Showcasing the field of bottle boring

In the third part of the ‘Internal machining expertise of WFL', Linz-based machining specialist, WFL Millturn Technologies covers ‘bottle boring' techniques. Alongside special tools, WFL's machining centres enable complex machining, while keeping set-up costs to a minimum. In the first part of our three-part series, WFL focused on deep hole drilling (Aerospace Manufacturing, June 2015), whilst part two dealt with internal turning (Aerospace Manufacturing, September 2015). In the final part, WFL sheds more light on the showcase field of internal machining, bottle boring.

WFL divides this up into two essential types: bottle boring in the centre of a rotating workpiece, or away from the centre of a workpiece using a rotating tool. As expected, the second type is the one that poses the greater challenge.

What these two machining procedures have in common is the requirement to drive and control specialist tools in the correct manner. For processes that involve bottle boring, the tools produced by Cogsdill and ITS are the primary tools available on the market. The solutions on offer from both manufacturers make use of a tool carrier with a defined base diameter enabling cutting tool carriers to be controlled radially on the head end of the tool. This is usually performed using a mechanism that is driven by an additional, rotating axis in the centre of the tool. The spindle axis of the tool carrier unit is generally available for this when using turning/milling centres.

In the case of second type of bottle boring referred to above, an additional U-axis is required. In addition to the requirement to incorporate both functions within the tool, clamping these tools safely presents an additional challenge. In the case of large projection lengths, additional compensation for the tool sag is required. In addition, it is also worth mentioning that bottle boring procedures are usually pull machined to prevent the tool from deviating as a result of compressive stress in the tool sleeve. Additional centring of the tool in the workpiece is also carried out by pulling.

Supporting stable working

As described in Part two, the WFL prism tool interface offers a solution that ensures stable tool insertion and significant processing reliability. Due to the mechanics contained within modulation tools, they are heavier by comparison than similar internal turning tools. These mechanics also reduce the internal rigidity of the tool. As a result, long tools for bottle boring are fitted with support devices in the form of guide bushes, called pilots. These pilots stabilise the tool close to the cutter.

The guide bushes must match the diameter of the base hole exactly. The material from which the guide strips are made must therefore be selected in such a way that on the one hand, the material must, in itself, suffer the lowest possible wear, whilst on the other, the surface of the base hole is not damaged.

The key to ensuring the smooth guiding of the modulation tool therefore lies in a suitable counterboring procedure for the guide drilling. In the case of particularly long base holes, bottle boring and step bottle boring being carried out along considerable lengths, several supports may occasionally be required.

“In our experience, the majority of processes involve base hole diameters of 35mm to 100mm,” begins WFL's sales manager, Martin Kaukal. “In doing so, we can achieve a machining diameter of up to 180mm with a base diameter of 100mm.

Swarf removal presents a particular challenge when bottle boring. If machining is carried out in a through-hole, the swarf can be simply flushed out of the hole by the workpiece spindle using cooling lubricant. The situation is different in the case of blind holes, however. Here, it's imperative that flow swarf, which can build up around the tool during the machining procedure, is prevented.

This flow swarf occurs particularly frequently with materials such as nickel-based alloys or titanium and is often impossible to prevent, despite making adjustments to the cutting geometry. To counteract this problem, WFL has now developed a new solution with this problem in mind.

“We break up the swarf using extremely high cooling lubricant pressure directly onto the cutter,” states Kaukal. “In addition, we ensure an especially high volume of lubricant so that the swarf is lead away via suitable channels.”

Tool service-life issues

The subject of large bottle boring and special geometries can be especially tricky.

“In this instance, we specifically struggle with the issue of tool service life,” explains WFL's applications engineering manager, Reinhard Koll (pictured). “Often we cannot help but machine in several stages. If the cutter is worn down, the tool must be withdrawn and a new cutter reinserted. Beginning again in a seamless way then requires a wealth of expertise in defining the machining strategy and compensating for changes. In this case, we rely on a measuring procedure we developed ourselves, in order to accompany the process, which supplies information relating to the tool status, cooling lubricant supply and the workpiece dimensions at any time. Corrections to the process can therefore be made immediately.”

The tools for bottle boring can be stored in the boring bar magazine with ease and automatically substituted up to a length of 2,500mm and 180kg tool weight.

“It should be added that the support option is sometimes lost with large machining lengths and stepped bottle boring diameters,” Koll adds. “We can only compensate for this to a certain degree by using pilots and guide bushes that can be modified during the machining process. However, additional control of the pilots is still required.”

Lateral machining

Machining laterally in the direction of the workpiece spindle is stated as the third discipline in the field of bottle boring and special geometries. For this type of machining, WFL offers machining using the D'Andrea facing head. Using tools clamped in the facing head, drill finishing and facing procedures can be undertaken at various angles to the workpiece axis.

In summary, WFL machining centres are able to provide the full range of internal machining operations required. These are carried out on a single machine, in one clamping operation using specialist machinery. The provision of tools for all machining processes in the integrated tool and boring bar magazines facilitate smooth processing. Thanks to the definition of appropriate machining strategies, the application skills of this specialist company based in Linz ensures productive machining and processing reliability.

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