Boosting the production of inclined oil holes in crankshafts

Crankshafts themselves are inherently complex components, yet are required in high volumes for the passenger car market. In fact, due to increasing demand in rapidly growing economies such as China and India, demand is set to increase further.

Retaining this thought, any advances in the machining process that help expedite these critical parts are warmly welcomed by automotive production plants the world over. Among many current areas of automotive focus at cutting tool and tooling systems specialist Sandvik Coromant is the production of inclined oil holes in crankshafts, a notoriously challenging operation that has been hampered by inconsistent results and frequent drill breakage due to the high depth-to-diameter ratios and steep drilling angles involved. However, a new breakthrough in this area is providing extremely encouraging results with the potential to make large gains in both tool life and productivity.

The predominant materials for crankshaft manufacture are cast iron (ISO K) and steel (ISO P), with workpieces typically starting off as forged, cast or solid billets. Cast-iron grades are usually those containing nodular graphite, such as GGG60, GGG70 and GGG80, while forged steel blanks from 42CrMo4 (240-285 Hb) and C38 (900-1400 N/mm²) are also commonplace. Strength, weight and cost are among the factors that determine the selection of cast iron or steel, with the current market split standing at approximately 50:50.

The challenge

Inclined oil holes, commonly four per crankshaft, are required to lubricate the big-end journals. Each hole, which is angled at 27° to 29°, measures 5-8 mm in diameter (depending on the size of the crankshaft) and around 90 mm in depth, typically passing through two adjoining journals and a counterweight section. In many cases, the inclined oil hole will also intersect a straight hole. Due to depths of up to 25 times the diameter, inclined oil holes (and pilot holes) are normally produced on specialised crankshaft production lines using machines dedicated to deep-hole drilling operations.

Until now there have been many problems associated with the drilling of inclined oil holes, namely the angle, depth (chip evacuation) and intermittent break-out into other holes – all of which ensure tool life is a constant challenge. Elevating the difficulty levels even further is the automotive industry’s drive towards MQL (minimum quantity lubrication), rather than conventional through-spindle emulsion coolant.

The solution

There is clear demand for a drill that can offer high penetration rates, repeatability and process security. With these ambitions in mind, the company is set about developing CoroDrill® 865 especially for producing inclined oil holes.

The principal innovation here is brand new geometry with a new flute profile to improve chip formation and increase strength. Each flute has an excellent surface finish to aid chip evacuation and reduce thrust forces, while further edge preparation refinement ensures consistency and form, ultimately delivering a green-light machining process. Moreover, strong geometry and optimised point features support accelerated feed rates.

As a point of note, subtle differences in the geometry design of CoroDrill 865 are offered to suit either ISO K or ISO P crankshafts. Indeed, each CoroDrill 865 is tailored to precise requirements, allowing dimensional adjustments of flute and overall lengths for optimum hole-making performance.

The result

To highlight the potential gains on offer, a recent customer case study saw 5 mm diameter inclined oil holes produced in a GGG80 crankshaft. At a cutting speed of 50 m/min (0.164 ft/min), feed of 0.28 mm/rev (0.011 in/rev) and MQL at 17-19 bar pressure and 19 ml/hr flow rate, the customer witnessed a 140% increase in tool life and a productivity gain of 108%. Moreover, the robust CoroDrill 865 demonstrated predictable wear patterns, enabling full reconditioning of the tool to as-new performance levels.

With some major crankshaft manufacturers producing around half a million crankshafts a year, tool life and productivity gains of this magnitude can have a significant impact on bottom-line profitability.

Secrets of the success

Along with tool innovation, a carefully considered machining strategy contributes enormously to the successful drilling of inclined oil holes. For instance, manufacturers should always start by using the dedicated pilot drill with CoroDrill 865 to ensure precise and reliable cutting action and hole position. The pilot drill has a manufacturing body tolerance of p7 and 150° point angle, while CoroDrill 865 has an m7 manufacturing tolerance and 135° point angle.

Once the pilot hole has been produced and the CoroDrill 865 has been engaged in the pilot hole, full recommended spindle speeds and feed rates can be applied. When approaching a cross hole or breakout of the angled face, drill until 1mm from this point and reduce the feed to 0.1 of its recommended value. The entire drilling operation takes place in a continuous motion – no pecking cycles should be applied. Once the outer corners of the drill have fully cleared the angled face, retraction should take place at 500 rpm and 600 mm/min.

Speeds and feeds

In terms of the recommended cutting data for, say, a 5 mm diameter inclined oil hole, a cutting speed of 50 m/min (0.164 ft/min) and a feed of 0.28 mm/rev (0.011 in/rev) should be applied for ISO K materials. For ISO P, dependent on the application, a feed range of between 0.20 mm (0.007 in/rev) and 0.28 mm (0.011 in/rev) is recommended, although the cutting speed should be increased to 80 m/min (262 ft/min).

Machine set-up and tool holding are also critical for drilling inclined oil holes. With a maximum recommended tool run-out of 30 µm (0.00118 inch), always use a high-quality tool holder such as CoroChuck™ or shrink-fit variants.

The MQL factor

For optimised performance, it is vital that the MQL flow and pressure is well controlled. CoroDrill 865 comes with an MQL-compatible shank as standard, although MQL system selection needs to be carefully considered.

Single-channel systems mix the MQL oil and air at the back of the machine, before a coolant pipe delivers the mix through the spindle to the tool. Standard tool holders can be used, but oil drips out due to gravity accumulating in the spindle and can cause a surge of MQL delivery. In contrast, double-channel systems mix the MQL oil and air in the spindle or at the spindle nose. This gives greater air pressure, uniform droplet size and is best used when drilling deep holes or any other application where chip evacuation is crucial.

Ultimately, the correct application of CoroDrill 865 allows crankshaft production lines to run at full capacity, producing precise inclined oil holes up to 25 times the cutting diameter with high reliability and excellent chip control.