Fine Blanking of Helical Gears

Andreas Feuerhack, Daniel Trauth, Patrick Mattfeld, and Fritz Klocke

Fine blanking is a well-established process for the production of near net shape compo-nents with high quality. The produced parts are characterized by a smooth sheared edgeup to 100 %, excellent surface properties with good flatness and little burr as well as closetolerances for near net shape manufacturing. These process characteristics are suitablefor the efficient production of spur gears with large batch size. In this work, the applica-tion of fine blanking was extended for the production of helical gears. Therefore, the fineblanking process was modified with an additional rotary movement of the dies to realizethe manufacturing of helical gears. In this contribution the process idea, experimental andnumerical work as well as the potential of fine blanked helical gears is presented.

1 Motivation

In the automotive industry gears are an essential part for the efficiency of gear boxes.These gears are characterized by high precision with close tolerances, high balance, andapplication of high strength material. The general demand on thin helical gears in Europehas a volume of about 20 million components, trend rising. These thin helical gears areused in gear boxes for reduction of backlash and to achieve high balance. The conventionalprocess chain for the production of thin helical gears is time-consuming because of manymanufacturing steps. The motivation is the reduction of this process chain by applicationof fine blanking. Fine blanking has been successfully used for the production of spur gearsdirectly from coil. Fine blanked parts in general are characterized by a smooth shearededge of nearly 100 %, excellent surface properties with good flatness and little burr as wellas close tolerances for near net shape manufacturing [1].

Andreas Feuerhack Daniel Trauth Patrick Mattfeld Fritz Klocke RWTH Aachen University, Aachen, Germanye-mail: F.Klocke@wzl.rwth-aachen.de

187 Springer-Verlag Berlin Heidelberg 2015A. E. Tekkaya et al. (eds.), 60 Excellent Inventions in Metal Forming,DOI 10.1007/978-3-662-46312-3_29

188 A. Feuerhack et al.

One approach for the manufacturing of helical gears is the application of shear cuttingand the usage of round sheet metal blanks [2]. The use of round sheet metal blanks isuneconomical and time-consuming and shear cutting does not fulfill the needed near netshape properties. Therefore, kinematics of the fine blanking process was adapted by anadditional rotary movement to manufacture helical gears directly from coil with enhancednear net shape properties. By performing preliminary tests the rotary movement of thepunch was linearized and the sheet metal blank was cut with a defined angle to simulatethe real manufacturing process. Thereby, the main influencing parameters were identifiedand its correlations described [3]. Based on these insights a tool concept for fine blankingof helical gears was developed and verified under industrial conditions. The successfuldevelopment by WZL and project partners was patented and is applied in the industry [4].The innovative fine blanking process for the production of helical gears will be presentedhere.

2 Process Idea

Fine blanking is characterized by a linear vertical movement of the punch. Typically anunderneath drive press is used, where the cutting plate realizes the movement. The deci-sive advantage of fine blanking is the achievable high quality of the blanked part. To reachthis high quality in rotary fine blanking it is necessary to ensure a small die clearance.The typical die clearance is approximately 0.5 % of the sheet thickness. The interactionof the die clearance, v-ring and the superposition of compressive stresses are responsiblefor the achieved part quality [5]. To realize the fine blanking of helical gears tool kine-matics had to be redesigned. The linear movement of the active parts gets superimposedwith a rotational movement (Fig. 1). Thereby, the punch and counter punch are mountedpivoted and axially fixed. The upward movement of the cutting plate causes the punch tothe rotational movement that leads to a helical cut. The PhD thesis of Zimmermann givesa detailed process description for the fine blanking of helical gears made of 16MnCr5 [6].

Technological challenging was the development of the tool redesign. The rotation angle is only 1.4 and thereby the realization of a pivoted bearing is difficult to implement. Theuse of rolling bearings was not possible due to process related limitations. According tofeasibility study of bearing designs a hydrostatic bearing with an oil chamber was applied.This tool design in combination with the upward movement of the cutting plate initiatesthe fine blanking process (Fig. 2). Depending on the movement of the cutting plate, the oilchamber pressure build-up and in consequence the punch moves nearly frictionless [7].

Beside the tool development the calculation of the cutting force was adapted. Thecalculations of the cutting force Fs [N] is based on the sheet thickness s [mm], cuttinglength ls [mm], tensile strength Rm [MPa] and shear strength factor cs [], see Eq. 1 [7].

Fs D slsRmcs (1)

Fine Blanking of Helical Gears 189

Fig. 1 Principle tool concept for the fine blanking of helical gears

Fig. 2 Tool in industrial application

Taking into account the helix angle [] the cutting path increases in comparison tothe sheet thickness s. The calculation of the cutting force is modified as shown in Eq. 2:

Fs D scos

lsRmcs: (2)

190 A. Feuerhack et al.

The determined experimental data of the cutting force correspond very well with thecalculated cutting force and confirms the mathematical approach of Eq. 2.

