An Introduction to Mill cutters and Types
Milling
Milling is a machining process for generating machined surfaces by removing a predetermined amount of material progressively from the workpiece. The milling process employs relative motion
between the workpiece and the rotating cutting tool to generate the required surfaces. In some applications the workpiece is stationary, and the cutting tool moves, whereas in others the cutting tool and the workpiece are moved in relation to each other and to the machine. A characteristic feature of the milling process is that each tooth of the cutting tool takes a portion of the stock in the form of
small, individual chips
Milling methods
■ Peripheral milling (slab milling)
■ Face milling and straddle milling
■ End milling
■ Single-piece milling
■ String or “gang” milling
■ Slot milling
■ Profile milling
■ Thread milling
■ Worm milling
■ Gear milling
MILLING CUTTERS
The most suitable type of milling cutter for a particular milling operation depends on such factors as the kind of cut to be made, the material to be cut, the number of parts to be machined, and the type of milling machine available. Solid cutters of small size will usually cost less, initially, than inserted blade types; for long-run production, inserted-blade cutters will probably have a lower overall cost. Depending on either the material to be cut or the amount of production involved, the use of carbide-tipped cutters in preference to high-speed steel or other cutting tool materials may be justified.
Rake angles depend on both the cutter material and the work material. Carbide and cast alloy cutting tool materials generally have smaller rake angles than high-speed steel tool materials because of their lower edge strength and greater abrasion resistance. Soft work materials permit higher radial rake angles than hard materials; thin cutters permit zero or practically zero axial rake angles; and wide cutters operate more smoothly with high axial rake angles
Cutting edge relief or clearance angles are usually from 3 to 6 degrees for hard or tough materials, 4 to 7 degrees for average materials, and 6 to 12 degrees for easily machined materials.
The number of teeth in the milling cutter is also a factor that should be given consideration
1. Plain milling cutters are either straight or helical ones. Helical milling cutters are preferred for large cutting widths to provide smooth cutting and improved surface quality (Figure 1). Plain milling cutters are mainly used on horizontal milling machines
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| FIG 1 |
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| FIG 2 |
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| FIG 3 |
4. Interlocking (staggered) side mills (Figure 4) mounted on the arbor of the horizontal milling machines are intended to cut wide keyways and cavities.
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| FIG 4 |
5. Slitting saws (Figure 5) are used on horizontal milling machines.
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| FIG 5 |
6. Angle milling cutters, used on horizontal milling machines, for the production of longitudinal grooves (Figure6) or for edge chamfering.
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| FIG 6 |
7. End mills are tools of a shank type, which can be mounted on vertical milling machines (or directly in the spindle nose of horizontal milling machines). End mills may be employed in machining keyways (Figure 7) or vertical surfaces
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| FIG 7 |
8. Key-cutters are also of the shank type that can be used on vertical milling machines. They may be used for single-pass milling or multipass milling operations (Figures 8).
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| FIG 8 |
9. Form-milling cutters are mounted on horizontal milling machines. Form cutters may be
either concave as shown in Figure 9 or convex
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| FIG 9 |
10. T-slot cutters are used for milling T-slots and are available in different sizes. The T-slot is
machined on a vertical milling machine in two steps: Slotting with end mill .Cutting with T-slot cutter (Figure 10)
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| FIG 10 |
11. Compound milling cutters are mainly used to produce compound surfaces. These cutters realize high productivity and accuracy (Figure 11).
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| FIG 11 |
12. Inserted tool milling cutters have a main body that is fabricated from tough and less expensive steel. The teeth are made of alloy tool steel, HSS, carbides, ceramics, or cubic boron nitride (CBN) and mechanically attached to the body using set screws and in some cases are brazed. Cutters of this type are confi ned usually to large-diameter face milling cutters or horizontal milling cutters (Figure 11).
13. Gear milling cutters are used for the production of spur and helical gears on vertical or horizontal milling machines (Figures 12). Gear cutters are form-relieved cutters, which are used to mill contoured surfaces. They are sharpened at the tooth face. Hobbing machines and gear shapers are used to cut gears for mass production and high accuracy demands
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| FIG 12 |
GENERAL-PURPOSE MILLING MACHINES
Milling machines are employed for machining flat surfaces, contoured surfaces, complex and irregular areas, slotting, threading, gear cutting, production of helical fl utes, twist drills, and spline
shafts to close tolerances.
Milling machines are classified by application into the following categories:
1- General-purpose milling machines, which are used for piece and small-lot production.
2-Special-purpose milling machines, which are designed for performing one or several distinct milling operations on definite WPs. They are used in mass production.
The general-purpose milling machines are extremely versatile and are subdivided into these types:
1- Knee-Type Milling Machines
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| FIG 13 |
2. Vertical Bed-Type Milling Machines
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| FIG 14 |
3. Planer-Type Milling Machine
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| FIG 15 |
4. Rotary-Table Milling Machines
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| FIG 16 |
Cutter Mounting
The nose of milling machine spindles has been standardized. It is provided with a locating flange φ H7/h6 and a steep taper socket of 7:24 (1:3.4286) corresponding to an angle of 16° 35.6′
( Figure17)
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| FIG 17 |
to ensure better location of arbor and end mill shanks. Rotation is transmitted to the cutter through the driving key secured to the end face of the spindle. Large face milling cutters are mounted directly on the spindle flange and are secured to the flange by four screws, whereas rotation
is transmitted to the cutter through the driving keys on the spindle (Figure17).
Plain and side milling cutters are mounted on an arbor whose taper shank is drawn up tight into the taper socket of the spindle (2) with a draw-in bolt 1 (Figure 18).
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| FIG 18 |
Milling arbors are long or short (stub arbors). The outer end of the long arbor (3) is supported by an overarm support (5) in horizontal milling machines, and the cutter (4) is mounted at the required position on the arbor by a key (or without key in case of slitting saws) and is clamped between collars or spacers (6) with a large nut.
The system shown in Figure 19 is used in the duplex bed milling machines. On the stub arbors, the shell end mill or the face milling cutters are driven either by a feather key, as shown in Figure 19 a, or an end key (Figure 19b).
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| FIG 19 |
End mills, T-slot cutters, and other milling cutters of tapered shanks are secured with a draw-in bolt directly in the taper socket of the spindle by means of adaptors (Figure 20a). Straight shank cutters are held in chucks (Figure 20b).
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| FIG 20 |
Milling calculations
As a general rule, to give satisfactory performance the number of teeth in milling cutters should be such that no more than two teeth at a time are engaged in the cut. Based on this rule, the following formulas (valid in both SI and English system of units) are recommended:
For face milling cutters,
T = 6.3 D /W
For peripheral milling cutters,
where
T = number of teeth in cutter;
D = cutter diameter in inches (mm);
W = width of cut in inches (mm);
d = depth of cut in inches (mm);
A = helix angle of cutter
In high-speed milling with sintered carbide, high-speed steel, and cast nonferrous cutting tool materials, a formula that permits full use of the power available at the cutter but prevents overloading of the motor driving the milling machine is:
where
T = number of cutter teeth;
H = horsepower (kilowatts) available at the cutter;
F = feed per tooth in inches (mm);
N = revolutions per minute of cutter;
d = depth of cut in inches (mm);
W = width of cut in inches (mm);
K = a constant that may be taken as 0.65 for average steel, 1.5 for cast iron, and 2.5 for aluminum. For metric units, K = 14278 for average steel, 32949 for cast iron, and 54915 for aluminum. These values are conservative and take into account dulling of the cutter in service.
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