Tsch?tsch H., Koth A. Metal Forming Practise: Processes - Machines

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6.10 Methods and machines for rolling gears 71

Figure 6.18 Factors which influence the quality of rolled parts

6.10.2 c) Establishing the theoretical pre-turning diameter d

pre

One crucial advantage of manufacturing gear tooth profiles by forming is the fact that there is

essentially no material loss.

The technician forming the part is confronted

with the task of defining initial forms with a

volume the same as that of the final form. The

quality of the rolled part which can be achie-

ved depends strongly upon the selection of the

theoretical pre-turning diameter d

pre

of the

rotationally symmetric starting form.

When this parameter is established by mathe-

matical / geometrical means, two main criteria

must be weighed up. On one hand, the pre-

turning diameter is worked out according to

the material displacement which takes place

during the forming process.

Figure 6.19 d

pre

if volume is constant

On the other hand, the calculation must also take into account the fact that the spacing rolled

on the external diameter of the blank is determined by the spacing of the addendum circle of

the rolling dies. If there are substantial differences between the theoretical pre-turning diameter

according to the volume and the value after the exact spacing is set down, repeated corrections

must be made.

72 6 Thread rolling

When determining the theoretical pre-turning diameter, the following formula can be used as a

basis:

hd p

pre n

(0.35)





Example:

The aim is to roll a helical gear with an involute flank profile out of C45 with the following

gear tooth parameters. Determine the theoretical pre-turning diameter of the rotationally sym-

metric starting stock.

m = 1.5875

= 25.75°

z = 10

= 21.7 mm

= 14.9 mm

Solution:

pre

21.7 mm 14.9 mm 1.5875

(0.35) 18.92 mm

2 cos 25.75





6.10.3 Method and machines for rolling gear teeth

Figure 6.20 lays out the main rolling methods for the manufacture of gear teeth profiles by

forming.

Figure 6.20 Transverse rolling method for the production of gear teeth profiles

6.10 Methods and machines for rolling gears 73

6.10.3 a) Rolling with the flat die method

The principle of rolling with flat dies (Figure 6.21) is a movement of the upper and lower dies

in opposite directions, in order to roll a defined profile onto a rotationally symmetric initial

form. In the rolling process which takes place, the geometry of the die is moulded onto the

workpiece (teeth) and involute flank profiles are formed.

At the start of the rolling process, the blank is held between drive centres. The upper and lower

dies move against one another in a translatory movement, simultaneously come into contact

with the workpiece and set the blank revolving by friction. The sloped intake zones of the dies

mean that the die teeth penetrate the workpiece to increasing depth with a continuous feed. At

the same time the material is displaced at the points of contact and flows into the spaces of the

die profile.

After the full-depth profile has been rolled at the end of the intake zone, the teeth are calibrated

in two further rolling revolutions to improve surface finish, the shape of the flanks and radial

runout. In the subsequent sloped die runout zone, the deformation forces ease off and the

rolled gear tooth system is removed.

Figure 6.21 The principle of the flat-die method with upper and lower rack dies

6.10.3 b) Rolling with the cylindrical die method

When transverse rolling with the cylindrical die method, the rotationally symmetric blank is

held in an axial direction between tooth tips. Two or three cylindrical dies (depending upon the

method used) shape the gear teeth on the blank, moving in the same direction and at a constant

rotation speed. The die teeth penetrate the workpiece by means of a radial shortening of the

distance between the axes of the cylindrical dies. This infeed motion is controlled hydraulically

or with a movable rolling die. The second cylindrical die is fixed. It must be possible to move

the workpiece holding device in the direction of the infeed movement so that the blank to be

centred during the deformation process. Another viable solution is rolling with a support gui-

de.

74 6 Thread rolling

In this variant, the axis of the workpiece is slightly below that of the rolling die in order to

prevent the blank from being pushed out during the deformation process.

As it is possible to re-roll several times, the rotary rolling method can be seen as a forming

process with an infinite die length.

Figure 6.22

Principle of the cylindrical rolling

method with two rolling dies

The current state of technological standards mean that it is possible to roll good-quality gear

teeth with cylindrical dies up to a modulus of m = 1.5. When the modulus increases, the kine-

matic conditions involved in rolling mean that spacing, flank shape and symmetry defects

occur to a degree which has a substantial effect on the performance of the workpieces when in

use. Profile rolling machines using the cylindrical rolling method are basically designed with

either two or three dies. The design used most often in rolling gear teeth profiles is a machine

design following the proven two-roll principle (Figure 6.23) with two moving roller slides.

Figure 6.23 “Rollex” two-roll machine (Photograph from Profiroll Technologies works, Bad Düben,

Germany)

6.10 Methods and machines for rolling gears 75

Two main spindles, driven synchronously in the same direction, are used to take up the rolling

dies. One of the two rolling spindles is fixed, the other is moved mechanically or hydraulically

so that the rolling dies penetrate the rotationally symmetric blank.

Using CNC control it is possible to control

and visualise the processing of the values

involved in the rolling and their functions,

in a reproducible manner.

When rolling large profile depths with

cylindrical dies, forces of up to 800 kN can

be used in the deformation (PRZ80, Profi-

roll, Bad Düben, Germany). Gear tooth

profiles with a standard modulus are for-

med using forces in the region of 200 kN

depending on the workpiece.

Figure 6.24

The working area of a two-die machine (Profi-

roll Technologies, Bad Düben, Germany)

6.10.4 Rolling defects in processing

During the rolling process, the complicated kinematics mean that the following rolling defects

typical to the rolling of gear teeth profiles occur:

– undercut in the tooth root area of the gear,

– deviations in the shape of the flank,

– deviations in the flank alignment,

– spacing defects,

– deviations in radial runout.

