The latest cutting-tool innovations are better able to tackle part complexity while coping with heavy wear
Tool wear is undoubtedly one of the biggest threats to automotive component production. With this in mind, a familiar practice at automotive part shops is to change tools before maximum tool life is reached, just in case; the cost of disposing of an insert before it is fully spent is far less than enduring a catastrophic insert failure. Of course, there are many different ways to measure tool life. Some count parts or use elapsed time, while other methods rely on measuring the amount of material removed. Whichever works best for the specific application, it is a useful tactic that can save untold amounts of money and time.
In a volume environment, the temptation is always to run aggressively, at the fastest possible speed, but in fact, speed is the leading cause of tool wear as it creates more heat in the cutting zone. As a result, numerous automotive part shops are guilty of buying comparatively cheap, four-edge inserts. However, this is a false economy because there are far greater cost benefits to be had from eight- or 16-edge inserts; although the cost per insert is higher, the cost per edge is lower, sometimes significantly.
Cutting-tool suppliers are without doubt under pressure to provide technologies which can help meet annual cost-down demands. If these goals are not met, they could lose preferred supplier status. Sandvik Coromant is among the suppliers aware of this fact. Among the company’s latest tools intended to benefit automotive shops is the CoroMill 5B90 milling cutter for finish-machining aluminium parts such as cylinder heads, gearboxes, valve blocks and motor-block crankshaft and deck faces. In one example, when machining cylinder heads made from AlSi9Cu-1 at a cut depth of 0.7mm, CoroMill 5B90 (160mm diameter, nine cutting edges) achieved a tool life of 40,000 parts at a rotational speed of 7,000rpm and a feed rate of 12,500mm/min.
Since each cutter is tailor-made for the component, it enables an optimised number of cutting inserts to be deployed, where one is always a wiper to ensure high surface quality. High-feed machining is therefore possible with a reduced number of teeth compared with conventional cutters. Set-up times can also be reduced as the M5B90 is designed with a novel axial and radial positioning of the inserts which allows each insert to cut chips instantly without any adjustments.
The benefits are highlighted at a Spanish maker of cylinder heads for car engines, where several problems were being experienced. Firstly, the machining process was unstable and creating burrs, while another problem was unpredictable insert tool life, which depended on cartridge adjustments that differed slightly between set-ups.
Integrating chipbreakers
Another recently announced innovation for the finish- and semi-finish machining of aluminium alloy automotive parts, including engine blocks and gearbox casings, is the BreakMaster LD type and GD type sintered diamond cutting tools with integral chipbreaker from Sumitomo Electric.
One of the problems encountered when machining with softer materials like aluminium alloy is that the amount of chips or swarf produced tends to increase, and if chips get caught up in the material being worked or in the tools used during the machining process, it can result in production stoppages. The latest Sumitomo inserts overcome this by forming an integral 3D chipbreaker on the sintered diamond next to the cutting edge.
Retaining the cylinder block theme, Komet says the range of machining operations required on these challenging components demands specific solutions. For instance, cylinder boring and finishing with the company’s KomTronic M042 fine-boring system allows closed-loop machining, with automatic resetting and cutting correction taking place on the machine. The tool offers cutting edges for semi-finish machining, and one cutting edge for finish-machining during withdrawal.
According to Komet, other cylinder block machining operations which demand carefully considered solutions include: cylinder-bore roughing, perhaps using inserts with guiding geometry and a pitch up to 0.4 mm; pre- and finish-machining of core-plug bores using a single tool; and short, compact tools with triangular inserts for the fine boring of oil channels.
Of course, many cylinder blocks still use cast iron, but this has its own challenges, including a phenomenon known as ‘breakout’, in which the edge of the workpiece chips away rather than being cut cleanly. Cutting-tool inserts that can improve machining quality by reducing the cutting forces and preventing chipping are therefore in great demand.
Among the latest innovations here is Kyocera’s MFK indexable milling cutter, which features double-sided, ten-edge negative inserts. Although negative inserts offer cost benefits because cutting edges can be formed on both sides of the insert, they tend to increase cutting resistance, causing a decrease in sharpness or an increase in chattering.
To address this issue, Kyocera has developed specially shaped inserts using proprietary moulding technology, which, through the generation of a large axial rake, is designed to reduce both of these unwanted effects. Increasing the lead angle also reduces cutting resistance in an axial direction of the workpiece, while surface quality is enhanced through a double-edge structure in which two cutting edges are provided for the insert corners. These cushion the shock at the moment of workpiece contact, preventing inserts from experiencing fractures or breakout.
