Roll out the barrel

Partnerships in manufacturing are nothing new, but in this latest venture between tooling specialist WNT and CAM software developer, Open Mind Technologies, the results are claimed to be nothing short of revolutionary with up to 90% reductions in milling cycle times. Dave Tudor went along to a recent seminar at the AMRC in Sheffield to find out more.

Right off the bat, it’s important to outline the ingredients necessary for this strategy to work: circular segment cutters (often referred to as barrel milling cutters) from WNT; Open Mind’s hyperMILL MAXX Machining CAM package; and a machine tool capable of full 5-axis operation.

Following an introduction from Tony Pennington, managing director of the Ceratizit Group UK & Ireland (WNT is part of the Ceratizit Group) – Adrian Fitts, business development manager, Ceratizit Group UK & Ireland, and Adrian Smith, managing director of Open Mind UK made presentations on their respective areas of expertise.

“From our perspective, this collaborative strategy makes effective use of solid carbide circle segment tools or barrel cutters as they are sometimes known,” Adrian Fitts begins. “There are four types – taper, oval, barrel and lens – and essentially the cutters are used on semi-finishing and finishing processes. They’re often referred to as barrel cutters, but in reality, this is only one of four types.

“The cutters enable machining with a larger toolpath distance during pre-finishing and finishing operations and are commonly used in applications such as mould-making, the production of tyre moulds, turbine blades, impeller blades and blisks on non-flat surfaces where perhaps traditionally ball-nose cutters would be used,” he adds. “Furthermore, the cutters are ideal for machining hard to reach planes on a component, such as deep walls in a pocket on an airframe or mould tool that would be impossible to reach with conventional end mills.”

Depending on the size of tool selected, stepover can be as great as 7mm, which using a conventional ball-nose cutter would be just 0.4 mm. it is this increased stepover, without any loss in surface quality that sees the dramatic cycle time reductions being achieved.

Mr Fitts explains that the differentiating factor of these barrel cutter end mills is the large radii used in the cutting area which simulates a ball-nose end mill with a cutting diameter of 12 to 3,000mm and even larger on request. The larger radii enable a larger axial depth of cut (ap) and offer significantly better surface finishes than conventional ball-nose cutters because the large radius enables greater stepover distances to be achieved. This in turn results in a dramatic reduction in cycle times.

He offers a hypothetical example: “Using a typical ball-nose cutter, the stepover distance (or axial depth of cut) would be 0.5mm and surface finish would equate to around Ra 1.22µm. With a circular segment cutter this stepover can increase to 1.5mm – three times greater – while at the same time improving surface finish to Ra 0.46µm. Threefold improvements in both surface finish and cycle times.”

The software side

Open Mind’s hyperMILL MAXX Machining CAM package is the vital software ingredient in this collaboration. The complete package actually embraces three areas – roughing (trochoidal milling), drilling (5-axis helical drilling) and finishing, but this application focuses on the latter two modules.

“HyperMILL MAXX Machining, as far as drilling is concerned really comes into its own when machining difficult to reach areas,” explains Open Mind UK’s managing director, Adrian Smith. “Processes such as pocket milling with small radii traditionally has been accomplished by using EDM but this is very time consuming and messy. Many of our customers are now machining straight into solid billets when processing flat, curved profiles in difficult areas. We’re seeing many more parts like this in industries such as aerospace and medical, and the time savings are nothing short of dramatic.

“It’s a very stable, reliable process for removing material from deep pockets or the roughing of two holes where the second diameter is larger than the first. The tool is inclined forward at an angle so that the back of the tool doesn’t come into contact with the material and is protected through a special 5-axis tool orientation. The more difficult the material, the greater the benefits.”

But it’s the use of conical barrel tools with hyperMill MAXX Machining that is the most significant aspect of this application with WNT as Mr Smith reveals: “Our software was designed from the ground up to work most effectively with conical barrel tools but they simply were not widely available. Now, thanks to companies like WNT, we have the hardware to complement the power of our software and really push things to the next level.

“Basically, the bigger the radius, the bigger the stepover, but with an ‘old style’ tangential barrel cutter, if you increase the radius, you also increase the risk of a collision where the holder could hit the part. Consequently, these types of cutters are restricted to something like 50mm radius and this is the limitation.

“What we do differently is put the radius onto an angle face on the tool. This pushes the holder away from the workpiece allowing for a much larger stepover. Whilst a high-quality 5-axis machine tool isn’t a prerequisite for this application to be effective, it is true to say that the better the quality of the machine being used (repeatability), the greater the radius that can be used on the tool and the better the end result.”

He continues: “Compared to the 50mm maximum radius that was achievable with older style barrel cutters, with a modern 5-axis machine tool, it’s possible to use tools with radii of 1.5m, 2m or even 3m,” he enthuses. “That’s ground-breaking in terms of reducing cycle times; from the extensive trials we’ve carried out, 90% reductions in time savings are not uncommon. I have no doubt that this approach will, eventually, be as revolutionary as 5-axis machining. The results really are that game-changing.”

And if that’s not enough, there are a number of additional benefits from using conical barrel cutters: they enable a greater line increment with the same theoretical scallop height; they facilitate more efficient production through shorter machining times with comparable or better surface quality; they increase tool life; and, tolerance deviations due to spindle growth and heat warping at the tool are also reduced to a minimum.

Importantly, the hyperMILL MAXX Machining user interface simplifies complex movements making them easy to program: “All the power is in the software,” Mr Smith explains. “A fundamental requirement during its development was that it had to be really easy to use and intuitive.”

Similar strategies

Toolholding also plays an important role in maximising the benefits of circular segment milling and the initial recommendation from WNT is to use its Centro-P collet system, although its hydraulic and shrink fit systems will work equally as well.

The precision of the high-performance Centro-P chucks and collets means that users can maximise the productivity of solid carbide cutters. Runout of the system is guaranteed at 3µm. The latest OZ-style collets can grip cutters between 3 and 25mm diameter to fully utilise their metalcutting capacity thanks to the additional gripping power of up to 350Nm available.

[caption id="attachment_26896" align="alignright" width="201"] Barrel mills[/caption]

WNT and Open Mind have also worked together on a similar helical drilling strategy. Making use of standard solid carbide end mills, or indexable insert face mills, using the complex motion available from simultaneous 5-axis machining centres it is possible to produce deep cavities from solid material in a short space of time.

The cutter is presented to the workpiece at a tilt angle to avoid machining with the end face, thereby eliminating any issue with lack of surface speed at the centre of the cutter. There are a number of major advantages to using this process – the first being that the cycle time to produce holes, whether they be round or square (or any other shape) can be reduced by 20-25%.

Secondly, the process using Open Mind’s HyperMILL MAXX Machining means that only a single cutter is required to produce different diameter/shape holes in a component. As swarf removal is more efficient using this method the surface finish is improved while stress on the cutter is reduced.

A video highlighting the benefits of circular segment milling can be found at:

WNT (UK) www.wnt.com

Open Mind www.openmind-tech.com