Smith G.T. Cutting Tool Technology: Industrial Handbook

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•

‘Step-support milling’ (Fig. 88b – right) – this tech-

nique utilises a similar approach to the previous

method, but in this case, there is an overlap between

passes on opposite sides of the wall. is strategy

gives more support at the vicinity where machin-

ing occurs and the cutting forces are less likely to

distort the wall as it height increases.

NB F

or very large height-to-thickness ratios of

>30:1, an alternative milling strategy, is to alter-

natively mill either side of the wall – approaching

the desired wall thickness in stages in a so-called:

‘

Christmas tree routine’

(i.e. not shown), so that

the thinner sections are always supported by thicker

sections below them. is method is then repeated

as the step-wise milling operation moves down the

wall.

Milling Thin Bases

Unsupported thin-base features, such as the one il-

lustrated in Fig. 89, are dicult to produce once the

previous side has been machined, because of the lack

of support, particularly at the base’s central region.

One milling approach in the production of this unsup-

ported thin-base, is to ‘helically mill’ the feature (i.e.

shown in cross-section in the small inset diagram in

Fig. 89). is usually necessitates milling at the cen-

tre of the base region, spiralling-down to the required

depth, then milling outward in a ‘attened helical

anner’ from that point (Fig. 89 – main illustration

and plan view). Occasionally, one of the faces has al-

ready been machined and under these conditions it

must be ensured that the cutter’s ank makes minimal

contact with this face, for this operation it is usual to

employ tooling with the minimum number of utes.

Sometimes a component to be thin-based milled,

has a hole at its base’s centre, in such a situation it is

prudent to leave a support leg in place when milling

the rst side. en machine the second side, nally re-

moving (i.e. milling) this support leg aer both sides

have been completed, thereby minimising any base de-

viation due to the presence of the cutting forces whilst

milling the feature.

30 ‘Christmas tree routine’ , is so-called, because as it is being

step-wise milled and progressively develops, the silhouette re-

sembles the prole of the Christmas tree – hence its name.

4.3 Rotary and Frustum-

Based Milling Cutters –

Design and Operation

Rotating Insert Face-Mills

One of the novel face-milling cutters which is cur-

rently available includes rotating round inserts that

are self-propelled as they cut (Fig. 90a), promoted by

the chip-ow over the insert’s face. It has been claimed

by the tooling manufacturers of these interchangeable

rotating insert cutters, that their unique cutting ac-

tion provides greater cutting eciency and is less de-

structive to the inserts, than the conventional ‘locked’

milling inserts. e term that is used for this rotating

cutting action is ‘

roll shearing’

. e rotation of the

inserts continually introduces a ‘fresh’ cutting edge to

the workpiece, this, it is claimed, minimises any heat

build-up in the cutting zone, with much of the heat

being transferred to the milled chips. Any remaining

heat being easily dissipated along the entire length of

these round inserts (i.e. the insert circumference has

an eective total cutting edge length of approximately

5 mm). is rotating insert has an almost innite ef-

fective cutting edge length, enabling around a 10-to-1

improvement of insert life. Due to the increased tool

life, less down-time for changing cutting edges is re-

quired, thereby improving cutting eciency and im-

pacting on actual overall cycle-times, because faster

cutting speeds

can be utilise.

It has further been claimed by the tooling manu-

facturer, that with the very high cutting speeds the lo-

calised heat is of benet, as the heat within the cutting

zone is concentrated in the chip and not in the work-

piece, or the insert. is local heating of the metallic

workpiece, allows it to reach its plastic deformation

stage, causing the chip to ow freely away from the

31 ‘Roll shearing’ , is a combination of the rotating action of the

round cutting inserts, in combination with the angled axis

which slices, or shears through the workpiece.

32 Rotating insert cutting speeds – with these rotary insert cut-

ters has been increased dramatically, when compared to the

more conventional ‘locked insert’ face-mills. For example,

when the silicon nitride cutting inserts are face-milling cast

ron components 1,000 m min

–1

is possible, conversely, when

face-milling aluminium workpieces cutting speeds of 2,300 m

min

–1

have been successfully employed.

172 Chapter 4

Figure 89. A strategy for the milling of thin-bases. [Courtesy of Sandvik Coromant].

