Klocke F. Manufacturing Processes 1: Cutting

Подождите немного. Документ загружается.

9.4 Sawing 441

Workpiece

Tool

Tooth feed

Tooth height

Base radius

t Tooth pitch

Cutting speed

Feed rate

Active speed

Fig. 9.71 Title and part geometry on the cutting saw band, after RENG [Reng76]

Standard restriction (SD)

Group restriction (GS)

Right-left-restriction (RL)

Waves restriction (WS)

Fig. 9.72 Sawband

restriction types (Source:

Wi kus)

442 9 Processes with Rotational Primary Movement

passive force. In contrast to other cutting methods, the tool life is not given as the

measure for the tool life parameter of a saw, but the cut surface (corresponding to

the workpiece surface cut until reaching the end of tool life) [Reng76].

9.4.2 Hacksawing

Hacksawing is a process variant with a repeated, usually linear cutting motion

(Fig. 9.70). According to DIN 8589-6, hacksawing, gangsawing, and jigsawing are

all hacksawing processes [DIN8589f]. From this deﬁnition, we can already see that

hacksawing is a process with a discontinuous cutting motion, i.e. with machine

hacksaws, material is removed only in the forward stroke. In the return stroke, the

saw blade is mechanically or hydraulically lifted. The result is that the material

removal rate is lower compared with bandsawing or circular sawing. On the other

hand, the cutting loss is relative small.

The saw blades are made either of solid HSS or of HSS segments that are riv-

eted on a blade body. Blades made of tool steel are generally only used for manual

hacksaws.

9.4.3 Circular Sawing

Circular sawing is sawing with a continuous cutting motion using a circular rotating

saw blade (Fig. 9.73). This high-performance process is employed for linear cuts on

low-cost materials, as much material is lost due to the relatively wide kerf.

Fig. 9.73 Tooth segments of

a saw blade (Source: Ohler)

9.4 Sawing 443

Both tool steel and HSS as well as cemented carbides as used as cutting tool

materials. At the same time, there are fundamental differences among construction

types of saw blades. Saw blades made of one material are made of tool steel or HSS.

In the case of larger saw blades. For cost reasons, the blade body is made of con-

struction steel upon which the individual HSS segments are riveted (Fig. 9.73). The

material removal can be improved further by means of soldered cemented carbide

cutting edges.

In order to guarantee the cutting capacity of circular saw blades, it is absolutely

necessary to break the chips in such a way that they are narrower than the kerf. If

such measures are neglected, the chips jam in the chip space and can damage the

tool. Figure 9.74 shows the two most prevalent possibilities [Schm80].

By subdividing the teeth into pre-cutting and post-cutting elements, the chips are

fractured such that the pre-cutting teeth protrude at least by the amount of chip

thickness and their major cutting edge length is smaller than the total width of

undeformed chip.

One alternative is grinding-in offset chip breaker ﬂutes into the otherwise identi-

cally formed teeth. In this way, one narrow and one wide chip is produced per tooth

that can escape towards the centre into the chip breaker ﬂute without jamming in

the kerf. The fact that a tooth in the section of the chip breaker ﬂute of the previous

Front Incisor

Chip flute

Chip cross section of

the front incisor

Chip cross section of the

after cutting tooth

After cutting incisor

Fig. 9.74 Cutting edge geometries of circular saw blades

444 9 Processes with Rotational Primary Movement

tooth must remove a larger chip cross-section has only an insigniﬁcant effect on the

progression of wear. The advantage is that one tooth with a chip breaker ﬂute can

take on the same chip cross-section as the pre-cutting and post-cutting teeth of the

other lead version.

In order to increase the circular saw blades’ stability and running smoothness, it

is necessary to introduce characteristic stresses into the blade in a speciﬁc fashion.

This is done either by hammering or by rolling a concentric pressure zone into the

side surfaces. A laser radiation process is also possible.

For separating general construction steel with HSS saw blades, values of v

18−30 m/min and f

= 0.22−0.28 mm and with cemented carbide saw blades values

of v

= 90 − 150 m/min und f

= 0.12 − 0.18 mm are common as cutting data.

Chapter 10

Processes with Translatory Primary Movement

Cutting processes using a geometrically deﬁned cutting edge that carry out a

translatory main motion include, among others:

• broaching,

• shaving,

• planing and

• shaping as whose process variants.

10.1 Broaching

Broaching is a machining process with a multi-toothed tool whose cutting teeth

lie in a row, each being separated by the thickness of one chip. Tooth graduation

perpendicular to the direction of the cutting speed replaces the feed motion. The

cutting motion is translatory, in special cases also helical or circular [DIN8589e].

Broaching can realize a high material removal rate in one stroke, since usually

several teeth are simultaneously in engaged. Moreover, high surface qualities and

precision are obtainable and tolerances of up to IT 7 maintained. This method can

only be utilized economically in serial production due to the high costs of tool pro-

duction and preparation, as the tools can always only be used for one cross-section

of undeformed chip [Kraz77].

