Son of the Earth

The last of a 3-part series on the history of titanium, its many uses, and how to machine this important metal

Earlier in this series, we explored the history and uses of titanium, as well as some ways to achieve satisfactory tool life while machining it. Yet even the best cutting tools and toolpaths will fail without a robust way to hold those tools, grip the workpiece securely, and support toolpaths with a solid and accurate machine tool. Being successful with titanium requires all this and more; for example, predictable process control is as important as what equipment and tooling is used. 

In this final installment, we’ll put together the other pieces to this challenging material puzzle. 

Toolholders and titanium

Titanium is a “grabby” material, and can suck an end mill out of the toolholder during machining. This is especially true during roughing operations, but tool creep is often a problem on even lighter finishing cuts. Weldon flat toolholders have long been used to counteract this disastrous tendency, but the high forces encountered when machining titanium can loosen even the most tightly torqued set-screw. Worse, the small amount of necessary clearance between the cutting tool and a Weldon-style toolholder creates runout, and runout means poor tool life. At higher spindle speeds – 10,000rpm and above – it also creates unacceptable amounts of vibration, leading to chatter and possible machine damage. 

If your shop is serious about machining titanium, you need to avoid tool creep and eliminate runout. Unfortunately, opinions vary on the best way to accomplish this. A needle-bearing mechanical milling chuck is a flexible solution for many general purpose machining applications. Most use proprietary collets and are tightened by rotating a collar with a wrench or via an external hydraulic clamping device. They are suitable for spindle speeds of 20,000rpm or higher depending on the manufacturer, but are relatively expensive compared to comparable toolholding options. 

Similar to milling chucks, hydraulic toolholders are simpler to use but every bit as strong and accurate. Instead of a mechanical collar, they use a small cap-screw or rotating nut to activate a hydraulic bladder, which surrounds and compresses an internal collet. Runout of less than 4µm (0.00016″) at a distance of 2.5x diameter from the toolholder face can be expected, with balancing to 30,000rpm and higher possible. One downside to both mechanical and hydraulic toolholders is dirt. They must be kept clean, and routine maintenance is required to assure accuracy and clamping strength. 

For the most demanding titanium applications, shrink-fit holders offer the ultimate in high rpm capability and gripping force. The cost of the toolholders is slightly less than that of hydraulic ones, but this is offset somewhat by the cost for a shrink-fit machine, which quickly heats the toolholder to several hundred degrees Celsius, expanding it enough that a carbide cutting tool can be inserted into the bore. The toolholder is then cooled in the same machine, creating an interference fit of approximately 0.0015″ (0.038mm) on a 0.5000″ (12.7mm) diameter tool. 

Shrink-fit toolholders have no external screws or moving parts, and are therefore capable of 50,000rpm and higher spindle speeds. They require little to no maintenance. Runout is often better than 0.0001″ (0.003mm) with somewhere in the neighborhood of 10,000 lb of clamping force. If that’s still not enough to keep tools in place, many shrink-fit tooling manufacturers offer spiral-groove systems that mechanically lock the tool and prevent even the slightest amount of creep. Aside from the initial investment cost, about the only downside to shrink-fit holders is the risk of a burned finger.

Ways and means

Let’s start with the machine tool. In an ideal world, CNC lathes and machining centers wouldn’t deflect when subjected to tool pressure. There would be no expansion or contraction of the machine base when the temperature of the shop changes, and heat from cutting processes would have no effect on the spindle and machine guideways. Shops could buy a one-size-fits-all machine tool that performs equally when cutting aluminum as it does with steel, iron, and superalloys. Sadly, that’s a fantasy. As a rule, high-end equipment performs better under a wider range of cutting conditions than do commodity machines. Boxways and geared headstocks are generally more effective in heavy cuts, while linear ways and direct-drive spindles work best for non-ferrous alloys such as aluminum and magnesium, and for high-speed contouring work in mold cavities and medical components. 

Titanium weights roughly 50% more than aluminum but is twice as strong. Despite its superior qualities, however, titanium is expensive to manufacture and machine, so is used primarily for critical airframe structures, engine components, and other parts where high strength and low weight is needed. Both metals are a favorite for aerospace components, many of which require large amounts of material removal, making machining efficiency paramount. Unfortunately, the similarities between the two end there. 

Did you miss Parts 1 & 2?

Read Part 1, titanium’s history and uses, at http://goo.gl/Cp5BFa. 

And part 2, understanding cutting tools and machining strategies that tame titanium, athttp://goo.gl/Jo4Uoy.

Machine tools must be very rigid to withstand the extreme cutting forces encountered when machining titanium. Cutting speeds are generally low and feed rates relatively high. This calls for large amounts of spindle torque and axial thrust, and since abrupt tool movements and dwelling in the corners is a big no-no with titanium, responsiveness in the drive system is likewise a must. Also, a suitably sized HSK or similar spindle interface with dual face/taper contact is better able to handle heavy loads than one designed for CAT and BT-flange tooling. 

