3.1.9 Deep-Hole Drilling – Cutting Forces and Power In Deep-hole drilling operations, the underlying the-ory for the calculation of cutting forces and for torque are similar to that util
Trang 13.1.9 Deep-Hole Drilling –
Cutting Forces and Power
In Deep-hole drilling operations, the underlying
the-ory for the calculation of cutting forces and for torque
are similar to that utilised for ‘conventional’ drilling
operations The major difference between the hole
production calculations for Deep-hole drilling to that
of ‘conventional hole-making’ techniques, lies in the
fact that support pads create a sizeable level of
fric-tional forces, that cannot be ignored These increased
frictional effect contributions – by the pads – to the
overall Deep-hole drilling cutting forces and torque
values are somewhat difficult to precisely establish,
however, an approximate formulae can be used to
esti-mate them, as follows:
Feed force (N):
Fp + Fpµ = 0.65 × kc × ap × f × sinκr
Where:
Fp = Feed force, or drilling pressure (N),
Fpµ = Force and Frictional effects (N),
kc = Specific cutting force (N mm–),
ap = Depth of cut (mm),
f = Feed per revolution (mm rev–),
sinκr = Entering angle (°)
Torque, or Moment (Nm):
Mc + Mµ = kc � ap� f � D (. − ap �D)
Where:
Mc = Torque cutting (Nm),
Mµ = Torque and Frictional effects (Nm),
kc = Specific cutting force (N mm–),
ap = Depth of cut (mm),
f = Feed per revolution (mm rev–),
Relatively high speeds are utilised for Deep-hole
Drill-ing operations, in order to achieve satisfactory
chip-breaking, this necessitates having a machine tool with
a reasonable power availability
The underpinning theory for calculating the power
requirements, corresponds with that of ‘conventional’
drilling operations However, the friction forces that are
present, due to the employment of support pads, gives
rise to a torque contribution (Mµ), which in turn
pro-duces an associated contribution ‘Pµ’ to the total
Deep-hole drilling power Therefore, in order to estimate the machine tool’s power requirement (i.e ‘P’ in kW ), an allowance must be made for any power losses in the machine tool Hence, the gross power required can be established by dividing the Deep-hole drilling power (i.e Pc + Pµ), by the machine tool’s efficiency ‘η’ This efficiency indicates what percentage of the power sup-plied by the machine tool, that can be utilised, while Deep-hole drilling
Power (kW):
(Pc + Pµ) = kc � ap, � f � vc (. − ap �D)
Where:
Pc + Pµ = Power contributions of: cutting and friction
respectively (kW),
vc = Cutting speed (m min–)
∴P = Pc + Pµ/η Where:
η = Machine tool efficiency
3.2 Boring Tool Technology –
Introduction
The technology of boring has shown some important advances in recent years, from advanced chip-break-ing control toolchip-break-ing (i.e see Fig 59, this photograph illustrates just some of the boring cutting insert
ge-ometries that can be utilised), through to the ‘active suppression of chatter’ – more will be mentioned on the topic and reasons why chatter occurs and its sup-pression later in the text Probably the most popular type of boring tooling is of the cantilever type (Fig
59), although the popularity of either ‘twin-bore-’ , or
‘Chatter’ , is one of the two basic types of vibration (i.e
namely, ‘forced’ and ‘self-excited’) that may be present
dur-ing machindur-ing In the main, chatter is a form of
self-excita-tion vibraself-excita-tion.‘[It is]… due to the interacself-excita-tion of the dynamics
of the chip-removal process and the structural dynamics of the machine tool The excited vibrations are usually very high in amplitude and cause damage to the machine tool, as well as lead to premature tool failure’ [After: Kalpakjian, 1984].
Trang 2‘tri-bore-heads’ , with ‘micro-bore adjustment’ of the
ei-ther the individual inserts, or having a simultaneous
adjustment of all of the actual cutting inserts, is
be-coming quite common of late
Boring operations invariably utilise cantilevered
(i.e overhung) tooling, these in turn are somewhat
less rigid than tooling used for turning operations
Boring, in a similar manner to Deep-hole drilling and
Gun-drilling operations, has its rigidity decreased by
the ‘cube’ of the distance (i.e its overhang), as the
fol-lowing equation predicts:
�
� EI
L(Mt + .Mb)
Where:
fo = normal force acting on the ‘free end’ of the
can-tilever (i.e boring tool overhang),
*EI = flexural stiffness (i.e I = cross-sectional moment
of Inertia) (Nm),
Mt = boring bar mass (kg),
L = length of cantilever (mm),
Mb = Modulus of elasticity of the boring bar
(N mm–)
* E, relates to the boring bar’s ‘Young’s modulus’.