Multi-tasking tools for knuckles
Away from the engine, there are plenty of other complex machined parts that test the mettle of cutting tool technologies. Take steering knuckles, for example. During the drilling process, the exceptionally high ductility of the material causes the formation of long chips which wind around the tool. The challenge is to break the chips as fast as possible and keep them small. Equally high requirements arise from the complex shape and related precision needed to ensure accurate connection to the steering mechanism.
At the end of last year, a major automotive OEM wanted to produce steering knuckles for cars of three different classes at its factory in Germany. The objective was to produce the steering knuckles on a single machine with as few tools as possible. To achieve this aim, LMT Tools, in collaboration with a German machine tool manufacturer, set about developing a feasible, precise and economic processing concept that allows the customer to produce up to 2,000 steering knuckle pairs (left and right side) for each class, every day.
So, how has this been achieved? LMT combined several production steps and processing methods (drilling, grinding, milling) in a single tool. This allowed engineers to reduce the entire procedure to just 22 tools, from a PCD circular cutter for pre-milling of the hub unit (at 1,250m/min cutting speed) to a PCD disc cutter for face-milling of the suspension arms (1,300m/min) and a PCD spherical cutter for finishing the spherical cap (600m/min).
Steering knuckles are commonly made of grey cast iron, or sometimes steel. Due to the high loads, tight tolerances on shape and position must be achieved during machining – all at low cost, of course. For these reasons, Mapal is another tooling supplier witnessing strong demand for combination tools, which can save on magazine stations and inventory, not to mention cycle time.
Particularly advantageous for complete machining is the use of tangential indexable inserts. These have a soft cut because of their positive rake angle and, due to the compact mounting, are suitable for use on multi-insert tools with high feed rates.
Regarding the machining of steering knuckles, Mapal has also observed that the milling and drilling of the shock absorber mounting can be completed using a combination of a disc-type milling cutter and insertion drill to reduce the number of tool changes. Similarly, when internal machining the main bore, a multi-stage tangential combination tool means that semi-machining is saved, while for the external machining of the main bore, a multi-stage combination tool with fixed insert pockets (without pre-adjustment) is capable of tolerances of less than ±0.1mm.
Machine Tool & Gear (MT&G) makes over 112,000 truck shafts each year at its facility in Owosso, Michigan, where a work cell is built around a Mori-Seiki NH 4000 horizontal machining centre. However, a recent design change to one of the shafts created a challenge for the company’s manufacturing engineer, Jeff Ochodnicky.
“The new shaft design includes flat areas to be machined that are 90º from the other operations, so they’re not easily accessible,” he says. “The obvious solution was to use another machine to mill the flats, but that was a major expense we wanted to avoid. I was determined to find a better solution.
“Instead, we decided to use two 90º milling heads to machine the flats,” he continues. “However, we ran into a number of problems with the tool inserts, including excessive wear and unacceptable surface finish. Working with the insert supplier, we tried different depths of cut, various inserts, spindle speeds and feed rates without success. Finally I got in touch with Horn.”
The challenge was to mill a 38mm-long flat, 1.5mm-deep, on a shaft, using a 22mm diameter cutter protruding more than 250mm out of the spindle. Not an easy task, but one Horn felt could be accomplished with the right tool. The cutter MT&G was attempting to use featured a staggered flute design that caused pushing and pulling during the cut. After careful analysis, it was suggested that the company try a Mini Mill insert from Horn which, thanks to its positive axial helix, would apply all cutting forces downward and free up the cut.
“As a result of the tooling change, we’ve saved the expense of buying another machine tool and kept additional labour costs out of the process,” says Ochodnicky.
Adapting to material changes
So, what of the turbocharger and its associated components? Due to the unstable cost of nickel (from 2005 to 2013, fluctuations were between €5 and €13 per kg), producers of turbochargers now use materials with low nickel content. In response to these industry changes, three independent Iscar technology centres have been working on the development of new cutting materials, geometries and coatings. The fruits of this R&D are S845 SNHU 1305 ANR-MM MS32 inserts, which are said to achieve up to 25% longer tool life than previously available Iscar tools.
For the V-band contour, Iscar recommends turning as the most economic machining strategy – and because the potential for collision is also avoided. Here, the five-edge, star-shaped Penta insert is preferred, with size selection dependent on the depth of the V-band groove. The tool carrying the insert can be adjusted radially and axially, so that tight tolerances of the V-band contour can be achieved. A customised solution for contour turning by means of a U-axis or radial-face sliding unit, as well as for interpolation turning, is also offered.
There are clearly many things to consider when machining complex automotive parts, not least cost. However, while cost per insert may appear to offer the optimum cost efficiency, cost per edge (using the latest insert technologies) will have a far greater impact on profitability.