Milling Cutters and Associated Technologies Part 3 potx

• Insert wear uneven – possibly resulting from inad-equate pre-setting of the cutting inserts – allowing some to ‘stand-proud’ of the rest and as a conse-quence, being subjected to highe

• Insert wear uneven – possibly resulting from

inad-equate pre-setting of the cutting inserts – allowing

some to ‘stand-proud’ of the rest and as a

conse-quence, being subjected to higher wear than the

others,

• Insert shape irregularities – possibly the result of

poorly manufactured cutting insert geometries,

creating differing heights once secured and

accu-rately positioned in their respective milling insert

seatings,

• Irregular chip-flow – possibly the result of either the

insert chip-breakers operating inconsistently, or the

workpiece material having matrix inconsistencies

4.2 Pocketing, Closed-Angle

Faces, Thin-Walled and

Thin-Based Milling

Strategies

Pocket Milling

In particular and in the aerospace industries,

alumin-ium machining from: wrought, extruded stock and

forged parts is a regular practice and, to a lesser extent,

this also occurs in many precision machining

environ-ments Often both for shallow and deep pockets and

for ribs, it is necessary to relieve weight at critical

sec-tions on components

One of the oldest established techniques for

achiev-ing pocket features, is to drill a hole at the centre of the

pocket to a pre-set depth Then change tools and

plac-ing the millplac-ing cutter in this hole clear-out the pocket,

repeating this cycle until the pocket is ‘roughed-out’

Perhaps changing cutters and taking finishing cuts to

complete the feature (Fig 87ai) Rather than simply

plunging to depth with the cutter, ramping-down into

pockets is an effective way of reaching the ‘first-level’

for the pocket’s area clearance Both ramping and

‘double-ramping’ (i.e this latter technique is

particu-larly efficient for smaller pocket dimensions), are ways

of removing stock via a ‘diagonal plunge’ , while

tak-ing the milltak-ing cutter to its required depth (Fig 87aii)

This technique is an efficient machining strategy for

the milling of square and rectangular pockets – for

high stock removal

If the pocket is of non-uniform dimensions, then

perhaps a ‘lace’ , or ‘non-lace’ cutter path clearance

technique might be the preferred option, when having

to machine these type of component features

Milling Closed-Angle Faces

For the machining of so-called ‘closed angle features’

such as a re-entrant pocket, or ‘dovetail’, these latter features are typically utilised for drop-forging inserts

In Fig 87bi, the pocket has a land (i.e to impart ad-ditional mechanical strength to the corner), this land which would run around the base of the enclosed pocket, requiring a 5-axis machining centre to com-plete the milling operation In Fig 87bii, a normal end

mill cannot remove the excess material left in the base of

the re-entrant angle, necessitating either a ball-nosed,

or tapered ball-nosed cutter to reach in and mill the desired feature (Fig 87biii – in this case, the illustra-tion shows a tapered ball-nosed milling cutter)

 ‘Lace’ , or ‘non-lace’ cutter path, a ‘lace-cut’ is where the cutter

clears (i.e machines) an area with cutter paths that step-over

at regular pre-defined intervals, normally used when a sur-face has regular dimensions, such as a square, or rectangular

feature Conversely, a ‘non-lace’ cut is normally reserved for

the machining of irregular surfaces with the tool paths being non-linear in their step-over paths, for example, when milling

a triangular-shaped pocket/feature, or similar.

NB With the advent of sophisticated Computer-aided

Manu-facture (CAM) programming capabilities, much of the auto-mated generation of cutter paths, decision-making is under-taken by the software, to optimise area clearances for these component features.

 ‘Re-entrant pocket’ , is one where the base of the pocketed

fea-ture is somewhat larger than its top, meaning that the pocket

faces slope inward.

 ‘Dovetails’ , are normally open at both ends allowing the male

dovetail on the part to be held and its tapered key to easily inserted, positioned and locked in-situ Usually, such features are generated and formed on machine tools such as:

Plano-mills, Shapers, etc., but where such equipment is not available, then a ‘closed-angle milling’ operation is necessary.

 5-axis Machining Centre, normally has 3 linear (i.e X, Y and

Z) and 2 rotary (i.e A and B) axes The relationship of the rotary axis will depend upon the machine tool’s configura-tion, but they allow axis of a milling cutter into an otherwise closed-feature, negating the possibility of any cutter/spindle fouling on the workpiece.

