Many automotive parts require hardening to provide the long service lives demanded by consumers. The challenge for manufacturing engineers is how to finish-machine such parts to final geometric form after hardening. Until recently, grinding has prevailed as the most common way to process hardened automotive components such as bearings, transmission gears, injector nozzles, shafts, pinions and cams. However, because producers of machined components are continually challenged to reduce cost, improve quality and minimise set-up times, improvements in machine tool rigidity and the development of advanced cutting tools means that hard turning of materials in the 58-68 Rockwell (HRc) range continues to gain ground as a cost-effective alternative to grinding.
The major attraction to hard turning is the potential to side-step grinding, which can be prohibitive in terms of expensive tooling, long manufacturing cycles, costly support equipment (balancers and dressers) and lengthy set-up times. However, the predominant query harboured by most machine shops is whether hard turning can mirror the performance of grinding. In response, a fine tuned, single point hard turning process can deliver 5μm size repeatability, roundness of 0.38μm and 0.1Ra surface finish, i.e. comfortably matching the performance of a typical cylindrical grinding machine.
Of course, achieving a well-tuned process is the trick, and its secret can be revealed by examining the chips, where most of the heat is delivered. During a continuous cut, chips should be bright orange and stream off like a ribbon. If cooled chips effectively disintegrate when crunched in the hand, it demonstrates that the correct amount of heat is being transported in the chip.
A further advantage to machine shops is that hard turning can often be performed on the same machine that ‘soft’ turns the part prior to hardening, therefore optimising equipment utilisation. However, a common mistake involves trying to process parts that do not lend themselves to hard turning.
Ideally, parts should demonstrate a small length-to-diameter ratio, which for unsupported workpieces should be no more than 4:1, and no greater than 8:1 for supported components. Chatter is the likely result if trying to hard turn parts exceeding these ratios. All overhangs should be minimised, with shims and spacers eliminated as the aim is to keep everything as close to the turret as possible.
Another common mistake is deploying a turning machine lacking the necessary rigidity to accommodate cutting pressures that can be double that of conventional turning operations. Machine rigidity dictates the degree of hard turning accuracy. While even an old, well-maintained manual lathe can be deployed for hard turning, with automotive part tolerances getting tighter and surface finishes finer, machine rigidity has become increasingly important in recent years.
Machine tool specialist Hardinge, for example, has integrated several features in its latest high performance CNC lathes to increase rigidity and damping characteristics for hard turning. These include machine bases offering polymer composite reinforcement combined with wide-spaced, heavy duty linear guides, direct-seating colleted spindles that locate the spindle bearing near to the component, and hydrostatic ways. Also, super-finished tracks and centrally-located, short pitch ballscrews provide high dynamic stiffness, which helps reduce vibration and minimise stick/slip movement.
According to Hardinge, the linear guideways on its RS high performance lathes are typically 40 to 60% heavier duty units than those fitted to most machines of comparable size.
The patented spindle design on Hardinge RS and Quest GT series lathes ensures that the workpiece is seated as close as possible to the spindle bearings for maximum rigidity and accuracy. This greatly improves the hard turning process compared with competitive workholding solutions.
Even though hard turning consists of small depths of cut and relatively low feed rates, the process can offer up to six times higher metal removal rates than grinding, depending on the application. Hardinge says there are many additional advantages associated with turning hardened materials, including; single-point turning of complex contours eliminating the need for costly form wheels; multiple operations machined in a single set-up can result in less part handling and less opportunity for part damage; and reduced machine tool cost adds up to greater productivity, better production control, shorter throughput and greater profits.
The higher the resonant frequency (RF) of a machine tool’s structure, the greater its inherent stiffness and the more difficult it is for machining forces to induce vibration.
With this in mind, German CNC lathe builder, Index, has redesigned its patented SpeedLine C-type turn-mill centres to increase their RF above that of similar lathes on the market, including existing models in its own range.