Whether your machine is a spring chicken or an old geezer, regular tune-ups are a must. Spindle drawbar force should be measured frequently and adjusted as necessary, eliminating many problems with chatter and tool chipping. Regular ballbar or laser measurement to minimize axial backlash and runout is a good idea no matter what materials your shop machines. It is also important to keep it clean to assure proper guideway lubrication and prevent cutting fluid contamination. 

Predictable process

“Oh great,” you’re thinking. So much for using that old machining center for the titanium job you just quoted. Well, maybe so, but even ho-hum machine tools can be used to cut titanium. The trick is development of a predictable process. Eliminate variables by closely monitoring wear on cutters, drills, and inserts, and replacing them at regular, predetermined intervals long before dull tools negatively impact part quality or breakage occurs. And always use the best cutting tools possible. Perform cutting tests with your top vendors, document the results, and once the winner has been determined, continue to use that brand and style of tool for the duration of your project. 

For workholding, unpredictable part movement can be avoided by pre-machining workpiece blanks to make locating surfaces square and burr free. Ditching the traditional 6″ hand-tightened machinist’s vise in favor of hydraulic or even pneumatic workholding is an excellent way to eliminate process variables and reduce operator fatigue. If mechanical clamping must be used, always employ a torque wrench so the same amount of force is applied from part to part. Many shops achieve gripping success with dovetail-style vises or serrated jaws that bite into the workpiece, especially on 5-axis machining centers, where relatively small amounts of material along the bottom of the blank are used to secure parts. 

Toolholders are another make-or-break aspect of titanium machining. Horror stories of end mill pullout while roughing expensive billets of titanium are enough to keep veteran machinists and shop owners awake at night. Well-maintained hydraulic toolholders are one solution, as is shrink-fit tooling, but toolholders with dedicated anti-pullout devices are the best bet with heavy cutting, as these feature a positive mechanical stop that eliminates tool movement under side loads. 

Keep cool

Cutting fluid is something many shops take for granted. As long as there’s some green or blue liquid squirting out of the coolant line, no problem, right? Bad idea. Titanium takes machining physics to the max, and cutting fluid makes a huge difference between success and failure. 

Using clean, filtered fluid that’s been mixed to the manufacturer’s recommended concentration level is a good idea for any material, but doubly so with titanium. Highly lubricious water-based or semi-synthetic fluids with a generous amount of extreme pressure (EP) compounds designed for such work can greatly extend tool life. So, too, can high-pressure coolant (HPC). Pump pressures of 1,000psi are fairly common, and serve to blast chips away from the heat zone and prevent chip recutting – the number one killer of milling cutters and drills. 

With multi-insert, corncob-style cutting tools, smaller coolant pumps might struggle to keep up due to the number of fluid ports, so pumps should be super-sized in this case to assure adequate flow. The best advice is to plan ahead, and make certain you explain all current machining projects as well as anticipated future needs to potential vendors before purchasing a HPC system. 

And don’t forget about the toolholders and cutting tools – are they coolant-through capable? How about the machine spindle itself? HPC is an excellent way to improve tool life, control chips, and improve part quality when machining titanium and other metals. 

Look for cutter bodies with reinforced insert pockets and secure clamping mechanisms. These prevent the micro-shifting that often leads to chipped or broken tools and unpredictable processes. Integral shank tooling, though slightly more expensive, will provide better results than two-piece tooling solutions. Quick-change toolholders are a great way to save time, especially on CNC lathes, where tool changeover times are typically longer. Note that these should use a dual-contact design and plenty of locating surface area for maximum rigidity. 

Plan for success

The right cutting tools and toolholders, robust workholding, rigid machine tools, and righteous toolpaths – these are all important success factors when machining titanium. Just as important, though, is having a good machining strategy. For those job shops with general-purpose tooling and equipment, don’t despair. The lighter cuts necessitated by less than optimal conditions can still get you to the finish line, provided a stable process is developed first. 

It might be a pain in the neck, but good documentation is key to spotting trouble in advance. Keep track of what feeds and speeds were used, when they were changed and why, which cutting tools work and which ones don’t, and do a root-cause analysis of failures and near misses. 

And remember, even though a stable, predictable process is important with titanium, continuous improvement shouldn’t be discounted. With the aerospace industry’s increased emphasis on lightweighting and fuel-efficiency, titanium’s not going away anytime soon. This means cutting tool and tooling manufacturers will continue their endless search for a better mousetrap to tame both this and other tough materials. Availing oneself of these technologies can make the difference between a winner job and one you’d just as soon forget. Happy cutting.

First published here: https://www.aerospacemanufacturinganddesign.com/article/son–of-the-earth/