Radial runout, spacing, flank shape and alignment affect the quality of the rolled part and the

optimising of the gear tooth profile.

6.10.4. a) Undercut in the gear tooth root area

Undercut occurs in the gear tooth root area if the tooth number limit, t

, is not reached in the

profile to be rolled.

– at an engagement angle of

= 20°:

sin



76 6 Thread rolling

The engagement and rolling interrelations mean that the addendum radius of the die profile

penetrates the target contour of the gear tooth root area, undercutting the usable involute

flank. The ensuing rolling defect is considered to be damaging if the load-bearing flank of

the tooth is undercut so much that it impairs the load-bearing capacity of the gear teeth dur-

ing gear engagement. This effect during rolling can be counteracted with a positive adden-

dum modification. The peak value of this addendum modification is limited by the forma-

tion of a peak on the top land of the gear tooth.

7 Cold hubbing

7.1 Definition

Cold hubbing is a cold bulk forming process where a hardened punch (male hub) penetrates a

workpiece at a low speed (slower than extrusion).

7.2 Application of the process

For the production of cavities in dies for moulding, stamping, injection moulding, plastic

moulding and drop forging.

For example:

Screw and bolt production (Figure 7.1)

The hubbing of screw and bolt head shapes in

punches and dies.

Figure 7.1 a) cold hubbed bolt die, b) hub, c)

carriage bolt produced by upsetting

Production of cutlery (Figure 7.2)

The hubbing of cutlery cavities in stamping

dies.

Figure 7.2 Forging die for cutlery handle, a)

hub, b) hubbed stamping die

78 7 Cold hubbing

The production of forging dies (Figure 7.3)

Hubbing cavities in drop forging dies.

gure 7.3 Cold hubbed cavity in a drop forg-

ing die,

a) hub,

b) hubbed cavi

7.3 Permissible deformations

The limits for cold hubbing are derived from the deformability of the tool steels and the per-

missible maximum surface pressure of the hub. As yet there is, however, no precise method of

calculation for this.

7.4 Calculation of force and mechanical work

7.4.1 Hubbing force

max

pA 

equivalent diameter d

1.13 DA 

F in N hubbing force

A in mm

hub area

d in mm diameter of the hub

t in mm hubbing depth

max

in N/mm

specific hubbing force

(from Table 7.1)

For hubs which are not round, an equivalent diameter d can be calculated from the hub area A.

Table 7.1 Specific hubbing force

max

in N/mm material and

§·

¨¸

©¹

0.1 0.2 0.4 0.6 0.8 1.0

1700 2000 2300 2600 2800 2900

2400 2750 3200 – – –

Material

group

III

3100 [4000] – – – –

7.4.2 Hubbing work

WFt  W in Nmm hubbing work

7.5 Materials which can be hubbed

Table 7.2 Steels which can be hubbed

Material

group

Quality Short desig-

nation

Material

number

Special notes on use

Screw and bolt tooling

I or II C 100 W 1

95 V 4

1.1540

1.2835

Heading punches and bottom dies for cold work

II III

X 32 CrMoV 3 3

X 38 CrMoV 5 1

45 CrMoW 5 8

X 30 WCrV 5 1

X 30 WCrV 9 3

1.2365

1.2343

1.2603

1.2567

1.2581

Heading punches and bottom dies for hot work

Drop forging tooling

II C 70 W 1

1.1520

II 45 CrMoV 6 7

X 32 CrMoV 3 3

1.2323

1.2365

Forging dies for light metals Forging dies for non-

ferrous heavy metals and for steels being pressed

III 55 NiCrMoV 6

56 NiCrMoV 7

1.2713

1.2714

Forging dies for steels being hammered

Die-casting tooling

I X 8 CrMoV 5

1.2342

Zinc pressure die-casting moulds

II 45 CrMoV 6 7

X 32 CrMoV 3 3

X 38 CrMoV 5 1

X 32 CrMoV 3 3

1.2323

1.2365

1.2343

1.2365

Zinc and light metal die-casting moulds Light metal

die-casting moulds Light-metal die-casting moulds

Brass die-casting moulds

Stamping dies

I or II II

C 100 W 1

90 Cr 3

1.1540

1.2056

Coining punches (minting)

Moulds for jewellery, electronic parts and furni-

ture mountings

III

45 CrMoV 6 7

55 NiCr 10

X 45 NiCrMo 4

X 165 CrMoV 12

1.2323

1.2718

1.2767

1.2601

(minting)

7.5 Materials which can be hubbed

80 7 Cold hubbing

7.6 Hubbing speed

Hubbing speeds are between

v = 0.01 mm/s and 4 mm/s

with higher-strength steels and difficult shapes needing to be hubbed at the lower hubbing

speed.

7.7 Lubrication during hubbing

To prevent cold welding between the hub and the workpiece, the following measures are nec-

essary:

– The surfaces of the hub which come into contact with the workpiece must be polished.

– Brush the hub surface with copper sulphate solution (acts as a lubricant binder).

– Lubricate with molybdenum disulphide (MoS

7.8 Characteristics of the workpieces to be hubbed

– For the workpieces which are to be hubbed,

their external diameter and their height must

be in relation to the hubbing diameter and

the hubbing depth.

2.5 Dd 

1.5

Dd 

2.5 htt

1.5 to 2.5

– The workpiece to be hubbed needs to have

recesses in its sides and base. This makes the

material flow easier and the hubbing capacity

higher.

Figure 7.4 Measurements of the workpiece to be

hubbed

d hubbing diameter, t hubbing depth, D external

diameter, D

recess in the base, r radius for the

recesses in the sides, D slope angle # 1°

Figure 7.5 Recesses in the workpiece

Tsch?tsch H., Koth A. Metal Forming Practise: Processes - Machines - Tools

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