Boring a hole will achieve several distinct production
criteria:
• Enlargement of holes – a boring operation can
en-large either a single, or multiple series of diameters,
to be either concentric to its outside diameter (i.e
O.D.), or machined eccentric (i.e offset) to the
O.D.,
• Correction of hole abnormalities0 – the boring
process does not follow the previously produced
‘Eccentric machining’ of the bore of a component with respect
to its O.D., was in the past accurately achieved by
‘Button-bo-ring’ – using ‘Toolmaker’s buttons’ (i.e accurately ground and
hardened buttons of ‘known diameter’) that were precisely
off-set using gauge blocks (i.e ‘Slip-gauges’) This technique might
still be employed in some Toolrooms, but normally today, on
CNC-controlled slideways, a simple ‘CNC offset’ will achieve
the desired amount of bored eccentricity
0 Correction of hole abnormalities, as Fig 60 schematically
il-lustrates, how boring can correct for ‘helical wandering’ of the
drill as it had previously progressed through the workpiece
The drill’s helical progression would cause undesirable hole
eccentricity, resulting from minute variations in its geometry,
hole’s contour, but generates its own path and will therefore eliminate drill-induced hole errors by the subsequent machining operation (i.e see the sche-matic representation shown in Fig 60),
• Improvement of surface texture – the boring tool
can impart a high quality machined surface texture
to the enlarged bored hole
NB In this latter case, boring operations to
previ-ously drilled, or to any cored holes in castings, can be
adjusted to give exactly the desired machined surface texture to the final hole’s dimensions, by careful ad-justment of the tool’s feedrate and the selection of an
appropriate boring tool cutting insert geometry
3.2.1 Single-Point Boring Tooling
‘Traditional’ boring bars were manufactured as solid one-piece tools, where the cutting edge was ground
to the desired geometry by the skilled setter/operator, which meant that their useful life was to some extent restricted Later boring bar versions, utilised indexable cutting inserts, or replaceable heads (Fig 61) Boring bars having replaceable heads are versatile, with the same bar allowing different cutting head designs and cutting inserts (Fig 61a) Here, the insert is rigidly
clamped to the tool post, with replaceable ‘modular tooling’ heads with the necessary mechanical coupling
to be utilised (i.e Fig 61b), offering ‘qualified tooling’
dimensions
necessitating correction by a boring operation This ‘correc-tion’ is necessary, because the drill’s centreline follows the path indicated, ‘visiting’ the four quadrant points as it spirally progresses through the part Hence, hole eccentricity along with harmonic departures from roundness can be excessive,
if the drill’s lip lengths and drill point angles are off-centre The cross-hatched circular regions represent the excess stock material to be removed by the boring bar, where it corrects these hole form errors, while machined surface texture is also considerably improved
‘Qualified Tooling’ , refers to setting the tool’s offsets, with all
the known dimensional data for that tool, allowing for ease of tool presetting and efficient tool-changing – more will be said
on this subject later in the text.
Trang 3Figure 59 A selection of some tooling that can be employed for boring-out internal rotational features [Courtesy
of Seco Tools]
.
Trang 4Figure 60 The harmonic and geometric corrections by a boring operation, to correct the previous helical drift, resulting from
the drill’s path through the workpiece
.
In the case of the boring bar’s mechanical interface
(i.e coupling) example shown in Fig 61a- top, the
ser-rated V-grooves across the interface along with the
four clamping screws provide an accurate and secure fitment for the replaceable head, with internal tension adjustment via the interior mechanism illustrated
Trang 5Figure 61 Interchangeable cutting heads for boring bars utilised in machining internal features [Courtesy of
Sandvik Coromant]
.
