Associated Technologies And Milling Cutter by J Smith_2 doc

Equally, an insert with an 45° approach angle, will spread the load over a longer cutting edge Fig.. 4.1.4 Face-Milling Engagement – Angles and Insert Density Face-Milling Engagement

Figure 82 Face-milling cutters, having inclined approach angles, with either high-shear (i.e tangentially-mounted) inserts, or

cutter insert density variations [Courtesy of Ingersoll, Courtesy of Sendrik Coromant]

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• Chip flow may be hampered – the vectored angle

for exhausting chips may be compromised,

• High radial force component – i.e in relation to

the axial force, produces unfavourable loads on the

spindle, creating vibration tendencies, hence the

feeds must be restricted,

• Positive geometry triangular inserts should not be

used – these weaken insert corners, whereas

rhom-boid-shaped insert geometries, or similar, offer

much stronger insert cutting edges

For general-purpose milling operations,

intermedi-ate approach angles such as inserts having an 75°

ap-proach are common (i.e see Fig 82a), as they provide

good edge strength in combination with a favourable

relationship between insert size and cutting depth

Moreover, if these 75° approach-angled inserts are

tangentially-mounted (i.e as depicted in Fig 82a), the

edges are even stronger because of the ‘body’ of the

in-sert is more fully-supported, than is generally the case

for most of the radially-mounted variants Equally, an

insert with an 45° approach angle, will spread the load

over a longer cutting edge (Fig 83b) This 45° insert

approach geometry, provides good chip-flow for

long-chipping materials, with a low radial force component

in comparison to that of the axial force, what is more a

strong insert edge allows higher feedrates to be utilised

(Fig 83a)

When roughing-cuts are necessary, or

difficult-to-machine workpiece materials must be milled

requir-ing strong insert edges, then a round insert might be

the answer In general, round inserts have a positive

geometry with no sharp edges and as a result, offer

very strong cutting edges and chip-loads are relatively

evenly distributed along the rounded contact region

(Fig 83b – right) Furthermore, a round insert

usu-ally has a positive insert geometry, which can then be

turned in its seating to simply provide additional

cut-ting edges

 ‘Square-shoulder cutters’ , can produce a high radial force

component, which means that feedrates must be limited, as

they may cause ‘edge frittering’ (i.e see Fig 81 – bottom left,

illustrating an edge break-out condition), this unacceptable

machining condition is particularly prevalent on brittle-types

of workpiece materials, such as: (most) Brasses, (many) Cast

irons, (some) Powder Metallurgy compacts, together with

(many) non-metallic materials – Plastics, Perspex, Tufnol,

Carbon-fibre, etc.

4.1.4 Face-Milling Engagement –

Angles and Insert Density

Face-Milling Engagement

In any milling machining operation involving the po-sitioning of the cutter in relation to a workpiece, some thought should be given to not only the cutter’s: diam-eter; number of teeth, or cutting inserts; width of the workpiece; but also how the resultant cutting force(s) might influence the overall effectiveness of the pro-duction process This latter point, not only influences method of component clamping, dictating: how, where

and what will be the optimum method of ‘location and restraint’ of the workpiece, but on exit from cut, the

sudden disengagement and release of cutting forces will potentially create not only a exit-burr on ductile materials, or frittered edge on a brittle material The cutter’s exit can influence the type of stress induced into the workpiece surface – a compressive stress be-ing preferred (more will be said on this topic later, in

the section dealing with ‘Machined Surface Integrity’)

In most milling operations the term: ‘engagement’

con-cerns the relationship of cutter-to-workpiece position-ing and, in virtually all face-millposition-ing operations, one tries to prevent at the exit of the cut, the chip being at

its thickest, as this is an unfavourable machining

strat-egy The objective when milling, is to always try to get

the thinnest possible chip at exit from the cut Some

of these engagement positioning relationships are de-picted in Fig 84, indicating where the most favourable cutter/workpiece relationships are present Also in Fig 84, are depicted some unfavourable engagements that should be avoided, this may be possible by either changing the milling cutter’s diameter, or its tool path

if possible, to avoid such engagements

The milled cut length is influenced by the position

of the cutter with respect to the workpiece, with tool life being related to each cutting insert’s amount of time engaged in the actual cut For example, in Fig

84g, a cutter has been positioned centrally over the workpiece, this produces the shortest possible time

 ‘Location and restraint’ , are important factors when work-holding A component needs to be not only accurately located

with respect to either a ‘datum’ , or held on a ‘grid-plate’ in a known relationship to that of the cutter’s position, but it must

also be properly restrained – to prevent any, compliance of

its ‘degrees of freedom’ while it is clamped during machining

Hence the term: ‘Location and restraint’.

