Milling tool

A milling tool is a rotating cutting tool for milling , which in technical terminology is called milling cutter for short . It has at least one, but mostly several cutting edges, which are usually referred to as cutting edges. It is used on milling machines and machining centers. In contrast to the drill , which only cuts in the direction of the axis of rotation, milling cutters are used for machining perpendicular or at an angle to the axis of rotation. Some milling cutters have cutting edges that extend beyond their center point and are therefore suitable for drilling with restrictions. Router exist in many different designs: With shaft or bore, with inserts or of solid material, for roughing or finishing , rolling cutters , shell end mill , face mill , and shoulder milling cutter plane for the preparation of surfaces, profiled for different grooves , narrow for the separation milling and many more.

Most milling cutters consist entirely of high-speed steel or carbide ; some instead have a steel body and screw-in or clamped indexable inserts. These can also consist of hard metal or of the much harder cutting ceramics .

Cutting edge geometry

Helical toothed end mills are the most commonly used milling cutters. They usually have a right-hand twist of the cutting edges in order to guide the chips upwards away from the workpiece via the chip spaces between the cutting edges. A large helix angle ensures that a cutting edge only disengages when the next cutting edge is already in the cut, and that the cutting edge slowly penetrates the material. As a result, the cutting force of the cutting edges is more even, the service life is extended and the machine runs significantly more smoothly .

See also : spindle fall

Milling cutters can have any number of cutting edges, whereby single cutting edge milling cutters are mainly used in plastics processing and sometimes also in aluminum processing and are used on high-speed milling machines in high-speed machining with motor spindles at up to 200,000 / min.

Milling tools cut on the circumference ( main cutting edge) and on the milling face ( secondary cutting edge ). Milling cutters that are suitable for plunging into the material have a secondary cutting edge extended beyond the axis of rotation, while all other cutting edges stop shortly before. These are called slotted holes (2-3 flutes) or end mills. When plunging vertically, however, you have to work with a reduced feed rate , since with the milling cutter there is no cutting on the face in the center of the axis of rotation, but the material is squeezed there.

Cutting materials

Since the cutting edges of a milling cutter are not constantly in the cut, as is the case with turning , but plunge into the material with each revolution and then retract again, the cutting material (the material of the cutting edges) must have high toughness under sudden loads and insensitivity to extreme temperature fluctuations.

High speed steel

2-edged and 4-edged end mills made of uncoated high-speed steel, below a radius end mill with a rounding at the tip. All have a cylindrical shaft with a thread.

Compared to hard metal, high-speed steel (HSS or HS according to the new standard) often has the decisive advantage of higher toughness and edge strength, which allows larger rake angles . A larger rake angle means a smaller wedge angle , as a result of which the cutting force decreases and thinner chips are possible. Thin-walled or soft workpieces can thus be processed much better. Due to the good shaping possibilities of the HSS, almost all imaginable milling cutters can be made from it. Depending on the application they can with different hard material coatings be provided.

Carbide, ceramic and cermet

• Solid carbide cutters are tools manufactured using powder metallurgical sintering for special milling work on high-strength stainless steels or hardened steels, where the use of indexable inserts is prohibited due to the lack of shaping options.
• Tools with indexable inserts made of hard metal or ceramics increase the cost-effectiveness considerably, since when the tool is worn , not the complete tool, usually a cutter head , has to be exchanged, but only the inserts mounted on it have to be turned or changed.
• Cermet cutters are wear-resistant, but have not caught on because of their low flexural strength .

In the case of hard metal cutting materials, cooling must be ensured, as their cutting ability decreases sharply at high temperatures, while wear increases. In practice, this means that coolant cools the milling cutter with up to 14 bar (saturated jet cooling). In many cases, however, a sufficient supply of coolant, especially during milling, cannot be guaranteed and there is a strong thermal alternating load. As a result, crest and transverse cracks occur in the hard metal. This can only be prevented by doing without coolants and lubricants.