Trang 6Possibly a more adaptable modular system to the
‘ser-rated and clamped’ version, is illust‘ser-rated in Fig 61b,
where the cutting head is held in place by a single
rear-mounted bolt and grub screws around the periphery
of the clamped portion of the boring bar securely lock
the replaceable head in-situ, enabling the cutting head
to be speedily replaced Some of these boring bar’s
have a dovetail slide mechanical interface, with the
dovetail coupling providing radial adjustment of the
cutting insert’s edge This ‘universal system’ (Fig 61b),
is normally used for larger bored diameters, that would
range from 80 to 300 mm Furthermore, it is possible
to add spacers/shims to precisely control the boring
bars overall length, this is particularly important when
medium-to-long production batches are necessary, in
order to minimise cycle time and its non-productive
setting-up times
In Fig 62a and b, are illustrated single-point
inter-changeable boring insert tooling, with Fig 62a giving
typical length-to-diameter (i.e L/D) ratios for actual
boring and clamping lengths The amount of boring
bar-overhang will determine from what type of
ma-terial the boring bar will be manufactured The most
common tool shank materials are alloy steel, or
ce-mented carbide, for L/D ratios of <4:1, with the
for-mer tool material in the main, being used here For
L/D ratios of between 4: to 7:1, steel boring bars do not
have adequate static, or dynamic stiffness, so in this
case cemented carbide is preferred One limitation of
utilising cemented carbide tool shanks, is its greater
brittleness when compared to steel, so careful tool
design is necessary to minimise this problem
‘Com-pound’ boring bar tool shanks have been exploited
to reduce both problems associated with either steel,
or cemented carbide tools A successful compound
tool used in cutting trials by the author, featured a
ce-mented carbide core surrounded by alloy steel, which
proved to be quite efficient in damping performance
and machining characteristics Fig 62b, illustrates the
internal mechanism of the boring bar, for potential
‘bar-tuning/damping’ – to reduce vibrational
influ-ences whilst machining Here, the mechanism consists
of a heavy slug of metal, held at each end by rubber
grommets, in a chamber filled with silicon oil
There-fore, as the boring operation commences the slug
vi-brates at a different frequency to the steel bar, which
counteracts the vibration, rather than intensifying
vi-brational effects Such ‘damped’ boring bars, have been
utilised with large overhangs, of between 10: to 14:1
L/D ratios More information on ‘damping effects will
be mentioned in Section 3.2.4
3.2.2 Boring Bar Selection of:
Toolholders, Inserts and Cutting Parameters
Boring Bar Toolholder – Decisions
Whatever the material chosen for the boring bar, its is
always preferable to use a cylindrical shank whenever
possible, as it offers greater general cross-sectional ri-gidity, to other boring bar geometric cross-sections Once the bar cross-section has been selected, the next decision to be taken concerns the tool’s lead angle Usually the first choice for lead angle would be a 0° lead, as the radial cutting forces are minimised, with the resultant forces being directed axially along the bar, toward the tool’s clamping point – which is ideal
If, a 45° lead angle is selected, then the cutting forces are split between the axial and radial directions This latter radial cutting force, can increase the probabil-ity of increased bar deflection and be a source for un-wanted vibrational effects
NB For more information concerning boring bar
se-lection, see Appendix 1b, for the ISO ‘code key’ for
‘solid’ boring bars.
Insert Selection – Decisions
Apart form the boring bar’s lead angle, an insert’s ge-ometry will also affect vibration during machining The two main types of insert inclination (i.e rake) an-gles are either positive, or negative – referring to their angular position in the bar’s pockets It is well known, that a positive insert shears workpiece material more readily than a negative style insert, as a result, the positive insert will generate a lower tangential cutting force This positive rake angle, is at the expense of de-creased flank clearance and, if too small, the insert’s flank will rub against the workpiece creating friction, causing potential vibrations to occur
Assuming that the insert’s edge strength will be adequate for the machining application, then when selecting an insert for boring, selection of a positive geometry with a small amount of edge preparation, having a suitable coating (i.e PVD, rather than CVD),
is a good start point Furthermore, the choice of a pe-ripherally-ground insert having a sharper cutting edge
in comparison to that of a directly-pressed and sin-tered insert, is to be recommended
Trang 7The insert’s substrate – if cemented carbide –
re-quires some thought, as if it is too hard, this type of
insert may chip via the effects of machining vibrations,
this is particularly so, if the tool geometry has an
ex-tra-positive and sharp insert cutting edge It might be
more prudent to initially choose a medium-hard
ce-mented carbide grade, as it tends to cope with a
poten-tial edge-chipping condition more readily, then, if this proves successful, a harder grade may be selected
Cutting Parameters – Decisions
Two complementary cutting parameters are the insert’s nose radius and the influence it has on the DOC For
Figure 62 Interchangeable cutting heads for machining internal features [Courtesy of Sandvik Coromant]
.