in-cut, conversely, in Fig 84h the cutter has been

moved just off-centre, causing a longer arc of cut for

each insert, which is likely to reduce the tool’s ‘cutting

life’ somewhat, but this is only part of the problem of

off-centre cutting Returning to the cutter positioned

centrally (i.e Fig 84g), here the direction of the radial

component cutting forces will fluctuate, with respect

to the cutting edges start and finish cutting, which

may create potential vibrational problems, or

prema-ture edge breakdown However, with off-centre

mill-ing (i.e Fig 84h), this machinmill-ing strategy introduces

a constant force direction, moreover, as the cutter is positioned not quite centrally over the workpiece, this central region produces the largest average chip thick-ness Just to complicate matters still further, if the cut-ter is positioned even further off-centre, this will allow even more inserts to be simultaneously brought into cut (i.e shown as ‘α’ , in Fig 85) There are often many

Figure 83 The importance of cutting insert approach angle inclination on the resultant chip shape [Sources: Fig a: Tooling

University, 2003; Fig b: Heuwinkel & Richter, 2005]

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milling strategy decisions and frequently some

com-promises that must be made, in order to obtain the

op-timum cutter/workpiece engagement for a particular

machining situation

Milling Cutter Density

In any face-milling operations the number of inserts

in cut (i.e see Fig 85), is a function of the quantity of

Figure 84 Face-milling cutter positioning over the workpiece – indicating favourable/unfavourable

cut-ter and workpiece placement – together with other important factors [Courtesy of Sandvik Coromant]

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inserts around the cutter’s periphery (Z) and the

en-gagement angle (α) An expression for these milling

cutter inserts and the cutter diameter’s relationship is

derived [Source: Isakov – Kennametal Inc and

pub-lished in American Machinist 1996)] from Fig 85, as

follows:

α = 90° + α

sin α= AB OA = W − .D .D = (W − .D) D = W − D D

α= arcsin W − D D

Z c = Z(�+ arcsinW − D�D)�

Where:

Zc (i.e see the following footnote)

D = Cutter diameter (mm),

W = Radial width of cut (mm),

α = Engagement angle (°),

α = Angle between cutter centreline and cutter

ra-dius to the peripheral point of either exit, or entry (°)

This above formula, can be simplified to the following

relationship:

Zc = Zα/360

This engagement angle is dependent upon the radial

width of the cut (W) and the face-milling cutter’s

di-ameter (D) Therefore, if the radial width of the cut

equals the cutter diameter (W/D = 1.0) and, the

en-gagement angle is 180°, then:

Zc = 180Z/360° = 0.5Z

 ‘Zc’ represents the number of inserts in-cut, which can be

found for any cut width (W), by applying the above formula,

derived from the schematic diagram, illustrated in Fig 85.

The values of ‘Zc’ can be obtained from the Table 7, for

various ‘W/D’ ratios, given below:

The face-milling cutter density must be such that

it allows the chip to correctly form and exhaust from the cut If inadequate chip space is provided, this will result in the chip remaining in the chip gullet This lodged chip is then carried around and merging with the succeeding chip and as a result, welding itself to it, potentially causing cutting edge breakage and possi-bly damage to the workpiece In any cutter/workpiece engagement, it is necessary to provide a cutting edge

density with at least one insert in-cut at all times

Fail-ure to achieve this cutter density could result in severe edge hammering, leading to one, or more of the fol-lowing conditions: chipped cutting edges; a damaged cutter; or excessive machine tool wear

For ‘coarse-pitched’ milling cutters, having between

1-to-1.5 inserts per 25 mm of diameter, this will allow for larger chip gullet spaces and as such, can be recom-mended to be used on either: soft workpiece materials that produce continuous chips; or, for wide cuts with

a long insert engagement Conversely, ‘fine-pitched’

milling cutters, with approximately 4-to-5 inserts per

25 mm of diameter, are normally utilised where lack of insert engagement is a problem These milling cutters

having ‘fine-pitches’ , will allow at least one insert to

be in-cut at all times, even when machining very thin workpiece cross-sectional areas These high-insert den-sity milling cutters, are usually recommended when machining high-temperature exotic alloys, or hard steels – where light chip loads are taken As a result of the smaller chips, less chip gullet space is necessary, al-lowing more inserts around the cutter’s periphery

4.1.5 Peripheral Milling Cutter

Approach Angles – Their Affect

on Chip Thickness

As has been previously discussed, a multi-point tool such as a milling cutter, will cut intermittently, as its cutting edges repeatedly enter and exit the workpiece’s arc of cut (i.e engagement) It was suggested in the previous section, that at least one, but preferably two,

