No twist when helical gear hobbing

Citation

(2009), "No twist when helical gear hobbing", Aircraft Engineering and Aerospace Technology, Vol. 81 No. 5. https://doi.org/10.1108/aeat.2009.12781eab.006

No twist when helical gear hobbing

Article Type: Aerospace technology From: Aircraft Engineering and Aerospace Technology: An International Journal, Volume 81, Issue 5

LMT's Fette Division reports that it has developed a combination hobbing and deburring tool for gear cutting that enables the functions of a roughing hob, the deburring of the top and bottom flanks of each tooth and the finishing of the gear into a single high-productivity operation that reduces the demand for other operational equipment and provides the means to significantly reduce production lead times.

Available through LMT (UK) of Coventry, the multi-functional Twist-Free Hob enables finished spur and helical gears to be completed in a single hobbing operation using a standard CNC hobbing machine with each of the cutters mounted on a single arbor. In addition, the new tool system that was developed in conjunction with gear cutting machine specialist Liebherr Verzahntechnik, eliminates tooth flank “twisting” or convexity error, one of the major drawbacks when hobbing helical gears, that creates the need for correction and separate finishing operations.

The new tooling system is said to improve production lead time by combining separate operations into one as well as any requirement for special gear finishing tooling. It contributes to smoother running of the gear as well as helping to reduce transmission noise.

A single arbour carries the assembly of cutters comprising the roughing hob; two chamfer/deburring cutters and the finishing hob – each separated by spacers. This arrangement provides the flexibility of choice when specifying the most productive cutters and settings for effective metal removal, for instance when roughing, enabling benefits of choice from the number of threads, gashes, tooth design, cutter material and surface coating. In addition, it enables the final finishing cutter to be selected to precisely meet the design profile and finishing requirement specification for the gear.

During the hobbing operation, the new tooling system allows the hob head of the machine to be programmed to swivel to different angles for roughing, deburring/chamfering and finishing. This ensures the lead and helix angles are correct for the individual hobbing cutters while still maintaining the appropriate lead angle for deburring and chamfering the roots and side edges of each flank of the gear tooth.

In developing the tool, LMT was also able to blend a gradient change into the pressure angle of the finish hob which creates a “corrected” tooth profile at the bottom of the gear tooth that is different to the top. When the finish hob is engaged in the cut, a diagonal axis feed under CNC control compensates for, and eliminates any tendency to create twist in the tooth.

As a result, especially on helical gears, by eliminating twist from the hobbing process, additional operations such as shaving to correct the convexity error are no longer required.

Details available from: LMT (UK) Limited, Tel.: +44 (0) 24 7636 9770; Fax: +44 (0) 24 7636 9771, Web site: www.LMT-tools.com