Smith G.T. Cutting Tool Technology: Industrial Handbook

Подождите немного. Документ загружается.

Figure 107. Threadmilling cutters and typical thread generation operations. [Courtesy of Sandvik Coromant].

202 Chapter 5

5.6 Thread Milling

Introduction

read milling geometry in contrast to that of a basic

tap (i.e. having a single spiral shaped tooth Fig. 107ai),

has a series of teeth which do not form a spiral, but

are congured without pitch (Fig. 107aii). is fun

amental dierence in tool design is attributed to the

dierent thread production processes, explained ear-

ier. Not only can a thread milling cutter have a similar

visual geometry to that of a machine tap (Fig. 107aii),

but it can occur with a single radially-mounted blade

for milling both external and internal threads (Fig.

108b). Albeit for an internal thread production, there

must be sucient working space for the cutter to be

able to perform the thread milling task.

Tooth Profile and Dimensions

e thread milling cutter prole usually conforms to

that of the thread to be milled. In certain cases, it may

be essential to correct the milled thread’s prole. is

being the case, when the diameter of the thread to be

milled does not have a denite ratio to the diameter

of the thread milling cutter. A major advantage of em

loying thread milling in the production of threads,

is that it can mill a range of threads of diering diam-

ters. e one limitation here

eing that modications

of the thread’s pitch is not practicable.

If one discounts the tool’s thread pitch, then the de

ign of a thread milling cutter is remarkably similar

to that of a machine tap (Fig. 107a). A typical thread

milling cutter (Fig. 107aii), is characterised by its

cutting section’s size and dimensions. e total tool

length and its associated thread length are also part of

the cutter’s dimensions. read milling cutter designs

can also incorporate either a collar, or not – as the

milling situation dictates, together with either a coun

ersinking chamfer, or not. erefore, the thread mill-

ng cutter’s cutting section

(

Fig. 107aii), consists of its:

ute length, ute prole, tooth form together with its

associated form relief. In a similar fashion to that of a

machine tap, the ute length usually incorporates run-

out of the utes, although this ute run-out does not

have to be as great as that found on machine taps, due

to the smaller chips that are produced. read milled

chips do not remain in the cutter’s utes during the

thread milling process, and as such, will not restrict

further chip development. e tooth width is larger

than that found on machine taps, with relief grinding

creating

he necessary clearance angles, required for

milling threads.

Interference Ratio

If the thread milling cutter diameter to that of the

nominal thread diameter ratio of 70% is adhered to,

then no milled thread prole distortion should take

place (i.e. see Fig. 109a), irrespective of the thread’s

depth – this fact has been consistently well proven by

industrial applications.

In Fig. 109a, the illustration depicts the fact that the

diameter of the thread milling cutter and its associ

ted prole depth, determine the pressure angle of the

thread’s diameter.

Helical Interpolation

Helical interpolation (Fig. 108a), is the amalgama-

tion of two kinematic motions, these being: linear and

circular interpolations. erefore, in thread milling,

dierent threads can be manufactured by the form of

overlaying the pitch direction with that of the direc

ion of rotation of the circular movement.

read milling cutters are normally designed for

right-hand cutting, with the direction of rotation be

ng generally clockwise. However, by altering a range

of kinematic motions, such as: the axial direction of

the feed, reverse cutter rotation, or by synchronous

milling, all thread combinations can be manufactured

– some of which are depicted in Fig. 107c. Depending

upon the component features to be thread milled, such

as into blind, or through holes and

hether horizon

al, or vertical machining techniques are to be incor-

orated, together with the lubrication type and chip

removal strategies, these will determine the correct

choice of milling procedure to be adopted. Generally,

for thread milling production,

synchronous milling

methods

(i.e. Fig. 109b) should be applied whenever

possible, as they achieve the following intrinsic ben-

ts: lower cutting forces, improved chip formation,

longer tool life and improved surface quality.

19 Synchronous milling methods, can be identied when the

thread milling cutting edge emerges with a chip thickness of

zero (i.e. h = 0).

Threading Technologies 203

Speed Ratio

When thread milling, the cutter edge’s speed is cal-

culated by the cutting speed (i.e. revolutions) and the

feedrate per tooth. With linear movement, the cutting

edge’s feedrate is identical to that at the tool’s centre.

