Micro-machining,small partsmachining,‘Swiss’machining – the

terms may vary but the theme is common– the dimensions involved are tiny.Applications,particularly in the medical,electronics,IT,telecommunications andjewellery industries,mean that cuttingtool manufacturers have their work cutout developing tooling technology that isin scale with the components.

At the West Midlands Micro-Machining Centre,which has recentlybeen established within the School ofEngineering at the University ofBirmingham,the principal objective is tohelp manufacturers develop and exploitmicro-machining capabilities in order todiversify and improve manufacturingtechnology in the region.The facility iswell equipped with several CNC machinetools including an LX-1 machining centrewith a 60,000 rpm spindle from partnercompany Matsuura. Collaboratingcompanies regarding cutting toolsinclude Miracle Engineering Europe andMitsubishi Carbide.

The Centre has been formed inresponse to the worldwide demand for

components having dimensions of lessthan 10 mm and features in the 1-100micron range or significantly largercomponents with micro features.Typicalparts include implants,pacemakerfeatures,fluidic mixing chambers,capillary units,fuel injectors,gears,pumps,cooling holes in enginecomponents,pressure transducer parts,gyroscope parts,mobile phonecomponents,heat exchangers and relays.

This list is expanding perpetually somany cutting vendors have wasted notime in sizing up the challenge. Stellram,for example,offers a significant selectionof precision solid carbide tools for micro-machining applications including twoflute,micro- ballnose and micro-endmillcutters. In terms of diameter these startas small as 0.3 mm.

According toStellram’sEuropeansolidcarbidemanagerJohn

Palmer,despite their size,these cutters areextremely tough,thanks mainly to aspecially designed,ultra-fine substrate.And despite popular misconception,theapplication is not always high-speedmachining.“Yes,spindle speed needs tobe as high as possible,but surface cutting

speeds are fairly conventional and feedrate is low,”he explains.“For example,a0.3 mm diameter cutter rotating at 50,000rpm would offer a surface cutting speedof just 47 m/min.”

ON THE EDGERunning at slower surface speeds tendsto cause built-up edge. Chips remainingin contact with the cutting edgeprecipitate heat transfer to the tool andinterfere with the cutting action when thecutting edge returns into contact with thematerial. Mr Palmer says that thepenetration,shearing and compression ofa chip produced during micromachining,combined with

friction forces acrossthe tool rake surface,produces

heat. Removing this heat with the chiphelps protect the cutting edge.The cuttersare coated (1.5 micron thick) usingTiA1N,but it is applied using a newprocess developed by Stellram thatresults in a ‘mono’ layer with a ‘multi’structure.The low co-efficient of frictionand high lubricity of the coating on theStellram cutters prevents the chip

DIMINUTIVE DIMENSIONS FOR CUTTING TOOLS

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It’s a small worldEver-decreasing product size means that cutting tool dimensions have also reduced. Steed Webzell

describes what some might struggle to see

adhering to the immediate cutting area.High-pressure coolant can be used,

but tends to create ‘pools’ in cavities thatretain the chips produced by the tool.Chips dragged back into the cutting pathcan create flute crowding and snatching,with the obvious result of tool breakage.The preferred coolant medium is air,which clears chips from the proximatecutting area,preventing both thermalshock and the re-cutting of swarf.

Stability and rigidity of machine toolis another critical factor.A rigid machinecan allow feed rates to be doubled anddepths of cut of up to six times greater.Using a 0.3 mm diameter cutter,feed ratesup to 0.006 mm/tooth,slot cutting depthup to 0.15 x D,and side (axial) cuttingdepth up to 1.5 x D can be achievedusing a rigid set-up. Mr Palmer alsohighlights that Stellrammicro-

cutters canhandle virtually any

material,including steel up to60HRc,stainless steels (with care) andgraphite by way of Stellram’s diamond-coated range.

Recent cutting tool trials usingInconel 718 and a speed increaser on aconventional CNC produced outstandingresults,witnessing tool life of 25 minutesusing a 0.8 mm diameter Stellramendmill. On an intricate,full slottingtoolpath,running at 10,000 rpm (25m/min) with a tooth load of 0.01 mm or80 mm/min feed,length of cut in excessof 2000 mm was achieved.

For high-volume applications someform of tool monitoring is probably

advisable and Stellram recommendsusing a vision-based system such as aninfrared camera rather than any form ofcontact probe.

BROADENING UPTAKESandvik Coromant has an extensivecutting tool range for small partsmachining resulting from its intensive in-house development programmes.“Thetraditional precision industry users ofSwiss-type cutting tools,such as watchand instrument manufacturing are nolonger alone with these applications,”says Steve Geddes,one of Sandvik’sturning specialists.“Suppliers to theaerospace and defence industries arenow users of small parts because of thegrowing role of electronics and

hydraulics for control units andmechanisms.”Mr Geddes addsthat components can be assmall as 0.5 mm diameter.