With the very warm-hard cutting ceramics, which react extremely sensitively to temperature changes and thermal shock loads, cooling is always dispensed with, except in rare special cases. In the case of cooled ceramics, hairline cracks occur as a result of the thermal stress due to the temperature difference between the uncooled cutting edge not reached by the cooling lubricant and the cooled remainder of the cutting plate, which significantly reduces the service life.

Polycrystalline diamond and polycrystalline cubic boron nitride

• Polycrystalline diamond (PCD / PCD) is a synthetically produced, extremely hard, intergrown mass of diamond particles with random orientation in a metal matrix. It is by co- sintering of selected diamond particles at high pressure and high temperatures produced. PCD is used as a cutting material in cutting tools for wood, plastic and non-ferrous metal processing. PCD cutting edges offer the longest service life and are usually the most economical solution, especially for larger quantities.

• Polycrystalline cubic boron nitride (PKB / PCBN) is a synthetically produced composite material made of cubic boron nitride (cBN) with a ceramic binder phase. To produce PCBN, cBN micron grains are synthesized from hexagonal boron nitride at high temperatures and pressures. Polycrystalline Cubic Boron Nitride is widely used when machining a wide variety of hard and / or abrasive FE workpiece materials. PCBN is chemically inert up to high temperatures and, unlike PCD, does not react with the iron in ferrous materials.

Differentiation according to the type of recording

The cutters are on the type of entrainment ( plug-on or end mill cutter ), according to the position and shape of the cutting edges (z. B. Walzenstirnfräswerkzeug ) and the purpose (. Eg rotary slot ) distinguished. Arbor milling cutters have a bore with which they sit on a milling cutter arbor between the adjusting rings and the final nut . End mills may offer more precision, as they are attached to the machine spindle using collets and collet chucks or by shrinking the milling cutter arbor, thus ensuring good coaxiality with the spindle axis .

The following milling cutters are generally in use:

Cylindrical milling cutter

End mill with roughing teeth, right with TiAlN coating, center with TiN coating

• End mill
  • Milling cutter with clamping shank at each end, for milling flat surfaces
  • Slot milling cutter , for milling keyways and pockets
  • End mills , deep for milling grooves, contours for chamfering and Gesenkfräsen
  • T-slot cutter , for milling T-slots
  • Slot cutter , for milling disk keyways
  • Angle milling cutter , for milling angular guides

• Shell mill
  • Milling cutter, for milling flat surfaces
  • Shell end mill , for milling corners and flat surfaces
  • Prism milling cutter , for milling prism guides
  • Half-round profile cutter , for milling half-round guides
  • Cutter heads , for milling corners and flat surfaces (milling heads with indexable inserts )
  • Side milling cutters, for milling slots, grooves and cutting

Differentiation according to processing type

A distinction is made between roughing and finishing cutters.

Roughing cutter

Roughing cutters can easily be recognized by the interrupted profile of the tool cutting edge, which enables the chip to be broken quickly and is therefore not suitable for producing a uniform and high surface quality. There is also often a chip breaker on the rake face, over which the removed chip flows. The aim in each case is to achieve a short-chipping behavior of the removed material, which promotes vibrations and thus poorer surface quality, but provides other significant advantages over long-chipping behavior - above all, the significantly better chip evacuation. Due to the good metal removal rate, roughing milling tools are ideally suited for operations in which it is important to remove material as effectively and quickly as possible with a finishing milling tool apart from a finishing allowance and when particularly high dimensional accuracy and surface quality are not required.

Finishing cutter

Finishing cutters usually do not have a profiling of the cutting edge or the rake face, which particularly favors short-chipping behavior. Due to the regularity of the tool cutting edge, the high speed at the same time as the low feed rate of the milling tool and the low chip volume due to the small finishing allowance, depending on the application from 1 to 0.01 mm, the result is high dimensional accuracy and a smooth surface. The climb milling , the one with the virtually play-free feeds of CNC machines with ball screw is possible to improve on earnings.

