The present invention relates to method of boring and burnishing of a bore in an automotive component, particularly a component requiring oil retention properties in a cyclic loading environment. The invention is applicable, for instance, to the small end of a connecting rod within an automobile engine.
A connecting rod requires a bored hole at its small end which is subject to very tight tolerances. Typically, a bore of nominal diameter about 17mm has these tolerances in the order of 0.011mm, with a cylindricity requirement of 0.003mm.
In order to achieve such tight tolerances, a two stage machining process is required. In the first stage, the small end of the connecting rod is bored using a boring tool having a tip such as a 060204 CCMT insert. The tool is operated at about 2000rpm, with a feed in the order of 110mm/min.
In the second stage, the internal walls of the bored hole are ground to the required dimensions. Typically, this is done using a 16mm diameter grinding wheel operated at about 40,000rpm, being finished at a feed rate of about 400mm/min. Lubricant channels of diameter about 3mm are also drilled to permit the supply of oil into the bore.
The grinding process is generally considered to raise environment concerns, particularly in relation to waste disposal.
The supply and retention of oil is a key operating concern of a connecting rod. This is generally governed by the surface texture {from grinding) of the connecting rod bore and of the pin.
It is considered desirable to provide an internal bore of a connecting rod which retains oil during operation. It is also desirable to provide a manufacturing process which is simpler than present methods.

STATEMENT OF INVENTION:
With these objects in view, the present invention provides a method for boring of an engine component comprising the steps of:
(1) forward feed boring of a hole through the engine component using a boring tool; and
(2) reverse feeding of the boring tool through the bored hole so as to form grooves in the hole walls due to vibration or deformation. The method may include the further step of:
(3) bui-nishing of the hole walls to smooth peaks of the grooves
formed in step (2).
Preferably, the engine component is one having a bore subjected to cyclic loading in the operation of the engine.
Preferably, the engine component is one having a bore which retains oil for lubrication under operating conditions.
The method may advantageously be used for the boring of a hole through the small end of a connecting rod. The advantage is obtained by eliminating the need for a separate grinding operation, as the hole bored using this method achieves required oil retention mechanics. Oil supplied through a drilled oil hole is retained between the bore and a pin for an acceptable length of time.
Also preferably, the burnishing is accomplished by a burnishing tool mounted on the same machining apparatus used for boring. This allows the connecting rod to be held in a substantially constant position whilst the tool is changed, increasing the speed and efficiency of the operation.
Preferably, steps (1) and (2) are conducted with a boring bar having a TCMT insert, such as a TCMT 090204 insert. In a preferred mode of operation, the tool is feed in the forward direction at about 120mm/min whilst rotating at

about 2200rpm. In the reverse direction, the feed is about 240mm/min at a rotation of about 2100rpm. Typically this results in a tolerance of+/- 0.005mm.
Burnishing is conducted using a tool having a 5mm diameter, rotated at about I500rpm and fed at about 800mm/min.
It will be appreciated that in the absence of machine tool vibration and connecting rod deformation, the reverse feeding of the boring tool would have no effect. In practice, it has been found that the presence of machine tool vibration leads to the formation of spiral micro-grooves in the walls of the bored hole during the reverse feed of the boring tool. The pitch of the spiral is proportional to the speed of the reverse feed. The depth of the grooves, typically in the order of 0.005mm, is dependent on the vibrational components of the machining tool system employed.
In an alternative embodiment, vibrations of suitable frequency and amplitude may be induced in the boring tool during reverse feed to achieve the necessary oil retention properties of the bore. This embodiment enables the exercise of greater control over the pattern of oil retaining groove on the surface of the bore.
Similarly, it has been found that a similar effect can be caused by deformation of the connecting rod and/or the boring tool, typically in response to temperature changes.
The burnishing is preferably done to a depth of about 0.002mm, but may be done to a depth of up to 0.005mm. The purpose of the burnishing is to partially flatten the peaks of the spiral groove. The internal wall of the hole following this step resembles a series of plateaus. It has been found that this provides an excellent degree of oil retention.
In the accompanying drawings:

Figure 1 is a representation of internal surface profile and roughness following the prior art boring-and-grinding approach; and Figure 2 is a representation of internal surface profile and roughness following the method of the present invention.
It will be apparent that the grooves present in Figure 2 represent a far more even arrangement for retaining oil than the more random nature of Figure 1.
In testing of the above method, connecting rods manufactured according to the above method and the prior art have been used in engines running for 100 hour endurance test on full throttle, and subsequently removed and tested. The connecting rods (and associated piston pin) of the present invention (Figure 2) showed no signs of abnormal wear when compared with those of the prior art (Figure 1)., By way of contrast, when step (2) of the above method was eliminated a much higher degree of wear was observed.
It is possible to employ the method of boring and burnishing explained above, to any component where oil retention properties of the bore are paramount for optimum operation of the engine. It is envisaged, for example, that a similar technique may be employed within a cylinder block.
Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.

WE CLAIM:
1. A method for boring of an engine component comprising the steps of:
(1) forward feed boring of a hole through the engine component using a boring tool; and
(2) reverse feeding of the boring tool through the bored hole so as to form grooves in the hole walls due to vibration or deformation.
2. A method for boring of an engine component as claimed in claim one,
comprising the additional step of:
(3) burnishing of the hole walls to smooth peaks of the
grooves formed in step (2).
3. A method for boring an engine component as claimed in claim 1 or claim 2, used for an engine component having a bore subjected to cyclic loading in the operation of the engine.
4. A method for boring an engine component as claimed in claim 1 or claim 2, used for an engine component having a bore which retains .oil for lubrication under operating conditions.
5. A method for boring an engine component as claimed in claim 1 or claim 2, used for boring a hole through the small end of a connecting rod.
6. A method for boring an engine component as claimed in claim 2, wherein the method is performed by a machining apparatus having both a boring tool and a burnishing tool mounted thereon.
7. A method for boring an engine component as claimed in any preceding claim, wherein step (1) is performed by a tool being feed in the forward direction at about 120mm/min whilst rotating at about 2200rpm.

8. A method for boring an engine component as claimed in any preceding
claim, wherein step (2) is performed by a tool being fed in the reverse
, direction at about 240mm/min whilst rotating at about 2100rpm.
9. A method for boring an engine component as claimed in claim 2 or claim 6,
wherein step (3) is performed by a tool rotated at about 1500rpm and fed
at about 800mm/min.
10. A method for boring an engine component as cfaimed in claim 2, 6 or 9,
wherein the burnishing is done to a depth of up to 0.005mm
11. A method for boring an engine component as claimed in claim 10, wherein
the burnishing is done to a depth of about 0.002mm.
12. A method for boring an engine component as claimed in any preceding
claim, wherein vibrations of frequency and/or amplitude is induced in the
boring tool during step (2).