Broaching tool

A broach

Various workpiece cross-sections and associated tools

A broaching tool is a cutting tool for broaching and is used on broaching machines. The elongated tools have several cutting edges one behind the other, each of which increases by the thickness of the chip . The tooth feed is therefore integrated in the tool; there is no feed movement. When broaching the tool, it is pulled through an existing hole or, when broaching outside, it is guided along the outside of it. High- speed steel is almost always used as a cutting material (tool material) ${\displaystyle h}$ ${\displaystyle f_{z}}$utilized. The tools consist of a shaft, a cutting part in the middle and the end piece as well as guides for centering. Broaching is very economical and productive, but because of the high tool costs it is only suitable for larger quantities.

Types

A multi-part broach

There are different types of broaching tools. Broaches are most commonly used. They are used for interiors and are pulled through the hole. In some cases, however, all broaching tools are also referred to as broaches. Tools that are pushed are called a broach . Broaches and mandrels are manufactured as one-piece tools up to a diameter of 150 mm. Multi-part pieces are manufactured with either broaching bushes or broaching inserts and are suitable for diameters up to 500 mm. Such large diameters are required for the interior of ring gears ; their length ranges from 100 mm to 10 m. Broaching tools, which consist of several bushings and are clamped on a mandrel, have advantages in terms of manufacturing technology: They bend less during hardening , which means that the allowance for subsequent grinding can be lower. There are also tools that consist of a simple elongated base body into which the cutting edges are inserted.

When broaching profiles , cutting disks are also used, which are screwed onto a block. If the cutting edges become blunt, they have to be reground and become smaller in the process. In the case of cutting discs, a new disc can be screwed on to its full extent and the front one removed.

For tube broaching (also called pot broaching), which is suitable for producing closed outer profiles, tools are used that consist of an elongated hollow cylinder. Inside there are guide strips and cutting edges. Usually the workpiece is pushed through.

For rotary turning broaching, there are special tools whose cutting edges are not arranged linearly, but on a circular path. The tool is swiveled during machining.

Most tools are made of solid material, mostly high-speed steel. For special applications, such as hard machining , rotary turning broaching or large-scale production, there are also tools with interchangeable indexable inserts . With changed insert geometries, they can be quickly adapted to other workpiece geometries.

Cutting materials

The standard cutting material for broaching tools is high-speed steel . When broaching, there are usually only low cutting speeds of a maximum of 30 m / min, which leads to low temperatures of no more than 600 ° C. High speed steel is therefore usually completely sufficient. Another reason for high-speed steel, in addition to its low cost, is its good grindability. Broaching tools usually have a rather complicated cutting edge geometry and are often reground. Tools equipped with carbide cutting edges are also used for machining gray cast iron . Cubic boron nitride can also be used for hard broaching or for gray cast iron . To increase the service life , tools for series production are often coated with titanium nitride or titanium carbonitride .

Components

Broaching tools consist of a middle part with cutting edges and a shank at the narrow end that is clamped into the machine to pull the tool. An insertion part is attached to the shaft and facilitates the insertion into the bore. This is followed by the cutting part, which can be divided into three areas:

• Roughing part for roughing (rough machining), with a relatively large chip thickness and correspondingly high tooth feed.
• Finishing part for finishing (fine machining), with less chip thickness and tooth feed. The last finishing tooth defines the final contour.
• Calibration or spare part. It consists of several reserve teeth of the same size, which compensate for the change in dimension of the finishing teeth due to wear. If the roughing and finishing teeth have to be reground due to wear, they will be smaller. The first calibration tooth becomes the last finishing tooth. Broaching tools are the only cutting tools with spare teeth, which is due to their high cost.

At the end of the cutting part there is another guide part to stabilize the position in the finished inner contour, as well as another end piece for clamping in the machine. Usually they are held on this part and inserted into the bore. Then another clamping device grips the front part from the other side of the hole and pulls the broach through the hole. After the workpiece has been removed from the machine, the first clamping device again grabs the end piece and moves back to the starting position.

The length of the cutting part results from the number of teeth and the tooth pitch, i.e. the distance between two teeth. He is an important constructive variable. If the pitch is too small, the separated teeth cannot be accommodated in the gaps; if the pitch is very large, machining takes a long time and becomes uneconomical. Usually, broaching tools in the roughing, finishing and sizing part have different tooth feeds and pitches.

