PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION (See Section 10 and Rule 13)
A METHOD OF MILLING A NON-ROTATING CRANKSHAFT
Applicant: BHARAT FORGE LIMITED
An Indian Company of
Mundhwa, Pune -411036,
Maharashtra, India.
THE FOLLOWING SPECIFICATION DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.

FIELD OF THE INVENTION
The present invention relates to a method for machining of eccentric work piece surface, especially a Crankshaft.
More particularly, the present invention relates to methods for machining of a Crankshaft’s fixed or non-rotating crank Pin using an elliptical orbital machining/milling method.
BACKGROUND OF THE INVENTION
Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
The crankshaft is a vital structural component of the internal combustion engine. It consists of cylindrical shafts i.e. main journals and pins, which convert reciprocating motion into rotary motion. To achieve desired conversion efficiently, narrow tolerance related to deviations in shape, position and dimensions is required on pins and main journals. The narrow tolerances are achieved on the forged/cast crankshafts by performing series of roughing and finish machining operations like turning, milling, grinding and super finishing.
Roughing operations like eccentric turning, chasing external milling and rotary or circular orbital internal milling are generally used to remove the excessive material (machining allowance) from crankshaft pins.
During the conventional eccentric turning method, crankshaft is mounted on a lathe/turning center by using special fixtures and then excessive material (machining allowance) from crankshaft pins are removed by a single-point cutting tool. The special fixtures are used to align the center/longitudinal axis of crankshaft pin with center axis of

the lathe/machine which ensures that the crankshaft rotates around the pin axis. Further, the special fixtures also counterbalance the eccentric crankshaft with suitable weight. The main disadvantage of this type of roughing operation is that the loading condition of the crankshaft on the lathe/turning center needs to be changed for every pin in order to align the axis of pin with lathe/machine center axis.
According to another known method i.e. chasing external milling method, crankshaft is mounted on a horizontal/vertical machining center where crankshaft pin, to be machined, is rotated about a given axis that is parallel to the axis of crankshaft pin. The excessive material (machining allowance) on crankshaft pins is removed using a cutter with inserts mounted on its external periphery. In this method, axis of the milling cutter is perpendicular to the axis of the crankshaft pins. The main disadvantage of this type of roughing operations is multiple numbers of passes are required to achieve circular cylindrical shape of pin which is time consuming.
Another known method is to machine the crankshaft pins by rotary or circular orbital internal milling method. In this method, the crankshaft is mounted on a special purpose milling center. Next, the crankshaft pin, to be machined, is fixed or rotated about an axis that is parallel to the axis of crankshaft pin. After this, the excessive material (machining allowance) on crankshaft pin is removed by a circular or disc type milling cutter with a number of inserts mounted on the circumference of its inner diameter which acts as a multipoint cutting tool. In this method, the axis of the milling cutter is parallel to the axis of the crankshaft pins. This circular or disc type milling cutter is capable of making heavy depth of cut during cutting. In Rotary or circular orbital internal milling operation, excess material (machining allowance) is removed in single pass of cutter to achieve circular

cylindrical shape of pin which takes less time as compared to other two methods, so rotary milling is more relevant and cost effective method for crank pin milling.
The main objective of the roughing machining operation is to ensure large depth of cut and provide uniform stock on diameter for subsequent finishing operation like grinding. Rotary or circular orbital internal milling is cost effective roughing method for crankshaft pin as compared to the eccentric turning and chasing external milling, as it allows to take heavy depth of cut and has lower cycle time as compared to these methods. Rotary or circular orbital internal milling or “CIM” generates circular cylindrical shape of the crankshaft pins by a circular orbital movement between the milling cutter and pin circumference.
CIM can be operated using one of the three different working principles. These working principles are differentiated according to the interpolation method used for definition of the movement path of the horizontal and vertical axis of the milling head. These three working principles are:
1. Movable or Rotating crankshaft pin (MCP) method;
2 .Fixed or non-rotating crankshaft pin method (FCP) and,
3. Rotating crankshaft pin and eccentric cutter method. The CIM method using the FCP principle used in prior art practice for machining of crankshaft pin involved use of a cutter which rotated along its center and simultaneously slides in vertical and horizontal directions perpendicular to its central axis (X and Y axis). During the movement of the cutter, the crankshaft pin is stationary which creates a circular orbital relative motion between crankshaft pin circumference and milling cutter.
Over the time, the CIM method using the FCP principle produces high material deformation and high cutting forces due to axis errors in

