TECHNICAL FIELD

The present invention concerns a. method for finishing hardened gears, preferably for dry finishing gears having a module range of 1.00 - 3.50 [mm] .

This kind of gears is particularly used for automotive transmissions requiring high performances in terms of transmitted power , low noise emission and efficiency. This result is achieved by means of hardened, ground gears (with a surface hardness of at least 54 [HRC] ) . In fact, this kind of gears presently provides the best quality/price ratio .

The gear performance is mainly determined by the ge- ometry and the surface structure of the gear flanks. Although the tooth root area as well as the transition area between the tooth root and the flanks (root fillet area) is of some importance , still today most transmission gears are not finished in these areas after the hardening process but are finished only on their flanks.

BACKGROUND AR

It is we11 known the fact of producing the aforesaid gears by means of: the. following operations: cutting teeth into a cylindrical workpiece made of soft metal ("green") (performed by means of one or more subsequent processes , in particular nobbing or shaping and shaving) and hardening.

With a reference to the aforesaid operations , the hardening process inevitably leads to changes in the gear profile, lead and pitch in particular : it alters the tooth macro-geometry. Therefore, given that the geometrical and the surface quality of the gear teeth depend on the sequence of processes they undergo, hardened gears intended for a high-performance use are inevitably subjected to subsequent finishing operations, in particular grinding operations , in order to obtain the best possible surface quality and accuracy. For a complete levelling of all imperfections of the gear geometry from the machining of soft metal "green machining" and due to distortion during hardening a minimum total stock removal normal to the flank of the gear of q ≥ 0.03xm [mm] is needed, wherein m is the module of the gear .

The grinding processes of the aforesaid hardened gears, besides producing chips, generate high amounts of heat. Only a minimum of this heat can be taken off by the chips and by the tool, and therefore lubricating oils or emulsions are used, aiming : to cool the component in order to avoid that the too hot gear gets damaged during the grinding process . In particular, if the gear gets too hot ("burns"), the hardening is ruined and the gear surface is no longer suitable for the required use .

However, the use of lubricating oil implies several disadvantages :

- complex and large systems to supply oil to and to return oil from the machine, filter chips from the oil and cool the oil to a certain temperature.

- management costs for keeping the area surrounding the machines clean;

- additional equipment to clean remaining oil from the ground gears ;

- maintenance costs for replacing the lubricating oil;

- disposal costs of used lubricating oil, mixed with metal sha ings ;

- protection and health care costs for the operators from contacting and inhaling lubricating oil.

In order to avoid the use of lubricating oil, dry nobbing processes of hardened gears are known. However, the surface quality obtained by these processes is not sufficient for automotive transmission applications. Moreover, also dry grinding processes with extremely long production times are known ( for example such processes are disclosed in "Innovative Zahnradfertigung", ed. Expert Verlag, 1986, ISBN- 13 : 978-3-8169-1871-4) but these processes are not suitable for the production of automotive transmission gears with very short machining imes ( few seconds) .

DISCLOSURE OF INVENTI_ON

The aim of the presen invention is the realization in competitive times (in a few seconds) of hardened gears having a high surface finish by means of dry finishing processes (namely wi hout lubricating oil ) . In particular, the aim of the present invention is to provide a method for grinding hardened gears without using lubricating oil and in competitive times .

The aim of the present invention is to provide a method for machining hardened gears according to Claim 1 and to the following Claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The pr sent invention will be now described with a. reference to the alleged drawings, showing examples of non- limiting embodiments , wherein :

-- figure 1 is a plan view of a hardened gear during a first machining step;

- figure 2 is similar to figure 1 and shows the hardened gear i a second machining step;

- figure 3 is similar to figure 1 and shows an alternative embodiment according to the present method; - figure 4 is a sketch of a gear tooth of an hardened gear before machining:

figure 5 and 6 are enlarged sketches of areas la and Ila of figure 1 and, respectively, f igure 2;

- figure 7 and 8 are schematics views of a hardened gear during a first and, respectively, second machining steps by means of disk-shaped tools; and

- figures 9 and. 10 are, enlarged sketches of areas lb and lib of figure 7 and, respectively, figure 8.

BEST MODE FOR CARRYING OUT THE INVENTION

In figures 1 and 2, 1 indicates as a whole a machine tool for finishing gears, which comprises a workpiece supporting table ^' 2 , resting in a known and schematically illustrated way on a supporting plane PI , a workpiece sup- porting spindle 3, having a rotational, axis C mounted (in a known and schematically illustrated way) on the workpiece supporting table 2, and a cutting unit 4. The axis C is substantially perpendicular to the plane PI .

