877-1

AN APPROACH TO DEVELOPMENT OF INVOLUTE SPLINE ON

LARGE PARTS USING CNC MACHINING

A.Velayudham1*, Prasmit Kumar Nayak2, A.M.Junaid Basha3

1*CVRDE, Avadi, Chennai, 600054, vel_sivam@yahoo.com 2CVRDE, Avadi, Chennai, 600054, prasmit1@gmail.com

3CVRDE, Avadi, Chennai, 600054, junaidbashaam@yahoo.com

Abstract

This paper describes the approach followed in machining involute spline on a crank shaped large size component using CNC machining centre. For spline milling shell type tool holder with throw away involute formed coated carbide inserts were used. To verify the involute geometry of the splines machined by CNC milling, on the similar size component involute spline form was generated on gear shaping machine with the same spline data. Comparative results of profile, lead and pitch errors are presented for involute splines obtained by spline milling and spline shaping processes. The accuracy of results is better for spline milling as compared to spline shaping. Keywords: Involute spline, CNC machining centre, Throw away carbide insert, Spline shaping.

1 Introduction

In a metal cutting production environment, with mixed production and limited time for optimal planning particularly for small batch runs is always a critical one. While optimised solutions are always the desired choice, in these environments the need is for a versatile, flexible and reliable solution, with reduction in setup time often demands a different solution [R.W. Jones, 2007]. Gear / Spline manufacturing is a relatively complex area, spanning over a broad spectrum of components, where volume, complexity, size, quality and costs are the main factors that determine the production methods and machinery. Moreover, it is an area where conventional methods have been applied to meet the demands that these factors make [Christer Richt, 2012]. Designing techniques to manufacture involute spline and gears on large sized components is always a challenging one. In this paper a conventional method of involute spline form milling using CNC milling machine on large part is discussed. The involute spline machining was also carried out on the similar component by spline shaping machine. The results of profile, lead and pitch errors are compared.

2 Description of component

Axle arm is one of the critical members in hydro-pneumatic suspension system (Figure 1) of combat vehicle. Its main role is to transmit the undulations encountered by the rim wheel to the suspension system to dampen the vibrations experienced by the vehicle while moving on different terrains. Basically axle arm consists of two important portions, viz. stub axle and crank pin besides its basic arm. The main function of the stub axle is to support rim wheel assembly and the crank pin is to connect the crank that is housed inside suspension housing. On the stub axle, rim wheel is supported by two bearings and the crank pin is supported by another two bearings assembled in the housing. The undulations of the terrain experienced by the rim wheel are transmitted via axle arm, crank, connecting rod and the dampening valves. The dimensional and geometrical accuracy of involute splined

crank pin is therefore, plays a vital role in sustaining the transmitting load and torque during movement of the combat

Figure 1 Hydro-pneumatic assembly

vehicle. In addition to transmission of torque, spline profiles are used to centralize the axis between a splined shaft and a splined hub located in the mating part [Norbert Weiss et al, 2004]. In a large sized component, controlling the geometrical tolerance is critical, since any small variation in the axis will get amplified when the centre distance is more.

The detail of the axle arm is illustrated in Figure 2. In the axle arm, the axes of the crank and stub axle are apart to a distanceof 550±0.5 mm and should be parallel within 100 µm. The bearing diameters Ø100g6, Ø140h6 and the diameter of involute spline are to be concentric within 20 µm and their axes must be perpendicular with the basic arm within 20 µm. While it is possible to achieve this with major diameter/minor diameter fits of the mating splines, the true centring effect can only be achieved by using involute splines.The involute, or "side fit" profile is the most commonly used spline in production today. Therefore to assure the assembly between suspension housing and rim wheel, side fit involute splines are specified in the design of the axle arm.

Suspension housing

Rim wheel

Axle arm

Crank pin

Stub axle

877-2

Ø139.5h11

Ø100g6

Ø140h6

91

550±0.5186

350.5

202.5Spline

R35

150

140

Crank PinArm

Stub Axle

Figure2 Axle arm

The raw material of axle arm is steel forging to BS: 970-1991, Part 1, 832M13 (En 36C). The detail of the composition of the raw material is given in Table1. Stub axle side of the axle arm is case hardened to a depth of 1.2 to 1.5mm with 50 to 55 Rc and finish ground. The remaining portion shall be hardened and tempered to a minimum hardness of 340BHN. The splinedata is as per DIN standard [DIN 5480-1, 2006] which is given in Table 2.

