard CHR, the tool needs to be disassembled, reconditioned, assembled, and set. Because this requires greater effort, reconditioning is most often done by the supplier or specialized tool companies rather than the user. Reconditioning of chamfer hobs is similar to that of standard hobs and can be performed by using existing hob-reconditioning facilities. With CCM, there is no reconditioning of tools as the standard indexable inserts at the end of the tool life will simply be exchanged.
5 Conclusion
There are a variety of chamfering/deburring options to meet the different requirements of gear manufacturers:
• Chamfer Hobbing with dedicated tools for automotive-sized gears, medium- to high-volume production and chamfer forms according to customer standards with tool-cost-per-part of less than 0.02 euros.
• Chamfer-contour milling with standard indexable carbide inserts for “truck-sized” gears providing a highly flexible process targeting low batch, small- and medium-volume production with cycle times longer than 1 minute and tool cost-per-part of less than 0.03 euros.
• Chamfer rolling for medium- and high-volume production with minimum cycle times (less than 10 seconds), shaft-type parts with no or minimum clearance below the root diameter or a close contour, or cluster gears with tool-cost-per-part of about 0.04 euros with new design tools for automotive shift gears and dry processing.
• Every gear production is different. Depending on the specific requirements, it is very likely that one of these three chamfer technologies described will address the varying challenges customers may have.
Bibliography
- Herrmann Pfauter Werkzeugmaschinenfabrik, 1976, Pfauter-Wälzfräsen, 2nd edition, Springer Verlag Berlin Heidelberg New York, Chap. 3.
- Augsburg, M. Development of a Method and Procedure for the Chamfering of Bevel Gears Based on Nominal Data. Diploma Thesis, Technical University of Ilmenau, April 2010.
Printed with permission of the copyright holder, the American Gear Manufacturers Association, 1001 N. Fairfax Street, Suite 500, Alexandria, Virginia 22314. Statements presented in this paper are those of the authors and may not represent the position or opinion of the American Gear Manufacturers Association. (AGMA) This paper was presented October 2019 at the AGMA Fall Technical Meeting in Detroit, Michigan. 19FTM17
