When choosing a milling cutter suitable for the machining task, various issues of the geometry, size and workpiece material of the workpiece to be machined must be considered

Milling cutter entering angle

The entering angle is the angle between the cutting edge and the cutting plane. The entering angle has a great influence on the radial cutting force and depth of cut. The magnitude of the radial cutting force directly affects the cutting power and the anti-vibration performance of the tool.

Main declination effect:

The smaller the entering angle of the milling cutter, the smaller the radial cutting force and the better the vibration resistance, but the depth of cut is also reduced.

Example: Shoulder Milling

Use a 90° entering angle when milling planes with square shoulders. This type of tool has good versatility and is used in single-piece and small batch processing. Since the radial cutting force of this type of tool is equal to the cutting force, the feed resistance is large, and it is easy to vibrate, so the machine tool is required to have high power and sufficient rigidity. When machining a plane with a square shoulder, a milling cutter with an 88° entering angle can also be used. Compared with the 90° entering angle milling cutter, its cutting performance is improved to a certain extent. It is also common for face milling with 90° shoulder milling cutters. In some cases, this choice is justified. For workpieces with irregular shapes to be milled, or casting surfaces where the depth of cut varies, a shoulder cutter may be the best choice. But in other cases, the standard 45° is selected. When the penetration angle of the milling cutter is less than 90°, due to the thinning of the chip, the axial chip thickness will be smaller than the feed rate of the milling cutter, and the milling cutter penetration angle will be suitable for it. The feed per tooth has a big effect.

Example: face milling

In face milling, a face mill with a 45° plunging angle results in thinner chips. As the plunge angle decreases, the chip thickness becomes less than the feed per tooth, which in turn increases the feed rate by a factor of 1.4. The radial cutting force of the 45° entering angle milling cutter is greatly reduced, which is approximately equal to the axial cutting force, and the cutting load is distributed on the longer cutting edge, which has good vibration resistance and is suitable for the overhang of the spindle of the boring and milling machine. longer processing occasions. When machining planes with this type of tool, the blade breakage rate is low and the durability is high; when machining iron castings, the edge of the workpiece is not prone to chipping.

How to choose the size of the milling cutter:

The standard indexable face milling cutter diameter specification is Φ16～Φ630mm. The diameter of the milling cutter should be selected according to the milling width and depth. Generally, the larger the depth and width before milling, the larger the milling cutter diameter should be. When rough milling, the diameter of the milling cutter should be smaller; when fine milling, the diameter of the milling cutter should be larger, try to accommodate the entire processing width of the workpiece, and reduce the tool pick marks between two adjacent feeds. When face milling large parts, smaller diameter cutters are used, which leaves a lot of leeway for improving productivity. Ideally, the milling cutter should have 70% of the cutting edge involved in cutting.

Tool size becomes especially important when milling holes with milling cutters. If the diameter of the cutter is too small relative to the hole diameter, a core may be formed in the center of the hole during machining (below).

When this core falls, it can damage the workpiece or tool. If the diameter of the milling cutter is too large, it will damage the tool itself and the workpiece, because the milling cutter does not cut in the center and may collide at the bottom of the tool (as shown in the figure below).

Choice of milling method:

When programming face milling, the user must first consider how the tool will plunge into the workpiece. Usually, the milling cutter simply cuts directly into the workpiece (as shown in the figure below). This type of cut is usually accompanied by a lot of impact noise, because when the insert exits the cut, the chip generated by the milling cutter is the thickest. The high impact of the insert on the workpiece material tends to cause vibration and create tensile stresses that shorten tool life.

A better way to feed the tool is to use the rolling plunge method, that is, the milling cutter rolls into the workpiece without reducing the feed rate and cutting speed (as shown in the figure below). This means that the milling cutter must be rotated clockwise to ensure that it is machined in a climb milling fashion. The resulting chips are thicker to thinner, reducing vibration and tensile stress on the tool, and transferring more cutting heat into the chips. By changing the way the milling cutter cuts into the workpiece each time, the tool life can be extended by 1-2 times. To achieve this type of infeed, the programmed radius of the toolpath should be 1/2 the diameter of the milling cutter, and the offset distance from the tool to the workpiece should be increased.

While the rolling plunge method is primarily used to improve the way the tool plunges into the workpiece, the same machining principles can be applied to other stages of milling. For face milling of large areas, it is common to program the tool to make successive passes along the full length of the workpiece (below) and complete the next cut in the opposite direction. In order to maintain a constant radial engagement and eliminate vibration, a combination of helical lowering and rolling of the workpiece corner is usually better.

We are familiar with the noise associated with machining. It usually occurs when the tool cuts into the workpiece, or when the tool makes a sharp 90° turn in the state of engagement. Roll milling workpiece corners can eliminate this noise and extend tool life. Generally speaking, the corner radius of the workpiece should be 75%-100% of the diameter of the milling cutter, which can shorten the arc length of the milling cutter and reduce vibration, and allow higher feed rates.

Other notes:

In order to prolong tool life, in face milling, the tool should be avoided (if possible) through holes or interruptions in the workpiece. When the face mill passes through the middle of a hole in the workpiece, the tool is down-milling on one side of the hole and up-milling on the other side of the hole, which causes a lot of impact on the insert. This can be avoided by bypassing holes and pockets when programming the toolpath.

More and more manufacturers are using milling cutters to machine holes with helical or circular interpolation. While this method is slightly slower than drilling, it has advantages for many jobs. When drilling on an irregular surface, the drill bit can have difficulty penetrating the workpiece along the centerline, causing the bit to drift off the surface of the workpiece (below). In addition, the drill requires about 10 horsepower for every 25mm of hole diameter, which means that when drilling on low-power machines, the required power may not be optimal. In addition, some parts need to be machined with many holes of different sizes. If the tool magazine capacity of the machine tool is limited, the hole milling method can avoid frequent machine downtime due to tool change.

By choosing the right cutter angle, size and feed so that the cutter cuts into the workpiece material with minimal vibration and tensile stress, and knowing when milling is more efficient than drilling, manufacturers can be efficient , The workpiece blank can be processed into beautiful parts at low cost.