Milling Thin-Walls

For the machining of thin-walls (Fig 88), such as

when milling rib-sections on aerospace components,

the machining strategy will vary, depending upon the

respective height and wall thickness In every case of

thin-walled machining, the number of passes will be

determined by the component’s wall dimensions and

axial depth of cut, in the following manner:

• Height-to-thickness ratios of <15:1 – then possibly

the most favoured milling strategy is to machine

one side of the wall in non-overlapping passes, fol-lowed by a repetition on the remaining side – as depicted in Fig 88a In all cases of thin-walled

ma-chining a ‘finishing allowance’ is left on both sides

and the base for subsequent machining,

• Height-to-thickness ratios of <30:1 – there are two

basic milling techniques that are usually employed, these are:

• ‘Waterline milling’ (Fig 88b-left) – this is where

either side of the thin-wall feature is milled to

pre-determined depths, in non-overlapping passes,

Figure 87 Pocket and closed-angle feature milling [Courtesy of Sandvik Coromant]

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Figure 88 Thin-walled machining strategies [Courtesy of Sandvik Coromant]

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• ‘Step-support milling’ (Fig 88b – right) – this

tech-nique utilises a similar approach to the previous

method, but in this case, there is an overlap between

passes on opposite sides of the wall This strategy

gives more support at the vicinity where

machin-ing occurs and the cuttmachin-ing forces are less likely to

distort the wall as it height increases

NB For very large height-to-thickness ratios of

>30:1, an alternative milling strategy, is to

alter-natively mill either side of the wall – approaching

the desired wall thickness in stages in a so-called:

‘Christmas tree routine’ 0 (i.e not shown), so that

the thinner sections are always supported by thicker

sections below them This method is then repeated

as the step-wise milling operation moves down the

wall

Milling Thin Bases

Unsupported thin-base features, such as the one

il-lustrated in Fig 89, are difficult to produce once the

previous side has been machined, because of the lack

of support, particularly at the base’s central region

One milling approach in the production of this

unsup-ported thin-base, is to ‘helically mill’ the feature (i.e

shown in cross-section in the small inset diagram in

Fig 89) This usually necessitates milling at the

cen-tre of the base region, spiralling-down to the required

depth, then milling outward in a ‘flattened helical

manner’ from that point (Fig 89 – main illustration

and plan view) Occasionally, one of the faces has

al-ready been machined and under these conditions it

must be ensured that the cutter’s flank makes minimal

contact with this face, for this operation it is usual to

employ tooling with the minimum number of flutes

Sometimes a component to be thin-based milled,

has a hole at its base’s centre, in such a situation it is

prudent to leave a support leg in place when milling

the first side Then machine the second side, finally

re-moving (i.e milling) this support leg after both sides

have been completed, thereby minimising any base

de-viation due to the presence of the cutting forces whilst

milling the feature

0 ‘Christmas tree routine’ , is so-called, because as it is being

step-wise milled and progressively develops, the silhouette

re-sembles the profile of the Christmas tree – hence its name.

4.3

Rotary and Frustum-Based Milling Cutters – Design and Operation

Rotating Insert Face-Mills

One of the novel face-milling cutters which is

cur-rently available includes rotating round inserts that are self-propelled as they cut (Fig 90a), promoted by the chip-flow over the insert’s face It has been claimed

by the tooling manufacturers of these interchangeable rotating insert cutters, that their unique cutting ac-tion provides greater cutting efficiency and is less de-structive to the inserts, than the conventional ‘locked’ milling inserts The term that is used for this rotating

cutting action is ‘roll shearing’  The rotation of the inserts continually introduces a ‘fresh’ cutting edge to the workpiece, this, it is claimed, minimises any heat build-up in the cutting zone, with much of the heat being transferred to the milled chips Any remaining heat being easily dissipated along the entire length of these round inserts (i.e the insert circumference has

an effective total cutting edge length of approximately

85 mm) This rotating insert has an almost infinite

ef-fective cutting edge length, enabling around a 10-to-1 improvement of insert life Due to the increased tool life, less down-time for changing cutting edges is re-quired, thereby improving cutting efficiency and im-pacting on actual overall cycle-times, because faster cutting speeds can be utilise

It has further been claimed by the tooling manu-facturer, that with the very high cutting speeds the

lo-calised heat is of benefit, as the heat within the cutting zone is concentrated in the chip and not in the

work-piece, or the insert This local heating of the metallic workpiece, allows it to reach its plastic deformation stage, causing the chip to flow freely away from the

 ‘Roll shearing’ , is a combination of the rotating action of the

round cutting inserts, in combination with the angled axis which slices, or shears through the workpiece

 Rotating insert cutting speeds – with these rotary insert

cut-ters has been increased dramatically, when compared to the more conventional ‘locked insert’ face-mills For example, when the silicon nitride cutting inserts are face-milling cast iron components 1,000 m min– is possible, conversely, when face-milling aluminium workpieces cutting speeds of 2,300 m min– have been successfully employed.

Figure 89 A strategy for the milling of thin-bases [Courtesy of Sandvik Coromant]

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Figure 90 A range of rotary milling cutters [Courtesy of Rotary Technologies Corp.]

