Deep drilling

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Deep drilling or deep hole drilling is a special type of drilling with drilling depths that are many times greater than the diameter.

Definition of deep drilling

In the sense of VDI guideline 3210, deep drilling processes are machining processes for the production and processing of bores with a diameter between D = 0.2 ... .2000 mm and the depth of which is usually greater than three times the diameter. With small bore diameters, length to diameter ratios of up to l / D ≤ 100, in special cases even up to l / D = 900 can be achieved. With large diameters, the I / D ratio is usually limited by the travel path of the machine or by its bed length.

The deep drilling

Soldered, single-edged BTA solid drilling tool (left); symmetrical twist drill (right)

Deep drilling also differs from short drilling in that , depending on the drilling process and the drilling diameter , cooling lubricant has to be pumped to the cutting point in large quantities and under high pressure. This results in good cooling and, at the same time, good lubrication of the active areas between the workpiece and the tool cutting edge on the one hand and the workpiece and guide strips on the other. In addition, the cooling lubricant leads to a constant removal of chips from the active zone, which saves time and eliminates the need for surface-damaging and time-consuming machining strokes. A distinction is made between two different types of tools for producing deep holes. On the one hand, these are tools with an asymmetrical, single-edged design. These include drilling with Einlippentiefbohrern , drilling with tools of the single-pipe system ( BTA deep hole drilling ) and deep drilling with tools of the double pipe system ( ejector deep drilling ), referred to as the "classic" deep hole. On the other hand, these are tool types with symmetrically arranged main cutting edges, which are used in helical deep drilling and two-lip deep drilling, which can also be assigned to the deep drilling process due to the drilling depths to be achieved with them.

The tool types mentioned differ in terms of the realizable diameter range, the achievable I / D ratios, the surface quality and their productivity . With symmetrical tools, only in the small diameter range of D = 0.2 ... 32 mm, bores with an l / D ratio up to a maximum of l / D = 85 can be produced, the standard is an l / D ratio of l / D = 30. Asymmetrical tools can be used to produce bores in the diameter range from D = 0.5 ... 2000 mm and the upper limit of the l / D ratio is usually limited by the machine dimensions. The figure shows selected deep drilling processes with their usual application diameters, whereby it becomes clear that deep drilling processes do not compete with one another in all diameter ranges. The advantage of the symmetrically structured tools compared to the “classic” deep drilling tools in the small diameter range is the realizability of significantly higher feed rates f, which can be 6 times as high as the usual values ​​for single-lip deep drilling.

Deep drilling processes with their usual application diameters

In addition to the high l / D ratio, the “classic” deep drilling processes are characterized by high productivity and high surface quality compared to the drilling process with HSS twist drills . The high hole quality is characterized by a high surface quality, a small diameter deviation and a geometric shape accuracy . The asymmetrical structure of these deep drilling tools is important for the good surface quality. With a few exceptions, the tools used in the "classic" deep drilling process, ELB drilling, BTA drilling and ejector drilling are single-edged and have a secondary cutting edge (round bevel) and guide strips. Due to this structural condition, forces are transferred to the bore wall and thus to the workpiece wall via the guide strips during the process . For certain force components on the tool head, there is a "closed" frictional connection that supports the tool in the hole just created and thus guides it in it. The force curve during deep drilling is therefore different from conventional drilling, in which the forces are largely absorbed by the tool shank and thus by the machine spindle. Due to the different nature of the force profile during deep drilling, the drill itself guides itself in the hole and thus has a comparatively small center profile, and on the other hand, due to the "support" between the hole wall and the guide rails, a deformation process takes place that the hole wall ( ideally) smooths. The surface roughness caused by the machining decreases by about 70% as a result of the forming process under the guide rails and leads to a high contact ratio. Thus, through deep drilling, a very high surface quality can be achieved with a hole tolerance of IT 9 to IT 7. Post-processing steps can often be reduced or eliminated entirely. Another advantage is the minimal formation of burrs when drilling out and when overdrilling cross bores. Due to the high surface quality combined with a high cutting performance, the use of conventional deep drilling methods can also be economical with shallow drilling depths.