DIN 8589-5 draws a distinction between:

• face broaching,

• circular broaching,

• helical broaching,

• proﬁle broaching and

• form broaching.

We also distinguish between internal and external broaching, for which dif-

ferently designed machined tools and tools are required. The broaching tool is,

as shown in Fig. 10.1, pulled/pushed through a borehole (internal broaching) or

pulled/pushed along the external surface of the workpiece (external broaching). The

ﬁnal contour is usually created in one stroke.

445

F. Klocke, Manufacturing Processes 1, RWTH edition,

DOI 10.1007/978-3-642-11979-8_10,



Springer-Verlag Berlin Heidelberg 2011

446 10 Processes with Translatory Primary Movement

Workpiece

Tool

Internal broaching

External broaching

Workpiece

Tool

Undeformed chip

thickness

Workpiece holder

Fig. 10.1 Contact ratios and process variants of broaching

To process unhardened steel materials, in most cases tools made of high speed

steel are employed. In certain cases, such as in large-batch production of grey cast

iron or broaching hardened steels, cemented carbide or CBN are also used as cutting

tool materials. To increase performance and wear resistance, the tools can be coated

with hard materials (e.g. TiN, TiCN). The tools are either designed as solid units or

ﬁtted with indexable inserts [Wege85, Merk80].

The standard cutting speeds in steel-cutting are between v

= 1−30 m/min.

Powerful broaching machines can reach cutting speeds of up to v

= 120 m/min so

that the surface quality is improved by avoiding built-up edge formation [Schü65,

Opfe81]. Further cutting speed increase is only possible to a limited extent because

of the process kinematics, since acceleration and deceleration must take place in one

stroke.

Cutting ﬂuids are almost always used in broaching processes in order to guaran-

tee chip transport from the chip space in addition to the lubricative effect [Falk70].

To this end, usually oils are used due to the low cutting speeds.

When machining hardened steels, high cutting speeds of v

= 60−70 m/min and

cemented carbide tools are used to reduce the high cutting forces and to increase tool

life. Cutting ﬂuids are not used in this case [Klin93].

The following description will be restricted to the most important of process

variants. Technologically comparable methods will be treated jointly.

10.1.1 Face and Circular Broaching

Figure 10.2 shows the typical structure of a broaching tool. Broaching tools have

a roughing, a ﬁnishing and a calibrating section. The subsections differ in feed per

tooth f

In the roughing section, the feed per tooth f

is in the range of f

= 0.1−0.25 mm

depending on the workpiece to be machined, while in the ﬁnishing section is it

= 0.0015−0.04 mm and in the calibrating section equal to zero. The teeth in the

calibrating section possess the geometry required for the workpiece.

10.1 Broaching 447

Finishing tooth

Rear pilot

Sizing tooth (f

= 0)

Roughing tooth

Fig. 10.2 Internal broaching tool (schematic)

The calibrating section makes it possible for the expensive broaching tools to be

reground and thus be economically employed. Regrinding the tool shifts the entire

tool proﬁle by one tooth towards the calibrating section. In this way, the previously

ﬁrst calibrating tooth now becomes the last ﬁnishing tooth.

The tool rake angle γ and the tool orthogonal clearance α of the second ﬂank

face are adjusted to the material to be machined. The chamfer width b

fα

is parallel

to the axis in the calibrating section and inclined by the tool orthogonal clearance of

the ﬁrst ﬂank face α

in the roughing and ﬁnishing section.

The cutting pattern determined by the arrangement of the cutting edges along the

tool axis is called the offset. If the cut is perpendicular to the broaching surface, it

is called offset in depth. If the broaching surface is machined from the side on the

other hand, it is called lateral offset. Both offset types can be provided on one tool

simultaneously depending on the workpiece geometry [DIN1415].

The pitch t of the broaching tool and thus the size of the chip space as well depend

on the height of the workpiece, chip formation of the material and the maximum

potential tool length. A large workpiece height and unbroken chip forms require

large chip spaces. In the case of small machines and short tools, only a small pitch

is required, since only workpieces with a low height can be machined. Figure 10.3

provides an overview of various broaching tools for internal and external machining.

Possible tool lengths are in the area of L = 100−10000 mm. Internal broaching tools

are manufactured up to a diameter of D = 500 mm (e.g. for the internal gearing of

hollow wheels). To obtain chip fracture, the individual teeth are equipped with chip

breakers in the form of tooth grooves [Weul85, Hoff76].