Trang 8example, when a finish boring operation is required,
then it is recommended that both a small nose radius
and DOC is used This smaller boring insert nose
ra-dius, minimises contact between the workpiece and
insert, resulting in lower tangential and radial cutting
forces For fine-boring applications, a good start point
is to choose an insert with a 0.4 mm nose radius, with
a 0.5 mm DOC It should be noted that the DOC ought to
be larger than the nose radius, this is because if it was
the other way around, cutting forces would be directed
in a radial direction – increasing potential vibrational/
bar-bending (i.e push-off ) problems
Feedrates should be identical regardless of tool’s
overhang, as any feed selection is normally based upon
the insert’s chip-breaking capabilities Avoidance of
very high feedrates when rough boring is necessary, as
it can significantly increase the tangential cutting force
component For finish boring operations, it is normally
the workpiece’s surface texture requirement that
dic-tates the maximum feedrate that can be utilised More
will be mentioned on the machined cusp height’s effect
on surface texture, this being created by the remnants
of the partial nose arc (i.e radius) of the cutting insert
and the periodic nature of the selected feedrate on the
bored workpiece’s surface, later on in the relevant
sec-tion in the book
A mistake often made by setters/machinists in
order to attempt to minimise vibrational tendencies, is
to reduce the rpm This strategy will not only decrease
productivity, but the lower rotational speed can lead
to BUE formation, which in turn, modifies the insert’s
cutting geometry and could change the cutting force
directions Instead of rpm reductions, modification
of other cutting data variables is suggested, in order
to improve these adverse vibrational/chatter effects
Sometimes even increasing the rotational speed, can
eliminate unwanted chatter
Although it is not a specific cutting performance
parameter, an often disregarded measure is that of
boring bar tool clamping In many circumstances,
cy-lindrical boring bars are simply clamped with several
setscrews, this is a poor choice of clamping method, as
at best, setscrews only contact about 10% of the boring
bar Conversely, a split-tool block, clamps along almost
‘Tool push-off’ – often termed ‘spring-cuts’ , are the result of
tool deflection, particularly when light cuts are used To
mini-mise the ‘push-off’ , very rigid workpiece-machine-tool setup
with a smaller nose radius to that of the D is recommended.
all of the boring bar’s periphery in the toolpost, allow-ing much greater tool rigidity and cuttallow-ing stability, al-leviating many of the potential problematic in-service machining conditions
3.2.3 Multiple-Boring Tools
Twin cutting insert tooling, usually consists of a cy-lindrical shank with slides mounted at the front (Fig 63a), or a U-shaped bar with cartridges (Fig 63b) The slides and cartridges can be radially adjusted, allow-ing for a range of various bored diameters to be ma-chined Normally, such tooling has a 7 mm maximum cutting depth recommended – for both edges simul-taneously in-cut With Twin-edged boring tools the
cartridges can be so arranged, that ‘Step-boring’ can
be utilised
When large diameter component features require
a boring operation, then the ‘Divided-version boring’
tooling can be exploited, but diametral accuracy is not
as good as for some of the other types of boring tool designs An advantage of the Divided-version’ boring tools, is the fact that a large diameter range can be
cov-ered, with this single tool If a ‘Universal fine-boring’
tool is utilised (Fig 63b), either internal (Fig 63b-top),
or external machining (Fig 63b – bottom), can be un-dertaken In this case, the fine-bore cartridges (1) are mounted on a radially-moveable slide (2), which is mounted on a bar (3) In the latter case of external com-ponent feature boring, there is a physical limit to the minimum diameter that can be machined – this being controlled by the bar’s actual size (i.e Here, it should be said that this particular tooling ‘setup’ can be thought
of as virtually a Trepanning operation with a boring tool) Moreover, with this external finishing operation,
the spindle must rotate in a left-hand rotation Tri-bore tooling often having individual micro-bore
cartridge adjustment (i.e not shown), as its name im-plies, uses three cutting inserts equally-spaced at 120° apart This boring tool arrangement of cutting inserts, offers very high quality bored diametral accuracy and
‘Step-boring’ , refers to using special shims with one of the
cutting inserts axially situated a little way in front of the other, while at the same time, the cartridges are radially adjusted en-abling the front insert to cut a slightly smaller diameter to that
of the rear one It should be noted that when ‘Step-boring’ , the maximum DOC is normally 14 mm, with an associated feedrate
of 0.2 mm rev–.
