Stop the Stringers
- Published: February 8, 2017
Achieving good chip control in CNC turning operations
Long, stringy chips are more than a pain in the neck. They’re also dangerous. Cut fingers are an ever-present possibility when handling the razor-sharp chips common with turning operations in stainless steel, superalloy, and even aluminum. And even if you’re lucky enough to avoid a trip to the emergency room for stitches, stringy chips can wrap around tools, clog up chip conveyors, and snarl spinning chucks, disrupting production and eliminating any chance for unattended operation.
Breaking up is (sometimes) hard to do
Tool life is also important. When inserts aren’t pushed hard enough to break the chip, rubbing occurs, spelling early failure. Built-up edge can be a problem as well, especially with superalloys and other metals high in nickel and chromium. Fortunately for lathe operators today, a wide array of chip-busting inserts is available, making easy work of even the most stubborn materials. Yet Gary Kirchoff, product specialist at cutting tool manufacturer Dormer Pramet notes that selecting the wrong chipbreaker is often just as bad as no chipbreaker at all.
“It’s very important to choose a chipbreaker that is designed to effectively curl and break the chip,” Kirchoff says. “If the depth of cut (DOC) and feedrate are proportioned correctly to the width of the T-Land and chipbreaker groove, the material enters the groove and is curled properly. The chip is broken and the chipbreaker does its job. But if the T-Land is too wide, or a DOC and feedrate too small in relation to it are used, then the material does not reach the chipbreaker groove, and it cannot do its job. Similarly, if too high a feedrate or DOC is used relative to the chipbreaker, the material will be crushed into the chipbreaker groove, causing most of the material to pass over the top and eliminating any chance for proper chip formation.”
Aside from selecting the proper chipbreaker groove, choosing the right carbide grade and coating is equally important. Grades are designed to be tough, wear resistant, or a combination of both, attributes determined largely by the amount of cobalt binder and the carbide’s grain size. Together they control where the grade falls in the spectrum between toughness and wear resistance. Finally, says Kirchoff, use the recommended feeds, speeds, and DOC; applying the wrong values with any of these three often leads to rapid breakdown of the insert’s cutting edge and catastrophic failure.
Water in a bucket
Iscar Tools is another supplier with its eye on the chipbreaking prize. Senior product manager Steve Geisel equates chip control to pouring water in a bucket: pour too much in and it’ll overflow. Aside from a wet floor, it’s no big deal. “Carbide’s not that simple–select a chipbreaker that can’t handle the volume of material you’re trying to put into it and you’ll destroy the insert,” he says. The volume of material removal is determined by feedrate and depth of cut. This makes it critical that manufacturers select inserts according to the specific material being machined and the cutting conditions used to machine it. For job shops, where wide material variety is the norm, this can mean cutting tool inventory levels that some might deem unacceptable.
“I can appreciate concerns over too many part numbers in the tool crib, but for aerospace shops, mould and die, and especially automotive, where you’re cutting the same kinds of materials on a constant basis, it’s perfectly justifiable to stock the exact inserts needed for a specific job,” Geisel says. “And for shops that do high mix, low volume work, it’s just as important to select the right insert, but sometimes you need to go with a more general purpose tool. This can create tradeoffs in performance, something that has to be weighed against the number of different inserts you keep in stock.”
For these shops, Geisel says a little feed and speed tweaking can go a long way towards effective chip control, even when the chipbreaker is less than ideal. Be careful, however, that the correct grade is used; applying TiC, TiN, or other coated inserts on titanium, for example, is a little like cutting wood with wood. “Most coatings contain titanium. This is why I always recommend uncoated inserts. I’ve gone to many shops struggling with titanium and given them an uncoated tool with the exact same chipbreaker and nose radius as what they were using, and ended up with three times the tool life.”
Also beware of pushing tools too hard in an effort to break chips. Not only can the workpiece come loose from the chuck or collet, but materials should be machined within a specific heat range, Geisel explains. Inconel and similar materials tend to work-harden when cutting temperatures exceed this range, creating difficulties on subsequent passes. Whatever the material, keep the cutting conditions within the carbide manufacturer’s recommendations, he says, and switch to a different chipbreaker if chip control is unachievable within those cutting parameters.
Riding the wave
Kurt Ludeking, director of marketing for Walter USA, says effective chip control often comes down to proper placement of bumps and waves on the surface of the turning insert. These serve to fold the metal, cold working it into a shape that causes it to break away. A specific amount of edge rounding is also needed, to reinforce the cutting edge according to the cutting conditions. A roughing insert, for example, requires a stronger edge than does one used for finishing, and therefore requires additional honing.
Chip control in extremely ductile materials such as low-carbon steel, commonly used for motor and hydraulic shafts, benefits from bumps placed directly in the chip groove. This creates folds in the material perpendicular to its direction of flow.
“Think of bending a paper clip,” he says. “Bend it enough and eventually it’ll break. But if you can move it in two directions simultaneously, you apply even more cold working to the material, making it break faster.”
Along these same lines, Ludeking says toolholders with built-in precision coolant channels are quite effective at chipbreaking, especially when high pressure cutting fluid (see sidebar) is applied. “Consider superalloys, where you not only need a very strong, sharp cutting edge, but also a geometry that can cold work the metal enough to make it break away,” he says. “Without it, you’ll end up with a giant bird nest of chips. This is one area where toolholders with integrated coolant really shine, as they remove heat very effectively from the cutting zone.”
If you’re concerned about having to buy a new style of insert for every job that comes through the door, Ludeking says most cutting tool suppliers can suggest four or five geometries that will handle 90 per cent of applications pretty well. For the rest, a few feed and speed tweaks should do the job. “Compared to the days of hand-ground brazed tools and bits made of high speed steel, cutting tools have come a long way.” SMT