Turning – Process of turning
Reexamination Certificate
2000-07-25
2002-10-29
Tsai, Henry (Department: 3722)
Turning
Process of turning
C082S047000, C082S106000, C409S132000
Reexamination Certificate
active
06470775
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of Use
The present invention concerns a method of machining rotationally symmetrical parts to put them into the condition in which they are capable of use, wherein the parts in the course of the manufacturing procedure are at least partially subjected to a hardening procedure. In particulars this concerns crankshafts, in particular the bearing surfaces (both of the big-end bearings and also the fitting bearings, the sealing bearings and central bearings) of crankshafts, in regard to which the condition in which they are capable of use is that condition in which the crankshaft can be fitted in the engine without further removal of material at the bearing surfaces. In that respect bearing surfaces is used to denote both the peripheral surfaces, that is to say the bearing diameter, and also the surfaces which are referred to as the mirror or thrust surfaces, that is to say the end faces which adjoin the peripheral surfaces generally in perpendicular relationship and which serve for example for axial thrust support. In this respects particularly in the case of steel crankshafts, the bearing surfaces are hardened in the regions thereof which are in the proximity of their surface. That serves both for increased resistance to abrasive wear of the bearing locations, and also serves to protect those bearing surfaces from damage in the course of handling throughout the entire manufacturing procedure as well as serving to influence the strength properties involved.
II. Technical Background
Crankshafts, in particular the crankshafts for private motor vehicles which have a large number of cylinders, are known to be workpieces which are unstable during machining and thus difficult to machine. Assessment of the dimensional accuracy of a finished crankshaft is effected primarily, besides the axial bearing width, by assessment of the following parameters:
Diameter deviation, which is deviation from the predetermined reference or target diameter of the bearing journal,
roundness, which is macroscopic deviation from the round reference or target contour of the bearing journal,
concentricity, which is a diameter deviation in the case of a rotating workpiece, that is to say, for example, the deviation from the reference positional contour which a main bearing location effects during the rotary movement of the crankshaft by virtues on the one hands of the non-ideal roundness of that main bearing journal and, on the other hand, of flexing of the crankshaft which in that case is clamped only at its ends,
roughness R
a
, which is a value which is ascertained by calculation and which represents the microscopic roughness of the surface of the bearing location, and
percentage contact area, which is the load-bearing surface proportion of the surface structure, considered microscopically, which comes into contact with a co-operating or counterpart surface pressed thereon.
In addition, in regard to the big-end bearing locations, there is an assessment of:
stroke deviation, which is the dimensional, percentage deviation of the actual stroke (spacing of the actual center of the big-end bearing journal from the actual center of the main bearings), from the reference or target stroke, and
angle deviation, which is the deviation of the actual angular position of the big-end bearing journal from its reference or target angular position relative to the main bearing axis and with respect to the angular position in relation to the other big-end bearing journals, the angle deviation being specified in degrees or as a length dimension in the peripheral direction, related to the stroke.
Observing the desired tolerances in regard to those parameters is made difficult less due to the available machining methods than the instability of the workpiece and the machining forces involved. The efficiency and economy of the method also play a large part in a practical context.
Hitherto the removal of material from the bearing locations on the crankshaft in its original form, that is to say as cast or forged, was effected in succession in four machining steps:
First Step:
Cutting machining with a given cutting edge; this involved using the processes of turning, rotary broaching, turning-rotary broaching, internal round milling and external milling, rotary milling, in particular in the form of high-speed milling or combinations of such procedures. The magnitude of the material to be removed was in the millimeter range.
Second Step:
Hardening of the desired regions, in particular the bearing surfaces, wherein heating and subsequent cooling of the crankshaft and the change in the material and configurational structure, in particular due to the irregular way in which those procedures take place, causes distortion and dimensional variation of the crankshaft or one of the regions thereof, and that has to be compensated by the subsequent operations of removing material, in consideration of the reference or target dimensions of the final condition.
Third Step:
Grinding by means of a hard, massive grinding tool, for example a grinding wheel, which generally rotates with its axis of rotation in parallel relationship with the axis of rotation of the crankshaft to be machined; the amount of material to be removed was in the tenths-of-millimeter range.
In the case of crankshafts which are difficult to machine, in particular crankshafts which are long and thus highly unstable, the grinding machining operation was also effected in a multi-stage procedure, for example in a two-stage procedure by preliminary and finishing grinding.
Fourth Step:
Finishing by generally a stationary grinding means (grinding belt or grinding stone) which is pressed against the external periphery of the rotating bearing location; the amount of material to be removed is at the present time in the range of hundredths of a millimeter or even mm.
In that respect, a distinction is also to be drawn in regard to the machining operation, in respect of the material of the crankshaft (steel or cast iron), in which connection in particular steel crankshafts which are preferably used for situations of use involving a high loading are hardened after the cutting machining operation.
In order to reduce the costs involved in crankshaft machining, the endeavour is to reduce the machining of the bearing locations from four to three different machining stages.
This means however when the cutting machining operation is omitted that in particular the removal of material which is to be implemented by the grinding operation must be greater than in the case of the four-stage method.
Removing material by means of grinding however involves the following disadvantages:
because of the cooling/lubricating agent which is to be added, the grinding slurry which is produced gives rise to problems and is extremely costly to dispose of,
because of the oil contained in the cooling/lubricating agent, for example in the case of CBN-grinding, there is always a latent risk of explosion,
in the grinding operation the amount of cooling/lubricating agent used is substantially greater than in the case of cutting machining procedures as the cooling/lubricating agent is additionally employed in order to remove the grinding dust and swarf from the surface of the grinding wheel again, by jetting the cooling/lubricating agent on to same under high pressure,
nonetheless the danger of the workpiece suffering from overheating is very high,
the machining pressures acting on the workpiece are higher than in the case of cutting machining, and
a microscopic surface structure is produced, in which the grain boundaries which are torn open by the grinding grain are smeared closed again by the subsequent grinding grains, with removed workpiece material, that is to say this is a surface structure with relatively few steep peaks, but with more or less flat, bent-over peaks which partly overlap in scale-like relationship.
SUMMARY OF THE INVENTION
a) Technical Object
A primary object of the invention is to simplify the removal of material when machining bearing locations on a crankshaft to be
Kohlhase Matthias
Santorius Rolf
Boehringer Werkzeugmaschinen GmbH
Tsai Henry
LandOfFree
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