The cutting unit 4 comprises a base 5 having a guide 6, a slide 7, which in turn comprises a guideway 8 slidably mounted ( in a known and schematically illustrated way) inside the guide 6, and an operating device 14 (of a known and schematically illustrated type) which can read the position of slide 7 along the guide 6. As illustrated, the slide 7 is slidably mounted on a plane P2 parallel to the axis C of the workpiece supporting spindle 3, and substantially perpendicular to the supporting piane PI . The slide 7 comprises two supporting elements 15, 16 which are sub- starit iai y perpendicular o the plane P2.

The cutting unit 4 comprises a shaft 9, which has a rotational axis B and is mounted with its ends on the supporting elements 15 , 16 ( in a. known and schematically i.1 - lustrated way) . The shaft 9 is rotatable about the axis B .

The cutting unit 4 comprises a motor 10 (of a known and schematically illustrated type) which can rotate the shaft 9 about the axis B. Preferably, the axis B is parallel to the plane P2.

The cutting unit 4 comprises a cutting tool 11 with defined cutting edges and a cutting tool 12 with non defined cutting edges which are both fitted on the shaft 9 and are mutually spaced along the axis B.

Cutting tool 11 with defined cutting edges is of a known type and comprises a plurality of cutting elements having preset profiles with known cutting angle , upper clearance angle and lower rake angles. This kind of tool is particularly suitable for removing large and unevenly distributed stock and the chips taken are capable of removing most of the process heat. Therefore these tools are well suited for dry cutting operations .

Cutting tool 12 with non defined cut ing edges is of a known type and comprises a plurality of cutting elements which have undefined shapes and distribution (generally having a negative rake, angle) . This kind of tools carry out abrasive processes for the surface finishing of the machined product. Taking chips with cutting tool 12 with non defined cutting edges is based on plastic deformation and frict ion between cutting tool 12 and gear I to be machined and therefore generates high amounts of heat depending on the stock amount removed.

Both cutting tools, i.e. cutting tool 11 with defined cutting edges and cutting tool 12 with non d.efined cu ting edges, can be cylindrical worms or can be of disk-type shape.

Furthermore, the machine 1 comprises a control unit 13 which is coupled (in a known and schematically illustrated way) with: the workpiece supporting spindle 3 , the operating device 14 of the slide 7, and the motor 10. The control unit. 13 adjusts the. translation of the slide 7 on the plane P2 , the rotation of the shaft 3 and the rotation of the 'workpiece supporting spindle 3 in order to synchronize and engage, in use , a gear I fitted on the workpiece supporting spindle 3 with the cutting tool 11 with defined cutting edges and, subsequently, with the cutting tool 12 with non defined edges (as better explained hereinafter) .

In figures 1 and 2 , 11a indicates a cutting tool with defined cutting edges and, respectively, 12a indicates a cutting tool with non defined cutting edges which are cylindrical worm tools. For example, cutting tool 11a is a hob and cutting tool 12a. is a threaded grinding wheel.

As shown in figures 5 and 6, the cylindrical worm tools 11a and 12a have straight profiles. Therefore , the gear involute is generated by an additional cinematic which is a continuous rotary rolling motion of the gear I to be machined synchronized to the rotation of each cutting tool 11a and 12a. Due to the absence of the non-productive indexing in respect of the machining with cutting tools lib and 12b of disk-type shape (as will be seen more in detail below) , cutting tools 11a and 12a increase, advantageously, the productivity and therefore justify the additional complexity of the rolling motion in mass production. In particular, cyli drical, worm tools 11a and 12a are advanta- geously for the mass production of small gears with module range m comprised between 1.00 and 3.50 [mm] .

In figure 7, lib indicates a cutting tool of disk-type shape and with defined cutting edges. I figure 8 , 12b indicates a cutting tool of disk-type shape and with non de - fined cutting edges.

For exam le , cutting tool lib is a milling cutter and cutting tool 12b is a grinding disk .

As can be seen in figures 3 and 10, cutting tools lib and 12b of disk-type shape have the exact profile of the tooth space of the gear I to be machined and operate on a tooth-by--tooth basis.

Because of the absence of the additional cinematic (i.e. the continuous rotary rolling motion of the gear I to be machined synchronized to the rotation of each tool 11a and 12a) the disk-type tools lib and 12b require a more simple cinematic but need additional non-productive time to index the cutting tools lib and 12b from tooth to tooth. Therefore disk-type cutting tools lib and 12b are advanta - geously dedicated to large machines and the production of large gears. Figure 3 shows an alternative embodiment 101 of the machine illustrated in figure 1 in a first working configuration. Components equal to those of the machine 1 maintain the same numbering in the hundreds .

According to the embodiment illustrated in figure 3, the cutting unit 104 comprises two slides 107a and 107b, which are slidably mounted (in a known and schematically illustrated way) on the base 105. Each slide 107a (107b) comprises, in. turn, a guideway 108a (108b) which is slidably mounted inside a guide 106 of the base 105 , and an operating device 114a (114b) which can read the position of the slide 107a (107b) along the guide 106. The slides 107a and 107b are driven, independently from each other, along the guide 106.