Cutting geometry and chipping sizes

In contrast to other cutting tools, the chipping sizes are firmly integrated in the tool. The designer of the tool thus also defines the process parameters. The distance between two consecutive teeth perpendicular to the cutting direction directly results in the tooth feed (feed per tooth) , which is identical to the chip thickness . The cutting width corresponds as when sawing the width of the tool and hence also the width of the groove produced in the plan areas. With the angle of inclination, which indicates the inclination of the cutting edges in relation to the cutting direction, the cutting width is obtained through ${\displaystyle f_{z}}$ ${\displaystyle h}$ ${\displaystyle a_{p}}$ ${\displaystyle \lambda }$ ${\displaystyle b}$

${\displaystyle b={\frac {a_{p}}{\cos \lambda }}}$.

The chip cross-section per cutting edge results from ${\displaystyle A}$

${\displaystyle A=b\cdot h=a_{p}\cdot f_{z}}$.

Chip chambers, which are used to hold the chips, are located between two adjacent teeth. As with sawing, the distance between two teeth in the cutting direction is called pitch. The arrangement of the teeth is called staggering. If each tooth removes material over its full width and each subsequent one penetrates deeper into the material, the tool is designed in depth graduation. If, on the other hand, individual narrow teeth are laterally offset, one speaks of side staggering. Combinations are also possible.

The choice of cutting edge geometry depends on numerous factors. This includes the material, the clamping of the workpiece, the cutting material and the coating of the tool, the cutting speed, the machine dynamics and the cooling lubricants . The designations of the cutting edge geometry are specified in DIN 1415-1 and DIN 1409.

Tooth feed / chip thickness

The tooth feed and thus also the chip thickness depends on the material and the type of graduation. It is also greater in the roughing part than in the finishing part. When broaching steel, the thicknesses are between 0.01 mm to 0.15 mm in the roughing part and 0.003 mm to 0.025 mm when finishing. The following table provides reference values.

material	Chip thickness h in mm
	Depth graduation		Page staggering
	Roughing	Finishing
Steel / cast steel / cast iron with spheroidal graphite	0.01 to 0.15	0.003 to 0.025	0.8 to 0.25
Cast iron with flake graphite / non-ferrous metals	0.02 to 0.2	0.01 to 0.04	0.1 to 0.5
plastic	0.02 to 0.06		0.1 to 0.5

division

The distance between two teeth is the pitch . The larger it is, the longer the chip chambers lying between the teeth. However, the tool becomes longer and the machining time therefore increases. With a smaller pitch, more teeth are in mesh at the same time, so that the machine's performance can be better utilized. However, the chip chamber requires a certain minimum cross-section to accommodate the chips. If the depth of the chip chamber is increased, this weakens the stability of the cutting edges. ${\displaystyle t}$

The chip requires more space the longer the broaching length (cutting path, chip length , path that the cutting edges cover in the workpiece), the greater the chip thickness and the larger the chip space factor (also the number of chip spaces ). Experience has shown that the necessary division can be estimated using the following formula: ${\displaystyle l}$ ${\displaystyle R}$

${\displaystyle t=2,5{\text{ bis 3 }}\cdot {\sqrt {A}}_{k}=2,5{\text{ bis 3 }}\cdot {\sqrt {h\cdot l\cdot \mathbb {R} }}}$

The factor 2.5 applies to finishing, 3 to roughing.

The following table contains reference values ​​for the chip space factor for tools made of HSS. If tools with other cutting edges are used, the pitch must be adjusted accordingly in order to obtain the appropriate chip space cross-section. The small values ​​apply to favorable circumstances such as low cutting speed, good chip removal from the chambers, small chip thickness, angle of inclination greater than zero or brushes to remove the chips. The desired surface quality, tool wear and the machines also play a role.

material	Chip space factor R
	Internal broaching tool		External broaching tool
	Flat	Round	Depth graduation	Page staggering
Steel , cast steel , cast iron with spheroidal graphite	5 to 8	8 to 16	4 to 10	1.8 to 6
Cast iron with flake graphite , non-ferrous metals , plastics	3 to 7	6 to 14	3 to 7	1 to 5

Rake, clearance and bevel angles

The rake angle influences chip formation, the forces that occur and the stability of the cutting edges. The clearance angle has an impact on wear and friction. The higher the cutting speed, the larger both angles are chosen. With a larger rake angle, the chip curls up better and with a larger clearance angle, less material sticks to the flank. When choosing the angles, it should also be noted that they influence the shape of the tool after grinding.

