FORM - 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)
MACHINING FIXTURE FOR BACK SPOT FACING
Applicant:
MAHINDRA & MAH1NDRA LIMITED
Automotive & Farm Equipment Sector,
Mahindra Towers, Dr. G.M. Bhosale Marg,
Worli, Mumbai.
Inventors:
1. DHANALE SWAPNIL 2. BANE UTTAM
3. SAVANT KEDARNATH 4. WAGHODE RAMESH
5. GUMASTE SANJEEV (all Indian Nationals)
Of
Mahindra & Mahindra Ltd. Farm Division,
Gate No. 4, Plant Vehicle Engineering Deptt, HI Office Building,
lst Floor, Akurli Road; Kandivali (E),
MUMBAI - 400101 (M.S.) - INDIA.
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE USED.

FIELD OF THE INVENTION
The present invention relates to a machining fixture used during manufacturing operations. In particular, the invention relates to a safe and user-friendly machining fixture.' More particularly, the invention relates to a machining fixture for accelerate back spot facing, back chamfering and back counter boring operations of the work pieces.
BACKGROUND OF THE INVENTION
A spot face or back spot face is a machined feature in which a certain region (a spot) of the work piece is faced for providing a smooth, flat and accurately located surface. This is especially relevant on cast or forged work pieces, wherein the smooth, flat and accurately located surface of the spot face stands in distinction to the surrounding surface, the roughness, flatness, and location of which are subject to much wider tolerances and thus not assured with a precise level of machining. The most common application of spot facing is while facing an area around the bolt hole, where the head of the bolt sits. This is often accomplished by cutting a shallow counter bore, just deep enough "to clean up", i.e. only that much material is removed to remove any irregularity and to make a flat surface.

Some other common applications of spot facing include: facing a pad on a boss, creating planar surfaces in known locations which can orient a casting or forging in position in the assembly or allowing part marking, such as stamping or nameplate riveting; or even offering machine-finish visual appeal in spots, without any need for finishing all over (FAO).
Prior Art
US4710070 A discloses a device in back spot facing tools of the kind comprising a spindle 11, at one front end portion 12 of which, at least one wing 14 is arranged in a recess 13 and pivotaJly mounted parallel with and eccentrically with respect to the spindle about an axle 15, 16. The wing is at the rotation of the spindle arranged to pivot from an inactive position within the circumference of the spindle to an active position outside it. It is at the part of the wing located outside of the spindle provided with at least one cutting edge and/or a cutter 17. The pivoted part of the wing 14 outside the spindle 12 is at least at its one preferably its front end surface 21 designed with an inclined ramp 22, which in the extended position of the wing, has its maximum height, as seen in the axial direction of the wing, in a portion located closest to the circumference of the spindle 12 and is arranged to incline towards the externa!, curved envelope surface 23 of the wing. The transition section 29 of the ramp 22 in the envelope surface 23 of the wing is slightly chamfered, so that at the insertion of the spindle in a boring 20 of a work piece 19, the portion of the wing 14 which during the rotation is pivoted outwards is in contact with the hole edge 27 of the boring and is pivoted to the inactive position at the continued displacement in the boring.

US3381579 A discloses a self-powered tool for spot facing the far side of a work piece including an axially movable motor-driven pilot shaft adapted to have a tool demountably secured to its outer end, a piston for retracting the shaft, a casing with an adjustable foot-piece tp bear against the near side of the work, and an adjustable stop means for limiting the stroke of the piston.
US3827821 A pertains to a back spot facing tool whose cutting blade is pivotally . mounted on a spindle body and is axially actuated by a plunger rod so as to be moved to either a cutting or a concealed condition in response to either a manual manipulation or to a hydraulic actuation of this rod. The back spot facing cutting blade is mounted with its retaining pivot point offset so that one side of the blade in a cutting condition extends a greater distance from the spindle body causing a greater pressure to be exerted on this blade cutting portion than on the opposite side blade cutting portion. The blade portion developing the greater pressure is supported by an adjustable set screw which is adjusted to provide means to establish the exact face angle of back spot face cutting desired by the tool. A precise grinding fixture is also disclosed and carries the blade at a determined condition to its support surface so as to establish the desired cutting blade angles.
US3017791 A relates to improvements in tools used for debarring, chamfering, back spot facing, grooving and similar operations. It discloses a device comprising a pilot adapted to be inserted into a hole in a piece of work and rotated therein, a cutter normally occupying a recess in the pilot and rotatabie therewith, means engageable with the work for holding the cutter against longitudinal movement relatively thereto, means for' moving the pilot longitudinally relatively to the cutter, and mutually engaging means between the -.. cutter and the pilot located closely adjacent the cutting edge of the cutter for moving the cutter laterally relatively to the pilot so as to project the cutting edge therefrom into engagement with the work.

