There is a growing demand among auto parts manufacturers for turning machines with integrated B-axis functionality

Multi-spindle automatics remain hard to beat for high-volume turned parts, but with designers seeking to combine parts or introduce part ‘families’ to reduce assembly time, the resulting added complexity often means components are transferred to turn-mill centres. These innovative machines typically feature dual spindles and up to three turrets – each with Y-axis functionality. The turrets are loaded with a myriad of driven tools for prismatic machining operations, but the latest trend for many astute manufacturers is the addition of a B-axis (defined as rotation about the Y-axis), which effectively gives five-axis positioning to allow the production of compound angles.

A turn-mill centre with twin spindles allows both milling and turning operations on both the front and back of the workpiece. This means that a job which previously required four set-ups (front-turn, back-turn, front-mill and backmill) can be reduced to a single set-up. Machines featuring a lower turret increase machining capability even further. In this configuration, the B-axis head operates at the same time as other turret-based cutting tools are being used on the opposite spindle, enabling simultaneous machining on both the front and back of the part.

Rapid evolution

The evolution of turning machines over the past 20 years has been nothing short of remarkable. From CNC lathes with back-working spindles and machines with true opposing spindles and twin turrets, through to platforms with a Y-axis to produce features not on centreline - and now the B-axis spindle.

The B-axis is essentially a rotating tool spindle, such as one on a machining centre, with high torque and the ability to change tools in the same manner. This technology has also introduced the tool magazine to the machine, allowing for 60, 80 and 100 or more tools. This design of multi-tasking machines allows for a reduction in set-up time because the tools stay resident in the magazine and do not need to be removed. Complex parts can be machined complete, including intricate angle features, because a B-axis tool spindle can change to any angle as easily as a new program position.

Until recently, these machines have been the domain of machine shops producing low volume, high-complexity parts, but when a B-axis works in conjunction with multiple turrets, cycle times are remarkably quick – a factor being spotted by a growing number of automotive parts manufacturers.

So what parts lend themselves to B-axis turn-mill machines? The simple answer is any part with turning, eccentric turning, drilling or deep-hole boring, OD or ID threads and a few milled features, such as a square or hexagon, even a bolt-hole pattern or gear profile.

To maximise profits further, automotive machine shops can run machines unattended, making costs even lower using simple automation, such as magazine bar feeders, pallet pools and robotics. The software will take care of issues such as tool life management.

If it all sounds too easy, well it probably is, particularly as the use of CAM software removes the headache of knowing in which order to machine components features. With a turn-mill centre, the transfer of the workpiece to the opposing spindle is one of the most critical movements and the CAM system will need to ascertain what features must be done on the same side to maintain tight tolerances.

Most software relies on wait codes (M-codes) to synchronise the programs for the left and right spindles.

These are used to avoid interference between the two programs, such as crossing over from one spindle to the other or conflicting turrets between the two while working on the same spindle. With the programming software, wait codes can be used to optimise the process and verify that there is sufficient time to add in a second operation while another is in process.

A new entrant in this technology area is the CTX beta 1250 TC 4A from Germany’s Gildemeister, a turn-mill machine with an upper B-axis spindle and a conventional lower turret with live tool stations and direct drive. The B-axis, with its 12,000rpm motor spindle, swivels by ±100° and is supplied with HSK-A63/Capto C6 tools from a 24-position disc magazine, although an 80-position chain magazine is an option. The bottom VDI 40 direct-drive turret has a ±40mm Y-axis and 12 driven stations capable of speeds of up to 10,000rpm, while rapid traverse speed is 45m/min.

There are many subcontract machine shops prepared to endorse the use of B-axis turn-milling, and among their number is Unicut Precision of Welwyn Garden City, UK. The company’s recently-installed £370,000 ($520,000) Integrex 100-IVST from Japan’s Yamazaki Mazak includes an Iemca Master 19 barfeed, identical 11kW main and second spindles with automatic part transfer, a 12,000rpm B-axis milling spindle with Y-axis cross-feed for off-centreline machining and a separate 10-station turret for simultaneous cutting capability to either spindle. Also included is a high-pressure 2,000psi coolant system with ultra-fine filtration and Renishaw laser tool probing and setting.