machine parts that is a result of large depth of cut and high contact area between cutter inserts and workpiece surfaces, which further leads to dimensional and geometrical inaccuracies on crankshaft pin and reduced productivity. These surface inaccuracies further leads to issues like unclean diameter, wheel wear, grinding burns, throw out etc. in the subsequent grinding stage.
The conventional method of CIM using FCP has following disadvantages.
1. Quality of milled crankshafts is not consistent and good.
2. Due to axis error in milling machine, the roundness profile of the crankshaft pins is not within spec in some of the milled crankshafts. Due to this roundness error, some of these parts get rejected/scrapped. Typically, due to roundness error, the pin diameter becomes elliptical in shape.
3. Due to the axis errors and roundness problems in milled crankshaft, the cutting parameters have to be modified or fine-tuned regularly which leads to lower productivity of milling operation.
4. The roundness error in milling output leads to non-uniform stock or machining allowance on diameter for subsequent grinding operation as the shape of pin becomes elliptical instead of circular.
5. The non-uniform machining allowance on the milled crankshaft leads to grinding issues like unclean diameter, uneven wheel wear, grinding burns, throw out and so on.
6. These issues lead to loss of productivity in grinding, increase in cost of process due to frequent change in grinding tool and inconsistent and bad quality products.

Accordingly, it is required to provide a method which can overcome the above stated drawbacks of rotary or circular orbital internal milling (CIM) method using the fixed or non-rotating crank pin (FCP) principle.
OBJECTS OF THE INVENTION Some of the objects of the present invention, which at least one embodiment herein satisfies, are now disclosed.
It is an object of the present invention to provide an improved elliptical orbital internal milling (EIM) method of the fixed or non-rotating crank pin (FCP) principle to ensure large depth of cut and uniform stock on diameter for subsequent grinding operation.
It is another object of the present invention to eliminate surface and geometrical inaccuracies on the crankshaft pin.
It is still another object of the present invention to produce the crankshaft pins with desired fit and tolerance.
It is yet another object of the present invention to provide a flexible interference to user that can handle family of the crankshaft pins with least input.
It is further object of the present invention to provide a flexible interference to user that can easily optimize the product specific parameters for achieving best quality.
Other objects and advantages of the present disclosure will be more apparent from the following description which is not intended to limit the scope of the present disclosure.
BRIEF DESCRIPTION OF DRAWINGS The accompanying drawings are included to provide further understanding of the present disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the

description, serve to explain the principles of the present disclosure. The drawings are for illustration only, which thus is not a limitation of the present disclosure.
FIG. 1 illustrates an exemplary flow diagram for a method of elliptical orbital internal milling (EIM) using the fixed or non-rotating crankshaft pin (FCP) principle, in accordance with an embodiment of the present disclosure.
FIG. 2 illustrates an exemplary block diagram of crankshaft loading for a method of elliptical orbital internal milling (EIM) using the fixed or non-rotating crankshaft pin (FCP) principle, in accordance with an embodiment of the present disclosure.
FIG. 3a and 3b illustrates an exemplary conceptual cycle of a method of elliptical orbital internal milling (EIM) using the fixed or non-rotating crankshaft pin (FCP) principle, in accordance with an embodiment of the present disclosure.
SUMMARY OF THE INVENTION
In accordance with the present invention, an EIM using the FCP principle is disclosed for producing specified form, fit and tolerance on a crankshaft pin. In particular, a mathematical formula for obtaining co-ordinates, in the directions perpendicular to cutter’s center axis, to generate tool path is provided which compensate the ovality produced by the inherent behavior of the machining center. EIM method ensures large depth of cut i.e. high material removal rate with low cost and uniform stock on pin diameter for subsequent grinding operation.
Further, invention provides flexible interference to user that can handle family of crankshaft pins with least input and easily optimize the product specific parameters for best quality.

