Drilling and Associated Technologies Part 5 pps

Vibraopera-tions can occur in any number of ways that could affect the boring operation, from the constructional elements of the machine tool, through to slideways, or their recirculatin

also be reduced by utilising a different boring bar

ma-terial, as this will improve its coefficient of elasticity

In boring-out roughing operations, any vibrations

present are only a problem if they lead to insert

dam-age For finish-boring operations, vibrational

condi-tions that may occur could be the difference between

success and failure for the finished machined part

So, the boring bar’s ability to dampen any vibrational

source becomes imperative, once a fine-boring

opera-tion is necessary Vibraopera-tions can occur in any number

of ways that could affect the boring operation, from

the constructional elements of the machine tool,

through to slideways, or their recirculating ball

bear-ings, etc Hence, the joints in a machine tool can be

regarded as a complicated dynamic system, with any

slideway motion of vibrating contact faces,

necessitat-ing lubricatnecessitat-ing oil to not only reduce any stiction and

frictional effects, but to help dampen these structural

elements Machine tool builders are acutely aware that

certain machine tool materials ‘damp’ more readily

than others Cast iron and in particular ‘Granitan’ (i.e

a product of crushed granite and epoxy resin), can

pre-dominantly act as built-in dampening media for any

vibrational sources present The main source for any

vibrations in boring, results from the long overhangs,

necessary to machine the hole depth of the

compo-nent’s feature Therefore, the magnitude of vibrations

in the overall system result from the dampening

capa-bilities of the actual boring bar

Tuned Boring Bars

A boring bar that has been ‘tuned’ , has the ability to

dampen any generated vibrations between the

work-piece and the cutting edge while machining The

‘dampening effect’ is achieved through a vibration

ab-sorbing device (i.e see Figs 61a and 62b), this has the

consequence of increasing the bar’s dynamic stiffness,

giving it the ability to withstand oscillating forces The

 Coefficient of elasticity, for a steel boring bar composition,

E = 21 × 10 (N mm–), conversely, using a cemented carbide

material for an identical boring bar, E = 63 × 10 (N mm–),

giv-ing three times greater stiffness, allowgiv-ing much greater borgiv-ing

bar overhangs.

NB In reality, the boring bar’s deflection will be higher than

the values given in these examples, as the formula is based

upon the assumption that the bar is absolutely rigidly clamped,

which is impossible to achieve.

method of achieving this bar damping has already been mentioned in Section 3.2.1, with the relationships be-tween the size of the bar’s body, suspension, viscosity

of the liquid media, being carefully designed by the tooling manufacturer During the boring operation, the vibrations set the body in oscillation Hence, the body and the liquid alternate, taking each others place

in the space within the actual boring bar A pattern is established during boring, where the oscillations of the body are not in harmony with the vibrations resulting from machining This out-of-harmony, means that the

vibrations are virtually neutralised – to an acceptable

level – via the kinetic energy being transformed by the

‘system damping’ Any vibrations present during bor-ing, are relative to the amount of bar overhang, there-fore on longer boring bar lengths, they are normally fitted with some means of adjustment, so that they can

be ‘tuned’ to the frequency occurring within its range The simplest manner of achieving adjustment, is by

a rotation of a lockable set screw, which when either tightened, or slackened, affects the suspension of the body in the liquid, thus ‘tuning’ the boring bar to the actual machining conditions present

3.2.5 ‘Active-suppression’

of Vibrations

As has been stated at the beginning of Section 3.2.4, if boring bars have an L/D ratio >5:1, then vibrational ef-fects may result in tool chatter It has been observed in experimental work, that the boring bar’s tip produces

a vibration motion that follows an elliptical path in the plane normal to the longitudinal axis of the bar The ratio of the amplitude of vibration along the major and minor axes varies with cutting conditions, further-more, the inclination of these axes to the ‘radial line’

of the tool also varies Of significance, is the fact that the build-up of chatter will begin almost immediately, even before one revolution of the workpiece has oc-curred This build-up continues almost evenly until some limiting amplitude occurs, which suggests that the well-known ‘Orthogonal mode coupling’ is pres-ent, further, with the phase difference between the vi-brations causing an elliptical tool tip path, the vibra-tional energy is fed into the tool-workpiece system, promoting self-excitation