A serious disadvantage of finishing milling tools is the very rapid wear of the mostly particularly sharp tool cutting edges.

Differentiation according to purpose

Milling cutters can be used for a wide variety of purposes. The best known uses for milling cutters are:

• Face Mills for transferring milling plane surfaces
• Milling cutter for grooves
• Thread milling cutter
• Engraving cutter

and ball mills for mold making

Differentiation according to type

Essentially, a distinction is made between full milling tools and carrier tools for indexable inserts .

Full milling tools

A full milling cutter is a milling tool that carries the cutting edges on the tool body itself. They can be made of either HSS or hard metal . Worn cutting edges on full milling cutters can be refurbished for further use by tool grinding .

Carrier tools

Cutter heads with indexable inserts: shoulder milling cutter (left), face and copy cutter head (center) and face cutter head (right)

Carrier tools are mostly holders for indexable inserts made from medium-strength non-tempered steels. The cutting tips made of hard metal, ceramic or diamond ( PCD ) are either screwed on or clamped individually and can be reused up to eight times depending on the geometry of the cutting tip. In the case of carrier tools with an internal cooling system, coolant bores supply each cutting insert individually with coolant. They are available in different designs, for example as a cutter head . These recordings are very durable, expensive and in the event of damage it is usually worth repairing.

Side milling cutter

Side milling cutters have a disk-shaped body. The diameter is therefore significantly larger than the width. In the middle they have a hole for receiving a milling arbor to attach to the machine. The torque is transmitted via a longitudinal groove in the axial direction or via a transverse groove in the radial direction. They have cutting edges on the circumference and at least one of the end faces, mostly on both. Seats for cutting inserts are incorporated into the base body, which is usually made of steel, as well as chip chambers for receiving the chips and chambers that accept clamps for fixing the inserts. The diameter ranges from 50 to 550 mm and the milling widths from 4.2 to 45 mm. The number of teeth varies from 4 to 50 and depends on the circumference and the tooth pitch.

Side milling cutters are suitable for the production of deep, long, open grooves and for right-angled shoulders. With particularly narrow disc milling cutters, which are known as parting cutters, you can also cut off workpiece parts, similar to a circular saw . They are only 1.6 to 6 mm wide and 50 to 350 mm in diameter. Cutting cutters have driver rings to stabilize the axial position, as they tend to vibrate. Normal side milling cutters have an axially protruding collar in the area of ​​the bore, which widens the tool holder, thus ensuring lateral stability and ensuring reliable transmission of the torque. However, it also limits the groove depth that can be produced.

Side milling cutters can be used on both horizontal and vertical milling machines. Straight-tooth cutters are only suitable for shallow cutting depths; Helical teeth also for larger ones. The cutting edges are inclined alternately to the left and right so that the axial forces are balanced out as much as possible. In addition, the cutting edges penetrate the material only gradually if they are inclined, which leads to lower vibrations. Side milling cutters are suitable for all metallic materials. They are very productive: Often the performance made available by the machine is not enough to take advantage of the milling cutter's capabilities.

There are versions according to DIN 885 as full metal milling cutters made of high-speed steel in types H, N and W (for hard, normal and soft materials). They can be uncoated or coated with TiN or TiCN . Indexable insert milling cutters have a wide range of different cutting materials and insert geometries available. The plates are either screwed or clamped directly into the base body, or they are inserted into exchangeable cassettes. The plates are inserted alternately left and right on each tooth, but in such a way that they overlap on the circumference. Therefore, the minimum width is also limited to about 4 mm for panel widths of 2.4 mm. Wider plates can be used for wider grooves.

