The present invention relates to a method of manufacturing connecting rods, forged in one piece, each connecting rod having a big-end, comprising an upper bearing and a bearing cap, and a small-end, said ends being joined to each other by a connecting rod arm, said method comprising: cutting, from a bar, blanks of a cross-sectional area and length appropriate to each connecting rod; preheating the blanks; forging out each blank and dividing the connecting rod forged in one piece into two parts along a predetermined dividing plane through the big-end of the connecting rod.

According to a previously known method, the big-end is divid¬ ed into an upper bearing and a bearing cap, which are recon¬ nected by means of bolts, when the complete engine is asse b- led. The division can be effected in different ways, e.g. by milling apart which is, however, both labour and material consuming, since the connecting rod must be overdimensioned due to the material which is milled off. This in turn re¬ quires that the crankshaft hole in the big-end first be dril- led in circular shape and then be worked to an oval and that the milling off of material during division be so exact that the crankshaft hole, when the parts are re-assembled, will again assume a circular shape fitting the crankshaft. In order to make possible re-assembly of the bearing cap and the upper bearing with exact fit, the separate parts must also be specially worked. The working is, as a rule, carried out by deep grinding or serration grinding and cutting grooves, i.e. alternating ridges and grooves are made in the facing sur¬ faces of the upper bearing and the bearing cap to provide the required guidance for correct assembly of the parts.

Another known process means that the bearing cap is separated from the big-end of the connecting rod by buckling. In order to avoid deformation of the material adjacent the dividing plane, it must, as a rule, be pretreated prior to the buck¬ ling blow itself. The pretreatment is intended to alter the properties of the material surrounding the plane of division,

to increase its brittleness, which is advantageous to the buckling step. One method of treatment involves an electroly¬ tic process, by means of which hydrogen is caused to diffuse into the metal along the intended plane of division, where- after a sufficiently heavy blow or other force produces a fracture without deformation. Another method involves in¬ creasing the brittleness of the material in the area in question by means of induction hardening. To be sure, by means of this process, the buckling plane through the remain- ing material in the upper bearing and the bearing cap will form after buckling opposing irregularly profiled surfaces which have varying profile depths depending on the hardness of the material. The projections and indentations in the pro¬ file surfaces guide the upper bearing and the bearing cap into exact alignment with each other.

However, this division according to known methods is compli¬ cated and requires special tools for execution, and therefore it is desirable that at least some treatment or working step can be simplified or eliminated. The operations in question include draw reaming of bearing caps, separation by sawing, serration grinding, disassembly, washing and re-assembly of each connecting rod and finishing of the crankshaft hole and buckling in a special buckling machine. A reduction in the number of operations can also make it possible to reduce the number of operators.

The purpose of the invention is therefore to improve known methods of manufacturing connecting rods by a method which is simple, inexpensive and can be carried out with high preci¬ sion.

The purpose is achieved according to the invention by virtue of the fact that the division of the finished connecting rod is initiated before the beginning of the forging operations by a groove being made by cutting into the lateral surface of each blank, that the groove is disposed at a predetermined distance from the end of the blank which is to form the big- end and has a predetermined shape, that the groove in each

blank is coated with a substance which prevents formation of oxides and mill scales before the blanks are heated to for¬ ging temperature and that the groove during the various for¬ ging steps is caused to expand in a predetermined manner to finally form, in the dividing plane, a fracture initiation about a remaining homogeneous portion of the big-end, which is located in the area within which bolt holes for fastening bolts of the bearing cap are drilled.

According to a preferred embodiment, the bearing cap is separated from the upper bearing by the bolt holes required for remounting of the bearing cap on the upper bearing being drilled through at least a major portion of the homogeneous portion surrounded by the fracture initiation, whereupon any remaining material in the dividing plane is broken off by a slight separating force.

According to a particularly advantageous example, the cross- sectional area of the remaining material in the homogeneous portion, after hole drilling, is less than 5% of the dividing area between the connecting rod portion and the bearing cap portion. This makes for a deformation free dividing surface, which provides reliable guidance between the connecting rod portion and the bearing cap portion.

In order to prevent the formation of oxides and mill scales in the radial annular groove in each blank, and to achieve a brittle dividing surface (after forging) , it has proved effective to coat the groove with graphite or a ceramic material before preheating the blanks. A suitable ceramic material is feldspar glaze.

The invention will be described in more detail below with reference to the accompanying schematic drawings.

Figure 1 shows a blank cut from a bar and provided with an annular groove,

Figure 2 shows a plan view of a preformed blank,

Figure 3 shows a finishing die with a finally forged blank in plan view,

Figure 4 shows the finishing die with the finally forged blank according to Figure 3, in section along the line A-A,

Figure 5 shows a plan view of a finished connecting rod, and

Figure 6 shows a cross-section along the line A-A of the connecting rod in Figure 5.

The method of manufacture is initiated by clipping or sawing a bar material in cold state into lengths or blanks to an appropriate length. The diameter of the bar D and thus the diameter of the blanks or lengths varies depending on what type of connecting rod is to be manufactured. Blanks of both circular and rectangular or square cross-section can be used in the invention. Suitable diameters lie within the interval 43-65 mm. The length of the blank L also varies, depending on the type of connecting rod, and suitable values of L lie in the interval 125-400 mm. Suitable types of materials for the purpose are steel or lightweight metals, preferably precipi- tation hardenable steel, but the forging method is applicable to all types of materials, e.g. alloys of aluminium or tita¬ nium, and powder materials.

