By Frans Zuurveen for Mikroniek – August, 2016. Read more Mikroniek reports at http://www.dspe.nl/mikroniek/.
Burrs: manufacturers of precision machining equipment usually deny their existence. But even laser machining, wire erosion and precision punching cause burrs, however tiny they may be, making some form of deburring indispensable. Timesavers International in Goes, the Netherlands, is specialised in such deburring equipment, achieving rounding-off radii from nearly 0 to 2 mm. Another precision challenge addressed by Timesavers has been the design of a huge machine for calibrating metal sheets.
Figure 1: A Timesavers 8100 belt grinding machine for the accurate calibration of sheet metal up to 7,500 mm long.
The Timesavers 81 series sheet calibration machines can handle titanium, stainless steel, aluminium and other metal sheets up to 7,500 mm long and 2,100 mm wide (see Figure 1). By grinding the upper and lower surfaces three times in a row, and with the sheet upside-down during the second time, a thickness accuracy tolerance of 25 μm is attainable, making this calibration process real precision technology. The main application area of such calibrated sheets is the aircraft industry.
Belt grinding versus stone grinding
The 81 series sheet calibration machines, which will be dealt with further on in this article, may be regarded as a successful spin-off of Timesaver’s deburring expertise with grinding belts. In the precision engineering world, stone grinding is mostly preferred to belt grinding because stone grinding has the ability to machine 3D-curved surfaces. On the other hand, belt grinding is limited to the machining of flat surfaces, although cylindrical forms can be handled too.
Hermes Abrasives in Hamburg, Germany, is an important supplier of grinding and sanding materials. They claim some important advantages for the application of their belts: long lifetime, high cutting rates and dry as well as wet application. Base materials for grinding belts are paper, cloth or webrax, a non-woven cloth with a polyester web. A surprising property is the regular arrangement of abrasive grains – aluminium oxide, zirconium oxide, silicon carbide or special ceramic – on the base material. Because of the identical orientation of each abrasive grain, Hermes states that their grinding particles nearly always show the same cutting and clearance angles. This is contrary to the stochastic orientation of grains in stone grinding discs. And Timesavers claims that changing a grinding belt requires much less time than changing and adjusting a stone grinding disc.
Timesavers has more than seventy years of experience in the field of deburring technology, with its roots in the development and sales of wood machining equipment. Gradually, its expertise evolved from the rather rough removal of unwanted machining remnants to a sophisticated technology for applying the finishing touch to precision parts from sheet metal. This evolution originated from the understanding of primary and secondary burr formation. Figure 2 shows how a so-called primary burr can be removed by making a grinding belt move in ‘with feed’ mode. The primary burr disappears, but a secondary burr occurs inside the hole due to plastic deformation of the primary burr [If the belt was moving in ‘against feed’ mode, a secondary burr would occur at the other side of the hole].
Figure 2: Upper part: removing a primary burr with a grinding belt, with the unwanted creation of a secondary burr inside a hole. Lower part: removing a secondary burr with a barrel brush. (Published in The Fabricator magazine, www.thefabricator.com, May 2012)
The lower part of Figure 2 shows the solution to this problem: using a barrel brush. This is a rotating brush with slabs of grinding cloth, which tend to protrude outwards thanks to centrifugal forces. Those slabs penetrate into holes and remove the secondary burrs. Depending on parameters such as rotational speed, slab stiffness, type of abrasive, product material, etc., the edge of the hole is rounded off with a radius between nearly 0 and about 2 mm.
Grinding tool configurations
Deburring is very often an indispensable operation, but the customer-specified rounding-off radius is subject to variation. Electronic workpieces need a rather large radius because of the risk of damaging wires at sharp edges. Rounded-off workpieces are also preferred in the food industry and for parts that have to be powder-coated. Products for the aircraft industry need some rounding-off as well, because sharp edges may damage the rubber cushions applied in large presses for aluminium sheet parts. On the other hand, high-precision parts must often have well-defined sharp edges. Needless to say, the deburring process for precision sheet parts should reduce the product thickness as little as possible. These varying customer wishes require a thorough knowledge of deburring processes.
