Optimizing automotive engine manufacturing
- Published: March 16, 2017
Constantly changing factors such as unstable oil prices, ever more demanding environmental protection legislations and the evolution of more efficient technologies ensures a continually changing global automotive market place. These factors also increase the ongoing competition between carmakers and OEMs and dictate today's automotive industry manufacturing trends.
This article focuses on one of these trends: ICE Optimization (Internal Combustion Engine) / Engine Downsizing Revolution
Today's engines are becoming smaller, lighter, more economical and environment-friendly, increasingly refined and quieter, whilst delivering 25-30 per cent more power and torque than previous generation power units.
The now ubiquitous Turbocharger plays a key role in ICE Optimization. A turbocharger uses the engine’s previously wasted exhaust gases to rotate a turbine that activates an air compressor. When propelled into the engine’s combustion chambers, the resulting air/fuel mixture significantly increases the engine's performance, and vastly improves its efficiency.
An unwelcome consequence of the use of a turbocharger is that the heat generated increases turbine housing temperatures to 900°C in diesel engines, and up to 1100°C in gasoline powered units. As it is crucial that these components function efficiently at such high temperatures, turbine housings are manufactured from austenitic, heat-resistant cast steels, which have relatively high-creep strength, good thermal stability and excellent castability.
This material solution would be perfect if turbine housings could be machined easily, however many turbocharger manufacturers face problems when using standard tools for machining turbine housings. Standard carbide inserts are often only able to machine a few parts before failing. In many cases these tool breakage problems can lead to crash downs and machine and other expensive equipment damages.
Iscar's automotive department was called upon to assist in rectifying the above issues. Essentially, there were two main problems to solve: prolong the life of the tool’s cutting edge and design special cutting tools to minimize the machining times of these complicated parts, which are being produced in millions all over the world.
Longer tool life minimizes machine downtime and makes the process much more efficient. As a result Iscar's R&D department has developed several new carbide grades which are able to run at extremely fast cutting speeds and to last much longer than those of other brands. An combination of advanced new carbide grades, innovative cutting edge geometries and revolutionary pre and post-coating treatments guarantee that the tools’ cutting edges last much longer and that machining times are slashed.
The MS32, one of Iscar's new grades, is intended mainly to be used in rough and finish milling. A carbide substrate provides an excellent balance between hardness and toughness, in combination with a superior CVD coating MS32 provides new levels of abrasion resistance. This advanced new grade has been proven in dry, wet and even MQL machining environments.
For example, Iscar's Ø100mm face milling cutter SOF45 8/16-D100-10-32R (top image), equipped with 10 standard inserts S845 SNHU 1305…MS32 easily removes up to 6 mm stock of a Heat Resistant Austenitic Cast Steel at Vc=150 m/min and f=3mm/rev and reaches a tool life of 25-30 parts. Competitors’ products barely achieve 12 parts per an edge.
Additional time savings are gained from the elimination of several standard operations by the provision of a single, combined and multifunctional tool. For example, the tool shown below on the right is able to perform 5 different operations; rough boring, filleting, finish boring, counter boring and chamfering in one single axial move. Assuming that each operation takes an average of 5 seconds off the machining time by using the illustrated tool, it can save 20 seconds per cycle.
As a consequence of the above, additional unforeseen savings are also achieved by the elimination of tool changing times. Assuming that each tool change takes approximately 5 seconds, another 20 seconds from the cycle time is cut.
Approximately 10 to 15 years ago, the most commonly used cast iron cylinder blocks were largely replaced by bi-metal blocks (aluminum blocks with inserted cast iron liners). Today, more and more car makers have replaced this method with thermal spray processes (or CBC – Cylinder Bore Coating), i.e. a special coating layer, which is applied directly on aluminum cylinder walls. There are a few different thermal spray methods: PTWA (Plasma Transferred Wire Arc Spraying), APS (Atmospheric Plasma Spray), TWA (Thermal Wire Arc Spraying), etc. These coatings deliver many advantages to engine/car performance, the two most important being:
Weight - Engines are much lighter without the presence of heavy cast iron liners.
Lubrication – Friction between cylinders and pistons is reduced due to the coatings’ microstructures.