End mill

End mills in various sizes with titanium nitride coating (black) and round chip breaker grooves on the circumference for roughing

End mill without coating, without chip breaker grooves with a low helix angle

End mills have an integrated shank that is clamped in the milling cutter holder of the machine. The diameters are about five to ten times smaller than the cutting length. They exist in many different designs and are suitable for the production of slots, grooves, pockets, recesses, dies and hollow forms. The end face is typically designed in such a way that one or more cutting edges cut “across the middle”, which is why they are also conditionally suitable for drilling. This is necessary for making elongated holes , closed grooves and pockets. End mills are intended to produce deep shapes and work with a large axial depth of cut, which is why the feed force can be very large. This leads to a lateral, elastic bending of the milling cutter, which limits the tolerances that can be achieved. When milling in solid material, the bending takes place against the feed direction, so that the accuracy that can be achieved is high. With circumferential and shoulder milling, however, the forces can become very large, so that the surfaces have to be milled over again with a finishing step or with lower feed rates. The available diameters range from 1 to 30 mm; Indexable insert tools are available from 15 mm. There are the versions "short", "long" and "extra long". Small solid carbide milling cutters are available from a diameter of 0.05 mm, for so-called micro - cutting or micro-milling . Both finishing and roughing cutters can contain internal bores for the supply of cooling lubricants and enable the use of minimum quantity cooling lubrication .

Roughing cutter

When roughing, as much material as possible should be removed at a time, so the metal removal rate should be as high as possible. Therefore, large cutting depths are used in the axial and radial directions. In the axial direction they are approximately 0.7 × D to 1.5 × D and in the radial direction up to 0.4 × D. The spiral angle is between 20 ° and 50 °. In the case of soft materials, chips are produced, the length of which exceeds the axial depth of cut. Therefore, chip breaker grooves are provided on the circumference , which break the chips into smaller parts and thus improve chip evacuation. Side effects are the lower stress on the cutting edge and the better access of cooling lubricant to the cutting edges. In terms of the shape of the chip breaker grooves, a distinction is made between a round profile for heavy-duty roughing of very tough material and a flat profile for medium loads and harder materials. In the area of ​​the corner of the cutting edge, the transition from the face to the circumferential cutting edge, there are high loads. The corners are therefore often rounded or chamfered.

Finishing cutter

Finishing cutters are usually made of high-speed steel or hard metal. Cermets and ceramics are only used in rare exceptions. Cermets are used for finishing steel up to a hardness of 50  HRC , ceramics for cast materials. Silicon nitride ceramic is used for nickel-based alloys at cutting speeds of 500 m / min and up.

The finishing cutters are available in numerous geometries: with a cylindrical or conical cutting part, with a flat or rounded face cutting edge , various helix angles, numbers of cutting edges, cutting part geometries and cutting edge rounding. A suitable milling cutter is therefore available for every practically relevant finishing task.

Cutting materials

Most end mills are made of solid material or high-speed steel (coated or uncoated), some also made of hard metal or cermets. Ceramics are also used for milling cutters with an indexable insert. Since the cutting lines overlap on the circumference of multi-row tools, only lower surface qualities are possible with indexable insert cutters. High quality is also possible with single row.

Shafts

There are smooth cylindrical shafts and cylindrical shafts with a thread on the end. There is also the Weldon shank (a cylindrical shank with a flat recess for better torque transmission) and the Whistle Notch shank, which also clamps laterally. Cylindrical shanks are clamped in collet chucks and shrink fit chucks . No lateral force is transmitted when clamping. There are also special chucks for internal cooling lubricant supply.

Milling heads / cutter heads

Milling heads, which are also called cutter heads , are attachable milling tools for face milling. Cutter heads consist of a basic tool with the machine interface and the cutter holder. Older cutter heads had brazed-in cutting edges made of hard metal, modern ones have replaceable indexable inserts made mostly of hard metal, sometimes also of ceramic. Depending on the arrangement and angle of the cutting edges, they are classed as face milling cutters or end face milling cutters but not perimeter milling cutters.