Table 7: The ratio of cut width-to-diameter (W/D) Number of inserts in-cut Zc

[Source: Isakov – Kennametal Inc./pub in American Machinist, 1996]

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Figure 85 Typical facemilling cutters and their inserts, with a schematic representation of a milling cutter engagement angle

and the number of inserts in-cut [Source: Isakov/American Machinist, 1996]

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cutting edges should be in-cut at all times Tooling

manufacturers design and test their cutters during

engagement, carefully determining both the feed and

speed of a milling operation, ensuring that the cutting

forces are effectively balanced-out around all of the

teeth The machining objective here, is to discover the

optimal chip thickness As has been previously shown,

milling cutter flutes can be either helical, or straight,

with the replaceable cutting inserts being located and

secured by either a wedge, or screw clamp With each

adjacent cutting edge around the cutter’s periphery

be-ing referred to as its pitch Durbe-ing a face-millbe-ing

oper-ation, a chip is formed at the two cutting edges,

where-upon, it slides up the tooth face and into the flute,

striking the fillet, or rounded corner of this flute

The approach angle is a key milling geometry

fac-tor, being formed the tool’s axis and by the peripheral

edges of either a solid cutter, or its cutting inserts This

approach angle describes how far the top of the insert

inclines away, from that of being parallel to the cutter’s

axis (i.e as shown in Fig 83a) In most general milling

operations, the ‘approach’ ranges from 0° for creating

square shoulders (Fig 83a-left), to 45° in

finish-mill-ing (Fig 83a-right) Usually, millfinish-mill-ing cutter approach

angles ranging between 15° to 45° are the norm, with

a 15° approach enabling deeper cuts to be taken As

the approach angle of the cutting edge inclination

in-creases, the chip becomes both longer and thinner for

the same DOC, or ‘ae’ (Fig 83b), with the load being

spread over longer edge length – resulting in smoother

cutting Larger insert inclination, enable higher

fee-drates to be employed, although it must be

empha-sised, with shallow cuts (Fig 83a-bottom)

Taking a different milling operational premise, the

objective when ‘rough-milling’ , is to remove the

maxi-mum workpiece stock in the shortest possible time

The material removal rate being limited by the

‘avail-able’ spindle power, although this condition can be

op-timised by ‘radial chip-thinning’ The chip thickness

is based upon the calculated feed per tooth (fz) and it

diminishes as the radial width decreases and in reality,

creating a lighter actual ‘fz’ This ‘lessening effect’ of the

chip thickness, causes the cutting edges to rub, rather

than cut the workpiece material, as a result, the feed

per tooth (fz) should be increased as the radial depth

 ‘Radial chip-thinning’ , is the effect of taking a radial DOC (ae)

of less than 25% of the milling cutter’s diameter

decreases This cutting strategy ‘boost’ in the effective feed per tooth, provides the twin benefits of longer tool life, with shorter cycle-times

For any operation in milling involving a ‘chip-thin-ning exercise’ , of paramount importance is the cutter’s

approach/inclination angle (χ) Therefore, as the ap-proach angle (χ) become more inclined from say, 90°

to 45°, the chip thickness, its ‘h-value’0 decreases (i.e as

schematically-demonstrated in Fig 83b) The optimal chip thickness for a given set of cutting data, can be entered into a machine tool’s CNC program, by utilis-ing the followutilis-ing formula:

fz = hm/sinχ The chip thickness (fz) is always constant, regardless of the approach angle inclination, be it operated at 90°, or

down to 30°, or indeed, at a flatter approach (see Fig 83b) The exception to this ‘chip thickness rule’ be-ing when utilisbe-ing a round, or button-type insert (Fig 83b-right), as it does not have either a top geometry, or

an edge chamfer, thereby creating the strongest type of cutting edge Round inserts without the straight cut-ting edges associated with other milling inserts, cre-ate chips that increase in thickness as the DOC becomes deeper Hence, for round inserts, the average chip thickness (i.e its ‘hm’ – value), relates to the thickness

of cut this being based upon the insert’s radial engage-ment of the workpiece via the milling cutter’s diam-eter If a comparison is made between a round milling insert to that of an insert with a 90° approach (i.e Fig 83b-right and Fig 83b-left, respectively), an identical volume of chips will be removed for both at a set feed

0 ‘h-values’ , for a material group are represented as a range,

with a lower number being the starting value For example, if utilising a machining centre with a 35 kW spindle power avail-ability for the milling of non-ferrous, or aluminium alloys,

the ‘h-value’ , or chip thickness ranges between 0.050 mm to

0.076 mm Alternatively, using this same machine tool to mill, either: stainless steels, titanium alloys, or heat-resistant

super-alloys, the ‘h-values’ will range from 0.076 mm to 0.152 mm,

whereas, for: plain carbon steels, cast-/nodular-cast irons the range will be between 0.152 mm to 0.254 mm

NB Do not attempt to mill thicker chips than is recommended

in the literature, as this action could result in over-loading the cutting inserts and breaking their edges.