The deep drilling method

Single-lip deep drilling

Principle of single-lip deep drilling

Single-lip deep drilling is usually used to produce bores in the diameter range from D = 0.5 ... 40 mm. This area of ​​application is currently limited by the technical implementation of the cooling lubricant channels as well as the increasing challenges in the grinding process with decreasing tool diameters. The upper limit results from the more economical use of alternative deep drilling methods. The internal cooling lubricant supply through one kidney-shaped or two circular cooling channels is characteristic of single-lip deep drilling. The chip / coolant mixture is discharged in a V-shaped longitudinal groove on the tool, in the so-called bead. The cooling lubricant mass flow is the only transport mechanism for removing the chips. For this reason, a diameter-dependent high-pressure coolant supply is necessary. The general structure of single-lip tools is divided into three parts: drill head, shank and clamping sleeve. The drill head is usually joined to the shaft by brazing. The clamping sleeve is the clamping element of the tool and forms the interface to the tool holder and thus to the machine tool. Solid carbide tools are often used for smaller tool diameters and high-performance tools. In these more powerful tools, the drill head and the shaft are made from a hard metal rod. The drill head is usually made of hard metals from ISO machining application groups K 10 to K 20 and is coated if necessary. In special applications, PCD, cermets, ceramics or high-speed steels are also used. The choice of the drill head geometry depends on the machining situation. In this regard, a distinction is made between different cutting edges and the circumferential shape of the guide strips. With the usual standard grinds for single-lip drills, the main cutting edge is divided into an outer and an inner cutting edge, which differ depending on the bore diameter through different setting angles. The choice of the shape of the circumference also comes into play. H. the number and arrangement of the guide strips on the circumference of the single-lip drill, an important meaning. Compared to conventional drilling with twist drills, single-lip drilling is characterized by its suitability and high process reliability with large length-to-diameter ratios. In addition, single-lip drilling achieves a comparatively high hole quality, which can save post-processing.

Tools

Single-lip drill with interchangeable cutting edges and guide elements from TBT
Single-lip deep drill made of solid carbide (VHM) from the company TBT

As can be seen in the pictures, a single-lip drill consists of a tool holder, a shank and a piece (usually hard metal), which forms the drill head. In terms of structure, it can generally be said that the shaft is kept a few 1/10 mm to 1 mm smaller than the drill head. It can also be seen that 1/4 of the shank surface is free, in which the chips are flushed out of the bore by the coolant flow. The cutting head itself has beveled guide surfaces in which the drill guides itself and thus, unlike a twist drill, follows the guide axis of the machine.

The actual cutting edge is the top tip to the middle of the drill. This means that the drill scrapes its way through the material to be drilled on one side. The chips produced on the cutting edge are flushed with coolant from the center and from the outside by at least one, from about 10 mm drill diameter, sometimes two or more channels and at the same time flushed away from the machining point via the free space in the shank.

BTA deep drilling

Principle of the BTA deep drilling process

The disadvantages of single-lip deep drilling, such as the chips coming into contact with the bore surface or the low torsional moment, were the motivation to develop a modified deep drilling method in which these problems are circumvented and the good properties are retained. A new deep drilling method, which was called the BTA method in the early 1950s, was developed for the above-mentioned occasion. BTA stands for "Boring and Trepanning Association" which was dominated by the now liquidated Bremen company Gebrüder Heller . Under her leadership, the new process was created during the Second World War by merging its own developments with those of Burgsmüller and Beisner. Burgsmüller replaced the grooved drill pipe that had been used until then with a pipe with a closed cross-section, which was more torsion-resistant and for the first time transported the chips through the inside of the pipe. Burgsmüller used a double-edged tool and an air-oil mixture, which is used nowadays in production with minimum quantity lubrication. Beisner improved the tool design and introduced oil as a cooling lubricant. Heller, which was the first company to introduce carbide-tipped ELB tools, had the patent for the cutting edge guide strip constellation which was then also used for the BTA tools.

During the machining process, with the help of the drilling oil supply device (BOZA), the cooling lubricant is fed to the working point through the annular gap between the bore and the drill pipe, as can be seen in the figure. The BOZA also takes care of the sealing between the workpiece and the drill pipe. For this purpose, this has a conical rotating workpiece holder that is directed towards the workpiece and is pressed against the workpiece with high pressure. The workpiece is centered and a sealing surface contact is created. In most cases, the back of the BOZA is sealed by a stuffing box, which also takes on the function of guiding the drill pipe. The tapping bush is usually integrated in the BOZA, which means that working with a pilot hole in the BTA process is rarely carried out.