In accordance with the presently claimed invention there is provided a method of milling a non-rotating crankshaft, said method comprising the following steps:
- placing a crankshaft (201) having a first end, second end, main journals (202) and crankshaft pins (203), on a milling machine adapted for machining of the crankshaft pins (203), wherein the milling machine comprises a milling head with an internal milling cutter (208) which surrounds the crankshaft (201),
- holding said crankshaft (201) between the machine centers (204) along the longitudinal direction (205), wherein angular position of crankshaft pin (203) is aligned by aligning jaws (209) and the first and second ends of crankshaft (201) are clamped by clamping chucks (206) mounted along the longitudinal axis (207) of the machine,
- circumference milling said crankshaft (201) using the milling machine, wherein said milling comprises positioning the milling cutter (208) longitudinally at a pin (203) location, rotating the cutter (208) at a pre-defined RPM along its center and simultaneously sliding in a direction perpendicular to its central axis (207) to create an elliptical orbital path or motion around the crankshaft pin (203) circumference and perform pin circumference milling; and
- unloading the crankshaft (201) from the machine,
wherein, the rotation and simultaneous sliding of the cutter (208) in particular direction corresponds to co-ordinates obtained from a numerical control system.
DETAILED DESCRIPTION OF INVENTION The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments

are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
The CIM method using the FCP principle used in prior art practice for machining of crankshaft pin of a crankshaft involved the use of a cutter which rotated along its center and simultaneously slides in vertical and horizontal directions perpendicular to its central axis (X and Y axis). During the movement of the cutter, the crankshaft pin is stationary which creates a circular orbital motion between crankshaft pin circumference and milling cutter.
Over the time, the CIM method using the FCP principle produces high material deformation and high cutting forces due to axis errors in machine parts that is result of large depth of cut and high contact area between cutter inserts and workpiece surfaces, which further leads to dimensional and geometrical inaccuracies on crankshaft pin and reduced productivity. These surface inaccuracies further leads to issues like unclean diameter, wheel wear, grinding burns, throw out etc. in the subsequent grinding stage. The axis errors in the milling center leads to error in the roundness of the crankshaft pins. This error manifests itself in the form of elliptical shape of the crankshaft pins. This elliptical shaped pin may lead to rejection at milling stage or in the subsequent grinding stage.
One way to overcome this disadvantage of the conventional CIM using FCP would be to provide a method to compensate for these axis errors. This compensation can be provided in the milling tool motion through the numerical control of the machine. For this, the coordinates of the elliptical path has to be calculated accurately based on the axis

errors present in the machine and then provided to the NC of the machine. Currently, there is no provision in numerical control to provide compensation for axis errors and thus, improve the geometrical shape of the pin diameter caused by above errors
Accordingly, in order to eliminate the drawbacks of CIM operation, the present invention provides an elliptical orbital machining (EIM) method. In one embodiment, these tool paths are used for the milling/machining of crankshaft’s pins in order to offset or compensate the ovality produced by inherent behavior of the machining center.
In accordance with the presently claimed invention there is provided a method of milling a non-rotating crankshaft (refer figure 1 and 2), said method comprising the following steps:
a) placing a crankshaft (201) having a first end, second end,
main journals (202) and crankshaft pins (203), on a milling machine
adapted for machining of the crankshaft pins (203),
wherein the milling machine comprises a milling head with an internal milling cutter (208) which surrounds the crankshaft (201),
b) holding said crankshaft (201) between the machine centers
(204) along the longitudinal direction (205),
wherein angular position of crankshaft pin (203) is aligned by aligning jaws (209) and the first and second ends of crankshaft (201) are clamped by clamping chucks (206) mounted along the longitudinal axis (207) of the machine,
c) circumference milling said crankshaft (201) using the
milling machine,
wherein said milling comprises positioning the milling cutter (208) longitudinally at a pin (203) location, rotating the cutter (208) at a pre-defined RPM along its center and simultaneously sliding in a direction perpendicular to its central axis (207) to create an elliptical

orbital path or motion around the crankshaft pin (203) circumference and perform pin circumference milling; and
d) unloading the crankshaft (201) from the machine,
wherein, the rotation and simultaneous sliding of the cutter (208)
in particular direction corresponds to co-ordinates obtained from a
numerical control system. In one embodiment, the method
further comprises a step of moving the cutter (208) to the original position post milling the crankpin (203) followed by positioning on another pin location and performing circumference milling.
In one embodiment, the coordinates are obtained by numerical control system using a mathematical code corresponding to input parameters used in the following mathematical equation:

wherein,
Xn : milling cutter X axis coordinate;
Yn : milling cutter Y axis coordinate;
a : Major axis of Pin ellipse;
b : Minor axis of Pin ellipse;
T : Crank Throw;
R : Cutter radius;
r : Crank pin radius;
Ǿn : Pin angular position; and
βn : Cutter interaction angle.

