An apparatus for forming a blank for finish forging for a forged crankshaft for a three-cylinder engine in which third and fourth crank arms connecting with a second crank pin in a center have no balance weights and remaining crank arms have balance weights, the apparatus configured to form, from a preform blank, in a process of manufacturing the forged crankshaft for a three-cylinder engine, the blank for finish forging to be subjected to finish forging by which a final shape of the forged crankshaft is obtained,
- the preform blank including:
- rough journal portions having an axial length equal to an axial length of journals of the forged crankshaft;
- rough crank pin portions having an axial length equal to an axial length of crank pins of the forged crankshaft;
- third and fourth rough crank arm portions corresponding to the third and fourth crank arms of the forged crankshaft, having an axial thickness equal to an axial thickness of such crank arms;
- and
- weighted rough crank arm portions corresponding to weighted crank arms having the balance weights of the forged crankshaft, having an axial thickness greater than an axial thickness of such crank arms,
- the rough crank pin portions of the preform blank having a smaller amount of eccentricity in a direction perpendicular to an axial direction than an amount of eccentricity of the crank pins of the forged crankshaft,
- the forming apparatus comprising:
- a reference crank pin die disposed at a location of the second rough crank pin portion, the reference crank pin die configured to be brought into contact with the second rough crank pin portion, the reference crank pin die configured to move in a direction perpendicular to the axial direction, but be constrained from moving axially, while being in contact with side surfaces of the third and fourth rough crank arm portions through which the third and fourth rough crank arm portions connect with the second rough crank pin portion;
- movable crank pin dies disposed at locations of the corresponding first and third rough crank pin portions at opposite ends, the movable crank pin dies configured to be brought into contact with the first and third rough crank pin portions, the movable crank pin dies configured to move axially toward the reference crank pin die and in the direction perpendicular to the axial direction, while being in contact with side surfaces of the rough crank arm portions through which the rough crank arm portions connect with the first and third rough crank pin portions;
- stationary journal dies disposed at locations of the rough journal portions connecting with the third and fourth rough crank arm portions, the stationary journal dies configured to hold and retain such rough journal portions therebetween in the direction perpendicular to the axial direction, the stationary journal dies configured to be constrained from moving axially while being in contact with side surfaces of the third and fourth rough crank arm portions;
- and
- movable journal dies disposed at locations of the corresponding rough journal portion excluding the rough journal portions connecting with the third and fourth rough crank arm portions, the movable journal dies configured to hold and retain such rough journal portions therebetween in the direction perpendicular to the axial direction, the movable journal dies configured to move axially toward the reference crank pin die while being in contact with side surfaces of the rough crank arm portions through which the rough crank arm portions connect with such rough journal portions,
- wherein, in a state where the rough journal portions are held and retained by the stationary journal dies and the movable journal dies, and the rough crank pin portions are contacted by the reference crank pin die and the movable crank pin dies, the movable journal dies are moved axially, the movable crank pin dies are moved axially and in the direction perpendicular to the axial direction, and the reference crank pin die is moved in the direction perpendicular to the axial direction, thereby compressing the weighted rough crank arm portions in the axial direction so as to reduce the thickness thereof to the thickness of the weighted crank arms of the forged crankshaft, and pressing the rough crank pin portions in the direction perpendicular to the axial direction so as to increase the amount of eccentricity thereof to the amount of eccentricity of the crank pins of the forged crankshaft.