Method for finishing hardened gears

Method for finishing hardened gears comprising: a first dry removal step of a first stock amount by means of a first cutting tool with defined cutting edges; and a second dry removal step of a second stock amount by means of a second cutting tool with non defined cutting edges.

RELATED APPLICATION DATA

This application is the national stage entry of International Appl. No. PCT/IB2015/054604, filed Jun. 18, 2015, which claims priority to Italian Patent Application No. BO2014A000339, filed Jun. 18, 2014. All claims of priority to such applications are hereby made, and such applications are hereby incorporated in their entirety by reference.

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 geometry 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 ART

It is well 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 hobbing 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≥0.03×m [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 shavings;protection and health care costs for the operators from contacting and inhaling lubricating oil.

In order to avoid the use of lubricating oil, dry hobbing 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 times (few seconds).

DISCLOSURE OF INVENTION

The aim of the present 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 without 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 Claim1and to the following Claims.

BEST MODE FOR CARRYING OUT THE INVENTION

InFIGS. 1 and 2, 1indicates as a whole a machine tool for finishing gears, which comprises a workpiece supporting table2, resting in a known and schematically illustrated way on a supporting plane P1, a workpiece supporting spindle3, having a rotational axis C mounted (in a known and schematically illustrated way) on the workpiece supporting table2, and a cutting unit4. The axis C is substantially perpendicular to the plane P1.

The cutting unit4comprises a base5having a guide6, a slide7, which in turn comprises a guideway8slidably mounted (in a known and schematically illustrated way) inside the guide6, and an operating device14(of a known and schematically illustrated type) which can read the position of slide7along the guide6. As illustrated, the slide7is slidably mounted on a plane P2parallel to the axis C of the workpiece supporting spindle3, and substantially perpendicular to the supporting plane P1. The slide7comprises two supporting elements15,16which are substantially perpendicular to the plane P2.

The cutting unit4comprises a shaft9, which has a rotational axis B and is mounted with its ends on the supporting elements15,16(in a known and schematically illustrated way). The shaft9is rotatable about the axis B.

The cutting unit4comprises a motor10(of a known and schematically illustrated type) which can rotate the shaft9about the axis B. Preferably, the axis B is parallel to the plane P2.

The cutting unit4comprises a cutting tool11with defined cutting edges and a cutting tool12with non defined cutting edges which are both fitted on the shaft9and are mutually spaced along the axis B.

Cutting tool11with 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 tool12with non defined cutting 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 tool12with non defined cutting edges is based on plastic deformation and friction between cutting tool12and gear I to be machined and therefore generates high amounts of heat depending on the stock amount removed.

Both cutting tools, i.e. cutting tool11with defined cutting edges and cutting tool12with non defined cutting edges, can be cylindrical worms or can be of disk-type shape.

Furthermore, the machine1comprises a control unit13which is coupled (in a known and schematically illustrated way) with: the workpiece supporting spindle3, the operating device14of the slide7, and the motor10. The control unit13adjusts the translation of the slide7on the plane P2, the rotation of the shaft9and the rotation of the workpiece supporting spindle3in order to synchronize and engage, in use, a gear I fitted on the workpiece supporting spindle3with the cutting tool11with defined cutting edges and, subsequently, with the cutting tool12with non defined edges (as better explained hereinafter).

InFIGS. 1 and 2, 11aindicates a cutting tool with defined cutting edges and, respectively,12aindicates a cutting tool with non defined cutting edges which are cylindrical worm tools. For example, cutting tool11ais a hob and cutting tool12ais a threaded grinding wheel.

As shown inFIGS. 5 and 6, the cylindrical worm tools11aand12ahave 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 tool11aand12a. Due to the absence of the non-productive indexing in respect of the machining with cutting tools11band12bof disk-type shape (as will be seen more in detail below), cutting tools11aand12aincrease, advantageously, the productivity and therefore justify the additional complexity of the rolling motion in mass production. In particular, cylindrical worm tools11aand12aare advantageously for the mass production of small gears with module range m comprised between 1.00 and 3.50 [mm].

