Drill Fluting or Helical Milling
One of the important indexing operations to be performed on Milling Machine is the helical or spiral milling. Generation of flutes in twist drill, milling of helical and spiral gears, milling of worms and helical cutters, etc. are some of the examples of this class. In the case of helical milling, the job is rotated and moved forward by the movement of the table under the rotating cutter. This is done by connecting the worm shaft of the indexing head to the milling table feed screw with the help of a set of gears. Lead of the helix depends upon the rate at which the job is rotated with respect to table movement.
While performing helical milling, the following points must be considered:
- The table of the milling machine must be set at an angle (equal to the helix angle) to the normal position of the table. This is done so that when the job advances and at the same time revolves also, the impression left by the cutter in the job will be identical to the contour of the cutter. The direction in which the table is swivelled determines the hand of spiral, i.e. left hand or right hand
- There must be a proper relation between the rotation of the job and table.
- The job is to be fed to the cutter by the table movement.
Lead of milling machine is the distance through which the table moves when the spindle of dividing head moves through one revolution. Velocity ratio between the dividing head spindle and the table feed screw remain same.
Sometimes it is necessary to introduce change gears between the worm shaft and the table feed screw, because when the table travels a distance equal to the lead of helix to be cut, the job must have completed one revolution. This can be achieved by introducing change gears. The formula given below holds good for the change gears.
The reamer blank is set between the centres of indexing head and tail stock on the table and the fluting coterie, i.e., a form relieved shaped profile fluting cutter (right hand), is mounted on the arbor of the universal milling machine. A double angle milling cutter with the desired radius at the end of the teeth and with 55 angle could also be used. The following procedure is to be adopted for milling the flute:
- Position the blank with respect to the fluting cutter.
- Align the centre of the blank with the centre of the fluting cutter
- Swivel the table to the required helix angle to be cut in the right hand direction.
- Touch the reamer blank against the rotating fluting cutter using a paper feeler.
- Adjust the table to compensate for the thickness of the paper feeler.
- Mount the required change gears on the indexing head and table lead screw.
Table feed should be such that the load caused by the cutting action is directed against the dividing headstock because the headstock centre rotates with the work piece. The tail stock centre, on the other hand is a dead center and the cutting load can impose a heavy load on it, which can result in excessive wear. Usually two types of cuts, a roughing and a finishing cut, are taken through each flute.