How to optimize the feedrate during milling

In addition to directly affecting the machining efficiency, the feed rate is also one of the key factors that affect the tool life. Feed rate is calculated as follows:

Vf=n×zn×fz

Vf=table feed (mm/min)

n = spindle speed (r/min)

Zn = number of teeth

Fz=feed per tooth (mm/tooth)

It can be seen from the formula that, for a given milling cutter, the feed rate fz per tooth directly determines the feed rate in the case that the number of teeth and the spindle speed are fixed.

How to assign a reasonable value to each tooth feed fz? We must consider the relationship between the maximum chip thickness hex and fz, because for a given milling cutter, there must be a reasonable hex with it, and hex and fz Not a simple one-to-one relationship.

Here we take the square shoulder milling cutter as an example, starting from the relationship between the feed per tooth fz and hex, to explore ways to extend tool life and improve machining efficiency.

In the previous issue, we have already mentioned that milling is a cyclic process in which the cutting edge continuously cuts in and cuts out the workpiece. When a universal (radial) feed is used for the milling cutter, the resulting thickness of the chip is constantly changing (Plunge milling/Axial feed To form a constant chip thickness, and the thickest thickness of the chip is hex.

For milling, two factors affect the relationship between hex and fz. One is the ratio ae/Dc (cutting width factor) of the cutting (milling) width ae to the milling cutter diameter Dc; the other is the leading angle Kr of the milling cutter.

Let us first discuss the effect of ae/Dc on fz. For different ae/Dc, it can be roughly divided into the following three situations:

(1) Full-slot milling: When cutting to the center, the chip thickness is equal to the feed per tooth. At this time, hex=fz, the minimum hex value of most indexable inserts should be 0.1 mm.

(2) ae/Dc> 50%: ae/Dc = 70% is the recommended cutting condition of the face milling cutter. When the blade is cut in, the chip thickness is close to the feed per tooth (0.9×fz), and the insert cuts directly into the workpiece to avoid friction. Not only can the life of the blade be guaranteed, but also efficiency can be taken into account.

(3) ae/Dc<50%: At this time, the maximum chip thickness (hex) is in the cut-in position, the maximum chip thickness hex is less than the feed per tooth fz, and the tool center feed is fz=0.1mm, the real maximum Chip thickness is less than 0.1mm.

Comparing (2) and (3), it can be seen that the same feed fz per tooth does not bring about the same maximum chip thickness hex because of the difference in ae/Dc. For cases where ae/Dc is much less than 50%, fz must be increased accordingly to obtain a reasonable hex value.

We want hex to reach 0.1mm, and if ae/Dc is much less than 50%, if the feed is still fz=0.1mm, the hex at this time will definitely be farther away from 0.1mm. This problem can be solved by increasing the feed per tooth fz.

Therefore, the feed speed calculation formula we mentioned at the beginning of this article should be modified accordingly to obtain reasonable efficiency and tool life:

Vf = n × zn × (hex × correction coefficient 1)

When ae/Dc is less than 25%, the feed should be corrected to obtain the optimum chip thickness.

When using a three-flute cutter to mill the groove, or when the end mill finishes the side wall, because ae/Dc is small, fz must be adjusted to obtain a reasonable hex.

Another influencing factor is the primary angle Kr of the milling cutter. When the primary declination angle is less than 90°, the maximum chip thickness hex is less than the per-teeth feed fz. In order to obtain a reasonable hex value, the fz must be increased accordingly. Therefore, the feed formula needs to be multiplied by another correction factor 2 to get the correct fz:

Vf = n × zn × (hex × correction factor 2)

The feed speed calculation formula mentioned at the beginning of this article has been revised twice. The final formula is as follows:

Vf = n × zn × (hex × correction coefficient 1 × correction coefficient 2)

The optimization of the machining process includes both the optimization of the cutting path and the optimization of the cutting parameters. The purpose of optimizing the cutting parameters is to obtain a reasonable maximum chip thickness hex, which is also the key to the normal operation of the milling cutter.