Tools

BTA deep drilling tool for full drilling with a cutting edge from botek
BTA deep drilling tool for full drilling with split cutting edge from BTA deep drilling systems
BTA deep drilling tool for full drilling with split cutting edge from botek

The chips are removed through the openings integrated in the tool with the aid of the oil flow. This is why the openings are referred to as "clamping jaws". In this way, the chips can be removed without contact with the wall of the bore. Due to the circular cross-section of the tool and drill pipe, the process has a greater torque resistance than ELB drilling, which means that a significantly higher cutting performance can be achieved. The BTA process is used for bore diameters of D = 6… 2000 mm. For industrial processes it is used in a range from approx. D = 16 mm. It is possible to manufacture BTA drills with a diameter of D ≤ 6 mm, but there is still no known application.

Deep ejector drilling

Process principle for ejector deep drilling
Ejector drill head with soldered-on cutting edges and guide elements from Sandvik
Ejector drill head with interchangeable cutting edges and guide elements from Sandvik

The ejector method is used in a diameter range of approx. D = 18 ... 250 mm. It is a variant of the BTA process, in which the drill heads used are structurally comparable to the BTA tools. The only difference is in the additionally introduced at the periphery KSS outlet openings. The cooling lubricant is supplied through an annular space between the drill pipe and the inner pipe, which is what gives the process the name two-pipe process. The cooling lubricant emerges from the side of the cooling lubricant outlet openings already mentioned, washes around the drill head and flows back into the inner tube with the chips produced. Part of the cooling lubricant is fed directly into the inner tube via a ring nozzle. This creates a negative pressure (ejector effect) at the chip opening, which facilitates the return flow in the inner tube. The system can be operated via an external high pressure pump or the internal cooling lubricant supply. Since, in contrast to the BTA process, no sealing against escaping coolant is necessary, the ejector process can also be used on conventional lathes and machining centers. Since the pipe cross-section through which the chips are to be removed is reduced by the double pipe system, the cutting performance is lower than with the BTA process. Because of this, lower cutting speeds are usually selected for ejector deep drilling. In addition, the lower rigidity goes hand in hand with impaired concentricity properties (IT9 to IT11).

The prerequisite for the implementation of the process is the use of a connection piece that is inserted into the turret mount of the lathe or the spindle of the machining center. Through this connection piece, the cooling lubricant is fed from the connected pump unit into the annular gap between the inner and outer pipes. Two different versions are possible to enable this function. A rotating connector is required for machining centers and a non-rotating connector for lathes. The associated space requirement must be taken into account when selecting the processing machine.

Tools

The structure of the tools for ejector deep drilling is almost identical to that of the BTA deep drilling tools. The additional coolant outlet openings are shown in the figures.

Processes associated with deep drilling

In addition to the classic deep drilling methods, there are a number of other methods for finishing deep drilling. These can be reworked with regard to their surface properties or serve as a basis for the machining of complex and non-cylindrical contours.

Internal profiling

For various reasons there are components with deep bores, the inner contours of which are rotationally symmetrical, but not uniformly cylindrical. Such components can be contours without undercuts, such as centrifugal casting molds or conical bores in extruder cylinders, as well as with undercuts, such as propeller shafts or undercarriages. High quality pre-drilling is required in order to produce such chamber pockets. If the radially extendable cutting tool is controlled via an NC axis and connected to the NC drilling slide of the deep drilling machine, it is almost possible to produce any drilling wall contour in one cut over the entire length of the contour. The position of the cutting edge can be modified by an axial displacement, e.g. B. by using an inner thrust tube. The guide pads can also be adjusted hydraulically. Since the guide hole has already been completely rotated after the first cutting step using the so-called long chamber method, the guide pads must also be radially adjustable in order to support larger chambers. As an alternative to this method, the so-called short chamber method does not require any retractable guide pads, since the tool is only located in the pre-drilled guide hole.

Peeling and roller burnishing

The peeling improves the roundness and the dimensional accuracy of the drilling diameter. In the vicinity of the underground zone, an open surface profile is created, which is particularly suitable for subsequent machining processes such as roller burnishing or honing . In the field of machining hydraulic cylinders and cylinder liners, peeling and roller burnishing are regarded as a related manufacturing process, although it has a purely cutting and also shaping component. The reason for this is the wide use of combined peeling and roller burnishing tools.