Index says that using the RF measure, the new C-type turning machines are over twice as stiff (rigid), as the established Index G-series models of equivalent capacity. It means that there is more latitude for finding the ‘sweet spot’ for any combination of toolholder, cutter and workpiece, allowing depth of cut to be increased for maximum metal removal rate. High rigidity and the lowering of machine tool vibration also ensures good surface finish of the machined part, prolonged tool life and reduced noise. Better damping leads to higher accuracy machining, even for strenuous applications involving intermittent hard turning and reaming.
The defining characteristic of the Index lathes that provide such high RF and stiffness, hinges on the actuation of the turrets. Each is mounted on a front plate, while an interconnected rear retaining plate is driven kinematically from behind the machine bed. The critical advantage of the sliding plate system is that the distance between the turret centreline and the point at which it is driven is much shorter than for a compound slide. The turret and drive plates are fitted with ceramic-coated, hardened tool steel strips at the top and bottom that slide over a third plate of surfacehardened cast iron fitted into the bed, the latter plate being effectively sandwiched between the other two.
The parallel kinematic drive to the rear plate is now effected by two (rather than the previous three) ballscrewactuated rods at the back of the machine, one vertical and one horizontal.
Elsewhere, the rigidity, power and tooling design of a MAG Giddings & Lewis VTC series vertical turning centre also makes it an increasingly popular choice for hard turning operations. The VTC table bearing design and hydrostatic ram offer a rigid machining platform with good damping characteristics, while the 75-108kW motors produce the horsepower needed for hard turning. Finally, the 150kN clamping force and large gauge diameter of the WedgeLock tooling system produces a sturdy, rigid connection between the tool and ram.
One automotive supply chain company witnessing the benefits of hard turning first hand is Turner Powertrain Systems of Wolverhampton, UK, which is using the process to produce an increasing number of features on complex, carburised steel (58-62HRc) drive shafts. The transition commenced in the mid-1990s with the installation of a Traub 600 lathe.
At the time, Turner was finding it difficult to maintain form on the critical seal grooves of geared shafts and in addition, grinder burn and heat cracking occurred frequently on the component. The problems were solved by hard turning the grooves to the required 0.3μm surface finish, following which the company began hard turning features on the flange.
“The first Traub was a great success. It hit the ground running, proved very reliable, produced good quality parts within tolerance without heat cracking, and was quick to set between batches,” says Operations Development Manager, Stephen Cooper. “We were a bit worried initially that hard turning would cause lathe vibration, leading to loss of accuracy and high tool wear, but early trials on the TNA 600 proved these concerns to be unfounded.”
A more recently installed Traub TNA 500 is compatible with the earlier model in terms of operational ability, programming and tooling, allowing shafts to be swapped between machines. With its introduction, Cooper resolved to extend further the proportion of hard turning on shafts at the expense of grinding. He conducted trials to turn other previously ground hardened features on the shafts, such as seal and bearing diameters, in addition to the grooves and flange.
The results have been impressive. For example, on the CAD drawing for the seal diameter, required surface finish is 1.25μm whereas 0.58-0.70μm is achieved, while figures for run-out and cylindricity are 0.007mm and 0.005mm respectively.
Mitsubishi Carbide advised on the optimum CBN indexable inserts with wiper chip breaking technology to achieve best results, including when taking intermittent cuts. A Neidlein driver in the main spindle pushing the component against the tailstock is used for clamping in preference to a chuck, avoiding the latter’s longer changeover time and the possible introduction of inaccuracies, while Renishaw probing ensures correct orientation of the shafts prior to turning, allowing for any distortion during heat treatment.
“We used to plunge grind the flange and bearing diameters but there was always a compromise on speed, as they are of different diameters, and it was difficult to hold corner definition for subsequent electron beam welding. Hard turning the features has eliminated the problem,” says Cooper.
Fastener Tool, also in the UK, reports similar success. Launched 15 years ago with little more than two surface grinders and abundant enthusiasm, the company has since built an enviable reputation for specialising in the production of hard fasteners thanks to its investment in two Miyano LX-08 turning centres.
