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.
When approached by suppliers of sheet material — particularly titanium and other exotics, such as Zirconium and Molybdenum — Netherlands-based Timesavers International BV (www.timesaversint.com
) took on the challenge, looking at the pros and cons of the existing techniques and developing its 81 Series wide-belt reciprocating-table abrasive machine to deliver 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 from 0.15mm up to 100mm, with stock removal rates up to 0.2mm per pass achievable, and with each pass covering a width up to 2.1m on sheets up to 7.5m long.
The result is a significant time saving compared with conventional grinding; cycle times can be halved when removing the 3mm skin from Titanium (as is required by aerospace customers), and Timesavers has reduced a conventional 10hr grinding cycle on Molybdenum sheet to 25min.
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 with up to three spark-out passes, with the sheet (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 0.25μm, with the process creating a ‘short-scratch’ finish, which means there is no direct path from inside to outside, making it suitable for use in applications such as sumps and cylinder heads.
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 and high cutting rates, thanks to the regular arrangement of abrasive grains, whether they be aluminium oxide, zirconium oxide, silicon carbide or ceramic.
These grains 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 of 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-volume metal removal are its lack of influence on the material being processed — and on the environment.
Using abrasive belts puts less heat into the material, thereby eliminating any chance of metallurgical damage or creating stress in the part.
The reduced cutting forces also require less energy, thereby generating savings in power consumption.
The Timesavers 81 Series machine uses 80% less energy to remove a given volume of material than a conventional grinding machine, and the energy savings are increased further when grinding ‘less exotic’ materials such as steel, stainless steel and cast iron, with the latter showing an energy saving of over 88%.
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 (www.ellesco.co.uk
) are turning their attention to the wider application opportunities for this process.
Ability to grind tapers
Vincent Simonis, Ellesco’s managing director, said: “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 it 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 onto sheet if required — something that has proved popular in the aerospace market for the pre-production of blanks for fan blades.”
For the steel market, the flexibility of the 81 Series design also brings advantages, as the machine can be easily adapted at the design stage to suit individual requirements.
For instance, two abrasive belts can be fitted to give improved throughput, with the option of having a rough and a finish belt running simultaneously, or two roughing belts to improve the 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 no balancing is required when changing a belt, unlike with abrasive wheels or stones, speeding up the process considerably — and the 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 the customer’s specification, with features such as clockwise and anti-clockwise rotation of the abrasive belt, plus a table vacuum system that can run 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 the material that has been removed, plus variable feed on the reciprocating table (0.1-10m/min) and the abrasive belt (7-22m/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 managed using pre-programmable settings by Siemens PLC control panel located on a swing arm or slide.
One example of where calibration was a 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, but size and tolerance requirement was 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; this was 100-times more than was actually needed for the project.
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.
As the world of manufacturing changes, new opportunities arise; one such opportunity for the Timesavers 81 Series is the grinding of baseplates used in 3-D 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, so 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 the manufacturing costs where
3-D metal printing is concerned.