The creation of knowledge

The event, held at Agie Charmilles' Coventry headquarters on Thursday, 9 September, was a mix of presentations, taking in both academic research and more immediately practical ones from the coal face, so to speak. The three-year project (extended to 3 ½ years), which ended in September, was funded under the 'Advanced materials: Materials for extended first use re-use' (see extended web article for full wording – original document here and box item below). Funding from both the Technology Strategy Board (TSB) and the Engineering and Physical Sciences Research Council (EPSRC) - £750,000 - supported the project, which had a total cost of just over £1 million. Called ELMACT, Extended Life Microtooling by Advanced Coating Technology, it focused on extending the life of carbide micro tools - drills and mills - by better understanding the cutting mechanism, exploring novel coating technologies, and through coating removal and recoating of the tools (see box item below for more detail. This work was undertaken with particular reference to machining intricate patterns in hard die materials. The group of companies and organisations encompassed the whole value chain and, at the outset, comprised: Teer Coatings (lead partner – tool coating firm); University of Manchester; Hanson Thorpe Precision Toolmakers; machine tool and cutting tool supplier Rainford Precision; machine tool supplier Agie Charmilles; Innovation Biotech (micro-fluidic devices, for example); micro-machining specialist Microsystems UK; and aerospace firm Rolls-Royce. Changes in this group of companies saw Hanson Thorpe Precision Toolmakers drop out and Innovation Biotech go out of business, while subcontractor WLR Prototype Engineers, Nottingham, came on board. Teer Coatings, initially an SME, subsequently became part of the Austrian Miba group (www.miba.com). CUTTING MECHANISM Kicking off proceedings on the day was Dr Paul Mativenga of the University of Manchester, who revealed some new information about the cutting mechanism in micro-machining and process parameters for success. He immediately tackled what is now becoming an old chestnut; that of defining just exactly what micro-machining is. In quoting various other academics, he added his own definition – "When 1 to 999 µm diameter end mills are used at undeformed chip thickness comparable to the cutting edge radius or material grain size, this presents a challenge for machining and could be considered as a micro machining domain." So, rather than focusing on a size of feature alone, it is the relationship and interaction between the cutter, chip thickness, cutter material grain size (0.4-0.7 µm for carbide) and cut material grain size. So, where grain size, chip thickness and radius are all of the same order, due to what Dr Mativenga calls 'size effect' "existing knowledge and experience in macroscale machineability of materials cannot be assumed to be valid at the micro-scale". Key for those undertaking micro-machining, then, is knowing what the minimum chip thickness can be. This is relative to cutter radius, and recent studies suggest that the minimum chip thickness is a fraction of the tool edge radius, somewhere between 5 and 43 per cent of the tool edge radius, he offered. If cuts are taken below minimum chip thickness then, although the cutter may have a positive rake angle, the effect is of a cutter with negative rake, and 'ploughing' rather than 'cleaving' is the action, with failure to cut the result - this is 'size effect'. (While cleaving is best for ductile material, such as steels, ploughing is better for brittle materials, such as ceramics, incidentally). Image: Ploughing, shallow cut, leads to compressive stress, although for brittle materials such as ceramics, this is preferred Image: At the right depth of cut, cleaving takes place, required for cutting metal Consistency of tooling is, therefore, a key requirement. First, in terms of the radius, because this directly impacts what minimum cut can be taken. And Dr Mativenga says that users should inspect all tools prior to use because: "Despite all the developments in grinding technology, maybe about 30-40 per cent of the tools we buy will have defects, depending on the supplier." A second consideration is grain size and strength. "When you buy tools from different suppliers, they do not tell you how fine or how strong is the carbide, and that is one of the critical input parameters." Variation in either radius or grain size will mean that a process that works for one tool will not for another, he adds. The material that is being machined is another critical process input. If cutting takes place at close to the material grain size, with individual grains removed, this can lead to poor surface finish. Material variability in constituent grain size is a further challenge to micro-machining, such as for AISI 1045 steel, with its average ferrite grain size