Sandvik Coromant’s CoroDrill 860 is available in grades optimized for steel, stainless steel, non-ferrous materials and heat-resistant superalloys.  Sandvik CoromantClick image to enlargeby Kip Hanson

Forget about the price, solid carbide is the clear winner when it comes to holemaking

 

It’s tough to argue against high speed steel (HSS) drills. They’re inexpensive. They take a heck of a beating. They’re readily available in screw machine, jobbers, taper and aircraft lengths, with coatings and flute geometries and point angles galore to choose from. Given a little practice, anyone with a steady hand will have no problem re-sharpening one, or just toss it in the bottom of the toolbox and reach for a new one. 

These are just a few of the valid reasons why HSS drills have long been the preferred holemaking tool in machine shops everywhere. The question is: should they be? Customers grow more demanding each year, the competition a little more fierce—can anyone afford to use a cutting tool that provides lower productivity, predictability, and tool life than the alternative, regardless of cost? 

Dream on
And what is that alternative? Solid carbide, of course, and according to Steve Pilger, a senior application engineer for threading and holemaking at YG-1 USA, shops throughout North America are finally starting to see the writing on the carbide wall. “There's always going to be a use for high speed steel jobber drills, but we’ve seen tremendous growth over recent years in the number of shops using solid carbide, to the point that it now dominates the market.”

Hydra modular head drills are a lower cost alternative to carbide, especially with larger, more expensive diameters. Dormer PrametClick image to enlargeFor Pilger and his colleagues at YG-1, the solid carbide drill of choice is the Dream drill, an application-specific tool available in various lengths, diameters, coatings, and coolant capabilities, all fine-tuned for a specific class of material. “We have them for aluminum,” he says. “We have them for alloy steel up to 70 Rc. We have them for stainless steels, superalloys, cast iron, and even CFRP (carbon fiber reinforced plastic). And we have had a tremendous response from our customers and the value it's brought them compared to high speed steel drills.”

Much of that value comes from the fact that most carbide drills are available in coolant-fed versions, which eliminate the endless pecking needed to keep HSS tools lubricated and free of chips. When used with a high pressure coolant system, cutting penetration rates are “significantly greater," and tool life is best measured in thousands of drilling inches.

Achieving these goals requires a little effort, however, and perhaps some new toolholder technology. Aside from the high pressure coolant just mentioned, Pilger notes that several of his customers have achieved 25 per cent greater tool life using shrink-fit vs. hydraulic or ER-style toolholders. Along those same lines, proper alignment and rigidity are crucial to all holemaking operations, but especially with carbide drills, which are less forgiving than HSS. If your lathe turret is out of whack, align it. If your machining centre has seen better days, carbide might not be the best choice.  

The Dream drill is also available in a flat bottom version like the one shown here. YG-1Click image to enlargeUse the Force
“Some shops continue to stick with high speed steel and cobalt drills because it's what they've always done and they're comfortable with it,” says Gary Kirchoff, product specialist at Dormer Pramet. “But there are also a large number of shops that closely track productivity. These are the ones that make the switch to carbide. Yes, the drills are more expensive, but this is easily offset by higher machine utilization, better quality, and more predictable processes.”

To that end, Kirchoff recommends Dormer Pramet’s Force drills, which have a four facet, split point geometry for improved chip formation and self-centering capabilities, as well as a Continuously Thinned Web (CTW) design for reduced thrust. As with most solid carbide drills, a variety of grades, coatings, and lengths are available, and coolant-through is optional (though highly recommended). 

Kirchhoff also suggests replaceable head drills such as the company’s Hydra drills are a lower cost alternative, especially in larger diameters where solid carbide may become prohibitively expensive for some shops. Dormer Pramet is not alone in this—YG-1 has its I-Dream replaceable tip spade drills, while the majority of drill manufacturers provide some iteration of a solid carbide replaceable head drill.

Whatever path you take, Kirchoff says it’s important to learn all you can about applying the tool in question. Dormer Pramet and most cutting tool providers have extensive knowledge bases and technical information online and many offer online and in-house training. Customers should also set aside time to test thoroughly before committing to a solution and to document their results. 

“Our online tool selector contains a cost calculator page, so it’s pretty easy to compare different tools and tool materials to determine which will generate the greatest value for your application. And our forum has a section where you fill in some machining information and it gives you the end result in cost per part—we call it a DTR, short for documented test report. Most of the time, customers find that carbide saves them a lot of money in the long run.”

What’s your Tolerance?
Sandvik Coromant also offers a range of material-specific solid carbide drills. Randy McEachern, product specialist for holemaking and tooling systems at Sandvik Coromant Canada points to the company’s eleven different product lines and suggests that there are few reasons why a shop should settle for the lower performance of HSS drills, even for those with very tight budgets. 

“Our CoroDrill 460, for example, is an economical solution for customers who want to migrate away from high-speed steel,” he says. “It’s a multi-material, general-purpose tool designed for shops with lower volumes but that still need high-quality holes. For anyone that is running specific components on a regular basis and need to maximize their throughput, however, I generally recommend the CoroDrill 860.”

The CoroDrill 860, he says, is available in variants PM, MM, NM, and SM, grades optimized for steel (P), stainless steel (M), non-ferrous materials (N), and heat-resistant superalloys (S) respectively. Like most solid carbide drills, all share features such as internal coolant, self-centering geometries, and problem free holemaking as standard. Yet Sandvik Coromant is currently expanding the line with a new GM variant for general-purpose holemaking along the lines of the CoroDrill 460, only kicked up a notch.

The toolmaker has also introduced a new line of indexable drills, the CoroDrill DS20. Designed for hole depths up to 7 x D, McEachern notes that the DS20 is “truly an innovative design for indexable drilling, one that takes us where no other drilling company is today in terms of length to diameter ratios.”

With all these holemaking choices—not only with Sandvik Coromant, but across the entire spectrum of cutting tool manufacturers—and also considering the overlap between solid carbide, replaceable tip, and indexables, it begs the question: when would I use one over the other? Is it just a matter of cost, or is there more to the holemaking story than that? 

“It largely comes down to quality,” says McEachern. “Each type of drill has its niche area where it can drill a hole and maintain a certain tolerance. When using an indexable drill on a 16 mm hole (0.62 in.) in mild steel, for example, I would expect 4 x D and 5 x D drills to hold a tolerance of 0.00/+ 0.27 mm (0.011 in.) to 0.00/+ 0.35 (0.014 in.), with 6 x D and 7 x D drills able to achieve -0.1/+ 0.4 mm (-0.004/+0.016 in.) tolerances. 

“A replaceable tip drill like our CoroDrill 870, on the other hand, has a nice, self-centering chisel point and will hold H9 (+0.0000/+0.00169 in.) tolerances, while the CoroDrill 860 will achieve H8 (+0.0000/+0.00106 in.), assuming each is held in a high-quality chuck of shrink-fit holder and a rigid machine tool. Long story short, all provide far greater performance than high-speed, but solid carbide is by far the winner in terms of hole accuracy.” SMT

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