As has been suggested, the dynamic stability of the boring bar is of prime importance, with the onset of self-excited chatter, being governed by the ‘Multiple regenerative effect’ , which is a function of the so-called

‘space phase’ This ‘space phase’ condition, is the phase

of vibration around respective turns of work,

fluctu-ating between 90° and 180° and is equal to the phase

between the inner and outer modulation Moreover,

it has been shown that by modifying the workpiece’s

rotational speed, this disturbs the ‘space phase’ and,

consequently influences the ‘time phase’ , leading to

a reduction in self-excited chatter It has been

practi-cally demonstrated that by modifying the peripheral

speed of the workpiece, this technique is only partially

successful in alleviating chatter More success can be

made by utilising damped boring bars, such as the

‘Lanchester’ type, with dynamic vibration absorbers

(DVA’s), to really suppress vibrational influences

dur-ing the bordur-ing process

Some progress has been made on the development

of DVA techniques, but the potential ‘step-change’

will occur in vibrational suppression for boring bars,

when the improvement of production versions of

‘ac-tive’ dampers for such tooling becomes a reality Just

such a potential ‘active’ boring bar is shown

schemati-cally in Fig 64 Invariably, the boring bar has a supply

of energy to it – via an external source, that controls

the cutting edge’s position by monitoring the feedback

of the relative displacement of tool’s edge with respect

to the workpiece In later research work by Matsubara

et al (1987), chatter suppression was analysed for the

boring bar using ‘feed-forward’ control of the cutting

force Further, the cutting edge was positioned in

re-sponse to this force, with these type of ‘active’ control

systems being known as: ‘Cutting edge positional

con-trol systems’

Typical of a vibrational control approach is

illus-trated by the ‘active’ boring bar already mentioned and

depicted in Fig 64, where the forces are damped in

re-sponse to the vibrational velocity of the cutting edge,

which has been termed a: ‘Vibrational velocity control

system’ In this damping technique, the boring bar

sup-pression is by a series of piezo-electric elements that

act as ‘active dampers’ Such a ‘damper’ responds to

onset of chatter vibration (i.e the high-energy

com-ponents) Moreover, the damping force achieves

opti-mal phase difference, since the phases between both

 ‘Lanchester boring bars’ , normally utilise an internal metal

slug which is usually surrounded by some form of: liquid/fluid

medium, DVA’s, or more primitively, sprung-loaded and as

such, the slug is free to move out-of-phase with the cutting

conditions, dictated by the boring bar’s applied cutting forces,

thereby the onset of chatter will be potentially ‘cancelled out’.

the ‘damping’ and vibrational forces are controllable This type of ‘active’ boring bar arrangement, achieves directional damping characteristics via its ‘dampers’ ,

here they control two ‘degrees of freedom’  via the ‘Re-generative feedback loop’ , which diminishes oscillatory

motion (i.e harmonics), by careful control of energy losses

In recent years with the advent of artificial intelli-gence (AI) applications to major industrial engineering problems, and more specifically, in the performance

and robustness of certain types of ‘Neural networks’ ,

the goal of obtaining some form of real-time monitor-ing and control in the machinmonitor-ing process is now closer

to reality These AI systems have been successfully utilised for applied research applications to tool wear monitoring in turning tool operations – after suitable

‘training’ of a pre-selected neural network architecture These ‘networks’ could be successfully applied to bor-ing bar vibrational monitorbor-ing and control situations More detailed information will be said on how, where and when Neural network decision-making and, why these cutting tool monitoring applications should be utilised in the production environment, later in the text

3.2.6 Hard-part Machining,

Using Boring Bars

Although ‘hard-part’ turning has been utilised for some considerable time, with the advent of polycrys-talline cubic boron nitride (PCBN) tooling, etc., it has seen little in the way of exploitation for boring opera-tions, to date One of the major reasons for this lack

of tooling application, is because most hardened parts are in the region of hardness values ranging from 42

to 66 HRC Such high component hardness, requires considerable shearing capability by the tooling to suc-cessfully machine the excess stock from the workpiece Generally, the robust nature of toolholding for turning

 ‘Degrees of Freedom’ , the ‘free-body kinematics’ , exhibit 6

de-grees of translatory (i.e linear) motions in space, these are: back-ward/forward, upward/downward and leftward/rightward.