In one embodiment, the cutter (208) rotates in a clockwise/ counter clockwise direction w.r.t the axis of the crankshaft.
In one embodiment, steady rests (210) of milling machine clamps on main journal (202) adjacent to the crankshaft pin to be machined.
In one embodiment, said method is characterized by a synchronized movement of the cutter (208) around crankshaft pin (203) circumference in directions perpendicular to longitudinal axis (x and y directions) which, creates an elliptical orbital path or motion around crankshaft pin (203) circumference.
In one embodiment, the pin circumference is completely milled after completion of one complete rotation of the cutter around the pin circumference.
FIG. 1 illustrates an exemplary flowchart 100 for an elliptical orbital internal milling (EIM) method using the fixed or non-rotating crankshaft pin (FCP) principle, in accordance with an embodiment of the present disclosure.
In one embodiment, the EIM method using the FCP principle comprises:
• Step 102: Parameters Input, is an input to numerical control system about the design size of crankshaft (i.e. longitudinal positions of crankshaft pins, throw pitch circle radius, pin angular position and crankshaft pin radius) and technical requirements (i.e. process parameters, major and minor axis of pin ellipse and angular shift of the high point);
• Step 104: Crankshaft Loading (200), is providing a crankshaft (201) consisting of main journals (202) and pins (203) and placing said crankshaft (201) on a machine adapted for

machining/milling of crankshafts pin(s) (203). The said crankshaft (201) is held between the machine centers (204) along the longitudinal direction (205), angular position of crankshaft pin is aligned by aligning jaws (209) and then both ends of crankshaft are clamped by clamping chucks (206) mounted along the longitudinal axis (207) of machine. The said machine comprises a milling head with an internal milling cutter (208) which surrounds the crankshaft (201) and is concentric to machine longitudinal axis (207) and steady rests (210) which provides rigidity during crankshaft pin machining/milling. The said internal milling cutter (208) slides in directions perpendicular to longitudinal axis, i.e. x and y directions. FIG. 2 illustrates an exemplary block diagram of crankshaft loading for a method of EIM using the FCP principle.
• Step 106: Data Computation, is calculations of cutter axis data, i.e. coordinates, by numerical control system using mathematical codes/equations in accordance to the input parameter in step 102. Step 106 is carried out on the basis of mathematical equations,

wherein,
Xn : milling cutter X axis coordinate;
Yn : milling cutter Y axis coordinate
a : Major axis of Pin ellipse;
b : Minor axis of Pin ellipse;
T : Crank Throw;

R : Cutter radius;
r : Crank pin radius;
Ǿn: Pin angular position
βn : Cutter interaction angle • Step 108: Circumference Milling/Machining, is removal of material from crankshaft pins (203) circumference. In this step, crankshaft pins (203) are stationary along the throw pitch circle diameter (212). Initially, milling cutter (208) is positioned longitudinally at pin location and steady rests (210) clamps on main journal adjacent to the crankshaft pin to be machined. Then cutter rotates at predefined RPM along its center and simultaneously slides in directions perpendicular to its central axis (207) as per step 106 outputs. The synchronized movement of the cutter (208) around crankshaft pin (203) circumference in directions perpendicular to longitudinal axis (207), i.e. x and y directions, creates an elliptical orbital path or motion around crankshaft pin (203) circumference. Pin (203) circumference is completely milled after completion of one complete rotation of cutter (208) around the pin (203) circumference. Then steady rest (210) unclamps, cutter (208) returns to center position (213) and move longitudinally to next pin (203) position along with steady rests (210). This cycle is repeated until all pins are milled. FIG. 3a is a diagrammatic representation of the manner in which cutter (208) and crankshaft pin (203) are initially positioned with respect to each other and Fig. 3b illustrates the manner in which the cutter (208) is displaced during pin circumference milling/machining by EIM using the FCP principle.

