Smith G.T. Cutting Tool Technology: Industrial Handbook

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this creates potential tool deection which could

become a major problem.

Boring Bar Deflection

When any boring operations take place, even with a

very rigid tool mounting and a small boring bar over-

hang, some vibration and tool tip deection will in-

evitably occur, this is exacerbated by machining hard-

parts. e former problem of vibration has previously

been mentioned and methods of minimising it are

possible. However, tool deections are more dicult,

if not impossible to completely eliminate, with these

longer cantilevered tools. Of note regarding overhang-

ing tool deections, are that a tool tip deects in two

directions (i.e. see Fig. 66a), these are:

•

Radial deection (∆

) – aects the machined (i.e.

bored) diameter,

•

Tangential deection (∆

) – causes the tip to move

downward for the centreline.

In each of these tool tip deections, both the size and

direction of the cutting forces are inuenced by the

chip thickness and insert geometry selected (i.e. illus-

trated in Fig. 66b). e radial deection will be equal

to the dierence between the diameter which was orig-

inally set and the actual bored diameter, this can be

easily found by the simple expedient of measuring it,

then adjustment can be made for this apparent deec-

tion. e tangential deection of the boring bar’s tip

can be established by either ‘direct’ , or ‘indirect’ met-

rological techniques at the tool’s tip. In Fig. 66a, the

graph depicts deections ‘∆’ (i.e. both the tangential

‘∆

’ and radial deection ‘∆

’), as a function of the

cutting depth ‘a

’. Due to the fact that the tangential

deection (∆

) linearly increases with increasing D

), it is usually recommended that machining passes

are divided into a number of cuts when close toler-

ances are needed (i.e. in the region of IT7

) – see Table

for an abridged version of the IT tolerances, with

*Rmax values in µm.

e magnitude of radial deection as a function

of the cutting depth, is also inuenced by the ratio

between the insert’s nose radius and the D

), to-

gether with the boring insert’s entering angle. In some

cases, a boring bar is situated slightly above the work-

piece centreline, so that when it enters the cut at full

depth it will have tangentially-deected to the actual

48 ‘IT’ (i.e. in units of µm) – represents the average value of the

basic tolerance for the ‘diameter range’ in question. Hence, it

will vary according to the choice of diameter range selected.

49 ese values are related to surface texture expression of:

*Rmax (µm), which is: e maximum individual peak-to-val-

ley height. e Rmax values (i.e. in Table 5) can be calculated

from the IT value, using the following equation, rather than

the conventional equation: Rmax = (fn

) 125

this equation tends to give excessively high surface texture va-

lues, thus more practical values related to IT are to be found

from:

�

Rmax =

�

n � IT

(µm)

Where: n = e number of IT’s.

Table 5: IT values related to the basic tolerance for various diameter ranges

Dc (mm): Over /up to

-/3

Over/up to

3/10

Over/up to

10/50

Over/up to

50/180

Over/up to

180/400

Over/up to

400/800

IT5 0.6 0.8 1.3 2.2 3.2 4.5

IT6 0.9 1.2 1.9 3.1 4.6 6.4

IT7 1.4 1.9 3.1 5.0 7.4 10.1

IT8 2.0 2.9 4.7 7.8 11.5 15.8

IT9 3.6 7.5 9.4 12.4 18.3 25.2

IT10 5.7 7.6 12.1 20.0 29.8 40.5

IT11 8.6 11.8 19.1 31.4 46.2 63.8

[Source: Sandvik Coromant (1995)]

Drilling and Associated Technologies 131

Figure 66. Hard-part boring, can create excessive boring bar deections and potential vibrational problems – if not carefully

controlled. [Courtesy of Sandvik Coromant]

132 Chapter 3

workpiece’s centreline. Boring bar overhang is not a

problem when ‘Line-boring’

as the tool is supported

at both ends, or in the case of the novel ‘Telescopic line-

boring tooling’

e chip area (i.e. illustrated in Fig. 66b – right), has

an eect on the load on the insert’s cutting edge, par-

ticularly when hard-part boring, although with small

chip areas, this may not create a vibration problem,

unless high friction is present between the insert and

workpiece. However, the cutting forces substantially in-

crease if a large chip area is utilised, necessitating some

means ‘damping stability’ to the boring tool.