InFIG. 7, 11bindicates a cutting tool of disk-type shape and with defined cutting edges. InFIG. 8, 12bindicates a cutting tool of disk-type shape and with non defined cutting edges.

For example, cutting tool11bis a milling cutter and cutting tool12bis a grinding disk.

As can be seen inFIGS. 9 and 10, cutting tools11band12bof 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 tool11aand12a) the disk-type tools11band12brequire a more simple cinematic but need additional non-productive time to index the cutting tools11band12bfrom tooth to tooth. Therefore disk-type cutting tools11band12bare advantageously dedicated to large machines and the production of large gears.FIG. 3shows an alternative embodiment 101 of the machine illustrated inFIG. 1in a first working configuration. Components equal to those of the machine1maintain the same numbering in the hundreds.

According to the embodiment illustrated inFIG. 3, the cutting unit104comprises two slides107aand107b, which are slidably mounted (in a known and schematically illustrated way) on the base105. Each slide107a(107b) comprises, in turn, a guideway108a(108b) which is slidably mounted inside a guide106of the base105, and an operating device114a(114b) which can read the position of the slide107a(107b) along the guide106. The slides107aand107bare driven, independently from each other, along the guide106.

The cutting unit104comprises a shaft109a, which has a rotational axis B and is mounted with its ends (in a known and schematically illustrated way) on the supporting elements115aand116a. The shaft109ais rotatable about the axis B. The cutting unit104comprises a motor110a(of a known and schematically illustrated type) which can rotate the shaft109aabout the axis B. Preferably, the axis B is parallel to the plane P2. The cutting unit104comprises a cutting tool111with defined cutting edges which is fitted around the shaft109a.

The cutting unit104comprises a shaft109b, which has a rotational axis B and is mounted with its ends (in a known and schematically illustrated way) on the supporting elements115band116b. The shaft109bis rotatable about the axis B. The cutting unit104comprises a motor110b(of a known and schematically illustrated type) which can rotate the shaft109babout the axis B. Preferably, the axis B is parallel to the plane P2. The cutting unit104comprises a cutting tool112with non defined cutting edges which is fitted around the shaft109b.

In use, a hardened gear I is fitted on the workpiece supporting spindle3(103). Preferably, the gear I has a surface hardness higher than 54 [HRC] and a module m comprised between 1.00 and 3.50 [mm].

Moreover, the gear I has a total stock q (as illustrated inFIG. 4) to be removed, normal to the gear flank f. The total stock q is more than or equal to 0.03×m [mm] (q≥0.03×m [mm]) 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.

The method comprises a starting dry removal step (without lubricating oil) of an initial stock q1(as illustrated inFIG. 5 or 9) of gear I by means of a cutting tool11(111) with defined cutting edges, and a subsequent dry removal step (without lubricating oil) of a remaining stock q2(as illustrated inFIG. 6 or 10) of gear I by means of a cutting tool12(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 tool11(111) with defined cutting edges. The use of a cutting tool11(111) with defined cutting edges allows the removal of a remarkable amount of unevenly distributed stock. The use of a cutting tool11(111) with defined cutting 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 tool11(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 microgeometric surface imperfections. During the subsequent removal, the gear I engages the cutting tool12(112) with non defined cutting edges. The risk of thermal damage in processes by means of cutting tools12(112) with non defined cutting edges (grinding) is very much depending on the amount of remaining stock q2.

For an economic production, the total amount of material removed (Volume Vw) and the time used for this removal (cutting time tc) are important.

The capability of cutting and grinding processes are typically described by the specific volume Vwand the specific removal rate Qw.

