The first thing they teach you in Sandvik Coromant’s Metal Cutting Technology course—right after they tell you that Sandviken means “the sandy bay” in Swedish— is that heat is what destroys a tool. So let’s begin our review of recent turning tool triumphs with advances in coolant delivery.
Cooling At The Cutting Edge
Done properly, 80% of the heat in a machining operation exits through the chip, with the rest split between the workpiece and the tool. While dry machining works in many applications, others require coolant for additional heat removal, chip evacuation, and lubricity. Walter USA LLC (Waukesha, WI) has introduced a new Walter Turn toolholder that delivers what it calls “precision cooling” in which internal coolant travels through the toolholder, comes up through the clamp, and jets right at the cutting edge. Kurt Ludeking, Walter’s director of marketing for its World West Region, explained that “this approach allows us to target the coolant more effectively and with better coolant pressure than other approaches. The farther your nozzle is back from the edge, the more a coolant jet fans out and loses pressure and effectiveness.”
He said it’s not a new concept, but “the difference is that the Walter design is very easy to use. It’s a standard D-style clamp with a single screw. Securing it with that one screw makes the connections to the coolant passages inside the toolholder. Operators like it because they index the insert the same way as with a regular D-style holder. There’s no difference in terms of the number of screws they have to deal with, messing with O-rings, none of that. It’s a very easy system to use.”
The system is so good that even relatively low pressure coolant systems deliver impressive results. “We’ve been telling each other in the industry that you need a coolant system in the high pressure range of 600 to 1000 psi [4137–6895 kPa] or more in order to see a benefit in wear resistance and chip control,” said Ludeking. “But most machines come with a coolant pump that delivers about 150 psi. This toolholder design improves chip control and boosts tool life 20 to 30% at 150 psi [1034 kPa], especially in austenitic stainless steels or superalloys. Even in normal steels like 4140 we’ll see improvements in tool life of 25 to 30% with coolant pressures as low as 175 to 200 psi [1210–1380 kPa].”
Ludeking said the toolholders have already achieved success in “all kinds of industries, from automotive to aerospace to general engineering. We’ve even put it into special tools for aerospace.” The toolholders are available for Walter Turn, Walter Capto (C4-C6) and Walter Cut ISO turning tools.
Over-Coolant, Under-Coolant, or Both?
Walter’s precision cooling system delivers both over- and under-coolant. The coolant that passes through the clamp hits the tool on the top for chip control. Another channel sends coolant from the bottom of the toolholder to the flank side of the insert, under the cutting zone, for heat removal via the coolant itself.
“There are always exceptions, but we find that adding under-coolant almost always helps increase tool life,” Ludeking said. “And if you don’t include over-coolant you don’t get any chip control advantages. Of course if you’re machining cast iron, you don’t need anything for chip control. But with most other materials in which chip control is an issue, top coolant really helps.” So Walter’s system is designed to cover the majority of machining circumstances, yet be extremely easy to use.
Sandvik Coromant (Fair Lawn, NJ) takes a somewhat more complex view of things, arguing that over-coolant may actually be detrimental when machining steel at higher rates. Specifically, it recommends only under-coolant when the feed rate exceeds 0.2 mm/revolution and the depth of cut (DOC) is larger than the nose radius of the insert. (In general, Sandvik Coromant recommends choosing a nose radius smaller than the cutting depth.) Under these conditions, over-coolant “might cause minor edge wear and increase crater wear in steel,” according to the company, adding that it might be difficult to evaluate this wear, which would lead to unpredictable and shorter tool life compared to using under- or external coolant. Conversely, if the DOC is either smaller than the nose radius or the feed rate is slower than 0.2 mm/revolution, using both over- and under-coolant is recommended, according to the company.