JPS60259302 A discloses a rod 16 is press-moved by the cooperation of an operating ring 17, lead ring 18, and a slidable pin 15, through the revolution of a tapered shank 49 by the main shaft of a drilling machine, and a slidable rod 25 moves in slidable ways leftward in a guide bar 23, and the tapered surface 24 at the top edge contacts with the tapered groove 27 of a cutting tool 28. When the slidable rod 25 shifts further leftward, the cutting tool 28 shifts leftward against a cutting-tool pressing elastic body 32, and the cutting tool 28 is advanced to the work A side by the operation of the tapered surface 24 and a tapered groove 27, and set onto the back spot-facing surface on the work A. With such constitution, the vibration of the cutting tool due to the impact during cutting can be prevented.
GB2399781 A discloses a method for machining a flat area in a surface surrounding an aperture through a work piece, where the surface is located at the rear of the work piece, includes a machine tool comprising a shaft 7 adapted to be mounted to a mechanism for rotating the tool about a longitudinal axis 11 and a tip 9 having a cutting edge 10 extending outwardly from the axis, inclined to the axis by an acute angle, and facing towards the rear face of the work piece when the tool is inserted through the aperture. The method comprises the steps of: inserting the tool through the hole; rotating the tool about its longitudinal axis; tilting the longitudinal axis of the tool through the acute angle so as to engage the cutting edge with the rear face of the work piece; recessing the longitudinal axis of the tool around the hole so that the longitudinal axis describes the surface of a cone, whereby the cutting edge cuts a flat region in the rear face of the work piece around the periphery of the hole; returning the axis of the tool to substantially normal to the said flat region; stopping rotation of the tool about its longitudinal axis; and extracting the tool from the work piece.

All above patents disclose different arrangement for the back spot facing tools, however none discloses a fixture for safe and accelerated back spot facing, back chamfering and back counter boring operation for work pieces being machined. Back spot facing means that the tool reaches the far side of the work piece away
from the driving spindle side and cuts back towards the spindle. Such operations
can be done with the help of following options:
a) Back spot facing with boring bar, wherein boring bar reaches through a
hole while kept off-center, then brought to center for spot facing.
Disadvantages:
1. Requires a large hole to spot face diameter ratio.
2. Higher cycle time.
3. No practicable on conventional machines.
b) Back-deburring type tools (e.g. automatic back spot facing tool), the cutting
edges of which open and close like an umbrella or a check-valve.
Disadvantages:
1. Inefficient on hard and large work pieces.
2. Efficiency decreases with increase in hole to spot face ratio.
3. Costly as compared to all other processes.
c) Inverted spot face cutter and tapered/straight shank pilots.
Disadvantages:

1. Restricted to conventional machines.
2. Requires higher cycle time as compared to other processes.
3. Needs human intervention, thereby making the process hazardous.
However, this process is highly useful for heavy and hard work pieces. Moreover, it efficiently works at higher hole to spot face ratio, and therefore, it is the cheapest solution.
DESCRIPTION OF THE CONVENTIONAL PROCESS
As indicated above, the conventional inverted spot face cutter with tapered/straight shank pilot is most suitable for large and hard equipment industry having conventional Radial drilling machines, where other methods are difficult , to carry out. An inverted spot face cutter (Figure 1A) along with tapered/straight shank pilot (Figure 1B) is employed in accordance with the present invention and this cutter is retained by a solid shoulder on the pilot (Figure 1C). For the removal of this cutter, it is necessary to return the cutter backward on the pilot by a small distance, thereby detaching it from the pilot. According to the presently used method, the operator does not use hand gloves at all to avoid his hand gloves from getting trapped between high speed rotating cutter and work piece. This also causes serious safety concerns because the operator has to handle very hot cutter using his naked hands.
Normally back spot facing is preferred when there is no possibility of carrying out normal spot facing operation. In most of such situations, the regions to be back spot faced are inaccessible to operator's hands. Due to this inaccessibility of such machining locations, there is an alarming increase in human injuries. This