A major manufacturing issue with the CBC coating is that its hardness is relatively high and its thickness is relatively uneven. Therefore, a cylinder honing operation to achieve the final size can be a long and complicated process. Iscar's engineers have targeted the honing cycle to enable these times to be minimized. They did so by first replacing a few time consuming rough-honing stages with one very fast boring operation. The tool is equipped with four to six PCBN inserts, which are individually adjusted to a precise diameter.
PCBN enables operations to run at very fast parameters. For example, for boring Ø100mm cylinder we work at Vc=400-700 m/min and f=1-1.2 mm/rev.
In some cases, when the chip evacuation becomes an issue, the PCBN insert is designed with a dedicated chipformer on its top. When the boring operation is accomplished, the cutting edges move towards the cutter's center to prevent scratching the cylinder surface on exit. (fig.2)
There are two common mechanisms (depending on the machine): actuation by a linear draw bar, which has only two positions (‘on’ during the boring operation and ‘off’ during the feed out) and actuation by a fully numerically controlled rotation bar, which can change the tool diameter in real time. For example, for producing conical, barrel or other shaped holes for internal grooving or for bore diameter correction/compensation (due to the insert's wear).
A key factor in the success of these operations is the selection of the appropriate PCBN grade related to the material being machined. The correct balance between the hardness and toughness of the grade has to be considered. Although using PCBN with coolant is not recommended, some automotive manufacturers insist on a wet machining process. In these cases, the machining environment (emulsion or oil coolant, MQL, dry machining, etc.) has to be considered. The cutting edge geometry derives from the machined material, cutting parameters and a depth of cut (T-land, E-land, S-land, sharp or honed edge, etc.).
The gas exchange valves, particularly exhaust valves, are always under intensive thermal loads. As previously mentioned, the temperatures of the exhaust gases reach more than 900°C, which constitutes a big challenge for valve materials and can lead to excessive wear and premature fatigue.
A few leading companies have developed new technologies to solve this problem. One of these solutions is to gundrill the valve stem up to its head and to fill this cavity with sodium. During the engine's operation, the sodium absorbs the generated heat and melts.
Iscar's solid carbide gundrills (fig 3) deliver outstanding surface finish, which is crucial for hollow valve applications. Diameter range: Ø0.9 – Ø16 mm (full solid carbide).
•Drilling accuracy from IT7
•Excellent straightness and concentricity
•Maintains high precision hole center alignment
•Surface roughness of Ra 0.4 - 1.6 m can be easily obtained
•Reboring operations are often unnecessary
The shaking effect forces this liquid to move up and down along the stem, which dissipates the heat from the valve head to the stem and cools it. As a result, the valve head remains cooler and hence lasts much longer and the risk of valve burning, pre-ignition and detonation is reduced.
When undertaking these manufacturing operations, in order to enable the sodium to slide easily inside the valve stem, the surface finish of the internal cavity needs to be as fine as possible. For this particular application, Iscar suggests working with gundrills with an integral tip and body made of solid carbide with either steel or a carbide driver. These drills are designed for conventional machines, machining centers, lathes and dedicated gundrill machines. They are available from Ø0.9mm, while providing superior rigidity and optimal coolant flow rates. As a result of being made of solid carbide, when compared to brazed versions, these gundrills can work with up to 100% higher feeds and speed parameters.
Iscar offers a very wide variety of gundrill geometrical shapes, which are designated for different drilling rates, hole accuracy and surface finish quality. The drill's shape, together with its profile must be matched to the workpiece material. In fact, this is exactly what our specialists did in this particular case.
However, selecting the correct gundrill geometry is only one important step towards a successful result. A suitable cutting edge treatment (rake face polishing and edge honing to the right size) improves the surface finish even more. It also improves the drill's performance and prolongs tool life. In addition, the gundrill body itself is being polished. It becomes very smooth and enables the chips to slide easily inside the gullet on their long evacuation. The best results in gundrilling hollow valves have been achieved by using one of Iscar's finest submicron carbide grades IC08 that is protected by a AlTiN nano-layer PVD coating.
