News Stories
Milling Machine That Sets World Record With Longest X-Axis
12 May 2008 11:38
Company Profile
The high performance capability of the latest LMT Boehlerit Nanolock indexable milling inserts, that are also able to provide better than 50 per cent improvement in cutting life under arduous conditions than conventional coated inserts, are to play a key role in the mobile track milling operation to be employed by London Underground. Employed on the bespoke Rail Milling Train that is suitable for underground as well as standard high speed railway tracks, the Nanolock inserts will be used to re-profile the railway track of London’s Tube network that connects some 274 stations over a distance of 25 miles.
Due to the length of track to be machined in-situ, this high technology machining system claims to have the longest X-axis travel ever covered by the milling process. If all the individual manganese steel rails were laid end to end, the entire distance would be in excess of 1,900 miles.
There are also a host of problems associated with track maintenance and in milling the profile, the dedicated machinery has to be able to cope with excessively tight curves having a radii of just 45 m on some of the oldest parts of the network.
The machine, that has four independent milling heads, has to be able to correct the rail profile that may have become bald, crowned or flattened. The running edge is often worn, cold rolled by years of abuse from train wheels which leave the surface hardened and as the rails are only supported every 650 mm by ties, the profile can also be distorted. In addition, continuous bending loads can cause micro-cracks in the surface that can lead to spontaneous fractures if they remain undetected and all these problems have to be eradicated by the milling process. To ensure the process meets the specification laid down, each milling head, carries a 1.3 m diameter cutter designed to carry 540 LMT Nanolock coated, indexable inserts.
Prior to the development of the Rail Milling Train there were three options for London Underground to eradicate its track problems. The best, but least viable due to cost, was complete replacement with the alternative options of traditional methods involving replaning or grinding of a new profile. However, these refurbishing operations also have pitfalls of mounting expense and are exceedingly time consuming to perform. Also, if the track is ground, the removal of stock material from the surface of the rail is limited and cracks, that can be as deep as 3 mm, tend to be smeared over, making them very difficult to detect on inspection rides.
Further problems can occur from carbide matter being worked into the running surface and the array of sparks generated in the process could set off a fire or risk of explosion. To plane the rail surface also has its drawbacks, with the rate of stock material removed from the rail being low, the applied forces in the operation tend to be high and the cutting edges of the tooling, due to the hardened surface, wear very quickly leading to frequent stoppages.
Milling the rails in-situ, overcomes all these problems providing a solution of operational effectiveness, economic efficiency and speed. And, this is what led London Underground to place a contract for reprofiling its tracks with Schweerbau of Stadthagen in Germany that provides services to railway companies. Schweerbau then engaged MFL, the specialist mechanical engineers from Leizen in Germany and carbide tooling specialist LMT Boehlerit in Austria which is represented in the UK by LMT (UK) of Coventry to develop the solution. The two, MFL and LMT Boehlerit then worked closely together to create the highly automated milling machine process able to service both underground and the latest rapid-transit railway systems as well as standard and high speed rail tracks.
The Rail Milling Train comprises two modules each 31 m long with an all-wheel driven power unit coupled to the milling module. It has been designed to meet the tight clearance restrictions of London’s Tube system of 2.8 m height and width in the tunnels and also cope with the 45 m bend radius of its tightest curves. The system is able to work autonomously underground for one week processing the rails and collecting all the chips and swarf produced in its own bunker while the development of the milling process by LMT Boehlerit, had to meet the life expectancy and finishes from the tooling it was contracted to provide.
Within the drive unit of the train is the operator’s compartment with space for four people, the drive unit, hydraulic system, compressors and swarf storage. The milling module houses the rear operator’s compartment, the machine room, with its four high power milling units and the entire electronic control and measuring room.
Due to track and network restrictions, the mobile milling machine is only able to function for two hours every night between 1.00 and 3.30 am so it is estimated the rail refurbishment will take over a year to perform. It has a working speed of 2.5 miles/hr but in difficult sections it is restricted to a progress rate of 0.5 miles/hr. On the top surface of the rail, up to 3 mm of material will be removed and 5 mm on the edges so that the milling process reaches below any hardened layers. Any micro-cracking can then be eradicated following inspection using ultrasound measurement.
The milling section of the train has four totally independent milling heads two on each side running at cutting speeds between 250 and 300 m/min. Two heads are engaged in roughing cuts and the other two in finish milling cycles. However, such is the flexibility of the system that all four can be utilised for roughing or that only two are used while the others are held as stand by.
Each milling head, powered by 75 kW motors, moves automatically with height and side profiling controlled within a few hundredths of a millimetre. The height displacement is up to 20 mm and width, due to curves, up to 500 mm. To ensure a viable process, each of the LMT Boehlerit milling cutters are 1,320 mm diameter with a cutting width of 115 mm and are fitted with 60 quick-change cassettes giving a total of 540 tangential indexable Nanolock inserts with either four or eight cutting edges. Each insert can be adjusted to within hundredths of a millimetre in its cassette ensuring perfectly fine running and the production of a precise milling pattern on each rail surface.
Depending on the condition of the track, one cutting edge of the Nanolock insert is rated to cover 5 miles at a feed rate of 30 m/min while working to depths of cut between 0.2 and 5 mm.
The world patented Nanolock insert, developed by LMT Boehlerit’s own metallurgy department is a ‘world-first’ using nano-structure technology (1 nanometre is 1 billionth of a metre) to adhere the coating to the carbide substrate. The resulting microstructure in the form of a ‘composite character’ has a crystallite size of just 25 nanometres, some three times finer than ever previously achieved.
The original brief for its development that was spread over two years was to meet a requirement by the automotive industry for an insert capable of high performance whirling or turn-milling of crankshafts. The company’s solution was a nine micron thick Chemical Vapour Deposition (CVD) coating of TiN, A1203, TiCN and TiN that creates a ceramic thermal shield to protect the insert substrate. Meanwhile, the extremely fine needle-shaped, nano-structured TiCN ensures optimised coating adherence to any adjoining layer. As the insert surface is far harder than standard TiN it improves heat dissipation into the chip enabling machining without coolant.