manual back spot facing operation also causes extreme fatigue to the operator and slows down the production process as well. In order to overcome this safety and concerns of slow production, the production department has to use auxiliary machines, which also lead to higher production costs.
DESCRIPTION OF THE PRESENT INVENTION
Accordingly, the' present invention provides a machining fixture for back spot facing, which makes this process not only safe but also user-friendly. Since this machining fixture ensures the operator's safety, the production of back spot faced work pieces is accelerated.
According to the present invention, a back spot facing tool holding fixture is configured such that the cutter throughout is held during the back spot facing operation and therefore, completely eliminates any human intervention during this machining process.
Therefore, the safety afforded to the back spot facing machine operator by the use of the machining fixture in accordance with the present invention automatically results in a reduced cycle time as well and avoids any need for auxiliary machines necessary until now, discussed above.
OBJECTS OF THE INVENTION
Some of the objects of the present invention - satisfied by at least one embodiment of the present invention - are as follows:

An object of the present invention is to provide a safe machining fixture for back spot facing operation.
Another object of the present invention is to provide a low-cost machining fixture for carrying out back spot facing operation.
Yet another object of the present invention is to provide a machining fixture which substantially reduces the machining cycle time and thereby accelerates the back spot facing operation.
Still another object of the present invention is to provide a machining fixture for back spot facing operation, which dispenses with the need for using any other auxiliary machines.
A further object of the invention is to provide a machining fixture for back spot facing operation, which is user-friendly.
Other objects and advantages of the present invention will become more apparent from the following description, when read with the accompanying figures of drawing, which are however not intended to limit the scope of the present invention in any way.
SUMMARY OF THE INVENTION
In accordance with the present invention, a spot facing fixture comprises:

- a guide plate on top, including at least one aperture for through passage of a respective pilot adapted for engaging the spot facing cutter and to facilitate the downward and upward movement of the spot facing cutter for carrying out the back spot facing operation on at least one site;
- a supporting plate connected to the guide plate on top and having at least one window for holding and viewing purposes, the inner wall of the supporting plate adapted for closely touching at least one face of the component to be spot faced; and
- a bottom plate connected to the supporting plate, configured with at least one cutter holder adapted for detachably holding a respective spot facing cutter for back spot facing operation;
. wherein a plurality of holes through are configured on the guide plate and the supporting plate for holding and aligning the spot facing cutter in an accurate position.
Typically, the spot facing cutter is an inverted spot facing cutter for carrying out back spot facing, back chamfering, back boring operations etc.
Typically, the pilot is a straight shank pilot or tapered shank pilot with a solid shoulder for detachably seating the spot facing cutter thereon.
Typically, the cutter holder is configured with a chamfer along with a counter bore for easy transfer of spot facing cutter on the cutter holder after conclusion of spot facing operation.

Typically, the cutter holder is configured with a radius or fillets.
Typically, the cutter holder, supporting plate & guide plate are made in one piece. Typically, the cutter holder, supporting plate & guide plate are made detachable.
Typically, the fixture is made of a synthetic light weight material, such as cast nylon, plastic or carbon fiber.
Typically, a concentric hole is also provided in to the pocket in the cutter holder to allow extended entry of tapered/straight shank pilots for smooth operation.
In accordance with the invention, the method for spot facing of work pieces comprises the steps of:
- placing a spot facing cutter on the respective cutter holder;
- lowering the pilot for engaging the spot facing cutter; reversing the direction of rotation of spindle to lock cutter & pilot;
- raising the pilot with spot facing cutter mounted on the shoulder of the pilot up by a predetermined distance;
- lowering the pilot with spot facing cutter until touching the cutter holder and again reversing the direction of rotation of spindle for unlocking and disengaging the spot facing cutter from the pilot to be used for subsequent operation by repeating the above steps on another work piece or site.