Trang 9Figure 63 Twin-edged boring tooling [Courtesy of Sandvik Coromant]
.
Trang 10precision to the machined hole, but such tooling can
be somewhat more costly than when utilising a
single-insert tool
3.2.4 Boring Bar Damping
For boring bars that have an L/D ratio of <5:1, then
relatively stable cutting conditions with controllable
vibrational influences can be tolerated However, if L/D
ratios utilised are larger than this limiting value, then
potentially disastrous vibrational tendencies could
oc-cur, leading to a variety of unwanted machining and
workpiece characteristics, these include:
• Limited tool life – caused by forced and self-excited
vibrations, restricting both cutting efficiency and
tool life,
• Unacceptable machined surface texture –
vibra-tions in the form of workpiece surface chatter, can
be the cause for component rejection,
• Substandard machined roundness – vibration/
chatter effects creating high-frequency harmonic
effects on the roundness profile
Stiffness can be expressed in terms of either static, or
dynamic stiffness Static stiffness of a bar is its ability to
resist a bending force in a static condition, conversely,
dynamic stiffness is the bar’s ability to withstand
os-cillating forces (i.e vibrations) Dynamic stiffness is an
essential property for a boring bar, as it is a measure of
its capacity to dampen the vibrations occurring during
machining, being greatly dependent of its overhang As
one would expect in testing for dynamic stiffness, with
‘Harmonics’ – on a machined component are the product
of complex interactions, including method of manufacture:
component geometry, cutting data utilised, any vibrational
influences encountered and material composition and its
manufacture (e.g Powder Metallurgy parts can vary in both
porosity and density throughout the part, which may affect, or
locally destabilised the cutting edge).
NB Harmonics on the machined workpiece, can be thought
of as a uniform waveform (i.e sinewave) that is
superim-posed onto the part’s surface The part’s low frequency
harmo-nicoften has higher frequency harmonics superimposed onto
the roundness For example, a 15 undulation per revolution
(upr) harmonic, could have a 500 upr harmonic superimposed
onto it, requiring suitable a Roundness Testing Machine with
Gaussian filters to separate out the respective harmonic
con-ditions – for metrological inspection and further analysis.
a boring bar’s overhang increasing under standardised machining conditions, the amplitude will also increase However, if the boring bar was dampened in some way, perhaps by utilising a ‘shock-absorber effect’ , ma-chining could be undertaken at longer overhangs This
‘damping effect’ is indicated by the highly centralised amplitude of oscillatory movements quickly reducing with time, indicating a high level of dynamic stiffness, this being crucial for long L/D ratios Obviously, the boring bar’s cutting edge deflection at its tool tip, is directly related to the amount of bar overhang, this de-flection being the result of the applied cutting forces The magnitude of a boring bar’s deflection being de-pendent upon: bar composition, diameter, overhang and the extent and magnitude of tangential and radial cutting forces The rigidly clamped and cantilevered boring bar’s ‘free-end’ will deflect/deform by forces acting upon it and, some idea of the magnitude of this deflection can be gleaned by the simple application of
‘mechanics of materials’ , using the following formula:
Where:
∆ = Boring bar deflection (mm),
F = Cutting force (N),
L = Boring bar overhang (mm),
E = Bar material’s coefficient of elasticity (N mm–),
*I = Moment of Inertia (mm)
* For a boring bar of circular cross-section, the Mo-ment of inertia will be:
For example, assuming that if a φ25 mm steel boring bar has an L/D overhang of 4:1, with an applied cut-ting force of 100 kP, then the magnitude of bar deflec-tion, using the above formula, would be:
∆L = D = 0.083 mm
If the overhang of this boring bar was now increased
to L/D ratios of 7:1 and 10:1, respectively, this would produce tool tip deflections of:
∆L = D = 0.444 mm
∆L = 0D = 1.293 mm
Hence, these deflection values emphasise the impor-tance of reducing overhang as it increases by approxi-mately ‘cube’ of the distance Moreover, deflection can