Turning and Chip-breaking Technology Part 2 pps

entering angle of 45° and lead angle of 45° is utilised,

giving rise to equal axial and radial component forces.

In ‘case II’, the entering angle has changed to 75° and

lead angle is now 15°, these altered angles change the

component forces, with an increase in the axial force

while reducing the radial force. In ‘case III’, an or￾thogonal cutting action occurs, with only a 90° enter￾ing angle (i.e. the lead angle reduces to zero), showing

a large increase in the axial force component at the

expense of the radial force component which is now

zero10

. In ‘case IV’, an oblique cutting action has re￾turned (i.e. as in ‘cases I and II’), but here the entering

angle has changed to -15°, with the lead angle 75°, this

produces a large axial component force, but the radial

component force direction has now reversed. This last

tool plan approach angle geometry (i.e. ‘case IV’), is

similar to the geometry of a light turning and facing

tool, allowing cylindrical and facing operations to be

usefully undertaken – but the tool’s point is somewhat

weaker that the others, with the tool points becoming

of increased strength from right to left. Therefore, in

‘case I’, for a given feedrate and constant DOC, the cut

length/area is greater than the other ‘cases’ shown and

with this geometry, it enables the tool to be employed

for heavy roughing cuts. Returning to ‘case III’, if this

tool is utilised for finish turning brittle-based work￾piece materials, then upon approaching the exit from a

cut, if the diameter is not supported by a larger shoul￾der diameter, then the axial component force /pressure,

will be likely to cause edge break-out (i.e. sometimes

termed ‘edge frittering’), below the machined surface

diameter at this corner (i.e. potentially scrapping the

machined part). In mitigation for this orthogonal cut￾ting tool geometry, if longer slender workpieces re￾quire cylindrical turning along their length, then with

the radial force component equating to zero, it does

not create significant ‘push-off ’ and allows the part to

be successfully machined11.

A single-point turning geometry is subject to very

complex interactions and, as one geometric feature is

modified such as changing the entering angle, or in-

10 In all of these cases, it is assumed – for simplicity – that there is

no nose radius/chamfer on the tool and it is infinitely sharp.

11 In order to minimise the effects of the radial force component

when cylindrically turning long slender workpieces with ‘Case

I and II’ tool geometries, the use of a programmable steady,

or a ‘balanced turning operation’ (i.e. utilising twin separately

programmable turrets on a turning centre, with tools situ￾ated virtually opposite each other running parallel during the

turning operation – see Fig. 41), will reduce this ‘push-off ’.

creasing the tool’s nose radius, this will influence other

factors, which in turn could have a great impact on the:

type of machined surface finish produced, expected

tool life and the overall power consumption during the

operation. In fact, the main factors that influence the

application of tooling for a specific turning operation

are:

I. Workpiece material – machinability, condition

(i.e. internal/external), mechanical and physical

properties, etc.,

II. Workpiece design – shape, dimensions and ma￾chining allowance,

III. Limitations – accuracy and precision require￾ments, surface texture/integrity, etc.,

IV. Machine tool – type, power, its condition and

specifications,

V. Stability – loop stiffness/rigidity (i.e. from the

cutting edge to its foundations),

VI. Set-up – tool accessibility, workpiece clamping

and toolholding, tool changing,

VII. Tool programme – the correct/specified tool

and its tool offsets, etc.,

VIII. Performance – cutting data, anticipated tool-life

and economics,

IX. Quality – tool delivery system and service.

In order to gain an insight into the complex and im￾portant decisions that have to be made when select￾ing tooling for the optimum production of either part

batch sizes, or for continuous production runs, then

the following section has been incorporated.

2.1.6 Cutting Toolholder/Insert

Selection

When deciding upon the correct selection of a tool￾holder/cutting insert for a given application, a range

of diverse factors must be considered, as indicated in

Fig. 21. As can be seen by the diagram (Fig. 21) and

associated text and captions, there are many other

variables that need to be considered prior to selection

of the optimum toolholder/insert. Generally, the fixed

conditions cannot be modified, but by ‘juggling’ with

the variable conditions it is possible to accomplish the

best compromise toolholder/insert geometry, to opti￾mise these cutting conditions for the manufacture of

a specific workpiece and its intended production re￾quirements. Whenever toolholders and cutting inserts

are required for a specific manufacturing process, it

is important to view the tooling selection procedure

as a logical progression, in order to optimise the best

Turning and Chip-breaking Technology 43