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Home >> Technical FAQs >> cbn tool inserts for machining crankshafts

 

cbn tool inserts for machining crankshafts

The crankshaft converts the reciprocating movement of the piston to the rotational motion needed to turn the wheels and other parts of a vehicle. Crankshafts are made of various kinds of steel including forged carbon-steel and steel alloys like vanadium micro-alloy steel. Steel provides good resistance to the mechanical stress and high temperature that the crankshaft is subjected to, during operation of the vehicle. In order to machine the crankshaft to an excellent surface finish, it is vital to use cubic boron nitride CBN tool inserts. These inserts have very hard cutting edges for efficient cutting and high resistance to heat and wear.

External Turning of Counter Weight on CNC Lathe Machines
Turning of the counter weight on the crankshaft is a delicate process. Machining a single surface at a time will make the process long and inefficient. So an insert holder that can machine two surfaces at the same time and absorb vibration is more suitable for this operation. Preferably, use a cutting speed of 150 m/min, a feed rate of 0.25 mm/rev, and cutting depth of 2.0 mm. Since this process involves heavy interrupted cutting, the insert holder should have a double clamp that will keep the insert secure during turning. A good example of a suitable cbn insert is the SNMG 150612. It has a zero degree clearance angle and a double chip breaker for slow and effective removal of large chips.

Turning of Pulley shaft (Roughing) on CNC Lathe Machines
Roughing of the pulley shaft at the extreme end of the crankshaft must be done with care to prevent excessive vibration or distortion. With a cutting speed of 200 m/min, a feed rate of 0.25 mm/rev, and cutting depth of 2.0 mm or less, the operation will be easier and faster. It is advisable to use an insert holder with a double clamp and jaw type holder which minimizes vibration and guarantees long tool life even during interrupted cutting. The hexagonal tipped polycrystalline cubic boron nitride insert, WNMG 080412, which has a double chip breaker and zero degree angle at the cutting face is suitable for roughing of the pulley shaft.

Turning of Flange Outer Diameter (Roughing)
Roughing of the flange at the other extreme end of the crankshaft also requires slow and delicate machining. But it requires a longer cutting depth than the pulley shaft. The cutting speed should be about 180 m/min and the feed rate could be set at 0.25 mm/rev with a 2.5 mm depth of cut. A strong tool holder with a secure grip should be used for this operation. This will reduce the vibration and give a high quality surface finish. To provide the required cutting depth, use the diamond shaped insert, CNMG 120412. It has a longer cutting edge than the WNMG insert and it will remove more material in a single pass.

Conclusion
Due to the unique non-homogeneous nature of the crankshaft, it causes a lot of vibration during machining. Therefore, all machining, especially turning and milling, should be done at very low cutting speeds. For better absorption of vibration and faster cutting speeds, solid cbn inserts with dimpled clamping may also be used.

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