Working with HRSAs and other metals that are difficult to machine is only going to increase and they require the right cutting tools and toolholders.
By Kip Hanson
There’s no shortage of metals for machinists to complain about. Whether it’s Inconel 718, Nimonic, Hastelloy C, or some other nasty aerospace-grade superalloy, all are tough on cutting tools and machinery, never mind one’s patience. The good news is, these nickel- and cobalt-based metals are quite manageable, given the right setup and machining parameters.
Let’s start with the cutting tools. With indexable milling and turning holders, spend a few extra bucks for ones with replaceable carbide shims. This will prevent the deformation and erosion that’s sure to occur due to the tremendous cutting forces involved with HRSA machining. And ditch the plastic lines by moving to through-the-tool coolant, backed by a high-pressure pump of at least 70 bar (1000 psi). Only clean, well-maintained coolant at a concentration of at least 12% need apply.
On the toolholding side, it’s probably time for an upgrade. Most side-lock holders introduce runout, a tool life killer in any material. Go with a shrink fit, hydraulic, or high-quality mechanical milling chuck. An anti-pullout mechanism is a safe bet to prevent tool pullout in “grabby” materials like titanium. ER-style collets are okay for drills and small diameter end mills (say anything under 6 mm or so), but only if it’s a top-shelf brand. No bargain tooling.
Serrated jaws are a good idea for first operations on lathes and machining centres alike. After that, how to best grip the part depends on its geometry, although I’m a big fan of hydraulic or pneumatic workholding wherever possible, regardless of the material. They provide greater consistency than mechanical methods and are practically a must for unattended machining (an achievement that’s possible even in HRSA-land).
As for the cutting tool itself, here’s where you should familiarize yourself with the term “material specific.” Cutting tool manufacturers have done a spectacular job over recent years in terms of coatings, substrates, and chip breakers for these uber-tough materials. There are also solid ceramic end mills that might have you running for the fire extinguisher, but there’s really nothing to worry about, provided the way covers and sheet metal have been cleaned of any oil residue. If you haven’t checked in with your friendly cutting tool salesperson lately, now’s a great time. Maybe they’ll bring donuts.
Once you’ve done all that and decided on some cutters and/or inserts, here’s the most important thing: run them at the manufacturer’s recommended feeds, speeds, and depths of cut. If you think tool life could or should be better, resist the temptation to slow everything down, as it’s likely that a more aggressive approach will produce better results. If anything, ease off on the depth of cut and compensate with a heavier feedrate.
And don’t be afraid to ask for help from the supplier—most have a team of applications people willing to lend a sympathetic ear and some helpful advice. The same can be said for your CAM provider. We won’t go into trochoidal toolpaths, high-efficiency milling, and the science behind chip thinning, except to say that all that newfangled CNC programming stuff can make or break the profit margin on a HRSA job. Good luck. SMT
TECHNICAL EDITOR KIP HANSON has more than 40 years experience in the manufacturing industry. He is the author of Machining for Dummies and Fabricating for Dummies and has written over 1500 articles on a diverse range of metal manufacturing topics.