Turning and Chip-breaking Technology Part 5 pptx

wrap itself around either the tool, or workpiece, but

such a geometry is perfect for machining alumin￾ium, or non-ferrous materials.

• Radial top rake (illustrated in Fig. 4 middle and

to the left – three grooving insert sizes illustrated).

This radial top rake is designed to thin the chip.

Such chip thinning, eliminates the need to under￾take finishing passes on the groove’s side walls. Fur￾thermore, this type of grooving insert geometry be￾ing on-centre, enables axial turning of diameters for

wide shallow grooves33, or recesses.

• Raised bumps on top rake (see Fig. 27a – left).

This sophisticated grooving geometry is utilised for

materials where chip control is difficult, as it pro￾vides an ‘aggressive barrier’ to the curling chip. The

raised bumps force the chip back onto itself, either

producing a tightly curled watch-spring chip, or

causes the chip to break.

(ii) Surface speed of the workpiece – in order to ob￾tain full advantage of a grooving insert’s chip-form￾ing abilities, the chip must be allowed to flow into the

chip-former. This chip-flow can be achieved by either

decreasing the workpiece’s surface speed, or increasing

the feed – more will be said on this shortly. The former

technique of decreasing the surface speed, allows the

material to move slower across the top rake of the cut￾ting edge and as a result, has greater contact time to

engage the chip-former. This slower workpiece speed,

has the benefit of increasing tool life, through lower

33 A groove, or recess, can normally be considered as a straight￾walled recessed feature in a workpiece, as illustrated in Fig.

40. Typical applications for grooves are to provide thread re￾lief – usually up to a shoulder – so that a mating nut and its

washer can be accurately seated , or for retaining O-rings. As

the groove is produced in the workpiece, the tool shears away

the material in a radial manner, via X-axis tool motion. The

chip formed with insert geometries having a flat top rake, will

have an identical width as the tool and can be employed to

‘size’ the component’s width feature. However, this chip action

– using such a tool geometry, creates high levels of pressure

at the cutting edge as a result of the chip wall friction, which

tends to produce a poor machined surface texture on these

sidewalls. Grooving with an advanced chip-former insert ge￾ometry, reduces the chip width and provides an efficient cut￾ting action, this results in decreasing the cutting edge pressure

somewhat. Chip-formers offer longer tool life and improved

sidewall finishes with better chip control, than those top-rakes

that have not incorporated such insert chip-forming geomet￾ric features.

tool/chip interface temperatures. The negative factors

of such a machining strategy, are that the:

• Part cycle times are increased and as a result, any

batch throughput will be lessened,

• As the cutting edge is in contact for a longer du￾ration, more heat will be conducted into the tool,

than into the chip, which could have a negative im￾pact of inconsistent workpiece size control,

• Due to the lower workpiece surface speed, the ben￾efits of the insert’s coating will be reduced, as such

coating technology tends to operate more effec￾tively at higher interface temperatures.

(iii) Increasing the feedrate – by increasing the feed

allows it to engage the chip-former more effectively

– this being the preferred technique for chip control. A

heavier applied feedrate, produces a chip with a thicker

cross-section. Further, a thicker chip engages the in￾sert’s geometry with higher force, creating a greater

tendency to break. Hence, by holding a constant work￾piece surface speed, allows the faster feedrate to reduce

cycle times.

Tra n s ve r s a l, o r Fa ce G ro ov i n g

Transversal grooving geometry has a curved tear￾shaped blade onto which, the insert is accurately lo￾cated and positioned. The transversal insert follows

the 90° plunged feed into the rotating face of a work￾piece. These tools are categorised as either right-, or

left-hand, with the style adopted depending upon

whether the machine tool’s chuck rotates anti-clock￾wise (i.e. using a right-hand tool), or clockwise (i.e.

left-hand). The minimum radius of curvature for such

transversal grooving tooling is normally about 12mm,

with no limit necessary on the maximum radial curva￾ture that can be machined. For shallow face grooves,

off-the-shelf tooling is available, but for deep angular

face grooves they require specialised tools from the

tooling manufacturers.

If a relatively wide face groove requires machining

with respect to the insert’s width, then the key to suc￾cess here, is establishing where in the face to make the

first plunge. This initial face plunge should be made

within the range of the tool’s diameter, otherwise the

tool will not have sufficient clearance and will ulti￾mately break. Successive plunges to enlarge the face

groove should be made by radially moving the insert

0.9 times the insert’s width, for each additional plunge.

The rotational speed for face grooving is usually about

80% of the speed used for parting-off – soon to be

Turning and Chip-breaking Technology 73