3.3 Reaming Technology –

Introduction

e reamer is the most commonly utilised tool for the

production of accurate and precise holes, having high

surface quality being true to form and tolerance. Ma-

chine reamers can have either a single-blade design

(Figs. 67 and 68), or are produced with a multiple series

of cutting edges – of constant diameter (Fig. 69) or, ta-

pered (Fig.73b) across a diverse range of diameters and

lengths. e surface texture quality obtainable by ream-

50 ‘Line-boring’ , as its name implies is utilised for boring part’s

with concentric and oen varying diameters throughout the

overall component’s length. Normally, a ‘Line-boring tool’ is

supported by a steady with suitable bushing and a mating ex-

tension bar, some distance from the cutting edge and its re-

spective rotating toolholder. is additional support enabling

long bored features to be precisely machined to the part’s cen-

treline in-situ.

51 ‘Telescopic line-boring tool’ , One major machine tool builder

in association with a tooling manufacturer, produced a rather

novel and clever ‘Telescopic line boring tool‘, for the machining

of quite long cranksha bearing housings on both automo-

tive engine blocks and bored cam-seatings for cylinder heads.

is uniquely-designed ‘Telescopic line-boring tool’ , machined

the rst bore, then continued to extend (i.e. telescopically

feed-forward), whilst supporting its progress by mating with

each automotive-machined bore, as it progressed through the

large automotive component, thereby supporting the machin-

ing operation throughout its boring cycle, then retracting on

completion, allowing the tool to be held in the machine tool’s

magazine, allowing/facilitating an ecient and speedy multi-

ple in-line boring operation to be executed.

ing ranges from approximately ‘Ra’

0.2 to 6.5 µm, ac-

cording to recommendations of DIN 4766. Normally,

reamed nishes of about Ra 0.5 µm can be regarded as

satisfactory. In general, reaming achieves tolerances of

IT7, but if the reamer has been carefully ground, it can

achieve tolerances of IT6, or even to IT5.

52 Arithmetic roughness ‘Ra’ parameter – it is the arithmetic

mean of the absolute ordinate values Z(x) within the sampling

length. It is the most frequently quoted international surface

texture (i.e. amplitude) parameter, expressed in the following

manner:

Ra =



l r

�



� Z(x) � dx

NB In the past and specically in the USA, its equivalent

term was known as the ‘Arithmetic Average’ , denoted by sym-

bols: ‘AA’.

Figure 67. A sample of indexable insert reamer technology –

for solid and oating reamer applications. [Courtesy of Seco

Tools]

Drilling and Associated Technologies 133

Figure 68. Single-blade reamers oer superior hole geometry over conventional reamers.

[Courtesy of Shefcut Tool & Eng’g Ltd.]

134 Chapter 3

Prior to beginning the reaming process

, holes

have to be either pre-drilled, or holes cored-drilled

Due to the nature of the role of the burnishing pads on

the hole’s machined and highly-compressed surface in

Gun-drilling operations, it is not particularly suitable

for reaming.

Machine reamers can be divided into several cat-

egories, these are: multi-point reamers with either

a straight, or Morse taper

shank, these reamers are

usually either manufactured from: HSS, Tungsten

carbide (Solid), or with carbide tips. Typically, the

Tungsten carbide (solid) reamers can be run at 10%

higher feedrates, to their HSS equivalents and can

ream workpiece materials up to a tensile strength of

200 N mm

–2

Machine reamers are available with: straight utes,

le-hand (LH) spirals, or 45° LH ‘quick’ spirals this lat-

ter reamer version is oen termed a ‘Roughing reamer’

and is oen used for ‘long-chipping’ workpiece mate-

rials. Reamers with straight utes are usually utilised

to ream blind holes, but with the absence of chip space

at the bottom, this means that swarf must be evacuated

by the utes. For virtually all other machining tasks,

such as holes with keyways, or intersecting holes, etc.,

53 ‘Hand-reamers’ , are available for the reaming both cylindrical

and tapered holes.

NB A basic rule to be observed when hand-reaming, is to

only turn the tool in the cutting direction and, never reverse

it (e.g. is is the standard practice in cutting a thread with

hand taps), as the reamer’s cutting edges will immediately be-

come blunt.

54 ‘Core-drilling’ ,

this is normally undertaken with a multi-

uted drill, as the hole already exists in the cast component

and in the main, the drill cuts on its periphery, so needs more

cutting edges in contact with the cored hole. Coring is result

of employing a core, prior to casting and it stays in the cavity

as the molten metal is gently poured to cast the part (i.e. cores

are normally made from an appropriate sand and binder, or

another suitable material, that can be removed at the ‘fettling

stage’ – leaving the hole), hence, its name: cored hole.