Unicut use the machine to produce a family of 46 parts, with each requiring the milling of outside profiles and flats, and a series of high tolerance holes between 2 and 3mm having up to 15:1 length-to-diameter ratios. There is also a number of compound angle features and holes, a range of cross-holes and high tolerance sealing surfaces on undercuts, grooves and bore diameters. As a result, tool setting and adjustment is eliminated when the automatic sister tool replacement is initiated (automatically) from the tool magazine.

The family of 46 components have been rationalised by Unicut to be produced overnight (lights out) from just three bar sizes. Cycle times vary between four and seven minutes.

Tools and sister tools are held in the machine’s 40-tool magazine.

“If each part were produced on a conventional turnmill centre we would need up to 30 tools,” the company’s joint MD, Jason Nicholson explains. “However, a further advantage of the Integrex is that we can make multiple use of tools for both front and back machining, which means we only need a maximum of 16 per part. This has the added benefit of less setting and reduced tool inventory.”

Both machine spindles incorporate quick-release collets to accept the three sizes of bar and so take less than 40 seconds to change. Tools are checked by the Renishaw system so Unicut has the confidence to run and check key features once production has started. There is also non-contact programmable video measurement on-board.

Cranking up rigidity

For certain automotive parts, capability combined with machine tool rigidity is the order of the day, as Allens Crankshafts of Wolverhampton, UK, discovered recently.

The company’s relationship with Japan’s Mori Seiki began with an investment in an MT machine for heavy-duty turning and milling. As demand grew, the company needed extra capacity and invested in a mill-turn centre from a different supplier, rather than waiting for a new Mori Seiki one. However, the machine proved to be unsuitable for crankshaft manufacture.

“The machine we purchased was not solid enough for the production of cranks, so after two years we sold it and invested in a Mori Seiki NT4250 1500S,” says Nash Sharma, Managing Director of Allens Crankshafts. “The difference is considerable: the NT4250 weighs 24 tonnes compared with 11 tonnes for the machine we replaced and it produces parts 33% faster.”

Camshaft solutions

Part probing isn’t performed on B-axis turn-mill machines as often as it is on conventional machine tools despite the clear advantages offered by in-process probing. A good example is the machining of large diesel engine camshaft sections on a turn-mill machine: after machining, the sections are assembled together to form a completed camshaft.

Each segment has holes positioned around a circle to accept assembly bolts. The bolt circle (thus the centerline of each bolt hole) must align with the actual centerline of the camshaft segment for proper alignment during assembly. Here, WFL Millturn Technologies, based in Austria, has developed a canned probing cycle for its B-axis turn-mills that finds the centerline of the segment automatically as well as the true centres of the bolt holes, all of which have been pre-drilled undersize. If a bolt hole’s centreline position is not where it needs to be relative to the part centreline, then offsets are made to position the tool to the correct location before drilling to final size.

Over at Emag in Germany, which specialises in combined vertical and horizontal turning and grinding centers for chucked and shaft-type components, the company is applying the vertical grinding concept in a new way for long workpieces such as automotive camshafts. The concept uses dual, opposing grinding wheels that engage a workpiece simultaneously. The prime advantage is that the axial forces created by the wheels are directed toward each other, so they are cancelled.

The two grinding spindles move in the X and Z axes on independent, compound slides to complete camshaft lobes. Located vertically between the spindles, the camshaft is secured from below by a fixed tailstock centre and from above by a moveable workhead centre. This provides the grinding wheels with ample access to the workpiece to enable simultaneous grinding from both the left and the right sides.

The machine features a ±120° B-axis, sub-spindle, 100-station toolchanger, 70 bar coolant pressure, Mori Seiki’s proprietary ORC (octagonal ram construction) technology and turret with integrated milling motor to assist metal removal rates of around 1 litre per minute. Journal diameters are left +0.5mm before dynamic balancing, nitriding, finish grinding and final balancing.

“The Mori Seki produces consistent cranks that are concentric and require very little balancing,” says Sharma.

“If I produced a part now and then another 12 months later they would be exactly the same weight, giving us excellent repeatability. Furthermore, tool life is 100% better than we achieved on the old machine.”

There is no shortage of new B-axis models coming to market, particularly from Japanese machine tool builders. Recent cases in point are: the Integrex j-300 from Mazak with 220° B-axis; the Okuma Multus B-200W with 225° B-axis; and the Nakamura-Tome Super NTMX with 230° B-axis. The latter also features dual ATC magazines.

Considering how far turning machines have evolved from the first centre lathes, one could argue that the term ‘lathe’ is today a misnomer. The same can be said of modern sliding head automatics – it’s difficult to visualise the transition from cam-operated screw machines.

