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Deep-hole drilling machine with a high level of automation – the challenge of eccentric drilling
October 2021

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Many machining tools have internal cooling. When it comes to long tool shanks, the routing of the cooling channels is an important factor in terms of quality. When machining long workpieces, single-lip drills used on a deep-hole drilling machine deliver better results than other drilling procedures. TBT has optimised a machine for exactly this purpose.

Some indexable insert drills or milling cutters for deep cavities or internal machining are given longer-than-average shanks. Drilling coolant channels into the hardened and tempered steels is a job for conventional deep-hole drilling. One manufacturer of such tools has been drilling with single-lip drills on deep-hole drilling machines from TBT Tiefbohrtechnik in Dettingen/Erms for about 20 years. When they were delivered, the machines were still designed for manual operation. Now the time had come to modernise the drilling process to the latest state-of-the-art technology and to automate it with the help of a robot. To meet these requirements, the company opted for a new machine from TBT’s ML250 series with the corresponding special equipment. Equipped with four spindles and a carriage travel of 1200 mm, it is designated as the
ML250-4-1200.

The operator can vary the use of the spindles. For large quantities, all four spindles can be used; for one-offs or small batches, the operator can work with just one spindle or select and deselect specific spindles. Drilling tasks are full of variations. Tool shanks come in lengths from 150 to 600 mm. Bore or cooling channel diameters vary between 2.5 and 12 mm. The L/D ratios therefore vary between 50xD and 85xD. The machine allows drilling of both through holes and blind holes.

Eccentric drilling pattern

Each shank has two bores symmetrical to the centre axis. TBT designed a cross slide to correctly adjust the bore spacing by moving the workpieces horizontally when clamped. The machine drills one channel first, then retracts the drilling unit, moves crosswise, and drills the second channel. The great advantage of this solution: the workpieces can be machined in one clamping. Bores that are eccentric to the centre axis have significant impact on processing: “About 90 per cent of rotary parts are drilled centrically on deep-hole drilling machines”, says Simon Bazlen. Machining takes place with counter-rotation of the workpiece. “The runout of the drilling tool can be minimised as a result. With eccentric drilling, only the drilling tool rotates, so other measures may have to be taken to keep the runout within an acceptable range”, explains TBT’s Sales Manager. The challenge: A drilling tool does not runout randomly, but rather in the direction of the smaller wall thickness. The reason for this is the varying heat distribution throughout the workpiece. Between the drilling tool and the thinner wall thickness, the material heats up more and the microstructure becomes softer. TBT therefore integrated an additional cross adjustment solution on the rear clamping of the workpieces.This allows the parts to be adjusted by ±1 degree against the expected runout. “The exact value of the runout compensation depends on the material, the drilling tool and the cutting data”, explains Mr Bazlen. It has to be determined through experimentation and requires a lot of experience on the part of the machine operator. “This solution was a great challenge in terms of construction. However, we managed to keep the runout values lower than what the customer specified for all bore diameters.”

Blow-out on the machine

Another requirement placed on quality was the cleanliness of the bores. Since the tool shanks come into contact with a water/oil cooling lubricant during further machining and are later heat-treated, it is essential to avoid the transfer of deep-hole drilling oil. This applies in particular to blind holes, where larger quantities of oil can collect and ignite during heat treatment. As a rule, cleaning in such instances is carried out with a blowing lance outside the machining unit. Since in this case the workpieces are drilled eccentrically in different ways, an external blow-out station would be difficult to construct. TBT has developed a different solution. The oil supply can be stopped by the NC program in order to blow compressed air through the drilling tool instead of deep-hole drilling oil. The single-lip drills enter the hole a second time. The compressed air switches on at its end, the tool slowly retracts and the oil residues can escape via the outer bead on the drilling tool. The resulting oil mist is immediately extracted at the drill bush carrier and discharged from the machine. Another important topic for operators was the set-up process. A fully automatic solution for tool change in deep-hole drilling is complex and cost-intensive and was therefore ruled out. However, the alternative solution also offers considerable time and ergonomic advantages. In order to reduce downtime, TBT designed an exchangeable tool unit that is pre-equipped with tools, drill bush carriers or steady rests on a separate set-up table. Once the operator has fixed the tool unit in the machine – a zero-point clamping system ensures safe and precise positioning – all that has to be done is connect the deep-hole drilling tools to the spindles. The tool unit can be replaced in just a few minutes.

In addition, the ML250-4-1200 received the usual adaptations to suit its operating location. This included, for example, the installation of an automatic loading door in the rear of the machine, through which a robot inserts the workpieces. Clamping is performed via NC control. Automatic mode also required a robot interface, which is now part of the standard scope of delivery. The same applies to the user-specific fine-tuning of the control system required to integrate the machine into the existing IT infrastructure.

Text: Yvonne Nagel, PressCo – the Press Company

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