of 7 µm and average pearlite grain size of 32 µm. Micros Systems UK picked up on this point (see box item below) Best surface finish, incidentally, is achieved when chip thickness is the same as cutter radius, but the smaller the cutter radius, the larger the burr produced, although the machining parameters affect the actual burr size. On machining performance, uncoated ultra-fine grain carbide cutters lose their radius very quickly when machining tool steel. In 45 HRC tool steel, using a 700 µm tool diameter at 20,000 rpm at an axial depth of cut 50 µm, feed per tooth 8 µm and cutting speed of 44 m/min, the radius of 4 µm lasted but a few seconds, becoming something like 25 µm after a little over 30 seconds. So coatings are requirement for prolonged machining with a maintained radius, Dr Mativenga maintains. But the requirements for the coating are, again, different for micro-machining versus large-scale cutting. There is less heat generated during micro-machining than during large scale machining, so, he says, it is more important to have tool coatings that have a lower co-efficient of friction, rather than very hard coatings. Mist lubricant is also recommended, to prolong cutter radius life. When comparing coated tools to uncoated, using a 0.5 mm diameter cutter in H13 steel, while an uncoated tool lasted 8.7 minutes, Teer TiN-coated tools (see box item below for Teer coating development details and box item number 4 for details about coating removal technology) went for over 50 minutes. And when a self-lubricating element was added – TiN-MOST, the figure rose to 54 minutes. Moving to micro-machining's environmental benefits when compared to alternatives, specifically thermal processes such as chemical machining, Dr Mativenga says that data shows that metalcutting is a more energy-efficient process. So machining might be seen as an old activity, he observes, but it is in keeping with modern, carbon footprint thinking. Could all this knowledge be encapsulated in a model and packaged as a software program for users to generate process parameters? Yes, Dr Mativenga offers, and only half in jest invites interested parties to work with him, although Rainford Precision's managing director Arthur Turner cautions, offering that there are many variables, including those introduced by machine tools and cutting tools from different suppliers. A micro-machining expert, Mr Turner, certainly sees the need for research such as ELMACT and opened his presentation with an image of an iceberg – current knowledge is that which is above the water line and ELMACT is helping reveal what's below. Indeed, he acknowledges his level of knowledge has been boosted in the specific area of cutting dynamics, as revealed by the University of Manchester's work above. Rainford Precision is the UK agent for Kern, a German manufacturer of high precision machines suited to micro-machining and which also has a sub-contract micro-machining operation, plus a number of cutting tool suppliers of micro-tooling, starting at 60 micron diameter. Image: Acknowledging ELMACT's aims, he offered that there are already things people can do today to improve their micro-machining operations and save money. Like Dr Mativenga, he also talked about process inputs, such as the rigidity and quality of the machine tool. Kern machines can faithfully reproduce very small movements and demonstrate no 'pulsing' when in position, for example. This finesse of movement supports both the generation of thin wall thicknesses and prolongs tool life, Mr Turner highlighted. Other key machine features include spindle runout, for example. Tool runnout above 0.003 mm TIR will see reduced tool life, with tool life cut by up to 40 per cent at runnout of 6 µm, according to data supplied by tooling firm Nikken Kosakusho. "I suggest customers clock in tools to within a 2 µm tolerance. If not, tool wear is uneven, with, in the worst cases, cutting on a single edge only," says Mr Turner, adding that most machine shops will only have a 10 µm clock. Indeed, the company is offering customers a 2 µm clock when purchasing five or more tools from Rainford Precision. As for cutting tools, he noted that tolerances for various features can vary significantly – shank diameters from 0.005 to 0.050 mm; cutter diameters also from 0.005 to 0.050 mm (with concentricity tolerances also varying); and radii on ballnose cutters from 0.002 to 0.020 mm. So choosing a good supplier is key, but there are those that will guarantee tolerances to +/- 0.002 to 0.003 mm, as well as highlight what the specific deviations are around a cutter's radius, Mr Turner underlines. And confirming what Dr Mativenga says regarding quoted and actual quality of cutting tools, he reveals that Kern has had the same experience. In fact, the company has developed its own 50x tool measurement system to better inspect tools – 10x is typical. This has not only allowed this type of