Smith G.T. Cutting Tool Technology: Industrial Handbook

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Figure 74. High-performance reamers, having the ability for radial infeed (i.e. ‘feed-out inserts’) – when tted. [Courtesy of Cogs-

dill Tool & Eng’g Ltd.]

Drilling and Associated Technologies 141

is oen produced from an abrasive-resistant mate-

rial such as PCD, in order to maintain and extend the

tool’s life and holding a good cutting edge over many

machined parts.

3.3.3 Reaming – Problems

and Their Remedies

For any resultant reamed surface, its form, accuracy

and surface quality are tremendously improved by

dividing the machining process into either, roughing,

or nishing reaming operations. Low cutting speed

together with high feedrates, in association with good

lubrication agents oering adequate cooling poten-

tial, provide the basis for optimum reaming practice.

hile, observing these ‘rules’ , improves both the

reamed surface quality and its individual tolerance. It

is worth restating, that a reamer only follows the pre-

drilled hole, consequently it cannot correct for any

previous alignment errors that might be present (i.e.

see the schematic diagram in Fig. 70). Although er-

rors between the spindle’s axis and the axis of the pre-

drilled hole, can be adjusted with the aid of ‘oating

reamer’ toolholders (Fig. 72). In Table 6, the following

fault-nding chart may be useful in tracing the pos-

sible causes of some common reaming problems.

3.4 Other Hole-Modification

Processes

Once the hole has either been: cast, core-drilled, or

drilled into solid workpiece material, it oen requires

a further post hole-making operation to complete the

job, for example, a tapping operation. ere are a num-

ber of these pre- and post-drilling hole operations that

require specic tooling to nish o the hole-making

activities. e most popular of these are briey men-

tioned below, but this is by no means an exhaustive

account of the many oen hybrid operations that are

available to the potential designer, or machinist.

Countersinks

ere are several reasons why a countersink tool might

be employed when machining features on a compo-

nent, ranging from:

•

Countersinking a countersunk-headed screw

– for ‘ush-tting’ to the surface (Fig. 75a),

•

Short tapers – can be adequately machined on a

component,

•

Providing a lead – for a soon-to-be-tapped hole,

•

Deburring operation – on a previously drilled

hole.

Countersinks are available with a range of included

taper angles and come in a variety of dimensional

sizes, the most popular being either: 60°, 90°, or 120°,

or indeed ‘specials’ can be ground to suit any angular

and diametral workpiece features, of varying lengths.

Countersinks are available from simply HSS, through

to a coated cemented carbide matrix.

Counter-Boring

Counter-bored tooling (Fig. 75b) is available as either

a solid tool, or is designed to be modular in construc-

tion. is latter modular counter-boring tooling, oers

a range of exibility to machine a wide assortment of

component features, by simply changing the ‘pilot‘, or

cutting element’s diameter. e ‘pilot’ as its name im-

plies, follows a pre-drilled hole and guides the counter-

bored cutting element enabling it to remain concentric

with the hole’s axis. is is important for any cap-head

bolts that require to be recessed either ush to a part’s

surface, or sunk below its outer face. Counter-boring

is also employed to machined a clearance face in the

female part feature allowing for a stepped bar to have a

ush face to locate against, or simply to provide clear-

ance for such a workpiece feature. Again, as with most

of these tool materials, they are produced from HSS,

through to coated cemented carbides.

Spot-Facing

Spot-facing tooling is normally utilised to produce a

consistent and uniform seating on for example, a cast,

or forged component, allowing a washer, or bolt-head

to be ush across its contact face. Spot-faced tools

(Fig. 75c), are available as either a solid, or modu-

lar constructional design – the latter version, giving

greater exibility across a wider range of features to

that of the former counterparts. Materials for these

tools are similar to those mentioned for other post-

drilling tooling, namely, HSS through to coated ce-

mented carbides.

142 Chapter 3

Table 6: Potential reaming problems and their possible causes, with some remedies

Reaming problem: Possible causes and some remedies:

Holes to large i) Concentricity error of either: machine spindle, toolholder, or tool.

(ii) Damaged t between tool and toolholder (i.e. taper, chuck, or collet).

(iii) Bevel lead on tooling incorrect.

(iv) Cutting speed, or feedrate too high.

(v) If problem is the result of workpiece material, eliminate it by using a weaker coolant medium (i.e. by

increasing its cooling potential, sacricing some of the lubricating abilities).

Hole too small i) Tool tolerance incorrect.

(ii) Ductile material that contracts after reaming – possibly eliminated by using a quick spiral reamer.

(iii) Excessive heating during the reaming process: perhaps by the hole expanding, then subsequently

contracting.

(iv) Reamer blunt.

(v) Cutting speed, or feedrate too low.

(vi) Insucient stock left on for reaming: tool seizes in the hole.