Milling Cutters and Associated Technologies 173

Figure 90. A range of rotary milling cutters. [Courtesy of Rotary Technologies Corp.].

174 Chapter 4

milled surface. is localised plasticity allows the en-

ergy to be maximised and the cutting eciency to be

increased. Moreover, lower workpiece heat, results in

less component distortion. Yet another benet of this

‘roll shearing’ action, is that when conventional cutters

are used the tangential force component is high and it

is one of the primary causes for spindle bearing wear,

because of the side load it imparts into the spindle’s

bearings. Due to the rotary motion of these inserts,

they minimise tangential forces and as such, reduce

side loads on the machine’s spindle bearings.

Frustum-Based Face-Mills

When compared to some other milling cutter insert

geometries, the round inserts have two advantages:

•

Inherent strength – no sharp edges, minimising

potential points of weakness in the geometry, im-

parting high shock resistance and fracture tough-

ness. Hence, ‘frustums-based’ face mills have up to

10 times longer tool life, in comparison to conven-

tional milling insert geometries,

•

More cutting edges – they can be turned and locked

in their seatings, creating approximately twice as

many cutting edges per insert – giving up to 24 in-

dexes per insert – when compared to conventional

milling inserts.

NB L

ike conventional insert geometries, normal

round inserts oer the user two choices, whether

to choose a high eciency positive insert, or longer

insert life using a negative geometry round milling

insert.

e frustum-shaped (round) face-milling insert (Fig.

91), has a cutting edge which is reinforced by addi-

tional mass while at the same time oering a 60° posi-

tive shearing action (Fig. 91a). is frustum-designed

insert geometry, eliminates angles and straight lines,

allowing high stock removal rates to be utilised. Typi-

cally, these frustum-based insert designs, when mill-

ng grey cast iron can use peripheral speeds of >700 m

min

–1

at feedrates of 6.4 m min

–1

, whereas, for alu-

minium milling, the surface speed can be increased to

,650 m min

–1

, but with a feedrate of >8 m min

–1

Figure 91. A frustum-based milling cutter. [Courtesy of

Rotary Technologies Corp.]

Milling Cutters and Associated Technologies 175

Figure 92. A range of special tools (i.e. customised), catering for specic company production needs. [Courtesy of Ingersoll].

176 Chapter 4

4.4 Customised Milling

Cutter Tooling

Custom-built tooling is as its name implies, oers

quite considerably diverse tool designs (i.e. see Fig.

92 for just ‘snap-shot’ of a small range of these types

of tools). Some of this customised tooling can be rela-

tively simple, perhaps just manufactured to mill only

one particular feature, while others are very complex

and sophisticated in both their design and operation.

Of this latter type are the numerically-controlled, or

‘feed-out’ facing and boring heads (not shown). ese

programmable heads allow the machining of features

such as large bores with intricate proles, typically:

multiple diameters, grooves, tapers and even threads –

on a range of prismatic parts. Until such heads became

available, these workpiece features would have required

the knowledge by either a CNC programmer, or more

likely they would have been ‘routed’ to a conventional

jig-boring machine for a highly-skilled technician

known as a jig-borer to complete the complex machin-

ing task. ese numerically-controlled heads have

a programmable U-axis tool-slide that can be co-or-

dinated to that of the Z-axis, enabling it to produce

tapers and contoured bores, or even outside diameter

features. Once the head is located in the spindle, its

powered tool slide via a compact auxiliary d.c. servo-

drive motor (i.e. being a closed-loop system with feed-

back – to monitor its relative position at all times), will

control the radial motion as it rotates down a bore, or

around the outside diameter of a component.

Tooling can be designed to create virtually any

component feature on a workpiece and, with the

CAD/CAM soware available today, tooling designers

have a vast array of computing power to allow them

to eciently produce customised tooling within very

short lead-times. However, a word of caution here,

these customised tools are not inexpensive and should

only be purchased if the alternative tooling approach

is such, that cycle-times are otherwise lengthy, or there

is simply no other technique that will enable these part

features to be produced at economic cost.