problem to be picked up, but also supports the more efficient use of cutters in Kern's subcontract machining operation by accurately noting tool wear and avoiding premature replacement of tooling. Mr Turner said that tooling costs had been cut by 30-50 per cent, simply by not replacing tools at the start of each new job. On drilling, he offered this was a more difficult than milling, citing the multitude of drill point geometries and coolant starvation in holes, especially when through-coolant tools cannot be employed, typically below 0.8 mm diameter. Image: Micro-machining drills. Quality issues affect performance greatly at such small depths of cut In addition to the above considerations, there's system rigidity, which includes toolholders, cutter overhang and workholding. Old and worn toolholders or workholding cannot support accurate machining. Machining strategy is another input, with trochoidal machining a particular strategy underlined, which avoids full cutter width cuts in corners. As for potential results when taking a disciplined approach, Mr Turner cited two companies' experiences: WLR Prototype Engineers and Rutherford Appleton Laboratories. With a true running drill, WLR have, on a "not very expensive machine", been able to get over 400 holes of 0.7 mm diameter in a stainless steel component, with variation kept to within 4 µm. At Rutherford Appleton, 6,000 holes of 0.1 mm diameter are being drilled, with a critical 10 µm wall thickness maintained. Correct machine tool (Kern), cutting tool, toolholder, workholding, lubrication and cutting strategy are support this achievement. Micro-drilling was also the subject of University of Manchester's Muhammad Imran. His work was specifically targeted on the drilling of holes in aerospace nickel alloys, particularly turbine blades. Of specific interest to Rolls-Royce, this is an alternative to currently used EDM and demonstrated promise, it was reported. WLR supported the machining trials (see box item below). Agie Charmilles' Steve Burrows followed a similar theme to Mr Turner, highlighting that the major enemies of micro-machining, from a machine tool perspective, are heat generation and vibration. He underlined relevant existing Mikron machine tool attributes, such as rigidity, dynamic and thermal stability and spindle performance; plus control-related technology that support: smooth, responsive, dynamic machine movement; user-defined surface quality, speed and accuracy capability; vibration monitoring/process tuning; and thermal growth control (because all machines, even Mikrons, move). CCD on-machine tool measurement (see also Machinery, April 2010), a recent development, also benefits accurate small tool measurement. This research work as clearly pushed UK micro-machining knowledge on and has certainly benefited the industrial partners and, hence, their customers. Box item 1 The funding envelope Funding was available for collaborative research and developments in materials technologies that address current barriers to the development of more durable products or discourage materials/products to be re-used. The continuing growth in global consumption of materials has increased the pressure on the availability of scarce raw materials and secondary materials. UK needs to work harder to optimise its use of materials resources, including the pursuance of major re-use and extended product lifetime initiatives, if it is to remain competitive in a global market. In recognition of the importance to address this need, an indicative £9 million of funding has been allocated for materials applications in sectors such as aerospace, automotive, marine, construction, chemicals, electronics, food and drink, oil and gas, and general engineering. The global market for materials is large and the UK exports more than £50 billion worth of its total turnover of £200 billion. Production and consumption of materials continue to increase worldwide. In the developed world, annual growth rate in consumption is around 5 per cent, while in China it is growing by more than 10 per cent. The pattern is similar across most sought-after materials, such as high performance polymers, composites, titanium, tantalum, nickel, steel and aluminium. Climate change, demands for sustainable economic growth, uncertain energy supplies and an ever changing consumer demands for better products are constant challenges for materials businesses. In parallel with uncertainties about energy supplies, there are also uncertainties in the supply of some strategic feedstock for the manufacture of many of these materials. The continuing growth in global demand for materials will increase pressure on supplies of the raw materials required to make these finished materials. UK is highly dependent on imports of these increasingly expensive raw materials and reserves are not limitless. To ensure the best use of raw materials, the Materials Innovation and Growth