A relatively new concept for making much lighter (up to 45%) and remarkably cheaper camshafts, in comparison to the traditional method of machining from cast or forged bar stock, is assembling camshafts from modules. This system uses thermal expansion as the process principle, some OEMs fix pre-heated individual cams on to a pre-cooled precision steel tube. Others fix individual cams on to the steel tube then, by using hot air pressure, expand the tubes diameter in the places where it engages with the cams. In both cases, the lobes of each individual cam are precisely arranged in accordance with the geometry of the camshaft.
The individual cams are produced either from pressed and sintered powder metal or from hardened steels. As there are millions of these cam produced each year, manufacturers are eager to reduce machining cycle times to a minimum. As OEMs need to remain flexible, to react immediately to the frequently changing market and when possible - to spend less money - they prefer to invest in special cutting tools rather than purchase new machine tools.
To minimize cycle time in this area, Iscar has developed what it describes as a revolutionary concept – a single innovative insert that is able to complete the entire cam machining process. The remarkable insert is able to complete face turning, internal rough turning, internal finish turning and chamfering. The extremely durable, tangentially clamped insert faces all 4 operations, including cam lobe profile, at the highest possible cutting parameter with equal ease and completes the cam machining cycle within a few seconds.
For deep hole drilling in forged camshafts, Iscar offers a different approach – a deep drill with an exchangeable carbide insert. This new idea brings many advantages to OEMs. It makes the process much more cost-effective when compared to using conventional gundrills. The standard insert is always available in stock, it has 3 cutting edges and it negates the need for re-grinding. The insert has a positive pressed chipformer and serrated cutting edges that split chips into multiple small segments, which reduces the machining torque (i.e. enables it to work with higher feeds) and improves chip evacuation. In addition, a small wiper at the end of the cutting edge provides very fine hole surface.
(fig.4) Iscar's TriDeep drilling line (GD-DH…) holds IT10 tolerance field and covers a range of Ø 16-28 mm. A standard TOGT insert has three serrated edges that create thin and short chip segments for smoother cut.
These efficient, cost effective tools are highly recommended for deep drilling camshaft applications and can be used on both lathes and dedicated gundrill machines. The GD-DH drills are available at 10, 15 and 25 drilling lengths to diameter ratios. As tailored ‘specials’, the company can produce up to 2400 mm long drills.
Much shorter and thin walled (sometimes friction welded) steel pistons are lighter than the conventional examples and are able to withstand much higher loads than those made from aluminum. T-piston geometry becomes more complicated and requires new and creative engineering ideas for machining difficult to access surfaces.
Iscar's goals in machining steel pistons are:
- To reduce the number of tools needed to shorten expensive cycle times. This requires a high level of creativity due to the fact that the machined areas are relatively hard to access. Although the tool has to be thin enough to penetrate into the piston without collision, it has to be strong enough to withstand high cutting forces. The Grip line products provide the required rigidity and versatility. The user-friendly insert clamping concept that doesn’t have removable parts, generates very high gripping forces that secure the insert in the tool pocket even when cutting directions are being changed, i.e. the tool is able to perform face grooving, right- and left side turning and profiling operations (without vibrations) and to leave a smooth and shiny surface. Also, to efficiently evacuate chips from the complicated cavities, Iscar provides a wide variety of chipbreaking geometries that split chips into small segments and allow quick removal.
- To prolong the life of the cutting edge. A short tool life means a high number of machine stops, i.e. – inefficient machining. However Iscar has proven that the use of its Jet HP concept, which brings a high pressure coolant jet right to the cutting zone, has delivered much improved life per a cutting edge. In addition, the Jet HP coolant method contributes to an efficient chipbreaking process.
Automotive manufacturers’ timeframes for launching new platforms and models become shorter every year, therefore OEMs continuously pressurize Tier 2 and 3 suppliers with demands for ever shorter delivery times. Although the majority of ISCAR's automotive projects are designed at its headquarters, the company’s logistics coordination pays special attention to the requested lead times. ISCAR has production facilities all over the world, and in many cases, for the manufacturing of special tools in the shortest possible time, ISCAR chooses a facility that is closest to the customer's location. In addition to the time and logistics aspects, this concept brings many economic advantages (less tax, lower shipment costs, etc.).
The environmental restrictions for much cleaner manufacturing play an important role in today's market. ISCAR’s contribution to building a better world today and in the future includes offering efficient carbide recycling program, longer lasting tools, products with reduced power consumption characteristics and the supply of MQL compatible tools.