4.5 Mill/Turn Operations

On many hybrid machine tools today, the traditional

operations associated with one particular type of ma-

chine tool, are now being produced on others. Take for

example a turning centre, in the past it would simply

have been employed in the production of workpieces

with rotational features. Now, with the addition of a

turret equipped with live/driven tooling (i.e. rotating

spindles in some, or all of the turret’s pockets), it is

possible to lock the headstock spindle, mill a feature:

at, keyway, gear tooth, or spline, then angular index

the spindle and repeat, until all of the so-called ats

– oen known as ‘prismatic features’ – are completed.

Moreover, it is possible to purchase a ‘mill/turn centre’

with ‘full’ C-axis headstock spindle control giving it

the capability to generate contoured surfaces, or faces

on the previously turned part (i.e. see Fig. 93). is

diversity in the machining operations that can be un-

dertaken by simply one machine tool, means that the

so-called ‘

one-hit machining’

operations are possible,

thereby reducing the risk of loosing the accurate da-

tum initially set when the part was turned, so increas-

ing production consistency due to its more repeatable

machining precision and accuracy.

Some companies in England and elsewhere, are

now producing rotational features on prismatic parts,

these being produced on machining centres. is un-

usual reversal technique is achieved by tting turning

tools in a suitable xture on the machine tool’s bed

and rotating the part held in a appropriate manner in

the machine’s spindle. Moreover, it is now possible to

not only purchase a machine tool that can: turn, mill,

bore, thread, but it can even cylindrically grind as well

– truly showing the diversity over a range of machin-

ing operational processes.

For further information on the possible problems

that may be encountered in milling operations and the

anticipated solutions, these are given in the Trouble-

shooting Guide for Milling Operations, in Appendix 6.

33 ‘One-hit machining’ , refers machining parts from wrought

stock, etc., in one complete operation.

Milling Cutters and Associated Technologies 177

Figure 93. Driven/live tooling: milling a spiral groove (top) and face-contouring (bottom) under ‘full’ C-axis control, on a mill/turn

centre. [Courtesy of DMG (UK) Ltd.]

178 Chapter 4

References

Journal and Conference Papers

Chaplin, R. Tool Geometry: the Two R’s [Milling Geometry].

Cutting Tool Eng’g., 52–55, Dec. 1999.

Crockford, G. Application of Modern Tooling in Machining

Centres, Int. Conf. on Industrial Tooling, Shirley Press,

95–100, Sept. 1997.

Deren, M. A Tale of Two Metals [Milling Stainless Steel].

Cutting Tool Eng’g., 48–56, May 2001.

Deren, M. Deep Impact [Milling Deep Pockets]. Cutting

Tool Eng’g., 40–45, Oct. 2001.

Eacott, R. Cutting Geometries for Milling and a New Ap-

proach to Chip Control. Int. Conf. on Industrial Tooling,

Molyneux Press, 52–61, Sept. 1999.

Ekback, M. Cut Longer and Better [Milling Cutter Applica-

tions]. American Machinist, 51–54, Nov. 1996.

Godden, T. Tools for Multi-axis Machining. Int. Conf. on

Industrial Tooling, Test Valley Group, 255–266, Sept.

2003.

Heuwinkel, M. and Richter, A. Let’s Talk Radial [Approach

Angles – Milling]. Cutting Tool Eng’g., 64–70, May

2005.

Heuwinkel, M. As Easy as X, Y, Z [Helical Interpolation

– Milling]. Cutting Tool Eng’g., 58–62, Aug. 2006.

Isakov, E. e Mathematics of Machining. American Ma-

chinist, 37–39, Aug. 1996.

Isakov, E. Reassessing Power Factors [Milling]. American

Machinist, 43–45, Dec. 1996.

Isakov, E. Power Equations. Cutting Tool Eng’g., 66–71,

May 2001.

Kennedy, B. Facing Facts [Face Milling – Automotive

Parts]. Cutting Tool Eng’g., 29–37, Feb. 2002.

Kennedy, B. Wall Smart [in-wall Milling]. Cutting Tool

Eng’g., 26–33, Feb. 2007.

Kline, W.A., DeVor, R.E. and Lindberg, J.R. e Prediction

of Cutting Forces in End Milling with Application to Cor-

nering Cuts. Int. J. Mach. Tool Des. Res., Vol. 22, 7–22,

1982.

McNamara, D. Getting Down [Plunge Milling]. Cutting

Tool Eng’g., 44–47, Oct. 2003.