Typically, a series of fixtures are arranged on either side of work table for spot facing of a plurality of work pieces.
Typically, a series of fixtures are arranged on either with the help of some dowel or by taking reference of any machining surface.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The above-mentioned and other features and advantages of the various embodiments of the present invention will now be explained in more detail with reference to the non-limiting accompanying drawings thereof, in which:
FIG. 1A shows perspective views from two different sides of a spline drive inverted spot face cutter to be used on the machining fixture in accordance with the invention.
Fig. 1AA shows a top view and side view of the cutter shown in Figure 1A which shows the profile details thereof.
FIG. IB shows a perspective view of a tapered shank pilot for retaining an inverted spot face cutter (Fig. 1 A) as shown in Fig. 1C.
FIG. IBB shows another line sketch of a tapered shank pilot for retaining an inverted spot face cutter (Fig. 1A) shown in Fig. 1B indicating the profile details thereof.
FIG. 1C shows an inverted spot face cutter retained on a tapered shank pilot by means of a solid shoulder on the pilot (Fig. 1B).

FIG. 2 shows the perspective view of a machining fixture for back spot
facing in accordance with the present invention to accommodate a single inverted spot facing cutter shown in Figure 1 A.
Fig. 3 shows the perspective view of a multi-cutter machining fixture in
accordance with the present invention for back spot facing, which can accommodate a plurality of inverted spot facing cutters shown in Figure 1A.
Fig. 3A shows the side view of a multi-cutter machining fixture shown in Figure 3 for carrying out the back spot facing operation on various work pieces.
Fig. 3B shows top view of the inverted spot facing cutter shown in Figure
3A.
Fig. 3C shows a cross-sectional view of the inverted spot facing cutter shown in Figure 3 across the section line C-C shown in Figure 3B.
Fig. 3D shows the supporting plate 30 (Figure 3) when seen in the direction of the arrow D in Figure 3B.
DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG. 1A shows perspective views of an inverted spot facing cutter to be used on the machining fixture in accordance with the invention. The cutter profiles of the tool or inverted spot facing cutter are shown in Fig. 1AA in top view and side view respectively for indicating details thereof.
FIG. 1B shows a perspective view of a tapered shank pilot for retaining an inverted spot facing cutter as shown on Figure 1 A. A solid shoulder is shown on the right side end thereof, on which the inverted spot facing cutter is seated during

the downward movement of the tapered shank pilot. However, a straight shank pilot may also be used with necessary changes. Thereafter, this pilot along with the inverted spot face cutter is pulled upwards to do spot facing operation to the , desired depth/height. Subsequently, the pilot is again lowered and on touching the tool pocket formed on the machining fixture; this inverted spot facing cutter gets detached from the pilot and is stored there for subsequent spot facing operation at another site.
FIG. IBB shows another line sketch of a tapered shank pilot assembled with an inverted spot facing cutter clearly indicating a solid shoulder formed on the right side end thereof.
FIG. 1C shows an inverted spot facing cutter retained on a tapered shank pilot by means of a solid, shoulder on the pilot (Fig. 1B) for carrying out the back spot facing operation as discussed above.
FIG. 2 shows the perspective view of a machining fixture for back spot facing in accordance with the present invention to accommodate a single inverted spot facing cutter shown in Figure 1A. Here, the cutter holder 10 for holding the inverted back spot facing cutter shown in Figure 1A is clearly visible. A through hole provided on the guide plate 20 on top is used for guiding the pilot, for engaging the cutter and for facilitating downward and upward movement of the inverted spot facing cutter to carry out back spot facing operation on the work piece. The inner wall of the supporting plate 30 is made to closely touch the face of the work piece to be spot faced. This way, any direct contact of the operator's hand with the cutter becoming hot after the back spot facing operation is safely avoided. This enhances the safety during the back spot facing operation. Moreover, because of this enhanced safety, the operator is more confident of the operation and thus, in a position to conduct back spot facing operation at much

