Boosting productivity is all well and good, but Sandvik Coromant Co. finds that high-volume part manufacturers are generally more concerned about enhancing and achieving a predictable tool life, noted John Dotday, drilling product and industry specialist for the toolmaker. That’s especially the case when drilling steel workpieces, which can have slight variations between material batches that cause tool breakage, he added.
Sandvik Coromant developed the solid-carbide CoroDrill 860-PM to enable secure and productive steel drilling, including hardened and softer steels. Soft, gummy, low-alloy steels, such as 1018 and 1020, create the biggest drilling challenge because of built-up edge and chip control problems, according to Dotday. While it is difficult to break chips in those materials, the CoroDrill 860-PM creates small chips with tiny tails, he said.
The CoroDrill 860-PM is for steel holemaking applications. In cross-hole applications wher hole diameters differ, end users should always drill the largest diameter first to minimize burr formation and prevent the drill from walking to one side.
In addition to optimizing the drill’s geometry for steel applications, Dotday explained that the drill’s edge-rounding treatment and reinforced drill corner prevent corner chipping, giving strength and security to the drilling process. The treatment is performed by applying abrasive brushes on a grinding machine.
He added that the new edge-prep reinforced corner and tough 4234 carbide substrate prevent premature tool wear, enabling machinists to hold tight tolerances and impart fine surface finishes. “We want a controlled wear pattern,” Dotday said.
The drill not only provides a predictable tool life but a longer one. Compared to other drills, the Sandvik Coromant CoroDrill 860-PM lasts 20 to 40 percent longer, according to Dotday.
The drill’s through-coolant capability also enhances chip control, and Dotday recommends a coolant pressure of at least 220 psi. For end users without through-coolant capability on their machines, he noted that a limited assortment of external-coolant tools are available, but they should apply them only for making holes up to 3 diameters deep and reduce the cutting speed and feed 25 percent. For the through-coolant variety, the toolmaker recommends a feed of about 0.010 ipr and a speed of 400 to 800 sfm for soft steels and 200 to 330 sfm for harder steel grades.
once the drill produces manageable chips, post treatment brushing of the tool’s multilayer TiAlN coating enhances lubricity and chip evacuation, Dotday noted, adding that the coating appears as if it were polished.
Standard drills are from 3mm to 20mm, are available to drill holes up to 8 diameters deep and, depending on the diameter, have a tip point from 139° to 144°. The tip angle is one reason starter holes are not recommended even when drilling convex and concave surfaces, Dotday explained. “Unless you have the proper center drill with that geometry, a starter hole will create problems and break down the drill faster,” he said.
When a drill needs reconditioning, the toolmaker requires that it perform reconditioning to provide the same life as a new tool because of the special grinding and edge prep required, according to Dotday. He added that reconditioning reduces tool length about 1mm while slightly increasing the diameter when recoated. A drill can be reconditioned three times, but beyond that the additional coating layers will experience flaking issues. Because reconditioning reduces tool length, Dotday pointed out that a user needs to specify a drill that’s still long enough for an application after several reconditionings, not just when brand new.