Table1Chemical composition

Elements Composition (%)

Min. Max. C 0.16 0.18

Si 0.26 0.35

Mn 0.50 0.60

Ni 3.50 3.75

Cr 0.93 1.10

Mo 0.20 0.25

Cu - 0.35

S - 0.03

P - 0.03

Table2 Spline data

Reference diameter ‘d1’ 140 mm

Module ‘m’ 2.5 mm

No. of splines ‘Z’ 54

Reference profile DIN 5480

Profile modification X1.m +1.125 mm

Total Depth ‘h’ 2.5 mm

Quality zone 8f DIN 5480

Base tangent length ‘W’ across 10 teeth

Nominal 72.026 mm

Aoe 71.982 mm

Au 71.947 mm

3 Manufacturing approach

Over the decades, numerous methods and manufacturing processes have been used in making the various kinds of splines based on various criteria, including applications, reliability, life time, processing time and manufacturing cost. Milling, hobbing, grinding, shaping, broaching, slotting and

rolling are some of the common methods followed in industries to produce involute splines and each having its inherent merits and demerits and applications [Dan Seger, 2005]. Splines are generally produced in dedicated gear production machines, which use tools in the form of hobs and disc cutters. With the goal of producing a given batch of components to the required dimensional accuracy with minimum cost and in the least amount of time, advancements in technology need to be continually investigated to find tools that can have a direct impact on customer’s quality, cost, and delivery. For components having many features, including at least one gear element, there is an increasing trend towards 5-axis machines capable of multi-tasking. The ability to completely machine components in one or two setups, for quality and cost reasons, is a strong driver of change. Apart from quality, the size of the component also drives the production method.

In this study, the component is crank shape, and the size of the component is quite large with a swing of 700 mm from the centre axis of the crank pin. To machine involute spline on this crank shaped large part at gear hobbing machine is not possible. Although gear shaping process is quite appropriate to process external spline on such a large and crank shaped component, gear shaping machine with higher capacity rotary table of 1400 mm diameter and higher stroke length is not quite commonly available in gear manufacturing industries. Above all, achieving the exit radius of R35 mm at the spline end is difficult with gear shaping process. This situation calls for an alternate method of manufacture. Hence, to meet these criticalities a conventional approach of form milling with involute shaped form cutter was adopted. To meet the large swing and indexing requirement of the component, a precision CNC machining centre with large CNC rotary table was selected. To ensure the exit radius that is required at the exit end of spline, a suitable size form milling tool holder was chosen.

Gear milling is characterized by interrupted cutting action, and in addition to thermal variations on the cutting edge, mechanical interruptions make demands on high insert toughness and strength. In gear milling the demands for edge toughness are also influenced by the relatively small radial depths of cut in combination with the large tool diameter [Steven Van Dyck, 2011]. To meet the productivity needs with huge batch sizes, traditional high speed steel tools, such as spline rolling racks, hobs, broaches and shaving cutters, have done the job admirably. However, there is a push for increased flexibility. A leap forward on the flexibility continuum is happening now, due to present development in multi-axis and multitasking machines to perform operations given exclusively to hobs, broaches, spline rolling racks and other tools in the past. With multitasking machines and new insert developments, it is possible to have a single cutting tool accomplish several operations. This is especially crucial for small to mediumbatch production, which often requires frequent changes throughout the machining and handling process [Aaron Habeck, 2014]. When compared to traditional high speed steel tools for gear milling, the new generation

877-3

coated carbide inserts offer a completely new scope for reducing machining times, cost and improving the product quality [Christer Richt, 2010]. In this study, considering the material composition and heat treatment condition of the component, suitable grade carbide insert was chosen. Stub Axle portion of the raw Axle arm forging was initially turned in Vertical Turret Lathe / CNC vertical CNC milling machine (by circular interpolation followed by external turning using Wohlhaupter facing and boring head). The Crank Pin portion was then machined as adopted at Stub axle side by holding on a milling fixture. To carry out the spline cutting, vertical CNC Machining centre which is having a CNC rotary table was used. Here a right angle head was utilised for carrying out spline cutting horizontally.Initially, to prove the process an end mill type involute form cutter that is made out of solid carbide was tried. The machining trials were conducted on a SIP 5000 Jig boring class double column vertical machining centre. After proving the process, a seven-flute shell mill holder with three cornered throw away TiCN coated carbide inserts, tailored to the involute geometry as per DIN standard (Vardex make) was used to carry out spline milling operation. The set-up of spline milling is illustrated in Figure.3. To compare the accuracy of form gear milling, a shaft to the size of crank was machined and spline was generated using shank type spline cutter utilising Mitsubishi gear shaping machine.The results are compared and discussed in the following paragraphs.

Figure 3 Spline milling set-up

4 Results and Discussions

All types of gears / splines require dimensional verification to ensure reliable, long-term performance. The spline on workpieces can be inspected in different ways viz. inspection of the individual form on the measuring machine (profile, helix trace, pitch, and run out), double flank gear rolling inspection on a double flank gear rolling inspection machine, where proper master spline is mandatory. A variety of 3D coordinate metrology technologies are utilized to inspect and measure gears / splines from stationary coordinate measuring machines (CMMs) to portable laser tracking systems. Optical sensor technologies, as well as tactile measuring devices, are

commonly used solutions. CMMs can effectively measure both splines and other production parts, which is their primary advantage over conventional gear measuring devices [Belinda Jones, 2011]. In this study, the spline form milled component (marked as ‘Part_S_1’) and spline shaped component (marked as ‘Part_S_2’) are inspected on Carl Zeiss three coordinate measuring machine. The data was measured using Gear Pro gear measuring software and compared with the standard data given in the DIN 5480, part 1. The results with respect to profile, lead and pitch are illustrated in Figure 4 and Figure 5.The inspection report of the above results of profile, lead and pitch errors are summarized in Tables 3, 4 and 5.