In terms of the latter, synchronised sub-spindles, live tooling, attachments for thread whirling and broaching, and other such developments have pushed this platform beyond its initial limitations and many vendors now apply the term ‘multi-tasking’ to their range, particularly as some of these machines also offer B-axis machining.

Capability for continuous B-axis machining on the S207 from Japan’s Tsugami is one such example – a 20mm, gangtooled machine that enables users to produce complex, sculpted shapes on both the main and sub-spindles. The S207 accomplishes this via a servo-driven, B-axis toolpost that can move vertically and swivel in a 135° range around the part being machined. The B-axis works in conjunction with C-axis rotation to enable continuous cutting across virtually any area of the protruding barstock.

In addition to machining highly contoured geometry, users can perform angled drilling and tapping without attachments. The B-axis toolpost accommodates six 5,000rpm live tools, three on the front for operations on barstock gripped in the main spindle and three on the back for operations on the sub-spindle.

In addition to producing complete components, the contouring B-axis helps to avoid the need to spend an inordinate amount of time reconfiguring the tool zone to accommodate a different part or particular machined feature. The S207 also comes with a conversion kit that enables users to remove the guide bushing and operate the machine like a fixed-headstock turning centre. In addition to providing a more effective means of producing parts with length-to-diameter ratios of less than about 3:1, this enables users to perform separate roughing and finishing operations, as opposed to taking a full-depth cut. It also eliminates the need to use expensive, ground barstock.

Like many Tsugami models, the S207 also features modular tooling, which permits users to move tools quickly from one position to another according to their individual application requirements. In a matter of minutes, an operator can move a live spindle to a different location, replace face-oriented live tools with cross-tools, convert a tool position from live to static (or vice versa).

Atypical turning

Every machine tool acquisition must be matched to the application, but sometimes the best solution is not always the most obvious. Take Millennium Machining for example, a Tier Two auto supplier in Macedon, New York, which frequently turns important features of oddly-shaped castings rather than machining them on a mill.

Here, correct machine balancing plays a key role in the effective turning of these non-symmetrical components. One example is a transmission extension machined by Millennium on a Korean-built Doosan turning centre. Rather than performing boring and facing operations on a mill, the company balances the rotating elements of this turn-mill machine to allow the features to be turned. Millennium says this enhances surface finish and produces more precise bore alignment.

In the example of the extension, a counter-balancing weight has been fixed to the faceplate. Depending on the application, balancing may also be achieved by removing weight from various areas on the faceplate. While the extension doesn’t create a hugely unbalanced turning condition, another component does: a transmission front case. However, after balancing, the Doosan machine will perform similar turning operations on these unwieldy front cases too.

Opposites attract

So how about switching the idea around and introducing turning functions to a five-axis machining centre? Germany’s Hermle has done this very thing with its recently-introduced C50U MT dynamic. To enable turning operations, a workpiece is rotated at up to 500rpm on the integral table (C-axis), which is mounted on a +30/-115° swivelling trunnion (A-axis).

Taper options for the main spindle are HSK-T, sizes 63 and 100, providing rigidity for static tooling during turning operations. Unusually, these may be performed not only when the table is horizontal or vertical, but also at any intermediate angle, allowing short toolholders to be used and opening up a wealth of new machining options.

The MT is a derivation of the Hermle C50U machining centre and offers identical five-axis prismatic metalcutting functionality, but is not simply an upgrade. Technological and safety requirements that apply to lathes have been taken into account. Even the upper part of the machine enclosure and roof were considered when designing the guarding. Another new safety feature is tool measuring and breakage monitoring. Milling tools are scanned by laser and turning tools are measured from two sides by 3D touch probes.

The construction of the C50U MT lends itself to the addition of high precision turning. A modified gantry positions all guideways above the working area. The Y-axis has a tandem drive for high machine dynamics and features a patented arrangement of three staggered guideways, one midway between and set back from the outer two, halving the Y-axis span. Production potential is high owing to 6m/ s2 acceleration and traverse rates of 60m/min in the X and Y axes, and 55m/min in Z.

Historically, almost every machine shop had about the same level of technology – generally a lathe, a mill and a grinder. To stay ahead of the competition, manufacturing engineers needed to be more creative with the equipment than the next shop. Today, it is advancements in technology, such as applying B-axis turn-mill centres, which helps shops stay ahead of the competition.