In this article, you will learn What is up milling and down milling and its difference between up milling and down milling.
The various milling processes performed by the different milling cutters may be grooved under two separate headings peripheral milling and face milling. The cutting action of milling cutters to perform the above processes are described below.
Peripheral Milling or Horizontal Milling
Here, the finished surface is parallel to the axis of the cutter dis machined by cutter teeth located on the periphery of the cutter, as shown in the fig.
The quality of surface generated and the shape of the chip formed is dependent upon the rotation of the cutter relative to the direction of feed movement of the work. According to the relative movement between the tool and the work.
Peripheral milling is classified under two headings, milling and down milling, according to the relative movement between equipment and work. The cutting processes involved in up milling and down milling gear described below.
The up milling, which is also called conventional milling, is the process of removing metal by a cutter which is rotated against the direction of travel of the workpiece. The up milling operation is illustrated in Fig.
The thickness of the chip in up milling is minimum at the beginning of the cut and it reaches to the maximum when the cut terminates. As the chip thickness per tooth, is not uniform, the cutting in up milling increases from zero to the maximum value per tooth movement of the cutter. The cutting force is directed upwards and this tends to lift the work from the fixtures.
In up milling, due to the typical nature of the cut, difficulty is experienced in pouring coolant just on the cutting edge from where the chip begins. As the cutter progresses, the chip accumulate at the cutting zone and may carried over with the cutter spoiling the work surface.
The surface milled by up milling appears to be slight wavy as the cutter spoiling the work surface. The surface milled by up milling appears to be slight wavy as the cutter teeth do not begin their cut as soon as they touch the work surface.
The teeth slide through a minute distance at the beginning and then cut is started. The up milling process, being safer, is still commonly used in spite of having so many disadvantages.
The down milling, which is also called climb milling, is the process of removing metal by a cutter which is rotated in the same direction by a cutter which is the workpiece rotated in the same direction of travel. The down milling is illustrated in Fig.
The thickness of the chip is maximum when the tooth starts its cutting and when the cut is finished it becomes minimal. The cutter tooth starts removing metal immediately on reaching the work surface, without slipping as it can apply a sufficient bite on the work.
The cutting force in down milling is also variable throughout the cut, it is maximum when the tooth begins its cut and it reduces to the minimum when the tooth leaves the work. In down milling, the fixture design becomes easier as the direction of the cutting force is such that it tends to seat the work firmly in the work holding devices.
The chips are also disposed off easily and do not interfere with the cutting. The coolant can be applied directly to the cutting area where the cutting force is maximum.
This results in improved surface finish and diminishes the heat generated. The down milling operation having so many advantages cannot be used on old machines due to the backlash error that may be present between the feed screw of the table and the nut.
The backlash error causes the work to be pulled below the cutter when the cut begins and leaves the work free when the cut is terminated. The sea action is repeated as soon as the next tooth engages the work.
Difference Between Up Milling and Down Milling:
In up milling, the direction of the cutter rotation is opposite to that of work done. Whereas in the down milling, the direction of the cutter rotation coincides with the feed direction.
In up milling, the thickness of the chip is minimum at the beginning of the cut and it reaches to maximum when the cutting ends. Whereas in the down milling, the cutting forces vary as a maximum at the beginning and minimum at the end.
In up milling, cutting forces varies as a minimum at the beginning to a maximum at the end. Whereas in the down milling, the cutting force varies as a maximum at the beginning and minimum at the end.
In up milling, The force of cutting tends to lift up the work since it is directed upwards. Hence fixturing is difficult. But in the down milling, The force of cutting is directed downwards and keeps the job pressed against the table. Hence fixturing is easy.
The up milling is difficult for efficient cooling. But in the down milling, the coolant can easily reach the cutting edge.
In up milling, the chip accumulates at the cutting zone and interfere with cutting, spoiling the surface. Whereas in the down milling, the chip disposal is automatic and do not interface in cutting.
In up milling, the cutter doesn’t begin cutting as soon as it touches the job and hence the surface appears wavy after milling. Whereas in the down milling, it starts removing metal immediately it reaches the job, the material is removed without sliding.
The up milling is safer and most commonly used in spite of its disadvantages. As compared to down milling, it can be done only on rigid machines equipped with backlash eliminators.
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