higher rate, thereby also increasing the production of back spot faced work pieces.
Fig. 3 shows the perspective view of another machining fixture for back spot facing in accordance with the present invention to accommodate a plurality of inverted spot facing cutters shown in Figure 1A. In particular, the machining fixture of Figure 3 includes four holes for passing through four respective tapered shank pilots on the guide plate 20 and four numbers of cutter holders 10 for holding or accommodating four respective inverted spot facing cutters. This multi-cutter configuration of the machining fixture facilitates in increasing the speed of the back spot facing operation. In another configuration, a number of such machining fixtures used together in specific layout, e.g. two such machining fixtures may be used for back spot facing a plurality of work pieces, which fixtures may be placed opposite each other. Therefore, multiple back spot facing operations can be done by the operator in one single run. This configuration also . substantially increases the production of the back spot faced work pieces.
Fig. 3A shows the side view of a multi-cutter machining fixture for back spot facing shown in Figure 3. The guide plate 20 or top flange of the fixture includes four through holes and the bottom flange includes four respective cutter holders 10 for accommodating one inverted back spot facing cutter (see Figure 3). Two dowels DP are also provided to hold and align the cutter in a proper position for adequate support of this fixture on the work piece.
Fig. 3B shows top flange or guide plate 20 of the machining fixture shown in Figure 3, including four through holes for passing the respective tapered shank pilots for carrying out the back spot facing operation, as discussed above.
Fig. 3C shows a cross-sectional view of the inverted spot facing cutter shown in Figure 3 across the section line C-C shown in Figure 3B. It includes a section line

C-C for showing the cross-sectional view in Figure 3C as well as an arrow D for showing details of recess shown in Figure 3D.
Fig. 3D shows the supporting plate 30 (Figure 3) when seen in the direction of the arrow D in Figure 3B. A slot is visible which can be used for easy holding the machining fixture in accordance with the invention for carrying out the back spot facing operation of relevant work pieces.
Working Principle of the invention
In principle, the machining fixture in accordance with the invention can be made of any light weight material. Preferably, a synthetic material, polymer or hard plastic material is used for making the same. Accordingly, the present invention provides a machining fixture, which makes this process not only non-hazardous but also user-friendly. Figure 3 shows a machining fixture to hold the spot facing cutter throughout the back spot facing operation, which results in complete elimination of any human intervention during this operation. This novel machining fixture also automatically results in reduced cycle time and eliminates . conventionally required auxiliary machines. At least one cutter holder pocket 10 is made, which is joined to the guide plate 20 with the help of the supporting plate 30. An adequate chamfer along with a counter bore is provided in each cutter holder pocket for holding a respective inverted back spot facing cutter after conclusion of the back spot facing operation in a proper position.
Other alternate design can also be used adopted in place of this chamfer, e.g. radius or fillets. A concentric hole is also provided in to each pocket to allow extended entry of a respective tapered or straight shank pilot. This cutter holder can be made in any shape to suit work piece and it also can be arrayed for multiple back spot facing operation requirement as shown in Figure 3. Guide

plate 20 helps to rest the machining fixture on the work piece and to facilitate the entry of the guide tapered or straight shank pilot in the machining fixture. Guide plate 20 is configured bulky in comparison to the cutter holder 10 and the supporting plate 30, so that the overall center of gravity of the machining fixture shifts upwards, which results in self-balanced nature of the machining fixture on the work piece. Supporting plate 30 basically links the guide plate 20 and the cutter holder 10. The dimensions of supporting plate can be varied according to the work piece to be back spot faced.
For adequate support of the machining fixture on the work piece, two dowels are also provided to hold and align the cutter in a proper position. For further improving the operational effectiveness the machining fixture, it is made in cast nylon for making this much lighter. There are further alternative materials like plastic, carbon fiber, etc. in which this machining fixture can be made. The machining fixture is cast in a single body, which provides rigidity. However, the machining fixture can also be made in separate pieces according to the work piece to be back spot faced, which can be assembled at the time of operation.
Technical Advantages & Economic Significance
1. Hazardous back spot facing operation is converted into a user friendly and safe back spot facing operation.
2. Cycle time for back spot facing operation is greatly reduced, which in turn results in increasing the production of work pieces with back spot faced profiles.
3. Auxiliary radial drilling machine currently required for spot facing operation is totally eliminated.