The development ofCoromant’s CoroTurnprogramme for small,positive geometryturning tools has taken

high machining speeds,smooth cutting action and

accuracy into account.Toolholders with precision shanks

minimise variation in cutting-edgepositioning when,for example,theaccuracy of cutting edge centre heightneeds to be high because of the smallworkpiece diameters. Setting is alsosimplified through toolholders having nooffset,which minimises vibrations andallows the cutting edge to come up closerto the spindle collet.

CoroTurn 107 inserts for precisionsmall parts turning on sliding-headmachines employ a sharp cutting edgeand edge radii down to zero,whilemodern cermet and coated insert gradesprovide the means for generating moreexact small component shapes,accuracyand finish.The very latest CoroTurn XStooling solutions for internal turning,grooving and threading bring all theperformance of CoroTurn down to

smaller bores than ever before (as smallas 1.0 mm).The programme comprisesfour sizes,each including inserts for arange of hole diameters and lengths.These inserts fit into boring bars designedwith internal cutting fluid supply,directedat the cutting edge via two nozzles.CoroTurn XS is designed for high-precision small part machining usingsliding- head machines within theautomotive,watch,connector andmedical industries.

The minimum entry bore diameterfor Horn boring tools is 0.3 mm and theupcoming DS Series of solid carbidemicro-endmills (MACH 2004 launch) kickoff at 0.3 mm diameter for steel (twinflute) and 0.6 mm (single flute) foraluminium.The company outlines that inthe mid ’90s,managing to bore within a1.0 mm entry diameter was prettyimpressive,but since then the lower limithas been constantly falling,aided andabetted by development of more resilient

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TOOLING/MICRO MACHINING

micro-grain carbon base material andimprovements in insert grindingcapability.

According to Horn,the drivers for thishave been the medical prosthetics,watchmaking and electronics industries,which impose tight tolerances forparallelism,finish and straightness onholes through which many wouldstruggle to thread a piece of cotton.Thesequality constraints allow progressiveminiaturisation of the end product andenhanced,reliable mechanicalperformance. For example,it is nowpossible to machine a replacementknuckle joint that not only works,but alsodoes not resemble a hand that looks likeit still has arthritis.

BORE DON’T REAMLikewise watchmakers have usedreamers for centuries but these alwaysfollow the path of the drill,whereas aSuper Mini boring tool,held in an adaptorfitted to a fine boring head,gives themanufacturer much more control. Hornhas an agreement to supply customisedadaptors to Urma for its range of Intramaxand Fine boring heads. Horn insists thatthe secret of successful machining withSuper Mini is careful set-up on a well-maintained machine capable ofsufficiently high spindle speeds toachieve ‘proper’cutting conditions for thecarbide.

Kennametal has a micro-machiningcatalogue crammed with solutions formanufacturers with ‘small’headaches. Insome instances the company hasadvanced further ahead,even offering aquick-change tooling system for certainshank sizes.According to marketingmanager Gary Owen,almost any materialcan be processed,ranging from steel and

stainless steel through to brass,copper and high-temperaturealloys for applications such asaerospace fasteners. Solidcarbide Kennametalcutters start as small as0.4 mm diameter and thecompany recommendsthe use of neat cutting oilfor extra lubricity.

More and morecutting toolmanufacturers arejoining the market. NTKTechnical Ceramics,forexample,offers its LBMBmicro-boring tools forbores down to 1.0 mmdiameter.According toassistant generalmanager Peter Ward,thecompany is enjoyingsuccess with these in thecomputer,automotiveand optical networkingindustries.All NTK chip breakers andcutting edges are ground (not moulded)to reduce cutting forces. Other marketoptions include solid carbide and HSSEendmills from Prototyp starting as small

as 0.4 mm and 1.0 mm diameterrespectively,and Microcut groove toolingfrom ACT offering cutting widths as smallas 0.5 mm.

While it seems clear there is nostandard definition of micro-machining,itappears that whatever the size of part,creating micro-features is a challenge.However,the cutting tool market hasalready responded to demand and isevolving new tooling and techniques toensure that the future for micro-machining in the UK is big. M

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Concentricity is the keyMicro-endmills from Stellram are ground to very specific tolerances for accuracy.Starting with the shank, it is possible to hold an h5 (+0, -0.005 mm) tolerance on a 6mm diameter. Why 6 mm? Basically due to the method of holding, for exampleshrink fit, which tends to employ 6 mm as the smallest size to offer maximum grip.

From the accuracy of the shank, it is then possible to derive accuracy of thecutting edge, both from a dimensional accuracy on diameter, but moreimportantly, concentricity. Using Stellram cutters it is possible to hold better than0.005 mm concentricity, which is important considering the feed rates applied tosmall-diameter endmills.

Events with ‘small’ focusThe Micro Machining Centre within the School of Engineering at the University ofBirmingham is hosting two events for those looking to find out more about smallparts machining. On 30 June 2004, a seminar called ‘sharing tomorrow’stechnology today’ will focus on micro-/nano-manufacturing, while the centre isplanning to hold a micro-machining open day on 17 November 2004.