• Step 110: Crankshaft Unloading, is unloading of crankshaft (201) from machine. After unloading crankshaft pins (203) are inspected as per control plan.
In one embodiment, the crankshaft (201) is not rotating along its axis.
In accordance with the present invention, a mathematical formula for obtaining co-ordinates (X and Y coordinates) is developed and provided below which further helps in achieving effective EIM machining.

wherein,
Xn and Yn: Milling cutter X and Y axis coordinates respectively
a and b : major and minor axis of pin ellipse respectively
T: Throw pitch circle radius
R: cutter radius
r: crankshaft pin radius
0n: pin angular position
Pn: Cutter interaction angle In an aspect, mathematical formula for obtaining co-ordinates of EIM using the FCP principle provides flexible interference to user that can handle family of crankshaft pins (203) with least input.
In another aspect, mathematical formula for obtaining co-ordinates of EIM using the FCP principle easily optimizes the product

specific parameters i.e. major and minor axis of pin ellipse and angular shift of high point for best quality.
A comparative evaluation of CIM using the FCP principle and EIM using the FCP principle was conducted. The results are provided in Table 1.
Table 1: Comparative evaluation of CIM using the FCP principle and EIM using the FCP principle:

The present invention has the following advantages:
1. EIM using the FCP principle consistently provides better quality.
2. EIM using the FCP principle shows improved roundness profile of crankshaft pins.
3. EIM using the FCP principle leads to improved productivity of milling operation by improving cutting parameters.
4. EIM using the FCP principle provides uniform stock on diameter for subsequent grinding operation.
5. EIM using the FCP principle minimizes grinding issues like unclean diameter, uneven wheel wear, grinding burns, throw out and so on.
6. EIM using the FCP principle shows improved grinding productivity with consistent quality.
The TECHNICAL ADVANCEMENT of the invention lies in the use of basic crankshaft parameters in conjunction with mathematical models which have been specifically developed for the machining

center to generate tool path for elliptical cutting. These tool paths are then used for the machining crankshaft’s pinsin order to offset or compensate the ovality produced by inherent behavior of the machining center.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results.
The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.
While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. These and other modifications in the nature of the disclosure or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.

WE CLAIM:
1. A method of milling a non-rotating crankshaft, said method comprising the following steps:
a) placing a crankshaft (201) having a first end, second
end, main journals (202) and crankshaft pins (203), on a milling
machine adapted for machining of the crankshaft pins (203),
wherein the milling machine comprises a milling head with an internal milling cutter (208) which surrounds the crankshaft (201),
b) holding said crankshaft (201) between the machine
centers (204) along the longitudinal direction (205),
wherein angular position of crankshaft pin (203) is aligned by aligning jaws (209) and the first and second ends of crankshaft (201) are clamped by clamping chucks (206) mounted along the longitudinal axis (207) of the machine,
c) circumference milling said crankshaft (201) using
the milling machine,
wherein said milling comprises positioning the milling cutter (208) longitudinally at a pin (203) location, rotating the cutter (208) at a pre-defined RPM along its center and simultaneously sliding in a direction perpendicular to its central axis (207) to create an elliptical orbital path or motion around the crankshaft pin (203) circumference and perform pin circumference milling; and
d) unloading the crankshaft (201) from the machine,
wherein, the rotation and simultaneous sliding of the cutter
(208) in particular direction corresponds to co-ordinates obtained from a numerical control system.

2. The method as claimed in claim 1, wherein the method further comprises a step of moving the cutter (208) to the original position post milling the crankshaft pin (203) followed by positioning on another pin (203) location and performing circumference milling.
3. The method as claimed in claim 1, wherein the coordinates are obtained by numerical control system using a mathematical code/equation corresponding to input parameters used in the following mathematical equation:

wherein,
Xn : milling cutter X axis coordinate;
Yn : milling cutter Y axis coordinate;
a : Major axis of Pin ellipse;
b : Minor axis of Pin ellipse;
T : Crank Throw;
R : Cutter radius;
r : Crank pin radius;
Ǿn: Pin angular position; and
βn : Cutter interaction angle.

4. The method as claimed in claim 1, wherein the cutter (208) rotates in a clockwise/ counter clockwise direction w.r.t the axis of the crankshaft.
5. The method as claimed in claim 1, wherein steady rests (210) of milling machine clamps on main journal (202) adjacent to the crankshaft pin (203) to be machined.
6. The method as claimed in claim 1, wherein said method is characterized by a synchronized movement of the cutter (208) around crankshaft pin (203) circumference in directions perpendicular to longitudinal axis (x and y directions) which, creates an elliptical orbital path or motion around crankshaft pin (203) circumference.
7. The method as claimed in claim 1, wherein the pin (203) circumference is completely milled after completion of one complete rotation of cutter (208) around the pin (203) circumference.