The first Miyano LX-08 was acquired seven years ago for the turning of hard and heat-treated materials. The investment was initially aimed at alleviating capacity issues, although the business soon realised additional benefits. “We used to turn our parts and leave a considerable amount of material on the job for the post treatment machining of forms as well as polishing and finishing,” says Managing Director Mel Shorthouse. “Now, we have a turning centre that has the rigidity to accurately machine intricate forms and turn to finished size on steels that have been treated up to 66HRc. Not only does it turn the hard surfaces accurately but the surface finish is exceptional and this is credit to the robust build quality of the machine.”
The effect of the Miyano LX-08 soon was to remove a bottleneck in the finishing section, to the extent that three jobs within the company became redundant. As such, when the company received a large order for seal components in 2008, it turned immediately to Miyano for its second LX-08 turning centre, this time with a 10-station tool turret as the fixed tooling configuration with increased rigidity.
“Our second machine, which was delivered in December 2008, gives us the flexibility to improve our scheduling and relieve pressure on our often short lead times,” says Shorthouse. “The second Miyano is delivering an additional saving of over 15 hours per week in the finishing section, and the bottom line is that accurate and intricate turning of our hard parts is significantly cheaper and faster than grinding.”
While no one can question the important role played by machine rigidity in hard turning operations, correct cutting tool selection is arguably even more vital to process success.
Here, cubic boron nitride (CBN) cutting tool inserts have traditionally been the tool of choice for hard turning applications, and yet some machine shops are initially discouraged by the higher price tag. This is a false economy, as CBN inserts hold up well during interrupted cutting and offer good tool wear for continuous cuts.
According to Sandvik Coromant, the combination of its CBN grade CB7025 with Safe-Lok insert design allows machine shops to increase depth of cut from around 0.3mm using conventional tip solutions, up to an impressive 1.0mm.
When performing operations such as turning away the locally hard zone on case hardened steel components ready for welding, the CB7025 solution can save considerable time and money thanks to improved process security and reliability. As the thickness of hardened zones on such components is normally around 1.0mm, only one cut is required using CB7025, as opposed to three cuts with a conventional tipped insert.
CB7025 is a polycrystalline CBN grade of fine grain size with an innovative ceramic binder. In continuous to heavy interrupted cuts, this grade can run at high speeds and temperatures and still develop wear in a controlled way. Sandvik Coromant wiper technology allows operators to machine at higher feed rates yet maintain process reliability.
This is possible through a patented radius configuration that is less sensitive to misalignment than conventional wipers. Furthermore, big radii permit larger cutting depths when machining up to shoulders, undercuts and in other profiling operations, while mechanically interlocked (Safe-Lok) CBN corners brazed far from the hot cutting zone provide strength and security superior to conventional tip designs. Overall Sandvik Coromant says its CB7025 solution helps produce surfaces at ground quality at 40 to 50% of grinding costs.
One recent success story was the internal and external finish machining of a camshaft gear, where CB7025 was tested against an established solution in unstable conditions.
Not only did the results show an increase in the number of components produced per insert, the ability of CB7025 to maintain edge line properties under difficult conditions allowed for a significant productivity increase. In fact, machining time was cut by 40%, largely because cutting speed could be increased from 90 to 110m/min and feed rate from 0.09 to 0.12mm/rev.
ATI Stellram also says it is enjoying considerable hard turning success using a specially-developed ceramic insert. In tests at a gear manufacturer in Italy, 58HRc components were turned successfully using Stellram ceramic insert RNGN-120700 based on SA7402 grade and E040 edge preparation.
Excellent chip control and built-up edge formation were two of the distinguishing results that delivered extended tool life against the number of components produced. Cutting data attained on the Biglia B600 CNC lathe included 110m/ min cutting speed, 0.09mm/rev feed rate and 0.4-0.5mm depth of cut.