For milling heads, a distinction is made between face milling cutters with a tool setting angle between 45 ° and less than 90 ° and shoulder milling cutters with exactly 90 °. These are also suitable for face milling. The most common are plates with an angle of 75 °. With ceramic cutting edges, milling heads are also suitable for hard milling or hard cutting , with which high levels of accuracy and surface quality can be achieved. It can then be carried out as the last operation instead of grinding . Milling heads with round cutting inserts are also suitable for copy milling or free-form milling .

Milling heads have four to 50 cutting edges and a diameter between 40 and 500 mm. With large diameters, higher cutting speeds can be achieved at the same speed. Therefore, large diameters are used for high-speed machining . Since the cutting corners are generally subject to high loads during milling, inserts with four to eight corners and a large corner angle are usually chosen because they are more stable.

Shell mill and shell end mill

Milling cutter made of solid material

Shell cutters only have cutting edges on the circumference and are only suitable for circumferential milling ; Shell end mills also have cutting edges on the face and are therefore also suitable for face milling . The diameter of the plain milling cutters and the face milling cutters is between 40 and 160 mm; the widths between 30 and 150 mm. The cutting width is smaller than the milling width. The cutting edges and grooves have a twist. There are left and right cutting edges. These cause axial forces that can pull the milling cutter from the holder. It must therefore be ensured that this force points in the direction of the receptacle, which is achieved by a corresponding direction of rotation. In the case of particularly long cylindrical milling cutters, the cutting edges can also be divided in the middle, with an opposing twist in each half. The axial forces then cancel each other at least partially. Shell cutters and shell end mills are shell-type tools.

They are available in the standardized versions H (hard materials), N (normal) and W (soft). Similar to the classification of twist drills , they have different rake angles, helix angles and numbers of teeth. For roughing there are cutting edges with chip breaker grooves. As with the end mills, these can be rounded for particularly difficult machining of tough materials or flattened for hard materials. Type H is used for finishing hard, short-chipping materials and has many teeth and a small helix angle. Type N is used for finishing under normal circumstances. Type W is used for machining soft and ductile materials that tend to produce long chips, is coarse-toothed and has a large helix angle.

As solid tools, they are often made of high-speed steel, some also of solid carbide. There are also shell cutters and shell end mills with indexable inserts. They are used exclusively on horizontal milling machines . They are used to manufacture flat surfaces with a width of up to 150 mm, for right-angled shoulders and for grooves with a small depth (maximum 10 percent of the diameter).

Web links

Commons : Milling Tools  - Collection of images, videos and audio files

Wiktionary: Milling tool  - explanations of meanings, word origins, synonyms, translations

Individual evidence

1. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 257 f.
2. ↑ Dirk Kammermeier: Chapter: (Milling) tools , in: Uwe Heisel, Fritz Klocke , Eckart Uhlmann, Günter Spur (Ed.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014, pp. 423-425.
3. ↑ Dirk Kammermeier: Chapter: (Milling) tools , in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (Ed.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014, pp. 423-425.
4. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 257 f.
5. ↑ Dirk Kammermeier: Chapter: (Milling) tools , in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (Ed.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014, pp. 423-425.
6. ↑ Dirk Kammermeier: Chapter: (Milling) tools , in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (Ed.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014, p. 418 f.
7. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 263.
8. ↑ Dirk Kammermeier: Chapter: (Milling) tools , in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (Ed.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014, p. 419 f.
9. ↑ Dirk Kammermeier: Chapter: (Milling) tools , in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (Ed.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014, p. 418 f.
10. ↑ Dirk Kammermeier: Chapter: (Milling) tools , in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (Ed.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014, p. 419.
11. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 263.
12. ↑ Dirk Kammermeier: Chapter: (Milling) tools , in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (Ed.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014, p. 420.
13. ↑ Dirk Kammermeier: Chapter: (Milling) tools , in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (Ed.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014, p. 420.
14. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 250 f.
15. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 251 f.
16. ↑ Dirk Kammermeier: Chapter: (Milling) tools , in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (Ed.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014, pp. 421–423.
17. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 251 f.
18. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 254.
19. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 253.
20. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 254.
21. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 254 f.