Investigations on Surface Defects in Gear Hobbing

Journal: ELSEVIER

Author: F. Klocke, C. Gorgels, A. Stuckenberg

Abstract:

An important property of manufacturing processes is the process reliability. This refers to the macro geometry and to the achievable surface quality. In dry gear hobbing as the most productive and common manufacturing technology for the soft machining of cylindrical gears sometimes surface defects are noticed. These defects like welded-on chips and smeared areas on the flank are not acceptable.

The mechanisms leading to surface defects are not known and understood in total. For the understanding, first the appearance and exact occurrence have to be investigated. Parallel, metallografic investigations are carried out for the characterization of the defects. Further on, the appearing of surface defects and characteristic values generated by a manufacturing simulation for gear hobbing are compared to find influences of the tool and process design on the tendency of dry hobbed gears towards surface defects.

Introduction and objective:

An important characteristic of production processes is the process reliability. This includes achieving the required quality of each individual work piece during single or multiple batches. The most common method for rough machining of external gears is hobbing. Next to the geometric quality requirements the surface on the flank has to be free of defects.

Experiments on surface defects in gearhobbing and reasons for the avoidance:

The upper picture shows a welded-on chip. The middle picture in vertical direction additionally shows a smeared area around a welded-on chip. These two defects are the characteristic defects for this point. The defects have a maximum height of hmax= 100 µm. These pictures show the proof that in case 1 the chip is welded on an already well generated flank, in case 2 the work piece material is smeared-up at the surface. In general every flank has slight surface defects, but these light defects are more an optical interference than a functional failure.

 The energy input of the process for the formation of one chip increases with a higher axial feed and cutting speed. For this part the defects are smaller with a higher energy input. The reason may be a warmer work piece and so a changed chip formation during the process. That means for a good cutting process a certain energy level or rather a minimal cutting length or chip thickness has to be exceeded.

A typical case hardening depth (CHD) distribution for small module gears based on Braykoff is drawn. The typical case hardening depth for a gear with a module of mn= 1.35 mm is 0.3 mm. With the knowledge of a maximum height of welded-on chips of hmax= 0.1 mm and a grinding stock of s = 0.05 mm the CHD is reduced in this area up to 50 %. This results in a surface hardness of about 660 HV instead of the required hardness of 720 HV +50.

According to Niemann and Winter that means a significant reduction of the allowed stress Hlimof 10 % and along with that a reduction of the local load carrying capacity. Additionally, the hardness is local out of the given tolerance field. This local decreased hardness can result in damages of the flank. Possible damages are on the one hand pittings and on the other hand flank breakage.

Investigations on reasons for surface defects:

The chip thickness, cutting length, kinematic clearance angle and number of cuts as function of the cutting edge for the variants with a lower axial feed and the conventional cut. The most significant fact is the higher chip thickness, cutting length and kinematic clearance angle for the conventional cut variant. The  higher values can be found especially in that area, where the conventional cut gears are only slightly defect loaded. The reason, especially for the defect smeared areas, may be a too small space between the tool flank and the work piece. This can be found as well at Winkel. There friction can be increased, which leads to a higher temperature on the work piece flank and therefore to smearing of material.

A further reason may be the accompanying increase of the rake angle with a reduction of the kinematic clearance angle. This increase leads to a worse chip flow and that to a longer duration of a chip in that area. This leads to a longer heat transfer into the work piece, which may result in the appearance of smeared areas.

Conclusion:

The investigations are focused on a planetary gear representing modern dry gear hobbing processes. The two major defects are welded-on chips and smeared areas.

The comparison shows a good correlation between the appearance of defects and the values cutting length, kinematic clearance angle and the compactness of chips. The results lead to an optimization potential for the process design; not in a predictive way but to optimize processes iteratively.