Team (Materials IGT) has recommended that the UK should work harder to optimise its use of materials resources including a major re-use or extended use initiative. By actively taking these steps now, UK businesses will be well placed to enhance their competitiveness in the global market, while addressing major environmental impacts in balancing the flow of materials through the economy. Government funding should act as a catalyst for industry and academia to come together to develop innovative technologies that will breakdown current barriers to a re-use culture, such as the need for better understanding of the impact of used materials, when used to manufacture new products. The materials design and user community should make greater use of lifecycle and design-for-life concepts, supported by access to trackable information on materials used to manufacture complex products. The Materials IGT supports this technology priority, with strong UK business and academic drive for efficient use of limited raw materials. Box item 2 Project description ELMACT exploited novel developments in PVD coatings and state-of-the-art laser processing to extend the life and facilitate the re-conditioning and re-use of hardmetal and other alloy micro tooling, more specifically micro- mills and drills used in demanding machining applications, such as the machining of intricate patterns in hardened die steel moulds, the direct machining of micro-scale features in difficult to machine alloy components, composites etc. ELMACT intended to benefit its industrial partners, which encompass the complete value chain, by improved tool performance, providing optimised machining conditions, and reduced cost. It will reduce the partners' demand for materials, such as heavy metals, that are increasingly strategic. The academic partner will increase understanding of the use of micro-tools and develop specific IPR in the area of femto-second laser technology. Box item 3 Developing coating technology Teer Coating's Shicai Yang explained that while PVD coatings can benefit micro-tools, the problems that arise for micro-tools: components preparation; handling; and surface quality (droplets,defects and non-uniform surface morphology) at surface of cutting edges. Dr Yang's work delivered solutions to these problems, and the process, using Teer CFUBMSIP (Closed Field Unbalanced Magnetron Sputter Ion Plating – Teer-developed, patented technology), was used to deliver a number of coatings – TiN; TiAIN; TiCN; CrN; CrTiAIN; TiMoN; TiN+MOST. Coating performance tests were given for a 6.25 mm diameter HSS drill, machining EN8 carbon steel on a Haas vertical machining centre at a feed rate of 690 mm/min (0.15 mm/rev) and cutting speed of 90 m/min (4598 rpm. Results showed that an uncoated HSS drill was unable to machine any holes, with TiMoN able to produce the most, over 950, while the least successful coating was CrTiAIN, producing 600 holes. Box item 4 Laser removal of coatings For repair and re-use of carbide micro-tools, it is beneficial to remove hard physical vapour deposited (PVD) coatings before re-manufacture/re-coating. In addition, mistakes can also be made in the coating process and tooling may need corrective action or deposition of a new alternative coating. Presently, these coatings are removed using wet chemical processes. These have some disadvantages, including the requirement for processing of waste residue, uneven removal, long cycle time and, most critically, environmental concerns. To overcome the existing problems an alternative technique was explored using short pulse laser ablation. Coatings successfully removed were TiN, CrTiAlN and TiAlN, with laser cleaning/decoating /ablation emerging as a promising non-contact method, because of advantages such as selective/localised removal. Box item 5 Micro-drilling aerospace alloys Essential requirements with an impact on fatigue life were: good hole roundness; no recast layer; no micro-cracking; no laps, tears and plucking; and better surface finish, while reduced cycle times were also required. The macro-scale drilling challenges in nickel alloys include: poor machinability of nickel based super alloys; material drag, lapping, micro cracks, residual stresses; metallurgical transformations; high surface roughness of 1-2 µm; drilling is considered as roughing process; micro-scale drilling challenges in nickel alloys; no work was reported in micro drilling of nickel alloys; drill wandering motions, tool rigidity and tool breakage; higher cutting forces (torque and thrust); size effect, higher specific energies, negative rake angles, tool wear; surface Integrity. Micro-scale drilling challenges are, additionally: no academic work was reported in micro-drilling of nickel alloys; drill wandering motions, tool rigidity and tool breakage; higher cutting forces (torque and thrust); size effect, higher specific energies, negative rake angles, tool