Metal working – Method of mechanical manufacture – Prime mover or fluid pump making
Reexamination Certificate
1999-12-28
2004-06-01
Jordan, Charles T. (Department: 3726)
Metal working
Method of mechanical manufacture
Prime mover or fluid pump making
C029S558000
Reexamination Certificate
active
06742252
ABSTRACT:
I. FIELD OF USE
The invention concerns a method of machining rotationally symmetrical parts, in particular crankshafts, in particular the bearing surfaces (both of the big-end bearings and also the central or main bearings) of crankshafts to put them into the condition in which they are capable of use, that is to say the condition in which the crankshaft can be fitted in the engine without further removal of material at the bearing surfaces.
In a practical context, the percentage contact area is ascertained by a procedure which comprises pressing against the surface to be determined, a counterpart surface of ideal shape, that is to say when dealing with flat surfaces, an ideally flat surface or, in the present case, when dealing with external round surface, a concave counterpart surface which ideally corresponds to a circular arc, under a given nominal loading, for example 0.1 N/mm
2
. By virtue of that nominal loading, the microscopic raised portions of the profile which without a loading would only bear against the counterpart surface with their tips and thus with a surface proportion of tending closely towards 0 are pressed somewhat flat so that the contacting surface proportion rises with respect to the total surface area and in practice can be satisfactorily ascertained by dyeing or tinting and so forth. At the given nominal loading mentioned, the percentage contact area is between 20% and 40% at a transfer between cutting machining and finishing operations. This same percentage contact area is less than 50% of the percentage contact area occurring after the finishing operation.
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=deviation from the predetermined reference or target diameter of the bearing journal,
roundness=macroscopic deviation from the round reference or target contour of the bearing journal,
concentricity=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 virtue on the one hand of the non-ideal roundness of that main bearing Journal and on the other hand by virtue of the out-of-center journal of the crankshaft which in that case is supported only at its ends,
roughness R
a
=a value which is ascertained by calculation and which represents the microscopic roughness of the surface of the bearing location, and
percentage contact area=the load-bearing surface proportion of the surface structure, considered microscopically, which comes into contact with a co-operating or counterpart surface pressed thereon, and in addition, in regard to the big-end bearing locations,
stroke deviation=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=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 longitudinal dimension in the peripheral direction, related to the stroke,
wherein 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 three 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:
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.
Third 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 &mgr;m.
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 at the surfaces of the bearing locations, after the cutting machining operation. That gives rise to renewed distortion of the crankshaft, and such distortion had to be compensated by grinding and finishing. Hardening of cast iron crankshafts is at the present time already omitted in many cases and can be completely avoided by using a cast iron material of relatively great hardness, for example GGG 60 or 70 or more and improved strength values.
In order to reduce the costs involved in crankshaft machining, the endeavour is to reduce the machining of the bearing locations from three to two different machining stages.
This means however that in particular the removal of material which is to be implemented by the grinding operation must be greater than in the case of a three-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.
III. SUMMARY OF THE INVENTION
a) Technical Object
Therefore the object of the invention is to simplify the removal of material when machining bearing locations on a crankshaft.
b) Attainment of the Object
That object is attained by the characterising features of
Kohlhase Matthias
Santorius Rolf
Boehringer Werkzeugmaschinen GmbH
Crowell & Moring LLP
Jordan Charles T.
Nguyen Trinh
LandOfFree
Crankshaft machining and finishing does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Crankshaft machining and finishing, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Crankshaft machining and finishing will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3353535