55 ‘Morse taper’ ,

was developed in the USA in the mid-to-late

1800’s by Steven Morse (i.e famed for his design and develop-

ment of the original geometry for the Twist drill). e Morse

taper is a ‘self-holding taper’ , which can be suitable sleeved ei-

ther upward, or downward in ‘ioned diameter’ to t the inter-

nal taper for the machine tool’s spindle/tailstock, requiring a

‘dri’ to separate the matching tapers upon completion of the

work. e Morse taper’s included angle varies marginally, de-

pending upon its Number (i.e ranging from 0 to 6). Typically,

a ‘No. 1’ is: 2° 58´ 54´ ´, with a ‘No. 6’ being: 2° 59´ 12´ ´.

LH spiral reamers are employed. e chip direction is

always in the feed direction and, for this reason, the

spiral ute geometry is virtually exclusively used for

through hole reaming operations.

3.3.1 Reaming – Correction

of Hole’s Roundness Profiles

Machine Reaming

In the ‘classical’ reaming operation, it is centre-drilled,

then the hole is through-drilled possibly producing

a variety of hole form harmonic out-of-roundness

errors present (i.e. see Fig. 70 ‘polar plots’ – bottom

le), including ‘

bell-mouthing’

at the entry and exit

of through drilled holes. Not only is there a possibil-

ty of ‘bell-mouthing’ , but a serious likelihood of the

drill following a helical path through the part, this is

termed: ‘helical-wandering’ (

i.e. see ‘Footnote No. 3’ ,

for an explanation of this drilling condition). By a fol-

lowing boring operation, this will correct for any prole

errors, while improving both the part’s overall out-of-

roundness

as exhibited by the ‘polar plots’ (ie. as il-

lustrated in Fig. 70 middle-le), but the hole’s ‘cylin-

ricity’

. Finally, the machine reamer is used to full

several functions: improve both the harmonic out-of-

56 ‘Bell-mouthing’ , is the result of the unsupported drill (i.e.

the margins as yet, not in contact with the drilled hole’s side

walls), producing the so-called ‘bell-mouth prole’ , upon hole

entry. At exit, if the drill is allowed to feed too far past the un-

derside of the hole, the drill has a ‘whipping-tendency’ , which

could introduce a smaller ‘bell-mouthing eect’ beneath the

part’s lower face.

57 ‘Out-of-roundness’ ,

was a term previously utilised, but today,

the term used has been changed to: ‘Departures from round-

ness’ , moreover, the term ‘polar plot’ has also been super-

seded by the term ‘displayed prole’ , however, in the current

context the former terms will be used.

58 ‘Cylindricity’ ,

is the term dened as: ‘Two, or more roundness

planes used to produce a cylinder where the radial dierences

are at a minimum’.

NB A more easily-understood appreciation of what ‘cylindric-

ity’ is, can hopefully be gained by the following ‘working ex-

planation’: If a perfectly at plate is inclined at a shallow angle

and, a parallel cylindrical component is rolled down this plate,

then if it is ‘truly round’ as it rolls there should be no discern-

ible radial/longitudinal motion apparent. In other words, the

component is a truly round cylinder, which can be equated to

a hole, or indeed, to a turned, or ground diameter.

Drilling and Associated Technologies 135

roundness (Fig. 70 top-le) and surface texture, while

‘sizing’ the hole’s diameter.

To further emphasise the point that drilling does

not produce a consistent harmonic out-of-roundness,

nor even a straight hole, Fig 71a, illustrates how the

‘polar plots’ are fundamentally modied at dierent

hole depths, here the ‘plots’ are shown near the top,

in the middle and close to the bottom of the drilled

hole. Correction of these roundness and diametrical

errors by machine reaming is not always the case, here

(i.e. shown in Fig. 71b), if the reamer is either not set

up correctly, or is slightly axially bent, in this case a

Figure 69. Types of solid reamer and their

associated geometry. [Courtesy of Guhring Ltd.]

136 Chapter 3

Figure 70. The exaggerated hole errors caused by an incorrect drill point geometry and the manufacturing techniques

for its subsequent correction

Drilling and Associated Technologies 137

Figure 71. Reaming can correct an assymetrically drilled hole – when correctly adjusted.

138 Chapter 3

large harmonic variation in the ‘plots’ is depicted, as is

the case when a ‘Floating reamer’ with roller drive has

been used inappropriately.

Floating Reaming

Solid machine reamers can be ‘oated-down’

a pre-

drilled hole, to produce a much straighter reamed

hole, than would otherwise be the case. When ‘oat-

ing’ reamers within their specially-located toolhold-

ers, two techniques are used to ‘oat reamers’ (i.e see

Fig. 72), these are:

. Radial play – where the machine reamer has lim-

ited movement laterally with respect to the princi-

pal axis,

. Composed radial and pendulum play – this has

both radial play, together with a degree of limited

angular movement (i.e. this motion is similar to

that of a Grandfather clock’s timing mechanism, via

its pendulum motion).