For a long time, the angle of inclination was almost always zero. Only in the outdoor area were inclination angles greater than zero. The reason for this was the high cost of grinding complicated cutting edge shapes. Since 2015 at the latest, grinding machines have been available with which inclination angles can be ground in at low cost. This means that the cutting edges do not immediately come into full width contact with the workpiece, but gradually, which leads to a slower increase in the cutting force. This also applies when the cutting edges exit the material. Via lower vibrations, this leads to better accuracy as well as less noise and easier chip evacuation.

For long chipping materials (mostly soft tough materials such as aluminum) which can chip break be problematic. If the chips don't break, they tend to get tangled. The cutting edges are then divided into two unequal halves. The chip breaker groove lies between them. This improves chip breaking. The chip breaker grooves are then arranged laterally offset on the next cutting edge.

graduation

The type of teeth that follow one another is called staggering. The standard is the depth graduation in which every tooth penetrates the material over its full width and every additional tooth penetrates deeper by the thickness of the chip. In the case of castings with a hard cast skin or forgings, on the other hand, staggered sides are preferred. Here strips are cut that are perpendicular to the tool. The tool load is then lower, but more cuts and thus longer tools are required. The finishing takes place in depth grading. A combination of depth and side graduation is called wedge graduation.

Individual evidence

1. ↑ Fritz Klocke , Wilfried König: Production Process Volume 1: Turning, Milling, Drilling , Springer, 8th Edition, 2008, p. 483.
2. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 324.
3. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 335 f.
4. ↑ Fritz Klocke, Wilfried König: Production Process Volume 1: Turning, Milling, Drilling , Springer, 8th Edition, 2008, p. 486.
5. ↑ Fritz Klocke, Wilfried König: Production Process Volume 1: Turning, Milling, Drilling , Springer, 8th Edition, 2008, p. 488.
6. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 336.
7. ↑ Fritz Klocke, Wilfried König: Production Process Volume 1: Turning, Milling, Drilling , Springer, 8th Edition, 2008, p. 488.
8. ↑ Herbert Schönherr: Machining , Oldenbourg, 2002, p. 336.
9. ↑ Fritz Klocke, Wilfried König: Production Process Volume 1: Turning, Milling, Drilling. Springer, 8th edition, 2008, p. 460 f.
10. ↑ Fritz Klocke, Wilfried König: Production Process Volume 1: Turning, Milling, Drilling. Springer, 8th edition, 2008, p. 491.
11. ↑ Herbert Schönherr: Machining production. Oldenbourg, 2002, p. 335.
12. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. Springer, 9th edition, 2010, p. 308.
13. ↑ Fritz Klocke, Wilfried König: Production Process Volume 1: Turning, Milling, Drilling. Springer, 8th edition, 2008, p. 484.
14. ↑ Herbert Schönherr: Machining Technology , Oldenbourg, 2002, pp 334-336
15. ↑ Alfred Herbert Fritz, Günter Schulze (ed.): Manufacturing technology , Springer, 9th edition, 2010, pp. 308-310.
16. ↑ Christoph Klink, Karlheinz Hasslach, Walther Maier: clearing , p. 469, 473 f. in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (Eds.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014.
17. ↑ Christoph Klink, Karlheinz Hasslach, Walther Maier: clearing. P. 469 in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (Eds.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014.
18. ↑ Christoph Klink, Karlheinz Hasslach, Walther Maier: clearing. P. 469 in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (Eds.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014.
19. ↑ Christoph Klink, Karlheinz Hasslach, Walther Maier: clearing , p. 470 in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (eds.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014.
20. ↑ Herbert Schönherr: Machining production. Oldenbourg, 2002, p. 332.
21. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer Vieweg, Berlin / Heidelberg 2015, p. 332.
22. ↑ Christoph Klink, Karlheinz Hasslach, Walther Maier: clearing , p. 471 in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (ed.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014.
23. ↑ Herbert Schönherr: Machining Technology , Oldenbourg, 2002, p 330th
24. ↑ Christoph Klink, Karlheinz Hasslach, Walther Maier: clearing. S. in: Uwe Heisel, Fritz Klocke, Eckart Uhlmann, Günter Spur (Eds.): Handbuch Spanen. 2nd edition, Hanser, Munich 2014.
25. ↑ Herbert Schönherr: Machining production. Oldenbourg, 2002, p. 330.
26. ^ Alfred Herbert Fritz, Günter Schulze (Ed.): Manufacturing technology. 11th edition. Springer Vieweg, Berlin / Heidelberg 2015, p. 332.