However, with helical interpolation, it follows a path

of a circle in the plane (Fig. 108a). All machine tool

CNC controllers will calculate speeds from the tool’s

centre, it is necessary to program a command for con

erting the cutting speed (i.e. a contour-related pro-

ram). When such a program does not exist, or the

central point is programmed, it is necessary to con-

ert the feedrate accordingly.

t should be mentioned,

that the interactive control at the CNC control panel

will always indicate the speed at the centre point of

the tool and, when running with no load (i.e. usu

lly termed a ‘dry-run’), this speed is simple to check.

Furthermore, if this speed is disregarded, the thread

milling cutter will run at a speed many times greater

than that of the feed, which shortly leads to the cut

er’s breakage.

Figure 108. Thread milling using a single-edged insert for either internal/external threading operations, can be achieved via a

complex simultaneous circular interpolation of the ‘x’ and ‘y’ axes and a ‘z’ axis linear motion. [Courtesy of Stellram]

204 Chapter 5

Figure 109. Threadmilling interference ratio, plus cutter

positioning and feeding. [Courtesy of Guhring]

Threading Technologies 205

Internal Thread Milling: Radial Positioning

to Nominal Diameter, Via Entry Cycles

e thread milling cutter’s radial positioning to the

nominal diameter at the start of the thread’s produc-

ion, is achieved by so-called ‘entry cycle’

(Fig. 109c),

while the movement following the thread’s milling op-

ration is achieved by cutter motion from the nomi-

al diameter to the hole’s centre, via a corresponding

‘exit cycle’. 

read milling cutter approaches to that of

the start of the thread, via suitable ‘entry cycles’ can be

achieved by several dierent ways, these are:

•

Linear plunging (Fig. 109ci) – of the thread milling

cutter into the workpiece material, creates a very

large contact angle at the cutter’s periphery, lead-

ng to the

ndesirable situation of high tool loading

and long chips. is problem is particularly acute

hen the dierences between the thread milling

cutter’s diameter to that of the hole’s size is small.

Moreover, this radial entry linear plunging tech-

ique can leave a small ‘delay mark’

on a portion

of the milled thread.

NB Linear plunging i

s not an advisable thread mill-

ing technique for the production of accurate and

precise small threads.

•

90° quarter circle entry cycle (Fig. 109cii) – allows

just a small dierence in the diameter between the

tool and the thread to remove a large part of the

chip volume, during the linear section of the entry

cycle. is particular entry cycle strategy, is nor

ally only utilised for relatively large dierences

in diameter between the hole size and the cutter’s

diameter.

NB 

e 90° quarter circle entry cycle has the advan-

tage of a relatively short e

ntry path, together with a

simple C

NC program.

•

180° semi-circle entry cycle (Fig. 109ciii) – the cut-

ting force loading of the tool is at its lowest when

20 Entry-cycles, allow the thread milling cutter to be moved in a

circular arc to the nominal thread’s diameter.

21 Delay marks, a

re the result of a slight dwell, prior to the next

command line activation in the thread milling program, caus-

ing cutting forces to ‘slightly relax’ and then impinge into the

machined thread’s surface.

the cutter is plunging, due to the contact angle be-

ing relatively small during the complete cycle en-

ry.

NB 

e 180° semi-circle entry cycle necessitates

a slightly more sophisticated CNC program, al-

hough it has been found to be the most cost-e-

ient thread milling technique overall. In Fig. 110,

is depicted a step-by-step visual interpretation of

this actual thread milling process, along with a typ

cal programming example.

One distinct advantage that utilising

thread milling

tooling g

ives to the quality and tment of matching

threads, is that minute variations i

n the pitch and to a

lesser degree its associated depth, can be programmed-

in b

y the operator to modify ‘working clearances’.

is has the distinct benet of providing control over

the ‘backlash’ between the two mating thread milled

parts.

