30079_33b.pdf

Template machining utilizes a simple, single-point cutting tool that is guided by a template.

However, the equipment is specialized, and the method is seldom used except for making large-bevel

gears.

The generating process is used to produce most high-quality gears. This process is based on the

principle that any two involute gears, or any gear and a rack, of the same diametral pitch will mesh

together. Applying this principle, one of the gears (or the rack) is made into a cutter by proper

sharpening and is used to cut into a mating gear blank and thus generate teeth on the blank. Gear

shapers (pinion or rack), gear-hobbing machines, and bevel-gear generating machines are good ex-

amples of the gear generating machines.

33.9.2 Gear Finishing

To operate efficiently and have satisfactory life, gears must have accurate tooth profile and smooth

and hard faces. Gears are usually produced from relatively soft blanks and are subsequently heat-

treated to obtain greater hardness, if it is required. Such heat treatment usually results in some slight

distortion and surface roughness. Grinding and lapping are used to obtain very accurate teeth on

hardened gears. Gear-shaving and burnishing methods are used in gear finishing. Burnishing is limited

to unhardened gears.

33.10 THREAD CUTTING AND FORMING

Three basic methods are used for the manufacturing of threads; cutting, rolling, and casting. Die

casting and molding of plastics are good examples of casting. The largest number of threads are

made by rolling, even though it is restricted to standardized and simple parts, and ductile materials.

Large numbers of threads are cut by the following methods:

1. Turning

2, Dies: manual or automatic (external)

3. Milling

4. Grinding (external)

5. Threading machines (external)

6. Taps (internal)

33.10.1 Internal Threads

In most cases, the hole that must be made before an internal thread is tapped is produced by drilling.

The hole size determines the depth of the thread, the forces required for tapping, and the tap life. In

most applications, a drill size is selected that will result in a thread having about 75% of full thread

depth. This practice makes tapping much easier, increases the tap's life, and only slightly reduces

the resulting strength. Table 33.13 gives the drill sizes used to produce 75% thread depth for several

sizes of UNC threads. The feed of a tap depends on the lead of the screw and is equal to I/lead ipr.

Cutting speeds depend on many factors, such as

1. Material hardness

2. Depth of cut

3. Thread profile

Table 33.13 Recommended Tap-Drill Sizes for Standard Screw-

Thread Pitches (American National Coarse-Thread Series)

Number

Diameter

J/4

3/8

3/4

Threads

per

Inch

Outside

Diameter

of Screw

0.138

0.164

0.190

0.216

0.250

0.375

0.500

0.750

1.000

Decimal

Equivalent

of Drill

0.1065

0.1360

0.1495

0.1770

0.2010

0.3125

0.4219

0.6562

0.875

Tap Drill

Sizes

5/16

27/64

21/32

7/8

4. Tooth depth

5. Hole depth

6. Fineness of pitch

7. Cutting fluid

Cutting speeds can range from lead 3 ft/min (1 m/min) for high-strength steels to 150 ft/min (45

m/min) for aluminum alloys. Long-lead screws with different configurations can be cut successfully

on milling machines, as in Fig. 33.24. The feed per tooth is given by the following equation:

'•-f

where d = diameter of thread

n = number of teeth in cutter

N — rpm of cutter

S = rpm of work

33.10.2 Thread Rolling

In thread rolling, the metal on the cylindrical blank is cold-forged under considerable pressure by

either rotating cylindrical dies or reciprocating flat dies. The advantages of thread rolling include

improved strength, smooth surface finish, less material used (—19%), and high production rate. The

limitations are that blank tolerance must be close, it is economical only for large quantities, it is

limited to external threads, and it is applicable only for ductile materials, less than Rockwell C37.

33.11 BROACHING

Broaching is unique in that it is the only one of the basic machining processes in which the feed of

the cutting edges is built into the tool. The machined surface is always the inverse of the profile of

the broach. The process is usually completed in a single, linear stroke. A broach is composed of a

series of single-point cutting edges projecting from a rigid bar, with successive edges protruding

farther from the axis of the bar. Figure 33.25 illustrates the parts and nomenclature of the broach.

Most broaching machines are driven hydraulically and are of the pull or push type.

The maximum force an internal pull broach can withstand without damage is given by

P = ^JL lb

(33.57)

where Ay = minimum tool selection, in.2

Fy = tensile yield strength of tool steel, psi

s = safety factor

The maximum push force is determined by the minimum tool diameter (Dy), the length of the

broach (L), and the minimum compressive yield strength (Fy). The ratio L/Dy should be less than 25

so that the tool will not bend under load. The maximum allowable pushing force is given by

Fig. 33.24 Single-thread milling cutter.

P — pitch of teeth

D - depth of teeth (0.4P)

L — land behind cutting edge (0.25P)

R — radius of gullet (.25P)

a — hook angle or rake angle

Y — backoff angle or clearance angle

RPT — rise per tooth (chip load) = ft

Fig. 33.25 Standard broach part and nomenclature.

P = -2-2 Ib

(33.58)

where Fy is minimum compressive yield strength.

If LIDy ratio is greater than 25 (long broach), the Tool and Manufacturing Engineers Handbook

gives the following formula:

5.6 X 107£>?

P =

(33.59)

sL2

Dr and L are given in inches.

Alignment charts were developed for determining metal removal rate (MRR) and motor power in

surface broaching. Figures 33.26 and 33.27 show the application of these charts for either English

or metric units.

Broaching speeds are relatively low, seldom exceeding 50 fpm, but, because a surface is usually

completed in one stroke, the productivity is high.

33.12 SHAPING, PLANING, AND SLOTTING

The shaping and planing operations generate surfaces with a single-point tool by a combination of

a reciprocating motion along one axis and a feed motion normal to that axis (Fig. 33.28). Slots and

limited inclined surfaces can also be produced. In shaping, the tool is mounted on a reciprocating

ram and the table is fed at each stroke of the ram. Planers handle large, heavy workpieces. In planing,

the workpiece reciprocates and the feed increment is provided by moving the tool at each recipro-

cation. To reduce the lost time on the return stroke, they are provided with a quick-return mechanism.

For mechanically driven shapers, the ratio of cutting time to return stroke averages 3:2, and for

hydraulic shapers the ratio is 2:1. The average cutting speed may be determined by the following

formula:

cs = ^c fpm

(33'60)

where N = strokes per minute

L = stroke length, in.

C = cutting time ratio, cutting time divided by total time

For mechanically driven shapers, the cutting speed reduces to

CS = — fpm

(33.61)

LVN

CS = -L- m/min

(33.62)

ouu

where Lj is the stroke length in millimeters. For hydraulically driven shapers,

CS = ^ fpm

(33.63)

L±N

CS = ~— m/min

(33.64)

OOO.7

The time T required to machine a workpiece of width W (in.) is calculated by

Example:

Material: Cast iron - HSS tools

Q - 12 Vc x w x dt inVmin

Chipload 0.005 in/tooth

. Qxp QxP

hPm a —— * ~rT~

Vc » 30 f pm

w - 1.5 in

E 0-7

dt - 0.040 in Q * 22in3/min

P - 0.7 hp/5n3/min hpm » 22 hp

Fig. 33.26 Alignment chart for determining metal removal rate and motor horsepower in sur-

face broaching with high-speed steel broaching tools—English units.

T = J^J nun

(33.65)

where / = feed, in. per stroke

The number of strokes (5) required to complete a job is then

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