The specific volume Vwi[mm3/mm] of the removed material is defined by the relationship:

wherein,i is an index which is 1 for the starting removal step and 2 for the subsequent removal step (FIG. 4);qiis 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 is 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 tci[s] to remove specific volume Vwiis defined by the relationship:

wherein,Δziis the process related to extra travel, which is function of the tool geometry, in particular tool diameter and numbers of starts [mm]; andfziis 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, tc1and tc2can 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 Vwiand cutting time tcican be combined to a specific removal rate Qwi±[mm3/(mm×s)] which defines the productivity of the process steps according to the relationship:

in other words:

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

in other words:

To be competitive with current gear hard finishing using lubricating oil a specific removal rate of at least Qw≥2.5 [mm3/(mm×s)] must be achieved.

Owing to the above, the combined specific removal rate Qw(the productivity) can only be achieved by an optimized combination of q1/tc1for the starting dry removal step and q2/tc2for the subsequent dry removal step.

Especially, during the subsequent dry removal step with non defined cutting edges (for example grinding) the risk of thermal damage of the ground surface is very much depending on the amount of the remaining stock q2. Therefore it is advantageous, to keep the remaining stock q2as small as possible.

Advantageously after the starting removal step, the remaining stock q2is less than or equal to 0.01×m [mm](namely q2≤0.01×m [mm]) and the initial stock q1of the initial removal step is more than 0.02×m [mm] (namely q1>0.02×m [mm]).

For example, to obtain the above mentioned advantages, the cutting tool11is a cylindrical worm tool11a(111) and the process data of the cutting tool11a(111) with defined cutting edge during the starting dry removal step comprise a cutting speed vc1more than or equal to 70 [m/min]; in particular, the cutting speed vc1is less or equal to 250 [m/min] (namely 70≤νc1≤250 [m/min]). Advantageously, the cutting tool11a(111) with defined cutting edge comprises a tool diameter d01which is more than or equal to 50 [mm] and less or equal to 100 [mm] (namely 50≤d01≤100 [mm]). Advantageously, the cutting tool11a(111) with defined cutting edge comprises a number of starts more than or equal to 1 and less or equal to 5 (namely 1≤ns1≤5).

For example, to obtain the above mentioned advantages, the cutting tool12is a cylindrical worm12a(112) and the process data of the cutting tool12a(112) with non defined cutting edge during the subsequent dry removal step comprise a cutting speed vc2more than or equal to 30 [m/s]; in particular, the cutting speed vc2is less or equal to 100 [m/s] (namely 30≤νc2≤100 [m/s]). Advantageously, the cutting tool12a(112) with non defined cutting edge comprises a tool diameter d02which is more than or equal to 100 [mm] and less or equal to 320 [mm] (namely 100≤d02×320 [mm]). Advantageously, the cutting tool12a(112) with non defined cutting edge comprises a number of starts more than or equal to 1 and less or equal to 7 (namely 1≤ns2≤7).

According to the aforesaid method, the starting stock q1removed by means of a cutting tool11(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 tool11(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 tool12(112) with non defined cutting edges takes place on a gear I having an extremely small remaining stock q2. As a result the heat in the dry process of the cutting tool12(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 tool12(112) with non defined cutting edges is longer than the cutting time of the cutting tool11(111) with defined cutting edges, but is sufficient to complete the whole machining process of the gear I in a competitive time (a few seconds).

Since the remaining stock q2is very small (q2≤0.01×m [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 detect such an amount of remaining stock q2accurately 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 q2to be removed, thus making impossible the correct adjustment/meshing of the cutting tool12(112) with non defined cutting edges.

Preparing on a same machine1(101) a cutting tool11(111) with defined cutting edges for the starting removal step and a cutting tool12(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 q1) and the subsequent removal step (grinding for removing q2) are adjusted according to the total stock q and to the process parameters detected by the control unit13(113).

Moreover, planning both machining steps on a same machine1(101) allows to reduce the machine preparation times related to the loading/unloading of gear I on the workpiece supporting spindle3(103).

Since the steps of the aforesaid method are dry (without lubricating oil), the machine1(101) is completely free from all economic and environmental drawbacks deriving from the use of lubricating oil.