5.7 Thread Rolling –

Introduction

It is normal to specify thread rolling when substantial

quantities of threads need to be manufactured. In es-

ence, the production process i

s one of ‘cold-forming’ ,

in which the threaded features on the

orkpiece are

formed by rolling a thread blank between hardened

dies (Fig. 111). is rolling action, causes the metal

to ow radially into the required V-form prole (i.e.

see Fig. 111a – inset schematic diagrammatic com

arison between a cut and rolled thread – indicating

the ‘directionality of the grain-ow’

). Due to the fact

that no workpiece material is removed in the form of

chips, there is no waste material – resulting in substan-

22 ‘Directionality of the grain-ow’ , this anisotropic behaviour

of the manipulated grain structure aer rolling is one of plas-

tic deformation of the local material (i.e. Fig.111a – inset dia-

gram, indicating the V-from rolled thread). is local plastic

deformation, raises the material’s: hardness, tensile and fa-

tigue strengths, together with its proof stress. However, there

is some ‘drop-o’ in both the thread’s creep strength and its

ductility as a result of rolling, but this is tolerated – due to the

major benets described.

206 Chapter 5

Figure 110. A typical threadmilling cutter operational sequence, with an illustrated series of cutter motions and a ‘practical’

word-address CNC program. [Courtesy of Guhring]

Threading Technologies 207

Figure 111. Thread rolling techniques – produce a strong thread form.

208 Chapter 5

tial economic savings as a result

. e main benets

of thread rolling on CNC machine tools, is that due

to this cold-working process, rolled threads have high

strength, are smoother and more wear resistant then

there machined counterparts. e thread rolling pro

uction rates are fast, typically a complete thread can

be formed in a second, with the thread quality being

consistently high.

A principal characteristic of a thread rolling op

ration is that the rolled thread’s diameter is always

greater t

han the original blank diameter. If the pro-

spective thread must have an accurate ‘class of t’ , then

its blank diameter is marginally increased by 0.05 mm

with respect to the thread pitch diameter. When it is

desired to have say, the body of a bolt larger than the

outside diameter of the rolled thread, then the thread’s

blank diameter is produced smaller than the body.

5.7.1 Thread Rolling Techniques

In Fig. 111, can be seen the three basic techniques

used to thread roll employed on CNC machine tools,

these are:

•

Two-roll tangential rolling (Fig. 111a), is a similar

process to that of ‘knurling’

. As the spindle turns,

the workpiece’s pre-rolled diameter is progressively

raised to its nal shape, normally over the course

of between

0 to 30 revolutions. e tangential

thread rolls are fed from the X-axis, at a tangent

to the workpiece. When the centreline of the rolls

line-up with that of the centreline of the workpiece,

the process is complete. Usually rolling a

φ2

0 mm

thread at 1200 rpm, takes about 1 second, con-

ersely, a single-point turned thread would require

23 read rolling, is known as a ‘chipless operation’ and as a re-

sult of the ‘cold rolling’ production process , the operation is

cleaner and material savings in blank stock weight are of the

order of between 15% to 20% – depending upon the size and

length of the threaded feature manufactured.

24 Knurling (

i.e not illustrated), utilises either two, or three

hardened rotating knurls which are pressed into the previ-

ously turned outside diameter, thereby giving a ‘gripping’ sur-

face pattern – and hence aids in purchase for one’s grip, with

normally either a straight-, or diamond-shaped knurl.

NB It is possible to utilise tangential sliding knurls to impart

the desired ‘imprinted patterned surface’ onto the workpiece’s

periphery.

10 times longer to manufacture the same threaded

feature,

•

ree-roll radial rolling (Fig. 111b), is similar in

operation to tangential heads, in so far as the work-

iece is normally approached from the side, per-

endicular to the major thread’s axis. e radial

rolls are sprung-loaded and when they are brought

over the workpiece, the tension is released, causing

the rolls to rotate and produce a thread. Flats on

the rolls allow for work to be inserted and removed.

In both the tangential and radial rolling techniques,

they are limited to thread lengths that are no greater

than the thread roll widths. e principal dierence

between these two heads, is that with

radial heads

he form is completed in just one r

evolution, as op-

posed to the 20 to 30 revolutions necessary with

tangential rolling methods. is fact, makes the ra-

dial rolling t

he fastest of all rolling techniques. For

example, if the workpiece spindle is rotating at 1200

rpm, and a φ1

0 mm thread is to be rolled, it would

take just 0.5 seconds to complete,

•

Two-roll axial rolling (Fig. 111c), these rolls engage

the workpiece from its front, along the workpiece’s

centreline (i.e. Z-axis). is rolling action is analo-

ous to a threading die, or thread-chaser, traversing

from one end of the workpiece to the other. Hence,

this rolling arrangement is capable of producing

very long threads, or threaded portions on the

workpiece, moreover, the axial heads support the

part during the thread’s manufacture, eliminating

the need of a supporting tailstock.