Figure 3: An arrangement of four barrel brushes, the two left ones as well as the two right ones mutually rotating in opposite direction.
Some decades ago, Timesavers’ deburring processes comprised the simple application of Scotch-Brite rotational discs. Such discs consist of non-woven polyamide mesh containing glue to which the abrasive grains are applied. These simple deburring processes sufficed to remove primary burrs.
Figure 4: An advanced configuration of two grinding belts and four barrel brushes, resulting in the removal of primary as well as secondary burrs.
Over time, customers and Timesavers specialists became aware of the existence of secondary burrs and the necessity to remove them. Figure 2 illustrates that barrel brushes, shown in Figure 3, are able to accomplish this. The only disadvantage is that secondary burrs with about the same direction as the circumferential brush motion do not vanish. This insight inspired Timesavers to invent several configurations of grinding belts and barrel brushes in gradually advancing deburring machinery, with the main objective of confronting every part of the product with a correct direction and speed of the brush and belt. Figure 4 is an example of this evolution and Figure 5 shows the configuration in practice.
Figure 5: The arrangement of Figure 4 in a Timesavers 42 series deburring machine.
The 8100 calibration machine
Timesavers offers an extensive range of deburring machines, each developed according to specific customer wishes. The Timesaver 8100 sheet metal calibration machine may serve as an interesting example of applied precision engineering. The huge machine shown in Figure 1 contains a stationary main grinding head with a moving slide supporting the metal sheet to be calibrated. In comparison, a configuration with a moving grinding head and stationary sheet would only require nearly half the machine length, but has the disadvantage that all grinding head connections must be flexible. These connections include the electric wiring of course, but also the cooling fluid supply and grinding waste removal. Moreover, the stationary head configuration guarantees a high sheet thickness accuracy, as this accuracy only depends on the position of the grinding belt with respect to the slide moving underneath.
Figure 6: The main grinding head of an 81 series sheet calibration machine with opened front panel. The (dark) vertical bars along which the grinding head moves vertically are clearly visible below.
For extra precision, the temperature of the cooling fluid – water – is thermostatically maintained at 20 °C with a tolerance of some tenths of a degree. The fluid can recirculate thanks to a filter that removes grinding waste. The workpiece, up to 7.5 m long, is clamped on the movable slide with atmospheric pressure, thanks to ‘vacuum’ suction below. The complete main grinding head has to be accurately adjustable in a vertical direction to meet the nominal sheet thickness and to compensate for grinding belt wear. Sheets with a thickness as low as 0.18 mm can be calibrated.
The engineering of the complete machine comprises rather well-known precision engineering design principles. The nearly 15 m long base frame consists of a stiff welded construction supporting stone-ground prismatic guideways on which recirculating ball units run. A rack-and-pinion unit with a pre-tensioned gearwheel drives the table.
The main grinding head, with separate display and control unit, is movable vertically thanks to four round precision bars on which ball bushings run (two on each bar, see Figure 6). Four ball circulating nuts running on carefully machined lead screws support the complete unit. The lead screws are positioned in-line with the four round bars. Driving the nuts synchronously makes the complete unit move.
Thanks to compensation of grinding belt wear during a machining operation, an ultimate tolerance field (over the full sheet area) of 25 μm results when processing a sheet of stainless steel, titanium, zirconium, aluminium or any other metal. Given the very large dimensions of a sheet, achieving this precision is quite a challenge.
Sheet metal deburring and calibration are the fields where the expertise of Timesavers is applied. Curiously, deburring is often being ignored by precision engineers. But the foregoing illustrates that awareness of the existence of burrs and careful sheet calibration both help to improve the performance of the sheet machining industry.