Each blank 2 is divided into a connecting rod portion 4 and a bearing cap portion 6 by means of a transverse annular groove 8, which is cut radially into the cylindrical lateral surface of the blank. The bottom of the annular groove de¬ fines a circular homogeneous core portion 10, the diameter d of which varies as a function of the values of D, and can assume values in the interval 10-30 mm. The groove 8 in the blank is located so that a fracture indication is obtained in the separation plane between the connecting rod portion and the bearing cap portion. Suitable distances Lδ from the end

of the blank to the groove vary depending on what type of connecting rod is to be manufactured. Tests have shown that a suitable value for Lδ is 40-50 mm. Depending on what tech¬ nique is used in making the groove, it is possible to allow the width b of the groove to vary from a few hundreds of a millimeter up to 5 mm. A particularly suitable groove width has proved to be 1.5 - 2 mm.

In order to prevent the formation of oxides and mill scales upon heating a blank with a wide groove to forging tempera¬ ture, it is coated with a ceramic material or graphite. For this purpose it is advantageous if each blank prior to the application is heated to a maximum of 200 C. This pro¬ vides better adhesion between the blank and the ceramic material than if the application were performed at room temperature. The material can comprise glazes of varying grain size and varying melting point. A feldspar glaze with a certain predetermined grain size has been shown in tests to function well for this purpose. The material can be applied in the groove with the aid of a nozzle achieving complete coverage of all of the surfaces of the groove.

The blanks are then charged in a furnace in such a manner that the groove is always turned in the same direction and is heated inductively to a forging temperature in the interval

1250 - 1280°C. After heating, the blanks are preformed in the conventional manner by means of stretch rolling, to obtain a blank 12 suitable for forging. The blank comprises a rolled portion 14 and an unrolled portion 16. The annular groove is located on the unrolled portion of the blank and is therefore not affected in any way by the preforming (Fig 2) . This preforming reduces the cross-sectional area and extends the length of the blank portion 14.

The subsequent forging is carried out in two steps in a thousand ton-crankshaft press, viz a preforming step and a final forming step. For preforming, the pre-rolled blank is placed in the preforming die 18, with the unrolled portion abutting against an abutment 20 in the preforming die, thus

assuring that the annular groove will always be placed at exactly the same location in the preforming die (Fig 2) . This first forging step in the preforming die 18 gives the blank the correct profile.

In the second forging step, which is carried out in the finishing die 22, the connecting rod 24 is finally formed, so that it has a big-end 26 comprising an upper bearing 28, a bearing cap 30, a small-end 32 and a connecting rod arm 34, which connects the big-end to the small-end (Fig 3) . The various working steps finally impart the connecting rod with such a section that the core portion 10 along a dividing plane 36, essentially moves out to the center of each of the areas 38, through which bolt holes 40 are to be drilled in the upper bearing and the bearing cap (Fig 4) . The core material can be allowed to be spread past the center of the bolt holes but may not continue past the outer edge of each bolt hole in order not to impede the subsequent division of the connecting rod. Controlled spreading of the material is made possible by virtue of the fact that the diameter d of the core portion 10 is balanced relative to a spew area 42 located in the center of the forging die. If the ratio be¬ tween the diameter d of the core portion and the size of the spew area 42 is correctly chosen, some of the core por- tion will always be spread out into the area 38 for the bolt holes. An optimally selected ratio will mean that each bolt hole area will be partially filled by the material of the core portion. This means that when the connecting rod is finally forged, it will have a fracture indication 44 sur- rounding the remaining portion 46 of the core portion.

After forging, the connecting rod is deflashed in a conven¬ tional manner both inside and outside in a crankshaft press. The deflashing is done in a heated state, directly after the final forging, to remove the material which has penetrated into the interior spew area 42 or the exterior spew area 48.

The next operation comprises heat calibration of certain surfaces of the connecting rod, e.g. the surfaces around the

crankshaft hole 52 and the surfaces around the gudgeon pin hole 54. The calibration is done in a heated state in the same press as the deflashing and directly after said opera¬ tion. After sand-blasting, the connecting rods are inspected optically, before they are subjected to a final machining. The bolt holes 40 are then drilled through the respective area 38 (Fig 6) . The deflashing and drilling operations have then removed the major portion of the remainder 46 of the core portion. What remains of the original core portion 10 is then remnants 50 remaining adjacent the bolt holes 40, which remnants 50 have not been affected by the deflashing and drilling operations. The connecting rod is thus now ready for division in the production line and is as shown in Figs 5 and 6. Each connecting rod is separated along the dividing plane A-A by a slight separating force, thus providing a guide between the upper bearing and the bearing cap by surfaces from the coated groove 56 enclosed by the forging. Only the small portion 50 needs to be pulled apart.

The invention simplifies or eliminates the following opera¬ tions in the manufacture of connecting rods:

The crankshaft hole 52 is forged directly in one step with a circular shape and not in two steps as is the conventional method, i.e. first forging a connecting rod with an oval hole. Draw reaming of bearing caps is unnecessary, since the new dividing area has an exact fit in the x-, y- and z-dimen- sions. Sawing apart is superfluous since the division is achieved primarily by drilling bolt holes. Deep grinding in the form of serration grinding can be eliminated, since a natural fracture surface is used, for exact localization between the upper bearing and the bearing cap.