Elsewhere, tooling specialist WNT says that developments in machine tool technology for hard turning have in turn led to a demand for improvements in cutting tool materials and how they are applied.
The latest development from WNT sees the combination of a tungsten carbide base layer being sandwiched by CBN.
While the method used to combine these two materials, brazing, has been around for longer than most care to remember, the use of the technology to create a sandwich effect for a CBN insert has not been achievable until now.
The problem before was that while it was possible to braze a small segment of CBN on to a carbide base, when attempting to braze a second piece, the original braze would melt and the CBN would move or be dislodged. The new, patented, manufacturing process used on the WNT inserts overcomes this problem, allowing multiple CBN ‘edges’ on a single insert.
Of primary importance to the user is the price advantage that the system brings to CBN insert manufacture. For example, CNGA- or DNGA-style inserts now have four CBN cutting edges as opposed to the single cutting edge found typically on these shapes of insert. Similarly, WNGA style inserts now have six CBN cutting edges as opposed to one.
The traditional method of manufacturing CBN inserts has been highly labour intensive, something that has been reflected in the price of such cutting tools. However, the new WNT process reduces labour content to such an extent that insert prices are typically 40 to 50% of those currently on the market. As a result the cost-per-part is reduced by a considerable margin.
For example, if a traditional, single-edge insert can machine 100 parts at a cost of £35 per insert, the cost per part is £0.35. With the new WNT insert it is possible to produce 400 parts but at a cost of just £17.50, or £0.044 per part.
“The financial argument of these new Sandwich CBN inserts is one that should be obvious, and welcome, by any company undertaking hard turning operations,” says Adrian Fitts, Business Development Manager at WNT (UK). “We are also confident that with the added benefits of newlydeveloped coatings, which improve tool life, reduce adhesion of swarf to the insert, and reduce tribo-oxidation (or surface wear), that we will also see an increase in the number of parts per edge when compared to conventional CBN inserts.”
When hard turning, usually the first question concerning coolant is whether or not to use it. For components such as gears, which have interrupted cuts, it is preferable to run dry because the thermal shock the insert experiences when exiting and entering cuts can cause breakage. For continuous cuts, the high tool tip temperatures that occur in dry turning anneal (soften) the pre-cut area, making the material easier to shear. This effect is why it is advantageous to increase speeds when cutting dry.
Coolant can be helpful for continuous cutting applications with respect to offering extended tool life and enhanced surface finish. The trick is to get the coolant to the tool tip, with high pressure coolant being the best bet. HPC also serves to minimise chip size, which reduces the chance that chips will prevent coolant reaching the tip. An exciting alternative to coolant when hard turning is Icefly, a project involving cryogenic (-160°C liquid nitrogen) cooling deployed on Hardinge CNC turning centres.
Icefly offers longer insert life, faster cutting rates and more affordable hard turning insert options.
The Icefly cryogenic coolant system was originally developed by Air Products and Chemicals and has since been refined for real-world machining by Hardinge. The system delivers a jet of -160°C liquid nitrogen directly to the insert during turning operations. The liquid nitrogen raises insert hardness, which reduces the thermal softening effect that an insert may experience as a result of the inherent high cutting temperatures. The steep temperature gradient between the chip/tool interface and insert body also helps remove heat from the cutting zone. In addition, significant cooling maintains insert edge integrity to prevent ‘smearing’ a part’s hot, compressed surface layer, thus providing enhanced surface finish.
While many hail hard turning’s advance into mainstream manufacturing, it is likely that grinding will always have a role in automotive machine shops, as not all components can be hard turned due to tolerance requirements and the surface integrity of the part. However, there is little doubt that hard turning is now a vital technology as manufacturers continue to seek ways to produce parts with lower cost, higher quality, rapid set-up, lower investment, smaller tooling inventory and the elimination of non-value adding activities. The migration of hard component machining from grinders to lathes can satisfy all of these ambitions.