Single-edged rubbing

Another machining process to increase the surface quality and dimensional accuracy of a hole is the use of single-edged reamers . Reaming is the countersinking of a pre-drilled hole, whereby the tool is supported by the guide pads. Therefore, the tool geometry of these reamers is very similar to single-lip drills. The difference to single-lip drilling with a shallow depth of cut is the mostly missing peripheral bevel, a long side cutting edge parallel to the milling axis and the low coolant quantities and pressures.

Deep drilling machines

Designs of deep drilling machines (examples with specification of feed and cutting movement)
Machine tools on which deep drilling is carried out (some examples with specification of feed and cutting movement)

For machining with deep drilling processes or processes associated with deep drilling, deep drilling machines are mainly used as standard (multi-purpose) or special machines. Single-lip drills are often used on machining centers when drilling holes with shallower drilling depths (up to approx. 40 × D). Ejector drilling is mainly used on conventional machine tools. Since deep drilling has a high productivity, only comparatively powerful machines are used. In principle, a cooling lubricant system is required that provides the cooling lubricant with (compared to other drilling methods) an above-average volume flow at higher pressures. A deep drilling system is understood to be the grouping consisting of the deep drilling machine and the cooling lubricant tank with other peripheral devices for cooling lubricant processing and chip handling. The ejector drilling process was developed as a deep drilling technology for use on conventional machine tools. The use of single-lip deep drilling is particularly common on machining centers in series production. On the right you can see schematic drawings of common deep drilling machines.

literature

  • VDI 3208: Deep drilling with single-lip drills
  • VDI 3209: Deep drilling with external supply of cooling lubricant (BTA and similar processes)
  • VDI 3209: Part 2 deep drilling; Guide values ​​for peeling and roller burnishing bores
  • VDI 3210: Part 1 deep drilling method
  • VDI 3211: Deep drilling on machining centers
  • VDI 3212: Acceptance conditions for single-spindle and multi-spindle deep hole drilling machines

Individual evidence

  1. ↑ a b c d e f g h i j k VDI guideline 3210: Guide values ​​for deep drilling with single-lip drills . Berlin, Beuth-Verlag, 1996.
  2. ↑ U. Heisel, R. Eisseler: Hybrid machining in single-lip deep drilling . Influence of the chip length through the coupling of vibrations . VDI reports 1987, Düsseldorf 2006.
  3. ↑ J. Steppan: Reduction of the center line of bores with an L / D ratio greater than 500 using an alternative manufacturing process . VDI reports 2142, Dortmund 2011.
  4. ↑ a b D. Biermann, F. Bleicher, U. Heisel, F. Klocke, H.-C. Möhring, A. Shih: Deep hole drilling . In: CIRP Annals . Volume 67, Issue 2, 2018, pp. 673-694 .
  5. ↑ a b D. Thamke: Possibilities and limits of dry machining, technical discussion between industry and university “Drilling and milling in the modern production process” . Dortmund 1997.
  6. ↑ P. Müller: High-performance twist drill for deep drilling . VDI reports 1897, Dortmund 2006.
  7. ^ T. Upmeier: Innovative process design for deep drilling . VDI reports 2142, Dortmund 2011.
  8. ↑ VDI guideline 3209: Deep drilling with external supply of cooling lubricant (BTA and similar processes) . Beuth-Verlag, Berlin 1999.
  9. ↑ U. Weber: Contribution to the metrological recording of the deep drilling process . Print Gräbner, Altendorf 1978.
  10. ↑ a b O. Webber: Investigations on the depth-dependent process dynamics in BTA deep drilling . Vulkan Verlag, Essen 2006.
  11. ↑ a b c d H. Fuß: www.tiefbohren.info, as of April 1, 2014.
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  25. ↑ U. Heisel, T. Stehle, R. Eisseler, P. Jakob: More productive in depth - higher process stability thanks to damping as well as longer tool life in extremely hard steels . In: workshop and operation . No. 12 , 2013, p. 68-71 .
  26. ↑ T. Ishida, S. Kogure, Y. Miyake, Y. Takeuchi: Creation of long curved hole by means of electrical discharge machining using an in-pipe movable mechanism . In: Journal of Materials Processing Technology . tape 149 , no. 1-3 , 2004, pp. 157-164 .
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  28. ↑ B. Greuner: The production of hydraulic cylinders according to the BTA process . In: Machine World . tape 4 , 1962.
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  30. ^ F. Pflegehar: Improvement of the drilling quality when working with single-lip deep drilling tools . Dissertation. University of Stuttgart, 1976.