The use of long and wide plate material, particularly in the aerospace sector brings with it its own set of issues, namely the consistency of the flatness and thickness of the material. While it is possible to grind these sheets using stone abrasives or to mill them, to bring them within specification; wide belt abrasives add a new dimension to the process, with the use of the 81 series high precision grinding machine.
When approached by suppliers of sheet material particularly Titanium and other exotics such as Zirconium and Molybdenum, Timesavers took on the challenge, looking at the pros and cons of the exiting techniques and developing its 81 Series wide belt reciprocating table abrasive machine that delivers considerable productivity gains and cost savings. While milling may produce a faster result, it is difficult to process thin sheet, accuracy is limited and the surface finish produced may require additional processing. Grinding using stones or abrasive wheels may deliver the surface finish and accuracy, but at the expense of cycle time. Timesavers’ alternative, the 81 Series, developed in collaboration with abrasive belt manufacturers Hermes and 3M, is a wide belt grinder that can process materials in thicknesses ranging from 0.15 mm up to 100 mm; with stock removal rates of up to 0.2 mm/pass achievable, and with each pass covering a width up to 2.1 metres on sheets up to 7.5 metres in length. The result is a significant time saving compared with conventional grinding where cycle times can be halved when removing the 3 mm skin from Titanium (as is required by aerospace customers) and on Molybdenum sheet where Timesavers has reduced a conventional 10 hour grinding cycle down to 25 minutes!
Timesavers describes the process as calibrating, an indication of the accuracy and quality that can be achieved. A typical 81 series process combines a fast rough grinding cycle followed by up to three spark-out passes, with the sheet, which is positioned on the powerful vacuum table, then rotated and the cycle repeated on the opposite face. The result is a thickness accuracy across the entire sheet of 0.25 μm with the major benefit of the process creating a ‘short-scratch’ finish, which means there is no direct path from inside to outside, making it ideal for use in applications such as sumps, cylinder heads, etc; the surface finish is such that potentially, gaskets can be eliminated. This is possible due to the combination of machine and belt technology, with new abrasives offering long life, high cutting rates thanks to the creation of a regular arrangement of abrasive grains, whether they be aluminium oxide, zirconium oxide, silicon carbide or ceramic. These can be applied to a wide range of backing media such as paper, cloth or a non-woven cloth with a polyester web. This regular orientation of the abrasive delivers constant cutting and clearance angles, unlike the random orientation of grains in grinding wheels or stones. The result being greater consistency over surface finish when processing a range of materials. Wide belt grinding also has advantages over milling, as thinner sheets can be processed, and the surface quality is far superior.
Further advantages of using the wide belt approach to high metal removal are its lack of influence on the material being processed and the environment. Using abrasive belts puts less heat into the material therefore eliminating any chance of metallurgical damage or imparting stress into the part. The reduced cutting forces also require less energy thereby generating savings in power consumption. For example: when using wide belt abrasives to grind Titanium on the Timesavers 81 Series machine the specific energy requirement is 90.3 kWh/mm3 of material removal. This compares with 441 kWh/mm3 for a conventional grinding machine, an 80 per cent reduction. These energy savings are increased further when grinding ‘less exotic’ materials such as steel, stainless steel and cast iron, with the latter showing a saving of over 88 per cent.
It is with these figures in mind, and having proved the process on more difficult materials, that Timesavers, and its UK and Ireland Agent, Ellesco are turning attention to the wider application opportunities for this process. “Having proved the advantages of the 81 series machine on these difficult materials the opportunity to deliver similar, if not greater savings to the stainless steel, steel and, cast iron segments of the market is one that we are addressing. These areas traditionally use stone grinding for this type of work, much of which is done on older, inefficient, machinery. We can now offer them a cleaner, more efficient and more controlled process, with added advantages, such as the ability to grind tapers on to sheet if required, which is something that has proved popular in the aerospace market for the pre-production of blanks for fan blades,” says Vincent Simonis, Managing Director, Ellesco.
For the steel market the flexibility in the 81 series design also brings advantages as the machine can be adapted to suit individual requirements very easily at the design stage. So, for instance, two abrasive belts can be fitted, this gives improved throughput with the option of having a rough and finish belt running simultaneously, or two roughing belts to improve cycle time. When finishing is required it takes a matter of seconds to change belts, as opposed to much longer to change a grinding wheel or segmental stones. Further benefits of the belt system are that it is the contact roller that is balanced, with the belt itself carrying relatively little mass. This means that when changing a belt, unlike with abrasive wheels or stones, no balancing is required, speeding up the process considerably. And, by default, that lack of balancing can be translated into higher speeds resulting in greater metal removal as more grit is presented to the component in the same time.
This versatility is enhanced by the fact that the machine can be built to match a customer’s specification, with features such as clockwise and anti-clockwise rotation of the abrasive belt; a table vacuum system that is capable of running under wet or dry operating conditions. When running wet the machine has integrated spray pipes, with separate coarse and fine filter systems to prevent any component damage from material that has been removed; variable feed on the reciprocating table between 0,1-10 m/min along with variable speed of the abrasive belt between 7-22 m/sec. As well as a stand-alone machine, Timesavers can integrate the 81 series machine into a fully-flexible machine cell, with workpiece handling, inspection tables, turn-over tables and washing systems, all of this is managed using pre-programmable settings using the centralised Siemens PLC control panel located on a swing arm or slide to create a simplified operator/machine interface.
An example of where calibration is the viable option was at Attewell, one of the world’s largest producers of shim stock used predominantly in the aerospace sector. A customer required shims with a thickness of exactly 0.3mm. This size and the tolerance requirement were not within the standard capability of the steel mill. To fulfil the contract Attewell was asked to place an order for 100 tonnes of sheet to justify the Mill’s involvement in making non-standard product, which was 100 times more than was actually needed for the project. Considering the economic implications, it was far more cost-effective to invest in a Timesavers 81 Series machine that could grind standard sheet stock to size and deliver the consistency that the customer demanded. Also, as the world of manufacturing changes new opportunities arise; one such opportunity for the Timesavers 81 Series is the grinding of baseplates used in 3D metal printing. Printed metal objects are often created from difficult to machine materials and removing them from the baseplate leaves behind a residue that cannot be easily removed, therefore the plate is written off. Grinding away that residue to bring the baseplate back to ‘as new’ condition using the Timesavers 81 Series significantly reduces manufacturing costs where 3D metal printing is concerned.