High Accurate Hobbing with Specially Designed Finishing Hobs

Introduction Load-carrying capacity of gears, especially the surface durability, is influenced by their tooth surface roughness in addition to their tooth profiles and tooth traces (Refs. 1 and 2). The harder the gears are, the smoother their tooth surfaces and the more accurately their tooth profiles should be finished in order to obtain high load-carrying capacity. Through research, the relationship between the cutting thickness (undeformed chip thickness), the size of built-up edge (BUE) and the tooth surface roughness has clarified that the number of gashes and the rake angle in hobs has an effect on the cutting thickness and the BUE formation, respectively. The cermet-tipped hobs developed by Ariura and Umezaki (Ref. 4) have excellent cutting performance with respect to wear resistance, tooth accuracy of hobbed gears and roughness of their tooth surfaces for gear blanks with hardness up to about 400HBW. For surface hardened gears (550N–800HV), some method of appropriate surface finishing is necessary for removing their distortion after heat treatment. One finishing method is skive hobbing with cemented carbide hobs, as shown by Yonekura and Ainoura (Ref. 5). However, most other finishing methods involve tooth grinding and honing (Refs. 6, 7, and 8). In this article, the optimal finishing is investigated from a point of view of economy and productivity. This article presents finishing hobs, which are different from skiving hobs. In finishing hobs, the following characteristics are required: (1) good wear and chipping resistance, (2) smooth finishing of tooth surfaces, and (3) high accurate hobbing. Finish Hobbing with High Speed Steel (HSS) Hobs High speed steel finishing hobs with many gashes and positive rake angles can reduce the size of built-up edges because of the thinner undeformed chip thickness and the positive rake angle. They make the tooth surface smooth, resulting in a few micrometers in peak-to-valley height (Ry). A cutting speed of about 20 meters per minute is used for medium hardness gears (300–400HBW) to make BUE small and flank wear rate slow. For this paper, we use AISI M35 high speed steel for our comparisons. Figure 1 shows the calculated result of undeformed chip thickness in hobbing. The vertical axis indicates divided layer numbers in vertical sections of the tooth trace. The horizontal axis indicates divided points of the hob tooth profile. The undeformed chip thickness is displayed in micrometers. When each hob tooth passes through many line segments planted in the tooth space, the length of each line segment cut is calculated and converted to an undeformed chip thickness (Ref. 9). The relationship between the maximum thickness removed by the cutting edge at the involute genHigh Accurate Hobbing With Specially Designed Finishing Hobs