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The adoption of high-speed machining has drawn attention to the advantages of shrink-fit semi-permanent toolholding for shank-style milling cutters of up to 32mm diameter.

This has led to the recognition of its potential in general machining in preference to traditional collet and hydraulic chucks.

Those that have already changed have quickly recouped the investment through higher levels of set-up rigidity and the capability to achieve lower run-out at the tool-tip, giving extended tool life, higher accuracy and improved surface finish.

They have also eliminated problems normally associated with collets: damage, tool slippage, wear and tools dropping out under cut.

Such has been the take-up of shrink-fit tooling according to milling specialists at LMT (UK) Coventry, that users are now looking for other uses and adopting the technology for drills and taps between M5 and M20, especially as holders are now available with built-in length adjustment and through-the-tool coolant feed.

Bob Osborne, tool holding manager of LMT (UK), said: ‘Many still view shrink-fit as the domain of industries such as aerospace, where very expensive high-speed machining applications using spindle speeds of 20,000 to 60,000 rev/min are the order of the day.

‘Here, shrink-fit holders also overcome problems where high spindle accelerations and the effects of centrifugal force would partially release a normal collet-based system, which allows the tool to slip or even drop out.

‘But today, even on general machining applications, faster accelerations and higher spindle speeds with improved tool performance are bringing shrink-fit into higher demand.’ Osborne also outlined how some potential users shy-away from shrink fit, believing holders are more expensive and the cost of a shrink-fit unit is prohibitive.

But he also pointed out that a shrink-fit holder costs half that of a hydraulic tool holder and can be used for thousands of gripping cycles.

Shrink-fit holders also overcome the problem of permanent damage to the internal membrane or a burst hydraulic bladder if closed without a tool being present.

According to managing director Trevor Tolley: ‘There have been many misnomers over shrink-fit tooling, which in the past have led companies to stick to the well-trodden path of conventional holders.

‘What is not generally realised is that the effective working life of cutters is being seriously shortened when using collet holders that enable the tool to run-out, which leads to uneven loading of its cutting edges.

‘A simple check of the run-out of a cutter in the spindle will reveal all.

‘The greater the eccentricity, the quicker the cutter will wear as one tooth takes the majority of loading.

‘This means tool costs escalate and quality levels in production suffer.’ Tolley estimated that for every 10 micron of run-out at the tool tip shown on the dial indicator, the effective cutting life will be reduced by around 10 per cent due to the uneven load.

This also has a direct influence on the machining process by intensifying noise and vibration and has a detrimental effect on surface finish.

These problems are exaggerated when longer-length tools are in use, such as when pocketing in mould and die and to achieve tool access to a feature of a component when five-axis machining is being carried out.

In applications of this type, a further positive advantage in using shrink-fit is that very slim yet rigid holders are available, which improve access to difficult-to-machine areas.

Osborne added: ‘Too often the choice and usage of the mechanism for holding the shank of the tool is down to custom and practice and is not seen in its true light as playing a critical part in optimising the cutting process.

‘The advantage of shrink-fit is that it provides a rigid monoblock connection between the cutting edge and the spindle.

‘Engineers go to great pains to design a rigid spindle with optimised bearing and load-carrying ability, which is then downgraded through the use of well-worn backends and adaptors and in many cases damaged and worn-out collets.

‘Companies will spend heavily on a new machine tool but give little consideration to the purchase or replacement of toolholders, the common theme being: “We already have sufficient in the stores or on existing machines with no consideration to investigate their condition”.’ To this he points out that ‘cost is relative’ but by changing to shrink-fit, which requires the installation of an automatic shrink-fit machine, this is not excessive.

He added: ‘A machine and a selection of backends would easily be justified in less than a year just on the payback generated from improvements in the life of the cutting tools without the additional benefits.’ The shrink-fit process is based upon applying heat to the outside of the toolholder.

In the LMT Thermogrip unit, the rise in temperature is created by a high-energy inductive coil that rapidly heats the locality of the clamping area of the toolholder up to 350C.

Due to the localised power, application only takes around 10 seconds and the level of heating is well below the point to initiate microstructural change in the toolholder material.

The shank of the tool, which is set by the operator to rest in a lead angle at the mouth of the holder, will simply drop to a deadstop position in the holder as the heat-affected zone causes the inner diameter of the holder to expand.

Due to the speed and location of heating, any heat transmission into the tool or other areas of the holder is minimal, due to the LMT Bilz Ferrite Plate System.

The holder and tool assembly are then cooled using either air, or in the latest ISG-3200WK machine development by LMT, by an automatic cycle that plunges the mated pair into a 30-litre capacity quenching tank housed in the machine base.

This immersion initiates the holder to immediately contract uniformly around the tool shank.

As a result, it forms a cohesive and extremely rigid connection between toolholder and shank that, when it is mounted in the spindle, should have a run-out within 0.003mm TIR (total indicated run-out).

As part of the automated cycle, the tool and holder are returned to the loading position and dried using a strategically positioned air knife ready to be removed by the operator.

To remove and replace a tool in the holder should take less than 30 seconds.

The assembly is re-heated, the tool to be replaced picked out and the new tool inserted.

It is then automatically quenched.

Tolley’s response to questions regarding the life expectancy of repeated heating and quenching cycles on the toolholders is that trials have shown that more than 5,000 application cycles reveal no degradation of the holder, loss of concentricity or elasticity due to the special heat-resistant tool steel used in the manufacture of LMT’s holders.

Shrink-fit technology is widely used in the aerospace and mould and die sectors, but really came to the fore in 2000 in the German and American auto industry for transfer line and flex-line type machining installations.

The adoption was in response to the trend to ensure predictable performance of the equipment and processes in order to maximise spindle utilisation, process reliability and machine uptime.

Along with tool pre-setting, it rapidly gained acceptance to ensure that minimal production time was lost due to tool changing and also that maximum rigidity was maintained in the tool-to-spindle assembly.

Rigidity of the tool/holder/spindle arrangement is even more important today as more exotic materials such as high-performance alloy steels, titanium, Inconel and high-strength steels are machined in ever-greater quantities.

He cites carbide cutters held in shrink-fit holders being used on titanium at 500rev/min, all a far cry from the ultra high speed cutting of aluminium.

Tolley said: ‘The production engineering logic is the same in determining security of process.’ There are other issues for consideration between shrink-fit and collet systems.

It is difficult to remove a broken cutter from a collet and, in any case, the collet should be immediately discarded.

Should a tool work become loose or drop out, the internal gripping area of the collet becomes damaged and again should be replaced.

Also, prevention is better than cure – collets wear, especially the critical first 10mm of the gripping zone, due to radial forces generated during the milling process – but collets are rarely checked until problems arise and that just adds further cost and downtime.

The ISG-3200WK shrinking machine has a 10kW high frequency generator with a tool capacity between three and 32mm diameter in solid carbide and between six and 32mm in high-speed steel.

Tools can be up to 450mm long requiring a single heating coil.

A fully programmable (menu with graphics display) automated process monitors the temperature ready for the tool shank to be inserted and manual override with full adjustment to the generator is included.

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