NB Of some interest but in the main, to machine tool

build-ers for the purposes of volumetric calibration, are the rotary motions of: yaw, pitch and roll, giving 18 degrees of freedom, together with the 3 squareness errors, totalling 21 possible de-grees of freedom.

Figure 64 An ‘active’ boring bar and their capacity to suppress vibrational effects on boring holes [After

Mat-subara; Yamamoto and Mizumoto; 1987]

.

tools with their modest overhangs, does not present

in-surmountable difficulties during machining, however

for the much longer overhangs associated with boring

operations (i.e see Figs 62a and 65a), then the cutting

forces generally dictate, short L/D ratios of <5:1 and

relatively large and robust boring bars (Fig 65b)

There are considerable difficulties to be

over-come when any form of hard-part machining is

required – particularly for boring operations, when

the components have been either case- or

through-hardened, these are:

• High temperatures in the cutting zone –

necessitat-ing high temperature resistant and

thermally-sta-bility of cutting insert materials,

• Cutting force magnitudes are both higher and more

variable – robust cutting edge geometry is

neces-sary to withstand these increased shearing/cutting

force demands on the insert,

• Small chip cross sections – these exert high

pres-sure near the insert’s cutting edge, often necessitat-ing an edge preparation to the insert’s corner,

• Greater tool wear rates – often more rapid cutting

edge wear, or the tendency to catastrophic break-down of the insert,

• Workpiece stresses during cutting – these stresses

are released during machining and may present localised geometric variations to the final shape of the part,

• Poor homogeneity in the workpiece material

– hardness variations across and through the part (e.g differential case hardened depths), can lead to significant and variable cutting force loadings on the boring insert,

• Insufficient stability – if the

‘machine-tool-work-piece loop’ is not sufficiently robust, then due to the greater cutting forces when hard-part machining,

Figure 65 Boring bar operational limitations and hard part boring at relatively high speed

[Cour-tesy of Sandvik Coromant]

.

this creates potential tool deflection 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 deflection will

in-evitably occur, this is exacerbated by machining

hard-parts The former problem of vibration has previously

been mentioned and methods of minimising it are

possible However, tool deflections are more difficult,

if not impossible to completely eliminate, with these

longer cantilevered tools Of note regarding

overhang-ing tool deflections, are that a tool tip deflects in two

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

• Radial deflection (∆T) – affects the machined (i.e

bored) diameter,

• Tangential deflection (∆R) – causes the tip to move

downward for the centreline

In each of these tool tip deflections, both the size and

direction of the cutting forces are influenced by the

chip thickness and insert geometry selected (i.e

illus-trated in Fig 66b) The radial deflection will be equal

to the difference 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

deflec-tion The tangential deflection 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 deflections ‘∆’ (i.e both the tangential

‘∆T’ and radial deflection ‘∆R’), as a function of the cutting depth ‘aP’ Due to the fact that the tangential deflection (∆T) linearly increases with increasing DOC

(aP), 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

549 for an abridged version of the IT tolerances, with

*Rmax values in µm

The magnitude of radial deflection as a function

of the cutting depth, is also influenced by the ratio between the insert’s nose radius and the DOC (aP), 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-deflected to the actual

 ‘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.

 These values are related to surface texture expression of:

*Rmax (µm), which is: The maximum individual

peak-to-val-ley height The 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/rε ) 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 IT (µm)

Where: n = The 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

[Source: Sandvik Coromant (1995)]

.

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

controlled [Courtesy of Sandvik Coromant]

.

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:

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]

.

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

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

.

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.]