The entering angle (KAPR) is the angle between the main, leading cutting edge of the insert and the workpiece surface. The entering angle affects chip thickness, cutting forces and tool life.
The most common entering angles are 90 degrees, 45 degrees, 10 degrees and those of round inserts.
When decreasing the entering angle, the chip thickness, hex, reduces for a given feed rate, fz. This chip thinning effect spreads the amount of material over a larger part of the cutting edge.
Smaller entering angles also provide a more gradual entry into the cut, reducing radial pressure and protecting the cutting edge. However, the higher axial forces increase the pressure on the workpiece.
Milling with a 90-degree entering angle
The main application area for a 90-degree cutter is square shoulder milling.
A 90-degree cutter generates mostly radial forces, in the direction of the feed. This means that the surface being machined will not be exposed to high axial pressure, which is advantageous for milling workpieces with a weak structure or thin walls, and in cases of unstable fixture.
Milling with a 45-degree entering angle
A 45-degree cutter is the general purpose choice for face milling. It generates well-balanced radial and axial cutting forces, which is less demanding on the machine power.
This type of cutter is especially suitable for milling in short-chipping materials that easily fritter if excessive radial forces act on the gradually reduced amount of material left at the end of a cut.
The smooth entry into cut limits vibration tendencies when milling with long overhangs or with smaller/weaker tool holders and couplings.
The formation of a thinner chip allows for high productivity in many applications because of the opportunity for higher table feed while maintaining a moderate cutting edge load.
Milling with 60–75 degree entering angles
These type of cutters are special-purpose face mills, offering greater depth of cut compared to general choice face mills. The axial forces are lower compared to a 45-degree face mill cutter and the edge strength is better compared to a 90-degree cutter.
Milling with a 10-degree entering angle
A 10-degree entering angle is used on high-feed and plunge milling cutters. A thin chip is generated that allows for very high feeds per tooth, fz, at small depths of cut and, consequently, extreme table feeds, vf.
The dominating axial cutting force is directed towards the spindle, which stabilizes it. This is favorable for long and weak set-ups, as it limits vibration tendencies.
This type of cutter is effective in hole making when using three axes and for plunge milling of cavities, or whenever the use of an extended cutter is required.
Milling with round inserts or cutters with a large corner radius
A round insert cutter is for general-purpose milling and is efficient for roughing operations.
The corner radius provides a very strong cutting edge, suitable for high table feed rates because of the thinner chips generated along the long cutting edge. The chip-thinning effect makes these cutters suitable for machining titanium and heat resistant alloys.
Depending on the cutting depth variation, ap, the entering angle changes from zero to up to 90 degrees, altering the cutting force direction along the edge radius, and consequently the resulting pressure during the milling operation as well.