The cutting unit 104 comprises a shaft 109a, which has a rotational axis B and is mounted with its ends ( in a known and schematically illustrated way) on the supporting elements 115a and 116a. The shaft 109a is rotatable about the axis B. The cutting unit 104 comprises a motor 1.10a (of a known and schematically illustrated type) which can rotate the shaft 109a about the axis B. Preferably, the axis B is parallel to the plane P2. The cutting unit 104 comprises a cutting tool 111 with def ined cutting edges which is fitted around the shaft 109a.

The cutting unit 104 comprises a shaft 109b, which has a rotational axis B and is mounted with its ends (in a known and schematically illustrated way) on the supporting elements 115b and 116b . The shaft 109b is rotatable about the axis B . The cutting unit 104 comprises a motor 110b (of a known and schematically illustrated type) which can rc- tate the shaft 109b about the axis B. Preferably, the axis B is parallel to the plane P2. The cutting unit 104 comprises a cutting tool 112 with non defined cutting edges which is fitted around the shaft 109b .

In use, a hardened gear I is fitted on the workpiece supporting spindle 3 ( 103 ) . Preferably, the gear I has a surface hardness higher than 54 [HRC] and a module m com- prised between 1.00 and 3.50 [mm] .

Moreover, the gear I has a total stock q (as illustrated in figure 4) to be removed, normal to the gear flank f . The total stock q is more than or equal to 0.03xm [mm] (q > 0.03xm [mm]) for a complete levelling of ail itnperfec- tions of the gear geometry f om the machining of soft metal "green machining" and due to distortion during hardening.

The method comprises a starting dry removal step (without lubricating oil) of an initial stock q (as illustrated in figure 5 or 9 ) of gear I by means of a cutting tool 11 (111) with defined cutting edges, and a subsequent dry removal step (without lubricating oil) of a remaining stock q ₂ (as illustrated in figure 6 or 10 ) of gear I by means of a cutting tool 12 ( 112 ) with non defined cutting edges .

The starting removal step substantially corrects the geometric imperfections of the gear flank f (macro- geometry) and removes almost all the total stock q. During the starting removal step, the gear I engages the cutting tool 11 (111) with defined cutting edges . The use of a cut- ting tool 11 (111) with defined cutting edges allows the removal of a remarkable amount of unevenly distributed stock . The use of a cutting tool 11 ( 111) with defined cut - ting edges is advantageous for the starting removal step, because the distribution of the total stock q is not known at the beginning. Furthermore, the cutting tool 11 ( 111 ) with defined cutting edges allows the easy removal of possible hardened burrs protruding from the edges of the hardened gear I .

The subsequent removal step corrects the micro- geometric surface imperfections. During the subsequent removal, the gear I engages the cutting tool 12 (112) with non defined cutting edges. The risk of thermal damage in processes by means of cutting tools 12 (112) with non defined cutting edges (grinding) is very much depending on the amount of remaining stock q ₂ .

For an economic production, the total amount of mate - rial removed (Volume V _w ) and the time used for this removal (cutting time t _c ) are important.

The capability of cutting and grinding processes are typically described by the specific volume V _w and the specific removal rate Q _w .

The specific volume V _wi [mm ³ /mm] of the removed material is defined by the relationship:

cos(E) wherein,

- i is an index which is 1 for the starting removal step and 2 for the subsequent removal step (figure 4) ;

- qi is the stock to be removed [mm]

- z is the number of gear teeth;

- b is the face width of the gear flank, f [mm] ;

- β ίβ the helix angle of the gear [deg] .

Parameters z, b and β are defined by the geometry of the gear I to be worked.

The cutting time t _ci [s] to remove specific volume V _wi is defined by the relationship:

(b + A _zi )

fzi

wherein ,

- A _zi is the process related to extra travel, which is function of the tool geometry, in particular tool diameter and numbers of starts [mm] ,- and

- f _zi is the feed rate which is function of the technology and the tool geometry, in particular tool diameter and numbers of starts [mm/min] .

Owing to the above , t _cl and t _c2 can be adjusted by the process data, in particular the cutting speed, the feed rate , the tool diameter and the number of starts ( for cylindrical worm tools) .

Specific volume V _wi and cutting time t _ci can be combined to a specific removal rate Q _wi [mm ³ / (mmxs) ] which defines the productivity of the process steps according to the relationship:

V ^■

in other words : q, x z x b

COS β X t _ci

The overall productivity defined by the specific removal rate Q _w of the starting dry removal step and the subsequent dry removal step is as follows:

Qw = Q _wl + Q _w2 __ + __

tcl ^L c2 in other words

9i * zxb q ₂ zxb _ zxb fq ₁ X f _zl q ₂ x f _z2

cos β x t _cl cos β x t _c2 cosβ K b + A _zl b + A

To be competitive with current gear hard finishing using lubricating oil a specific removal rate of at least Q _w ≥ 2.5 [mm ³ / (mmxs)] must be achieved. Owing to the above , the combined specific removal rate Q _w (the productivity) can only be achieved by an optimized combination of qi/t _cl for the starting dry removal step and q ₂ /t _c2 for the subsequent dry removal step.