NB is latter ‘oating’ technique has the potential for

a combination of both radial and pendulum motions

to the machine reamer. ese unrestrained kinematic

motions gives it free motion without lateral and angu-

lar constraint, to simply follow the ‘line of least resis-

tance’ along the spindle axis, as the reamer progres-

sively feeds down through the predrilled workpiece.

3.3.2 Radially-Adjustable

Machine Reamers

Special-purpose machine drill/reamers (Fig. 74a)

are oen utilised in high-volume production envi-

ronments such as in the automotive sector, for util-

ity engines which can account for >55,000 complex-

reaming operations per week. Conversely, for defence

vehicle engines the production volumes are quite low,

accounting for <300 operations per month. Typical

operations on such automotive components, using a

machining centre include the reaming of:

59 ‘Floated-reaming’ , relates to the reamer’s ability to have some

degree of lateral compliance, namely limited motion, allowing

it some ‘play’ to follow the hole’s path, but still correcting for

any previous ‘helical wandering’ by the drill.

•

Cylinder head tappet rail drill-reaming – in a sin-

gle operation,

•

Cylinder head valve seats and guides – machining

both features, in the parent bore and nish machin-

ing,

Figure 72. Solid machine reamers can be ‘oated-down’ a

pre-drilled hole, by two distinct ‘oating techniques’: (I) radial

play, (II) composed radial pendulum play. [Courtesy of Guhring

Ltd.]

Drilling and Associated Technologies 139

•

Engine block and crank bores and cheek faces –

nish machining, with this latter feature requiring

controlled ‘radial infeed’ of the cutting/reaming in-

sert.

NB e special-purpose ‘radial-infeed’ tooling neces-

sary for the satisfactory machining of the cheek faces

of this latter low-volume production engine block, will

now be briey discussed.

Case-Study of Engine Block Bore Features

In this novel, but interesting automotive ‘case-study’ ,

all of the challenges facing such special-purpose

reamers are present. Here, the machining application

consisted of the following: a six-cylinder diesel cast

iron engine block for an armoured personnel car-

ier, reaming at 70 m min

–1

, requiring a bore straight-

ness of 0.02 mm/m, tolerance on the bore diameter

of 0.025 mm, with >0.003 mm tolerance between the

individual journals. e solution to this demanding

industrial problem, was the machining with two tools

and three operations of the crank bore and the genera-

tion of two cheek faces – this latter operation was nec-

essary to minimise the tted cranksha’s end oat.

is particular special-purpose reamer had a ra-

dial feed-out/retract cutting insert requirement for

the nal-machining of the cheek faces. erefore,

the base-tool holder contained a thrust and feed-out

mechanism, in addition to the whole tooling assem-

ly ‘running-true’ , so that it could be ‘datum-out’ and

precisely and axially-set with respect to its potential

engine block machining features. e radial mecha-

nism would incorporate an actuator sha mechanism

which can be pulled-/pushed-back, thereby resulting

in either a radial infeed, or retraction, respectively, of

the cutting insert. is bi-directional control of the

feed-out/-in of the radial mechanism is achieved in

conjunction with the CNC feed spindle of the machin-

ing centre.

In general, these special purpose reamers, have two

guide pads and a blade (Fig. 74a) with the reaming

blade set with a back-taper, producing the well-known

characteristic ‘saw-toothed prole’ to the reamed sur-

face (Fig. 74b – right). Such reamed surface texture to-

pography has been highly magnied in the schematic

diagram (Fig. 74b – right) and, requires very high

vertical magnication of the surface topography (i.e.

x50,000) to see any trace prole details at all! e posi-

tion of the cemented carbide guide pads, with respect

to the blade is critical to the reamer’s performance, as

is the residual stiness of the whole cantilevered tool-

ing assembly.

For many automotive industrial reaming applica-

tions, the components are oen cast from high-silicon

aluminium materials, as the addition of the element

silicon, creates a micro-grained and harder cast struc-

ture, than would otherwise be the case. However, the

disadvantage from a machining viewpoint, is that the

resultant cast matrix is highly abrasive to the cutting

edge. Under these circumstances, the reamer’s blade

Figure 73. Reamers in action, reaming automotive parts.

[Courtesy of Shefcut Tool & Eng’g Ltd.]

140 Chapter 3