Table 3 Profile error between Parts S_1 and S_2

Sl. No

Tooth No.

Profile Error (in µm)

Allowable limit

S_1 S_2

LH RH LH RH

1 1 21 (As per

accuracy grade 8)

13 5 10 21

2 15 14 12 20 17

3 28 13 10 12 36

4 42 11 7 14 17

Table 4 Lead error between Parts S_1 and S_2

Sl. No

Tooth No.

Profile Error (in µm)

Allowable limit

S_1 S_2

LH RH LH RH

1 1 11 (As per

accuracy grade 8)

10 10 15 14

2 15 13 11 27 33

3 28 12 11 21 15

4 42 11 9 27 26

Table 5 Pitch error between Parts S_1 and S_2

Sl. No

Tooth No.

Pitch Error (in µm)

Allowable limit

S_1 S_2

LH RH LH RH

1 1 17 (As per

accuracy grade 8)

19 21 23 42 2 15

3 28

4 42

It can be seen that the profile deviation observed when

machining by the process of CNC milling is nearer to the tolerance grade of 7, which is well within the required tolerance grade of 8. But the profile deviation observed from shaping process is closer to the tolerance grade of 10, which clearly shows that the approach followed by CNC form milling meets the design requirement and thus suitable for machining involute spline on large part. In the case of CNC form milling, the involute profile is formed by direct replication of tooth space by the involute profile ground carbide inserts, due to which the required profile accuracy is achieved. However in the case of spline generation through gear shaping machine, the inherent inaccuracies in the gear kinematics and the inaccuracies resulted in the gears linkages

Shell mill holder

Axle arm

CNC Rotary table

877-4

Figure 4 Inspection report of profile, lead and pitch of

involute spline milled through CNC form milling process

877-5

Figure 5 Inspection report of profile, lead and pitch of

involutespline generated through shaping process

877-6

due to normal usage of machine would have caused this variation. Apart from the above, error in setting the blank axis in-line with the axis of machine table also could have resulted into the above variation. However it is to be noted that the variation between the tolerance grade of 8 and 10 is very minimum for the module of 2.5mm.

Based on the variation with respect to lead, the tolerance grade observed from CNC milling process is 9, whereas from the shaping process it crosses the tolerance grade of 12. Therefore with respect to lead, the tolerance obtained by the process of CNC milling is much closer to the design requirement of 8 tolerance grade. Based on the pitch variation, it is observed that the tolerance grade achieved by CNC milling is 9 on both left and right flanks, whereas by spline shaping process the left flank is grade 9 and right flank is grade 11. Therefore, based on the pitch, the variation is almost same in both the cases.

The above results clearly indicate that, approach followed by form milling using throw away carbide insert utilising CNC machining centre is quite suitable to machine involute spline on large parts. The results also suggest that to achieve closer side fit 8f tolerance grade, alternateprocesses like spline hobbing or rough machining by shaping followed by grinding can be resorted.

5 Conclusions

From the study on spline machining through form milling using CNC machining centre, the following conclusions can be drawn: � The profile deviation observed when machining by the

process of CNC milling is nearer to the tolerance grade of 7, which is well within the tolerance grade 8 specified in the standard. But the profile deviation achieved through shaping process is closer to the grade of 10. With respect to lead, the observed deviation through CNC milling process and conventional shaping process is 11 to 13 µm and 15 to 27 µm respectively.

� The measured variation of pitch while machining through CNC milling is 9 to 19 µm and by shaping process is 9 to 23 µm against the allowed variation of 8 to 17 µm.

� From all the results, it can be seen that approach followed by form milling through throw away carbide insert using CNC milling machine is quite suitable to machine involute spline on large parts.

� The results also suggests that to achieve the closer side fit 8f tolerance grade, alternate processes like spline hobbing or roughing through shaping followed by grinding can be resorted.

References

Aaron Habeck, (2014),Speedingup Splines, Modern Machine

Shop, March, 2014. Belinda Jones (2011), Putting teeth into gear inspection, Gear

Solutions, pp. 45-54, Aug 2011. Christer Richt (2010), Anew edge on gear milling, Gear

Solutions, pp. 22-30, Apr 2010.

Christer Richt (2012), Sophisticated simplicity, Metal working

world, pp. 26-27, Feb 2012. Dan Seger (2005), Inside Splines, Gear Solutions, pp. 32-35, Jan 2005. DIN 5480-1 (2006), Involute splines based on reference diameters – Part 1: General. Norbert Weiss, Rudolf Och and Jerry Kowalsky (2004), Side Fit Spline Profiles, Gear Solutions, pp. 40-43, Mar 2004. Jones, R.W (2007), A New Tool for Prototype and Production – Case Study, Gear Solutions, pp. 40-43, Mar 2007. Steven Van Dyck (2011), The Case for Cold Rolling, Gear

Solutions, pp. 46-52, May 2011.