Richter, A. e Right Angle [‘Angled’ Milling Heads]. Cut-

ting Tool Eng’g., 48–53, July 2004.

Rowe, J. Right Face [Face Milling]. Cutting Tool Eng’g.,

56–61, Sept. 2001.

Sandvik Coromant. Geometries at Work [Milling Cutters].

Cutting Tool Eng’g., 56–63, May 2003.

Smith, G.T. and Booth, S. e Manufacture [Milling] of

Propellers using ree and Five Axis Machining Cen-

tres. Proc. of LAMDAMAP Int. Conf., Computational

Mechanics Pub., 307–316, 1993.

Stabler, G.V. e Fundamental Geometry of Cutting Tools,

Proc. IMechE, Vol/165, 14–26, 1951.

Tlusty, J. and Masood, Z. Chipping and Breakage of Carbide

Tools, ASME J. of Eng’g for Ind., Vol. 100, 403–412, Nov.

1980.

Yamane, Y. and Narutaki, N. e Eect of Atmosphere on

Tool Failure in Face Milling – (1

Report). J. Jap. Soc.

Prec. Eng’rs. Vol. 49 (8), 521–527, 1983.

Books, Booklets and Guides

Application Guide – Aerospace Frames [Milling], Sandvik

Coromant Pub. No. C-2920:17 ENG/01, 2003.

Application Guide – Die and Mould Making, Sandvik Coro-

mant Pub. No. C-1120:2 ENG, 2000.

Application Guide – Face milling, Sandvik Coromant Pub.

No. C-1120:1 ENG, 1998.

Application Guide – Side and Facemilling, Sandvik Coro-

mant Pub. No. C-1129:022 ENG, 1996.

Boothroyd, G. and Knight, W.A. Fundamentals of Machin-

ing and Machine Tools. Marcel Dekkar (NY), 1989.

Childs. T.H.C., Maekawa, K. Obikawa, T. and Yamane, Y.

Metal Machining – eory and Applications. Arnold

Pub., 2000.

Ingersol Pub. Fine Tuning [Milling Cutter Rigidity], 14–15,

e Cutting Edge, No.3, 1987.

Ingersol Pub. Fine Tuning [Milling Optimisation], 14–15,

e Cutting Edge, No.2, 1988.

Ingersol Pub. Fine Tuning [Milling Cutter Insert Density],

14–15, e Cutting Edge, No.1, 1989.

Ingersol Pub. Fine Tuning [Milling Insert Edge Prep.], 14–

15, e Cutting Edge, No.1, 1992.

Kalpakjian, S. Manufacturing Engineering and Technology.

Addison Wesley, 1989.

Machining Data Handbook. 3

Ed., Machinability Data

Center, MetCut Pub. (Cincinnati), 1980.

Metals Handbook – Vol. 16 – Machining. ASM Int. Matls.

Park (Ohio), 1989.

Modern Metal Cutting – Part 6: Milling eory. AB Sandvik

Coromant Pub., 1980.

Modern Metal Cutting – Part 7: Milling Tools. AB Sandvik

Coromant Pub., 1980.

Milling Cutters and Associated Technologies 179

Modern Metal Cutting – A Practical Handbook. AB Sand-

vik Coromant Pub., 1994.

Oxley, P.L.B. Mechanics of Machining. Ellis Horwood Pub.,

1989.

Shaw, M.C. Metal Cutting Principles. Oxford Univ. Press,

1989.

Society of Manufacturing Eng’rs., Tool and Manufacturing

Engineers Handbook – Vol. 1 – Machining. 4

Ed., SME

Pub., Dearborn, Mich., 1983.

Smith, G.T. Advanced Machining – e Handbook of Cut-

ting Technology, IFS/Springer Verlag, 1989.

Smith, G.T. CNC Machining Technology. Springer Verlag,

1993.

Tlusty, G. Manufacturing Processes and Equipment. Pren-

tice Hall, 2000.

Usui, E. Modern Machining eory [i.e. In Japanese]. Kyo-

ritu-shuppan (Tokyo), 1990.

Zorev, N.N. Metal Cutting Mechanics. Pergamon Press

(Oxford), 1966.

180 Chapter 4

Threading Technologies

‘But I grow old always learning many things.’

SOLON

(640 – 558 BC)

[Plutarch: Solon, xxxi]