Especially, during the subsequent dry removal step with nori defined cutting edges (for example grinding) the risk of thermal damage of the ground surface is ve y much depending on the amount of the remaining stock q ₂ . Therefore it is advantageous, to keep the remaining stock q ₂ as small as possible.

Advantageously after the starting removal step, the remaining stock q ₂ is less than or equal to 0.01 X m [mm] (namely q ₂ < 0.01 x m [mm]) and the initial stock q of the initial removal step is more than 0.02xm [mm] (namely q _x > 0.02xm [mm] ) .

For example, to obtain the above mentioned advantages, the cutting tool 11 is a cylindrical worm tool 11a (111) and the process data of the cutting tool 11a (111) with defined cutting edge during the starting dry removal step comprise a cutting speed v _cl more than or equal to 70 [m/min] ; in particular, the cutting speed v _cl is less or equal to 250 [m/min] (namely 70< v _cl <250 [m/min] ) . Advantageously, the cutting tool 11a ( 111) with defined cutti g edge comprises a tool diameter d _Q1 which is more than or equal to 50 [mm] and less or equal to 100 [mm] (namely 50<doi<100 [mm] ) . Advantageously , the cutting tool 11a (111) with defined cutting edge comprises a number of starts more than or equal to 1 and less or equal to 5 (namely l<n _sl <5) .

For example , to obtain the above mentioned advantages, the cutting tool 12 is a cylindrical worm 12a (112 ) and the process data of the cutting tool 12a (112) with non defined cutting edge during the. subsequent dry removal step comprise a cutting speed v _c2 more than or equal to 30 [m/s] ; in particular, the cutting speed v _c2 is less or equal to 100 [m/s] (namely 30< v _c2 <100 [m/s]). Advantageousl , the cutting ool 12a (112) with non defined cutting edge comprises a tool diameter d ₀₂ which is more than or equal to 100 [mm] and less or equal to 320 [mm] (namely 100< d ₀ <320 [mm] ) . Advantageously, the cutting tool 12a (112) with non defined cutting edge comprises a number of starts more than or equal to 1 and less or equal to 7 (namely l<n _S2 7 ) .

According to the aforesaid method, the starting stock qi removed by means of a cutting tool 11 (111) with defined cutting edges is in percentage the larger portion of the total stock q to be removed . Cutting by means of a cutting tool 11 (111) with defined cutting edges allows the correction of geometric imperfections and. the quick removal of most of the stock q.

Therefore, the subsequent removal step by means of the cutting tool 12 (112) with non defined cutting edges takes place on a gear I having an extremely small remaining stock q ₂ . As a result the heat in the dry process of the cutting tool 12 ( 112 ) with non defined cutting edges is low enough so that the hardening is not ruined and the gear surface remains suitable for the required use .

Then, the cutting time of the cutting tool 12 (112) with non defined cutting edges is longer than the cutting time of the cutting tool 11 (111) with defined cutting edges, but is sufficient to comple e the whole machining process of the gear I in a competitive time (a few sec- onds) .

Since the remaining stock q ₂ is very small (q ₂ ≤0. Olxm [mm] ) , control sensors presently used in machine tools to determine the rotational position of the gear I in order to mesh it perfectly with the tool (112) are not able to de- tect such an amount of remaining stock q ₂ accurately enough to adjust the process accordingly. Therefore, the steps of the aforesaid process cannot be carried out on two separate machines , since the margin of error of known control systems is larger than the remaining stock q ₂ to be removed, thus making impossible the correct adjustment/ eshing of the cutting tool 12 (112) with non defined cutting edges.

Preparing on a same machine 1 ( 101 ) a cutting tool 11 (111) with defined cutting edges for the starting removal step and a cutting tool 12 (112 ) with non defined cutting edges for the subsequent removal step allows to overcome the problem related to the accuracy of control sensors for adjusting the grinding process , since the starting removal step (hobbing for removing q _x ) and the subsequent removal step (grinding for removing q ₂ ) are adjusted according to the total stock q and to the process parameters detected by the control unit 13 (113) .

Moreover, planning both machining steps on a same machine 1 (101) allows to reduce the machine preparation times related to the loading/unloading of gear I on the workpiece supporting spindle 3 (103).

Since the steps of the aforesaid method are dry (without lubricating oil) , the machine 1 (101) is completely free from all economic and environmental drawbacks deriving from the use of lubricating oil.