With milling, part of any speed increase can be offset by taking lighter and more numerous cuts, relaxing the strain on the overall process. Unfortunately, this method is unavailable when performing drilling operations: Whatever the drill speed is, the tool path will remain the same and one pass results in one hole. The upshot is that any challenges inherent to the slower process grow more pronounced as spindle speeds and feed rates are increased. Chief among the challenges is the need to remove heat and chips as the drill feeds further into the hole.
The dilemma, of course, is that machine shops want speed. They want high productivity without any detriment to process repeatability or hole quality, both of which increasingly are taken for granted. The bottom line is velocity — literally— because drilling going faster results in greater productivity and higher profit potential.
Cutting tool specialist Sandvik Coromant has studied the drilling process over the course of many decades, and each successive generation of drills has offered improvements over previous product families. Now however, the bar has been raised with Sandvik Coromant’s CoroDrill® 860, which it claimed represents the fastest solid-carbide drill now commercially available.
This high-performance drill can be applied to a range of holemaking tasks for maximum cost-per-hole reductions in steel workpieces, including long- and short-chipping steel materials, including: unalloyed steels, low-carbon steels, low-alloy steels, high-alloy steels and steel castings. For instance, aside from conventional drilling and stack drilling, CoroDrill 860 is highly proficient at producing chamfered holes and cross-holes, as well as holes through inclined or convex/concave surfaces. Excellent chip evacuation control makes it possible for the drill to provide consistently economical, problem-free drilling at high penetration rates, to boost hole quality and tool life.
Making the grade
Another enabling factor here is the use of a tough, new carbide grade, GC4234. At lower speeds, machine shops can drill effectively using tools made from high-speed steel, which offers relatively high toughness and bending strength. However, higher-speed applications demand specially formulated carbide that is able to promote greater wear resistance and heat resistance at high cutting data. The evenly distributed cutting forces resulting from low run-out are essential for boosting tool life as penetration rates climb.
Any tool used for high-speed drilling must be chosen not only for its hardness (which determines resistance to wear) but also for the extent to which it keeps its hardness at high temperatures. Both requirements tend to disqualify high-speed steel tooling. Instead, the vast majority of today's high-speed drilling applications call for a tool made from solid tungsten carbide in high-temperature grades. The tool's heat resistance becomes more important as the holes get deeper.
Shaped for success
The secret behind CoroDrill 860 and its ability to provide competitive and economical drilling is a completely new geometry combined with an novel flute shape that offers a cutting edge optimized for effective chip clearance, even at increased rates of penetration. An ER treatment helps protect the cutting edges further and makes possible controlled and evenly distributed wear formation.
The drill point is designed for self-centering, while a reinforced drill corner adds strength and process security far beyond that of more universal drills. A further benefit is that the 50% lower cutting forces also provide fewer problems when encountering issues such as weak fixtures, thin-walled components or undesirable loads on the cutting edge.
The automotive sector is one of the many industrial markets where machining operations will see the benefits of these developments. Here, customers’ anecdotes are already providing impressive results. For instance, drilling 6.90-mm diameter through-holes for thread tapping in engine plates made from low alloy steel (using MQL), CoroDrill 860 is offering 62% gains in both productivity and tool life against a competitor product. Penetration rates have increased from 387 to 627 mm/min.
Similarly, producing a chamfered 8.5-mm diameter through-hole in a wheel hub made from an unalloyed steel casting of 255HB hardness (using flood oil coolant), CoroDrill 860 is offering 98% greater productivity, while in another example of the tool’s strength for increased reliability at high cutting data, the same product is providing productivity gains of 100% when drilling 10.50-mm through-holes in low-alloy steel (stacked) drive axles.
The standard diameter range of CoroDrill 860 is 3-20 mm (0.118−0.787 inch) in lengths of 3-8 x D. The achievable hole tolerance is IT8–IT9, while through-coolant is available on all drills. Through-the-spindle coolant, coupled with drills offering internal ducts for delivering coolant directly to the tip of the tool, can extend drill life at almost any speed.
However, while MQL continues to be attractive in certain high-speed drilling applications, many others still demand a flow of coolant through the tool as this functionality can be critical to assisting the removal of potentially catastrophic chips and heat.