wear; surface integrity. Work undertaken employed a 0.5 mm diameter coated carbide drill with a TiAIN coating. A pilot drill with point angle 120° and a 30° helix angle was followed by a drill with a larger point angle of150°, with 30° helix. Work to discover the best cutting parameters followed, with these shown to be: spindle speed, 3,000 rpm; feed, 5 µm/rev; and peck depth of 0.1 mm. And with the .5 mm diameter drill, almost 400 holes were drilled in 2 mm thick nickel 718 alloy. Inclined holes were then drilled in an undisclosed specification nickel-based super alloy plate of 7.6 thickness by WLR, with holes inclined at 30° and at 60° - achieving a depth to diameter ration of 15:2. The result was that 1,300 holes were drilled, with both surface finish and sub-surface deemed acceptable. The overall conclusion is that there is evidence that the micro-drilling process can be exploited for repeatable hole making. Box item 6 Micro-machining specialist Micro-machining specialist Micro Systems UK specialises in the design and machining of hardened steel mould tools for micro-components for medical, optical and aerospace applications, even producing features of size 1 µm, although not through machining. Micro EDM, including wire erosion using >30 µm wire, plus 3 and 5-axis machining are processes undertaken by the company. Moulds features include: gates of from 0.050 – 0.200 mm diameter; small holes cored out in plastic parts by pins of 0.2 mm diameter machined in-house; surface features of > 0.050 mm machined by micro-milling/EDM; surface features of >100 nm produced in nickel via electroforming from master patterns (master patterns are made by special processes eg laser lithography and electron beam). Special processes for sub-micron features include: diamond machining for features >5 µm and with Ra range 3-30 µm; laser machining for holes and line features >10 µm dia/width; laser interference lithography for lines and interference patterns >1 µm; LIGA for high aspect ratio nickel structures >5um footprint (LIGA - X-ray Lithography, Electroforming (German: Galvanoformung) and moulding (German: Abformung); electron beam lithography for features 25 µm – 1 µm in size; focused ion beam for features down to 2.5 nm; and nickel electroforming for replication of microstructured master patterns for features >100 nm. Reluctant to divulge very much machining process detail, due to competitive advantage, business development manager, Paul Glendenning, concurred with the Messrs Turner and Burrows, and confirmed Dr Mativenga's 'size effect' findings regarding quality and surface finish of parts. Indeed, a machining example involving a 50 by 50 µm rib having a burr of height 2.5-5 µm was offered as an example of the 'size effect', where ploughing not cleaving is operating. The results vary "depending on material, cutter type and strategy", he explains, adding that the company has investigated suitable steels for micromachining – "regularity of grain size are very important; the finer the grain, the finer the surface finish, and surface finishes of 0.1-0.2 µm Ra on hardened steel are achieved, with 0.1 quite challenging". Coated ultra-fine carbide tools with radii of 0.8 -2 µm and with 2 or 4 flutes are employed, with 4 flutes giving a better surface finish, while the coatings taking in TiN, CrN, TiALN plus other commercial coatings employed. TiN-coated cutters give up to a 5-fold improvement in cutter life, as demonstrated by ELMACT. Micro Systems UK has also employed diamond tooling on non-ferrous materials, with cutters having edge radii of 0.0006 mm – about the size of carbide grain, so impossible for carbide cutters. Highlighting the effects of thermal machine tool growth, a surface finish issue – bright lines at regular pitch on a surface machined with ballnose cutters from 1 to 0.1 mm diameter - caused by height variation of less than 1 µm on a brass component was tracked to this and eradicated through better control. Process details that he was willing to share included that oil mist lubrication is "often preferred" and that higher spindle speeds deliver smaller burrs. An example of burr generation was shown, the subject being a 50 by 50 µm rib. The burr height was 2.5-5 µm at worst following final machining, although with correct machining parameters, it could be avoided. Results Increased in understanding of micro-machining – peer reviewed papers; critical evaluation of machining strategies to help partners improve current activities; successful demonstration of PVD and laser-generated sol-gel coating coating processes; new and modified coatings; successful coating removal with no tool damage; extension of work to new alloys, particularly aerospace alloys. Where next – partners will exploit results. Exploit outcomes individually with new partners. New call by TSB – high value manufacturing, could take this work forward. First published in Machinery, November 2010