3 Results and Applications

The feasibility study of the fine blanking of helical gears was carried out with a case-hardened steel 16MnCr5 and a sheet thickness of 3.2 mm. In Table 1 the helical gear isspecified in detail.

The fine blanked helical gears showed a very good quality, see Fig. 3. However, thequality of the tooth flank differs for the right and left flanks. The right flank shows asmooth sheared edge of 100 % and the left flank of only 81 %. Furthermore, the analysisof the die roll height gives different results along the tooth profile. For the left flank a valueof 0.12 mm was determined and in comparison to the right flank (0.65 mm) the differenceis nearly by factor 5. These results indicated a change of the die clearance during thefine blanking process [7]. The change of the die clearance is undesired and thereforethe experimental work was supported by the application of the finite element method todetermine the cause-and-effect relationships between the part quality and fine blankingparameter.

Figure 4 shows a fully elasto-plastic simulation model, tangential expansion of thepunch and the contact normal stress distribution at one tooth. The tangential displace-

Table 1 Specification of the fine blanked helical gearparameter valuehelical angle 26.74normal pressure angle 20normal module 3 mmnumber of teeth 40pitch diameter 134.37 mmaddendum modification coefficient 0

Fig. 3 Fine blanked helical gear and detailed view of the 100 % smooth sheared tooth flank

Fine Blanking of Helical Gears 191

Fig. 4 Fully elasto-plastic FE model with a detailed view of the tangential displacement of thepunch as well as the contact normal stress distribution at one tooth

ment of the punch as well as the contact normal stress distribution correlates well with theexperimental work. The shifting of the die clearance is related to the tangential displace-ment and finally leads to a non-uniform stress distribution at the tooth. This leads to aflank depending cutting quality [8]. Current research work is focusing the adaption of thedie clearance to achieve a homogenous stress distribution and thereby a uniform 100 %smooth sheared edge for the right as well as for the left tooth flank.

4 Potential

Fine blanking of helical gears is a promising technology and has the following potentials:

1. The helical gears have a good part quality with a smooth sheared edge and requiringless refinishing operations.

2. Manufacturing of helical gears directly from coil compared with conventional machin-ing is very economical.

3. Processing of a wide range of materials and sheet thicknesses is possible.4. Achieving of a homogenous 100 % smooth sheared edge by an optimization of the die

clearance.

5 Conclusion

Fine blanking of helical gears is an innovative process to manufacture helical gears di-rectly from coil. The major challenge in tool design was the bearing of the punch torealize the rotary movement of the punch. The approach with hydrostatic bearing wassuccessfully implemented and ensured a reproducible manufacturing of helical gears. Theexperiments showed a different smooth sheared edge for the left and right flank of thetooth. Experimental investigations were supported by means of the finite element method.

192 A. Feuerhack et al.

The tangential displacement of the punch leads to a displacement and thereby to a shiftingof the die clearance. The shifting of the die clearance is represented by different con-tact normal stress distribution at the tooth and correlates with the flank dependent smoothsheared edge. Current research is focusing on the adaption of the die clearance to achievea homogenous 100 % smooth sheared edge on both the right and left flank of the tooth.

6 Acknowledgement

This process has been investigated in several projects. The work has been kindly supportedby the German Research Foundation (DFG) under the grant number KL 500/126-1 andthe Industrial Collective Research under the grant agreement number [KF 2120301PK8].

References

1. Klocke, F., Knig, W., 2006, Fertigungsverfahren 4, Umformen, 5. Ausgabe, Springer VerlagBerlin.

2. Belcredi, L., 1986, Procedimento per la tranciatura di ruote dentate a profile elicoidale e prodotticosi ottenuto. Patent IT 1137113.

3. Klocke, F., Zimmermann, M., Bcker, V., Wegner, H., 2011, Finite Element Simulation of anAnalogy Process for the Fine Blanking of Helical Gears, Proceedings of International Sympo-sium on Assembly and Manufacturing ISAM 2011, Tampere, Finland, 16

4. Zimmermann, M., Watermann, M., Trump, K., 2011, Verfahren und Vorrichtung zum Fein-schneiden von Werkstcken, Patent EP2208552 B1.

5. Hoffmann, H., Neugebauer, Spur, G., 2012, Handbuch Umformen, Hanser Verlag6. Zimmermann, M., 2015, Feinschneiden von Schrgverzahnungen, Apprimus Verlag7. Klocke, F., Zimmermann, M., Mattfeld, P., Feldhaus, B., Trump, K., Watermann, M., 2011,

Feinscheiden schrgverzahnter Stirnrder. Innovatives Vorverzahnen schmaler Schrgverzah-nungen, wt Werkstattstechnik online, Volume 10 (10), 668672.

8. Zimmermann, M., Klocke, F., Schongen, F., Feldhaus, B., 2011, Fine Blanking of Helical Gears Finite Element Simulations and First Experimental Results, ICTP 2011, International Confer-ence on Technology of Plasticity, 10, 581585.

Fine Blanking of Helical Gears1 Motivation2 Process Idea3 Results and Applications4 Potential5 Conclusion6 AcknowledgementReferences