(vii) In most cases, eliminate problems using a more concentrated soluble oil mixture (e.g. 1:15 to 1:10,

alternatively use cutting oil).

Conical, non-circular and

other hole malfunctions

(i) Machine spindle not concentric.

(ii) Bevel lead not correct.

(iii) Axis of pre-drilled hole and reamer not in alignment – eliminate by using a ‘oating’ toolholder.

Unsatisfactory surface

texture of hole

i) Reamer blunt.

(

ii) BUE on edges, caused by ‘cold welding’ , eliminate by using high concentration coolant, possibly cut-

ting oil, or by a reduction in reamer’s land width – to almost zero.

(iii) Cutting speed too high, feedrate too low.

(iv) Stock removal allowance too small – caused by the pre-drilled hole being too large.

(v) Incorrect bevel length.

Reamer seizes and breaks (i) Reamer blunt.

(ii) Too high a cutting data employed (i.e. speed and/or feed).

(iii) Pre-drilled hole too small.

(iv) Poor coolant mixture – lubrication too dilute.

(vii) Reamer geometry requires modication.

[Courtesy of Guhring Ltd]

Drilling and Associated Technologies 143

Figure 75. Some alternative hole modication machining tooling. [Courtesy of Guhring Ltd.].

144 Chapter 3

Back Spot-Facing

Back Spot-faced tools (Fig. 75d), are usually employed

in ush-facing an internal hole’s face on either a cast-

ing, forging, or wrought stock. e Back Spot-facing

operation, enables a bolt-head, or nut and its washer

to be accurately seated. In some instances, it is possible

to generate, the back-face, rather than to form it, via

specially-modied tools that are fed to the other side

of the part, then circular interpolation techniques are

used to create the required back-face.

NB With most of these post-drilling operations, the

cutting data is restricted and calculated to the outer di-

ametral dimension of the part feature to be machined.

Solid post-drilling tooling can usually be operated at

higher cutting data to that of their modular tooling

counterparts.

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Drilling and Associated Technologies 147

Milling Cutters

and Associated Technologies

‘Mit der Dummheit kämpfen Götter

selbst vergebens.’

TRANSLATION:

‘Against stupidity the Gods

themselves struggle in vain. ’

FRIEDRICH von SCHILLER

(1759 – 1805)

[Die Jungfrau von Orleans, III.vi]

4.1 Milling – an Introduction

At its most basic level, a milling operation involves a

co-ordinated linear, or multiple-axis feeding motion

of the multi-edged cutter

as it rotates across and into

the workpiece. Milling cutters are usually tted into a

driven rotating machine spindle for a range of machine

tools. e type of machines available is quite diverse

and include: milling machines, machining centres

mill/turn centres

, plano-mills

, etc. In special cases,

1 ‘Fly-cut milling’ , is the exception here, as it is considered a

milling operation that will normally utilise only one cutting

edge, it is similar in design and operation to that of a two-cut-

ting edged trepanning tool. A ‘y-cutter’ is usually employed

for the machining of large diameter circular features, where

either the hole is the required component feature, or the cir-

cular blank is the necessary item from for example, a large

wrought plate/sheet.

2 ‘Machining centres’ ,

are milling machines equipped with an

automatic tool changer – for fast and ecient ‘tool changing’

(i.e. to reduce down-time to a minimum), having either a

vertical, or horizontal spindle conguration, although multi-

spindle machines can also be used. Predominantly, machining

centres have what is known as: ‘orthogonal axes’ – having each

axis at 90° in relation to each other. A ‘basic’ machining centre

has three orthogonal axes (i.e. X, Y and Z), but can have rota-

tional axis (i.e. A, B and C) incorporated onto them, or built

into the structure of the machine tool. Some of these machine

tools, may have 6-axes (i.e. 3 linear and 3 rotational), or more,

necessary for any complex free-form sculptured machining

work.

3 ‘Mill/turn centres’ ,

as their name implies, can turn parts and,

with controlled rotational axes, coupled with driven/live spin-

dles, they can accurately and eciently generate: prismatic

features, faces, splines, keyways, cams, etc., onto the work-

piece at one setting – termed ‘One-hit machining’. In order to

increase the versatility of such machine tools, these mill/turn

centres can also be tted with ‘co-axial spindles’ , where two

spindles are coincident with each other (i.e. their respective

spindle centrelines are in-line, but opposing each other), al-

lowing for example, twin tool turrets to work simultaneously

on two parts. ese machined components may have quite

distinctly dierent part geometries/dimensions and, this level

of complex and sophisticated costly plant, requires quite sig-

nicant linear and rotational CNC axes programming capa-

bilities. is type of conguration of mill/turn centres with

co-axial spindles, oer two machines in one, but on a quite

small shop-oor ‘footprint’ – an important and perhaps vital

benet when oor-space is at a premium.