In all these thread rolling processes, the operation of

thread rolling remains primarily identical in its nal

rolled threaded feature on the workpiece and the pro

ess of imparting threads on ductile and to a lesser

extent some work-hardening materials, should be en-

ouraged. ere are other techniques for the produc-

ion of rolled threads that have not been shown here,

including: reciprocating and at die designs, planetary

rolling, etc., they have not been incorporated into this

review, because of the diculty of utilising them on

CNC machine tools.

References

Journals and Conference Papers

Bolden, A. Tapping Troubles: the Hidden Causes. Cutting

Tool Eng’g., 20–25, April 1990.

Threading Technologies 209

Burns, S. Keep the Tool Cool during Tapping. Cutting Tool

Eng’g., 33–37, April 1990.

Hanson, K.

Roll your Own [

read Rolling]. Cutting Tool

Eng’g., 54–58, May 2002.

Hazelton, J.L.

Tapping the Hard Stu. C

utting Tool Eng’g.,

62–68, Mar. 2007.

Henderer, T.

Solid Synchronicity [

Solid Tapping].Cutting

Tool Eng’g., 58–63, Feb. 2006.

Jonah, A.K.

Standard Taps for Exotic Materials. C

utting

Tool Eng’g., 26–30, April 1990.

Kennedy, B.

What’s on Tap? [

Tapping Advances]. Cutting

Tool Eng’g., 26–35, May 2002.

Lewis, B.

Challenge on Tap [

Tapping Problems]. Cutting

Tool Eng’g., 44–48, April 2003.

Nelson. D.

Swiss reads [

Swiss-type, segmented thread-

ing]. Cutting Tool Eng’g., 56–62, April 2007.

Pontius, K.

Low-silicon Lowdown [

Tapping Si-Al Parts].

Cutting Tool Eng’g., 58–64, May 2001.

Restall, M.

e Ins and Outs of read Milling. C

utting Tool

Eng’g., 28–33, Aug. 2001.

Richter, A.

Know your Limits [

read Limits and Classes].

Cutting Tool Eng’g., 36–41, Jan. 2005.

Rowe, J.

e Lowdown on Laydown Inserts [

Laydown

threading systems]. Cutting Tool Eng’g., 45–48, Oct.

2002.

Books, Booklets and Guides

ltan, T. Oh, S-I. and Gegel, H.L. Metal Forming: Funda-

mentals and Applications. A

SM Int. Pub. (Matls. Park,

Ohio), 1983.

Burrows, L. and Hancox, D.

Cra Engineering Data Book.

Stanley ornes Pub., 1978.

Cottrell, A.

An Introduction to Metallurgy. E

dward Arnold

Pub., 1975.

Degamo, E.P., Black, J.T., Kosher, R.A.

Materials and Pro-

cesses in Manufacturing. J

ohn Wiley and Sons Inc.,

2003.

Precision Cutting Tools. G

uhring Pub. 8

Ed., 2004.

Inuence of Metallurgy on Hole Making Operations. A

Pub. (Ohio), 1978.

Kalpakjian, S.

Manufacturing Processes for Engineering Ma-

terials. A

ddison Wesley Pub., 1984.

Modern Metal Cutting – Part 11: Other Tools. A

B Sandvik

Coromant Pub., 1981.

Modern Metal Cutting. A

B Sandvik Coromant Pub., 1994.

Reed-Hill, R.E. Physical Metallurgy Principles. V

an Nos-

trand Reinhold 2

Ed., 1973.

Rollason, E.C. Metallurgy for Engineers. E

dward Arnold

Pub. 4

Ed., 1973.

Schey, J.A. Introduction to Manufacturing Processes. M

Graw-Hill Book Co. 3

Ed., 1999.

Wick, C. et al. Tool and Manufacturing Engineers Hand-

book – Vol. II: Forming. 4

Ed., Society of Manuf. Engrs.

(Dearborn Mich.), 1984.

210 Chapter 5

Modular Tooling

and Tool Management

‘A place for everything

and everything in its place.’

SAMUEL SMILES

(1812 – 1904)

[In: Thrift, Chap. 5]