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milled surface This localised plasticity allows the

en-ergy to be maximised and the cutting efficiency to be

increased Moreover, lower workpiece heat, results in

less component distortion Yet another benefit of this

‘roll shearing’ action, is that when conventional cutters

are used the tangential force component is high and it

is one of the primary causes for spindle bearing wear,

because of the side load it imparts into the spindle’s

bearings Due to the rotary motion of these inserts,

they minimise tangential forces and as such, reduce

side loads on the machine’s spindle bearings

Frustum-Based Face-Mills

When compared to some other milling cutter insert

geometries, the round inserts have two advantages:

• Inherent strength – no sharp edges, minimising

potential points of weakness in the geometry,

im-parting high shock resistance and fracture

tough-ness Hence, ‘frustums-based’ face mills have up to

10 times longer tool life, in comparison to

conven-tional milling insert geometries,

• More cutting edges – they can be turned and locked

in their seatings, creating approximately twice as

many cutting edges per insert – giving up to 24

in-dexes per insert – when compared to conventional

milling inserts

NB Like conventional insert geometries, normal

round inserts offer the user two choices, whether

to choose a high efficiency positive insert, or longer

insert life using a negative geometry round milling

insert

The frustum-shaped (round) face-milling insert (Fig

91), has a cutting edge which is reinforced by

addi-tional mass while at the same time offering a 60°

posi-tive shearing action (Fig 91a) This frustum-designed

insert geometry, eliminates angles and straight lines,

allowing high stock removal rates to be utilised

Typi-cally, these frustum-based insert designs, when

mill-ing grey cast iron can use peripheral speeds of >700 m

min– at feedrates of 6.4 m min–, whereas, for

alu-minium milling, the surface speed can be increased to

1,650 m min–, but with a feedrate of >8 m min–

Figure 91 A frustum-based milling cutter [Courtesy of

Rotary Technologies Corp.]

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Figure 92 A range of special tools (i.e customised), catering for specific company production needs [Courtesy of Ingersoll]

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4.4 Customised Milling

Cutter Tooling

Custom-built tooling is as its name implies, offers

quite considerably diverse tool designs (i.e see Fig

92 for just ‘snap-shot’ of a small range of these types

of tools) Some of this customised tooling can be

rela-tively simple, perhaps just manufactured to mill only

one particular feature, while others are very complex

and sophisticated in both their design and operation

Of this latter type are the numerically-controlled, or

‘feed-out’ facing and boring heads (not shown) These

programmable heads allow the machining of features

such as large bores with intricate profiles, typically:

multiple diameters, grooves, tapers and even threads –

on a range of prismatic parts Until such heads became

available, these workpiece features would have required

the knowledge by either a CNC programmer, or more

likely they would have been ‘routed’ to a conventional

jig-boring machine for a highly-skilled technician

known as a jig-borer to complete the complex

machin-ing task These numerically-controlled heads have

a programmable U-axis tool-slide that can be

co-or-dinated to that of the Z-axis, enabling it to produce

tapers and contoured bores, or even outside diameter

features Once the head is located in the spindle, its

powered tool slide via a compact auxiliary d.c

servo-drive motor (i.e being a closed-loop system with

feed-back – to monitor its relative position at all times), will

control the radial motion as it rotates down a bore, or

around the outside diameter of a component

Tooling can be designed to create virtually any

component feature on a workpiece and, with the

CAD/CAM software available today, tooling designers

have a vast array of computing power to allow them

to efficiently produce customised tooling within very

short lead-times However, a word of caution here,

these customised tools are not inexpensive and should

only be purchased if the alternative tooling approach

is such, that cycle-times are otherwise lengthy, or there

is simply no other technique that will enable these part

features to be produced at economic cost

4.5 Mill/Turn Operations

On many hybrid machine tools today, the traditional operations associated with one particular type of ma-chine tool, are now being produced on others Take for example a turning centre, in the past it would simply have been employed in the production of workpieces with rotational features Now, with the addition of a turret equipped with live/driven tooling (i.e rotating spindles in some, or all of the turret’s pockets), it is possible to lock the headstock spindle, mill a feature: flat, keyway, gear tooth, or spline, then angular index the spindle and repeat, until all of the so-called flats – often known as ‘prismatic features’ – are completed Moreover, it is possible to purchase a ‘mill/turn centre’ with ‘full’ C-axis headstock spindle control giving it the capability to generate contoured surfaces, or faces

on the previously turned part (i.e see Fig 93) This diversity in the machining operations that can be un-dertaken by simply one machine tool, means that the

so-called ‘one-hit machining’  operations are possible,

thereby reducing the risk of loosing the accurate da-tum initially set when the part was turned, so increas-ing production consistency due to its more repeatable machining precision and accuracy

Some companies in England and elsewhere, are now producing rotational features on prismatic parts, these being produced on machining centres This un-usual reversal technique is achieved by fitting turning tools in a suitable fixture on the machine tool’s bed and rotating the part held in a appropriate manner in the machine’s spindle Moreover, it is now possible to not only purchase a machine tool that can: turn, mill, bore, thread, but it can even cylindrically grind as well – truly showing the diversity over a range of machin-ing operational processes

For further information on the possible problems that may be encountered in milling operations and the

anticipated solutions, these are given in the

Trouble-shooting Guide for Milling Operations, in Appendix 6

 ‘One-hit machining’ , refers machining parts from wrought

stock, etc., in one complete operation.

Figure 93 Driven/live tooling: milling a spiral groove (top) and face-contouring (bottom) under ‘full’ C-axis control, on a mill/turn

centre [Courtesy of DMG (UK) Ltd.]

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