4 ‘Plano-mills’ ,

have the ability to generate large at surfaces

with their traditional range of single-point planning tools, but

with an additional milling capability of having one, or more

milling machine spindles tted, for large-part surface milling

operations to be conducted on the large part.

milling operations can be utilised for multi-axis free-

form ‘sculpturing’ of complex-curved workpieces, us-

ing equipment such as 5-, or 6-axis machining centres,

or even robots with ‘slaved’ (i.e. tted) spindles, all

these machines being equipped with specially-ground

milling cutters – necessary for ‘double-curvature’ ma-

chining operations. A milling cutter’s design and its

respective cutting edges come in a vast range of shapes

and sizes (i.e. see Fig. 76). Each of the individual ‘mills’

cutting edges – as the cutter rotates and is fed into the

workpiece, will mill a certain amount of material from

the part. Milling operations are an ecient way of re-

moving either excess stock from previously fabricated

parts, or by machining from wrought material. e

production of a milled surface, oers a machined sur-

face texture that is consistently good, having accurate

and repeatable dimensions, oering great exibility

in terms of the geometric types and shapes for these

milled components.

A milling operation is an ‘intermittent cutting ac-

tion’ ,

where each individual cutting insert continu-

ously enters and exit’s the cut, unlike turning, which is

basically a continuous machining operation, once the

cut has been engaged. It follows that with each cutting

tooth impacting onto the work’s surface (‘intermittent

cutting’), its operation will be aected by: the cutter’s

inherent robustness, the machine tool’s condition and

the spindle power availability. ese factors will have a

great inuence on the cutter’s ability to eciently ma-

chine the desired component features. Milling opera-

tions can vary considerably and can be performed by

a wide variety of machine tool congurations, with a

diverse range of tooling (Fig. 76) and across a large ar-

ray of workpiece materials, shapes and geometries.

At an early point once the engineering drawing(s)

have been designed and produced and, prior to ma-

chining the part, an in-depth study of the type of cuts

to be made to produce the desired component features,

either out of: wrought, cast, forged, or extruded mate-

rials should be instigated. Oen at this initial stage in

machining assessment, the ‘study’ should address new

approaches to machine the desired part geometries,

by attempting to manufacture the part in the shortest

possible cycle-time

. Moreover, when considering the

5 Cycle-times for the manufacture of parts, should include the

productive operations (i.e. all machining times) and non-

productive elements (i.e. including: tooling and workpiece

set-ups, tool-changing operations and any necessary form of

workpiece measurement), in producing the overall completed

component.

150 Chapter 4

individual part feature to be machined, judge whether

just one cut, or several passes are the best machining

strategy for its subsequent production. In scheduling

particular components for milling operations, the se-

lection of which CNC type of machine tool congu-

ration should be of prime importance, for example a

milling machine, or machining centre equipped with

either a: vertical, or horizontal spindle, or a univer-

sal mill, or even a large gantry mill. Once the CNC

machine tool has been selected, other secondary, but

perhaps nonetheless important factors should be ad-

dressed, such as the potential accuracy and repeatabil-

6 Scheduling of parts, is based upon a range of crucial produc-

tion decisions. Typically the nal decision may be due to a

number of interrelated factors: when the parts are needed,

the quantity of parts in the batch, their geometry and size,

the availability of the correct machine tool, any potential

cycle-time reductions when utilising a specic machine tool.

Other important machining economic factors may need to be

addressed such as: workholding methods, cutting tool moni-

toring, automated part loading/unloading, etc. In many large-

scale volume production environments, ‘line-balance’ deci-

sions that may have to be made could arise. Particularly when

a diverse sequence of operations on component features that

must be machined, these being part of a series of operations

across several machine tools.

ity of the production process including the attainable

milled surface texture, together with rigidity/instabil-

ity of the overall process for the selected machine. is

latter factor of milling stability, via the rigidity of the

machine-tool-workpiece loop, will dictate not only the

anticipated milling cutter’s tool life, but aects the to-

tal performance of the overall production process with

large-scale ramications for potential component part

economics.

4.1.1 Basic Milling Operations

Regardless of the type of milling cutter selected, a ma-

chining operation utilises one, or more of the follow-

ing production milling techniques, with any variations

in methods being related to feed directions in relation

to the tool’s rotational axis. e three basic milling op-

erations are:

•

Face milling (Fig. 77a, b and c – depicts some typi-

cal machined features produced by facemilling).

A facemilling operation is a combined cutting ac-

tion by the inserts, in the main on the tool’s periph-

ery and, to a lesser extent by insert edges on the

cutter’s face. In facemilling, the cutter rotates at 90°

to that of the direction of radial feed against the

workpiece. Facemilling has a D

in an axial direc-

Figure 76. Just a small selection of

the vast range of milling cutters avail-

able for both machining and mill/turn

centres. [Courtesy of Seco Tools]

Milling Cutters and Associated Technologies 151