Drilling and Associated Cutting Tool Technology Industrial Handbook_6 pdf

‘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 mac

Figure 66 Hard-part boring, can create excessive boring bar deflections and potential vibrational problems – if not carefully

controlled [Courtesy of Sandvik Coromant]

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workpiece’s centreline Boring bar overhang is not a

problem when ‘Line-boring’ 0 as the tool is supported

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

line-boring tooling’ 

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

an effect 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

The 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 The surface texture quality obtainable by

ream-0 ‘Line-boring’ , as its name implies is utilised for boring part’s

with concentric and often 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 This additional support enabling

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

cen-treline in-situ

 ‘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 crankshaft bearing housings on both

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

This uniquely-designed ‘Telescopic line-boring tool’ , machined

the first 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 bormachin-ing cycle, then retractmachin-ing on

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

magazine, allowing/facilitating an efficient 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 finishes 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

 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= 

lr

l r

�

 � Z(x) � dx

NB In the past and specifically 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 floating reamer applications [Courtesy of Seco Tools]

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Figure 68 Single-blade reamers offer superior hole geometry over conventional reamers

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

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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

1200 N mm–

Machine reamers are available with: straight flutes,

left-hand (LH) spirals, or 45° LH ‘quick’ spirals this

lat-ter reamer version is often lat-termed a ‘Roughing reamer’

and is often used for ‘long-chipping’ workpiece

mate-rials Reamers with straight flutes 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 flutes For virtually all other machining tasks,

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

 ‘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 This is the standard practice in cutting a thread with

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

be-come blunt.

 ‘Core-drilling’ , this is normally undertaken with a

multi-fluted 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.

 ‘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) The Morse

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

ei-ther upward, or downward in ‘ioned diameter’ to fit the

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

‘drift’ to separate the matching tapers upon completion of the

work The 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 The chip direction is

always in the feed direction and, for this reason, the spiral flute 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 left), including ‘bell-mouthing’  at the entry and exit

of through drilled holes Not only is there a possibil-ity 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 profile

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-left), but the hole’s ‘cylin-dricity’  Finally, the machine reamer is used to fulfil several functions: improve both the harmonic

out-of- ‘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 profile’ , 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 effect’ beneath the

part’s lower face.

 ‘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 profile’ , however, in the current

context the former terms will be used.

 ‘Cylindricity’ , is the term defined as: ‘Two, or more roundness

planes used to produce a cylinder where the radial differences 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 flat 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.

roundness (Fig 70 top-left) 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 modified at different

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.]

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

for its subsequent correction

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

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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 ‘floated-down’  a

pre-drilled hole, to produce a much straighter reamed

hole, than would otherwise be the case When

‘float-ing’ reamers within their specially-located

toolhold-ers, two techniques are used to ‘float reamers’ (i.e see

Fig 72), these are:

1 Radial play – where the machine reamer has

lim-ited movement laterally with respect to the

princi-pal axis,

2 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  This latter ‘floating’ technique has the potential for

a combination of both radial and pendulum motions

to the machine reamer These 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 often 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:

 ‘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 finish machin-ing,

Figure 72 Solid machine reamers can be ‘floated-down’ a

pre-drilled hole, by two distinct ‘floating techniques’: (I) radial play, (II) composed radial pendulum play [Courtesy of Guhring Ltd.]

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• Engine block and crank bores and cheek faces –

finish machining, with this latter feature requiring

controlled ‘radial infeed’ of the cutting/reaming

in-sert

NB  The 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 briefly 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-rier, reaming at 70 m min–, 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 The 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 operagenera-tion was nec-essary to minimise the fitted crankshaft’s end float This particular special-purpose reamer had a ra-dial feed-out/retract cutting insert requirement for the final-machining of the cheek faces Therefore, the base-tool holder contained a thrust and feed-out mechanism, in addition to the whole tooling assem-bly ‘running-true’ , so that it could be ‘datum-out’ and precisely and axially-set with respect to its potential engine block machining features The radial mecha-nism would incorporate an actuator shaft mechamecha-nism which can be pulled-/pushed-back, thereby resulting

in either a radial infeed, or retraction, respectively, of the cutting insert This 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 profile’ to the reamed

sur-face (Fig 74b – right) Such reamed sursur-face texture to-pography has been highly magnified in the schematic diagram (Fig 74b – right) and, requires very high vertical magnification of the surface topography (i.e x50,000) to see any trace profile details at all! The posi-tion of the cemented carbide guide pads, with respect

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

is the residual stiffness of the whole cantilevered tool-ing assembly

For many automotive industrial reaming applica-tions, the components are often 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.]

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