The total cutting force, which is comprised of the individual components of all

the cutting teeth in action, is crucial for the minimal cross-section of the broaching

tool shank, which in turn limits the maximum size of the chip space. The height

448 10 Processes with Translatory Primary Movement

Fig. 10.3 Internal and

external broaching tools

(Source: Forst)

Total cutting force

Broaching stroke

m = 3

Tool

= 0°

Total cutting force

m = 2,5

Broaching stroke

tan

(

)

tan

(

)

≥

m =

Pitch t

Cutting length l

Fig. 10.4 Total cutting force during straight and diagonal toothed external broaching

and variation of the total cutting force are highly contingent on the pitch t and the

inclination λ

; an example is shown in Fig. 10.4.

If the ratio m of the cutting length (i.e. of the workpiece height to be broached)

and the pitch t is an integer, we obtain a constant total tensile force. In the case of

a non-integer ratio, the tools are highly stressed dynamically by periodic variations

[Vict76].

10.1 Broaching 449

Tools with an inclination λ

not equal to zero show a slowly increasing cutting

force proﬁle and, if the construction is favourable, minimal cutting force variations.

In the case of external broaching, lateral forces can arise that sometimes cause tool

misalignment. Double helical gearing is characterized by the fact that the teeth of

one level have the opposite inclination, thereby compensating the lateral forces. On

internal broaching tools, an inclination λ

not equal to zero can be realized by a

coil-shaped cutting edge. In practice, inclinations of up to λ

= 5

◦

are used.

10.1.2 Proﬁle Broaching

One important variant of internal proﬁle broaching is gearwheel broaching of

involute-geared hollow wheels, which has become increasingly important because

of its high cutting performance and obtainable s urface quality. Figure 10.5 shows

a few application examples, including hollow wheels for automatic transmissions,

sliding sleeves and components with internal splined hub proﬁles.

In the case of the gearwheel broaching processes used today, all tooth gaps are

machined simultaneously, so that a partial device can be dispensed with, and the

quality of the broached gearwheel depends above all on the precision of the tool.

For diameters up to 150 mm, solid tools are used, in the 150–300 mm diameter

range primarily bushings clamped to a pin. This has the advantage that the tool can

be subdivided into several bushings to avoid excessive deﬂection during hardening,

so that the grinding allowance can be kept small.

The efﬁciency of broaching can be fully exploited by using broaching tools that

make it possible to broach involute proﬁles in one pass. Since, in the case of pure

offset in depth (in which the proﬁle of the tooth ﬂank is formed by the minor cutting

edges) the proﬁle’s dimensional accuracy is insufﬁcient, a lateral offset is provided

in the rear of the tool so that one can calibrate to the ﬁnished dimensions with a feed

motion in the direction of the tooth ﬂank.

Complex broaching tools can be composed of cutting discs that are bolted on a

holder as a block. The tooth thicknesses of the cutting discs are graduated so that the

Fig. 10.5 Examples for

internal broaching

(Source: Forst)

450 10 Processes with Translatory Primary Movement

Fig. 10.6 Broaching tools for ﬁnishing internal toothed centre gears (Source: Forst)

ﬁnal tooth cuts to the ﬁnished dimensions. It is possible to shift the discs, becoming

ever smaller by regrinding, forward always by one position and to append a disc

with full dimensions only on the respective end of the broaching tool. Each disc has

two cast iron plugs to keep the discs in perfect alignment. These plugs are perforated

in case of an exact seating of the discs so that two discs can always be pinned to each

other.

Pre- and ﬁnish broaching can be executed either on one machine in one clamp-

ing (ﬁnish broaching tool as an attachment on the pre-broaching tool) or in two

clampings with two separate tools (pre- and ﬁnish broaching in two clampings).

To increase output, one can also work parallel in several clampings if the machine

power is sufﬁcient. Pre- and ﬁnish broaching can be distributed among two separate

machines in order to exclude a mutual inﬂuence of the machining processes and to

guarantee improved quality [Schw71].

Another tool concept is the one-piece, full-dimension cutting broaching bush-

ing as shown in Fig. 10.6. It also has major cutting edges running parallel to the

involute form and is used separately or as an attachment on a broaching tool in

order to work in one clamping. The great advantage of calibrating bushings is that

they can be used to produce helical gearings among other things (spiral broaching)

[Bung74].

The full-dimension cutting broaching bushing is mounted in a ﬂoating fashion

so that it can centre itself in the pre-broached proﬁle. In the case of helical geared

broaching tools, the chip spaces can be arranged both in a ring-shaped or in a helical

manner. In the case of helical chip spaces, cutting force variations are much smaller,

but they cause considerably higher tool manufacturing and sharpening costs.

“Pot” or “tubular” broaching is used for the external proﬁle broaching of close

surfaces. The tool consists of a hollow body in which are inserted the strip-shaped

broaching tool segments. In this broaching technique, the tool is the moving element

as in conventional broaching. In another process variant, the workpiece carries out

the cutting motion. The workpiece is pressed by the broaching tool so that all con-

tours to be produced on the periphery of the workpiece are produced in one stroke

[Schw75, Spiz71]. In gearing technology, the use of this method is limited till now

solely to the manufacture of dog gears.