4. Uses a very simple arrangement requiring no special purpose machine, thereby providing huge capital cost savings.
5. Enhances the confidence of the operator by providing greater safety during the back spot facing operation, which was very hazardous until now.
It should be understood that the embodiment described herein is merely a preferred embodiment of the invention. Therefore, the person skilled in the art may practice it with any modification within the spirit and ambit or scope of the embodiment as described in accordance with the present invention.
The description provided herein is purely by way of example and illustration. Various features and advantageous details are explained with reference to this non-limiting embodiment in the above description of the seat frame structure in accordance with the present invention. The descriptions of well-known components and manufacturing and processing techniques are consciously omitted in this specification, so as not to unnecessarily obscure the specification.
The exemplary embodiments described in this specification are intended merely to provide an understanding of various manners in which this embodiment may be used and to further enable the skilled person in the relevant art to practice this invention."*
Accordingly, these illustrative exemplary embodiments should not be construed as limiting the scope of the present invention. It is to be understood that the phraseology or terminology employed in this specification is merely for the purpose of description and not for limiting the scope thereof.

Although the embodiments presented in this disclosure have been described in terms of its preferred embodiments, the skilled person in the art would readily recognize that these embodiments can be applied with modifications possible within the spirit and scope of the present invention as described in this specification.
While considerable emphasis has been placed on the specific features of the preferred embodiment described here, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiments without departing from the principles of the invention. These and other changes in the preferred embodiment of the invention 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 invention and not as a limitation.

We claim:
1. A spot facing fixture comprising:
- a guide plate on top, including at least one aperture for through
passage of a respective pilot adapted for engaging the spot facing
cutter and to facilitate the downward and upward movement of the ,
spot facing cutter for carrying out the back spot facing operation on
at least one site;
- a supporting plate connected to the guide plate on top and having at least one window for holding and viewing purposes, the inner wall of the supporting plate adapted for closely touching at least one face of the component to be spot faced; and
- a bottom plate connected to the supporting plate, configured with at least one cutter holder adapted for detachably holding a respective \ spot facing cutter for back spot facing operation;
wherein a plurality of holes through are configured on the guide plate and the supporting plate for holding and aligning the spot facing cutter in an accurate position.
2. Fixture as claimed in claim 1, wherein the spot facing cutter is an inverted spot facing cutter for carrying out back spot facing, back chamfering, back boring operations etc.

3. Fixture as claimed in claim 1, wherein the pilot is a straight shank pilot or tapered shank pilot with a solid shoulder for detachably seating the spot facing cutter thereon.
4. Fixture as claimed in claim 1, wherein the cutter holder is configured with a chamfer along with a counter bore for easy transfer of spot facing cutter on the cutter holder after conclusion of spot facing operation.
5. Fixture as claimed in claim 1, wherein the cutter holder is configured with a radius or fillets.
6. Fixture as claimed in claim 1, wherein the cutter holder, supporting plate & guide plate are made in one piece.
7. Fixture as claimed in claim 6, wherein the cutter holder, supporting plate & guide plate are made detachable.
8. Fixture as claimed in claim 1, wherein the fixture is made of a synthetic light weight material, such as cast nylon, plastic or carbon fiber.
9. Fixture as claimed in claim 1, wherein a concentric hole is also provided in to the pocket in the cutter holder to allow extended entry of tapered/straight shank pilots for smooth operation.
10. Method for spot facing of work pieces by the fixture as claimed in any of the preceding claims, the method comprises the steps of:
- placing a spot facing cutter on the respective cutter holder;

- lowering the pilot for engaging the spot facing cutter; reversing the direction of rotation of spindle to lock cutter & pilot;
- raising the pilot with spot facing cutter mounted on the shoulder of the pilot up by a predetermined distance;
- lowering the pilot with spot facing cutter until touching the cutter holder and again reversing the direction of rotation of spindle for unlocking and disengaging the spot facing cutter from the pilot to be used for subsequent operation by repeating the above steps on another work piece or site.

11. Method as claimed in claim 10, wherein a series of fixtures as claimed in claims 1 to 7 are arranged on either side of work table for spot facing of a plurality of work pieces.
12. Method as claimed in claim 10, wherein a series of fixtures as claimed in claims 1 to 8 are arranged on either with the help of some dowel or by taking reference of any machining surface.