 ‘hm value-ranges’ for various workpiece materials are identical

to those ‘h-value ranges’ previously mentioned.

per tooth and DOC Although, if the DOC is half that of

the round insert’s inscribed circle, this round

geom-etry creates chips that are 30% thinner to that of the

90° approach inserts This reduction in chip thickness

is the result of the round insert having a longer cutting

edge, which engages radially with the workpiece (Fig

83b-right) Alternatively, if chip volume for both the

round and 90° approach inserts were identical, then

the chip length generated by the round insert is

ap-proximately 50% longer and it is much thinner than

its counterpart Moreover, with the same feed for the

round insert, but the DOC is reduced so that it is 25%

of it’s inscribed circle, the chip thickness produced is

now 50% less for an identical chip volume Hence, to

achieve the desired productivity benefits that will

ac-crue from utilising a ‘chip-thinning strategy’ , the DOC

needs to be <25% of the round insert’s inscribed

cir-cle As the DOC has now become more shallow using

a round insert, the chip has now been ‘thinned’ , so in

order to compensate for this loss of stock removal, the

feedrate needs to be increased Therefore, as the round

insert’s DOC becomes more shallow the approach angle

flattens-out to almost ‘infinite length’ So, when the

average chip thickness and approach angle variables

are entered into the formula for ‘feed per tooth’  in the

CNC program they can be significantly higher –

al-most up to 100% greater

In order to establish either round, or button-style

geometries for their ‘effective approach angles’ , the

fol-lowing formula has been derived:

‘Effective approach angle’ Tan χ = ae/(ICeff/2)χ

Where:

χ = Approach angle (°),

ae = DOC (mm),

ICeff = Inscribed circle – ‘effective’ (mm)

If a 90° approach angle is used, the rate of advance

per tooth equals the chip thickness When there is a

decrease in the approach angle inclination, the chip

volume stays the same, but the length of cutting edge

engagement with the workpiece will increase This

re-sults in a chip which is both smaller and longer than

that programmed, hence it is necessary to raise the

 ‘Feed per tooth’ (f z), calculations will affect the chip-loading

for the milling cutter and be influenced by the spindle power

availability – see previous equation for this relationship.

feedrate to increase the chip thickness to its required level, when the DOC is less than the round insert’s ra-dius It can be said that although the chip created by the round insert geometry has an almost identical chip thickness to that of a 90° approach angled insert, the button-style insert geometry removes workpiece ma-terial at a considerably faster stock removal rate The only ‘down-side’ to that of utilising button-style milling inserts, is the spindle power requirement is greater With increasing insert inclination of the approach angles the chip thins, causing the cutting forces to be re-directed By way of an illustration of this effect, if a 45° insert approach is used, the axial force component will be identical to that of the radial force This radial force component, tends to make the tool deflect and may generate chatter, conversely, the axial component force is toward the direction of the spindle thereby re-ducing the potential risk of its damage via vibrational effects Therefore, if the insert inclination is such that the approach angle is almost flat, this has the advan-tage of the axial force component being in the spindle’s direction, this will minimise the likelihood of tool de-flection Thus the primary objective here, is to remove workpiece stock at high rates and speedily, keeping the approach angle low, with a light DOC, in this manner allowing chips to be thin and as a result the cutter will

‘fly’!

4.1.6 Spindle Camber/Tilt –

when Face-Milling

On some conventional and CNC milling machines, the spindle can be tilted slightly, this small inclina-tion in the direcinclina-tion of the feed, ensures that the cut-ter does not lie completely flat to the workpiece’s

sur-face This small spindle tilting technique avoids the so-called ‘re-cutting effect’  that normally if present

when utilising a large face-milling cutter In reality, the

spindle camber is very slight and generally amounting

 On many machining centres it is not always possible to tilt the

spindle and, in such situations, back-, recutting is an unavoid-able milling surface texture condition.

 ‘Re-cutting effect’ , this is the product of the cutting inserts

on the ‘back-edge’ of a large face-milling cutter scoring the recently machined surface, thereby affecting and slightly degrading the milled surface texture, while simultaneously avoiding additional flank wear on the inserts – prolonging the cutter’s life.

to between 0.1 to 0.3 mm over a length of 1,000 mm

When this is converted to angular measurements, this

equates to a value of between 20 to 60 seconds of arc

respectivly – as shown in the exaggerated diagram in

Fig 86 Often, when it is not possible to slightly tilt the spindle, back-cutting problems can arise through such factors as spindle, or workpiece deflections This problem can be minimised by:

Figure 86 The influence of spindle camber

(tilt) on the milled workpiece surface [Courtesy

of Kennametal Hertel]

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• Improvements in workpiece support – ensuring the

both packing and clamping are sufficient to support

the component,

• Modifying the cutter to a positive geometry – this

has the effect of reducing any back-cutting milling

by the cutting inserts,

• Reduction in cutting forces – though: feedrate

re-ductions, lighter DOC’s/cut widths, or by increasing

the cutting speed,

• Modifications to approach angles – this will have

the effect of reducing the axial force component,

• Reducing spindle overhang – will decrease cutter

deflection,

• Inspection of cutter milling mounting – this will

ensure that any burrs, debris, or misalignments are

minimised

If the spindle is slightly cambered when

face-mill-ing, a plain workpiece surface will not normally be

produced Under these conditions of a slight camber,

the machined surface is normally concave, due to the

angular tilt of the milling cutter (i.e shown in Fig 86

top/middle schematic diagrams) The surface

concav-ity generated by this camber, depends upon the

rela-tionship between the: cutter’s diameter; width of the

workpiece surface being cut; together with the DOC

The milled workpiece concavity ‘f’ , can be calculated

using the well-established Kirchner–Schulz formula,

as follows:

Milled concavity f =q [De� − (D

e� − e�)

]

Where:

f = Milled concavity (mm),

q = 1000tanθ where θ is the spindle camber (°),

De = Effective diameter of the cutting circle (mm),

e = Width of workpiece surface being milled (mm).

Alternatively, a reasonable estimate of the milled

con-cavity ‘f’ can be obtained from the graph in Fig 86

(bottom), that illustrates the variation in the concave

shape, for a variety of spindle cambers and

face-mill-ing diameters These concave surface modifications

produced by the spindle camber are never large

devia-tions from the ‘true’ plane surface For example, even

under the extreme conditions of employing a relatively

small diameter milling cutter of: φ100 mm; together

with a large spindle camber ‘q’ value of 0.05 mm, the

deviation in milled surface concavity only amounts to

25 µm over a workpiece width of 100 mm – this being within the accepted tolerances for many commercial situations However, for the generation of high-pre-cision milled surfaces, this minute level of concavity would not be tolerated In fact, when a milled surface

is metrologically inspected at high levels of

magnifica-tion, the ‘surface topography’  consists of both form

and surface variations These form and surface varia-tions are related directly to either the cutter-spindle ac-curacy, or the axial displacement of the cutting inserts, with the distance between wave crests (i.e sometimes termed the asperities – high points – on the machined cusps) frequently coinciding with the feed per tooth It

is possible to establish the reasons why a surface might deviate from the ‘true’ plane, with some of the possible factors being caused by:

• Machine tool condition – possibly resulting from

the fact that the spindle bearings are in poor con-dition, the slideways have appreciable ‘back-lash’ present, or poor ‘damping’ generating vibrational tendencies – showing-up as ‘chatter-marks’ on the milled surface,

• Workpiece clamping/stability – if the workpiece is

not sufficiently and correctly clamped, then it could flex, or move on the fixture/pallet/table, whilst be-ing machined, creatbe-ing unwanted surface devia-tions/fluctuations,

• Axial insert displacement – possibly created by

cutting inserts not precisely located in their respec-tive pockets, or resulting from movement during milling – due possibly to inadequate locking of the insert in-situ during pre-setting,

 ‘Surface topography’ , is a term that is often used to describe

the form, waviness and surface texture fluctuations from the

‘true’ plane A machined surface may exhibit some, or all of

these variables, together with its ‘lay’

NB Form errors are long-frequency components of a surface,

with waviness being medium-frequency components, while surface texture is normally associated with short-frequency

components Depending upon the relative size of the

work-piece, these variables are superimposed onto each other, but

each one can be ‘filtered-out’ by suitable magnification on

a Surface Texture Machine – for future analysis The ‘lay’ is the direction of the dominant surface pattern, created by the passage of the cutter over the surface When assessing a ma-chined surface with a anisotropic lay condition – this being a surface that has a significant lay (i.e clearly visible machining marks), it is normal procedure to assess the surface’s

condi-tion at 90° to the lay.