Hand-held, dry core drilling in masonry using polycrystalline diamond (PCD) cutters

Transcription

1 Hand-held, dry core drilling in masonry using polycrystalline diamond (PCD) cutters S. G. Moseley, L. F. Garcia, O. Griessinger and K-P. Bohn Hilti Corporation, P.O. Box 333, Feldkircherstrasse 1, LI-9494 Schaan, Liechtenstein Keywords Polycrystalline diamond; PCD; core drilling; masonry; performance modelling; indentation modelling Abstract This paper first describes the applications and base materials being drilled in the building construction, mechanical- and electrical installer trades in the European construction industry and highlights a selection of technological developments and the performance improvement made possible in the last 15 years by the use of polycrystalline diamond-tipped core bits. The second part addresses some important aspects from the integrated system development of the latest Hilti DD11-M tool and PCM New bits developed specifically for the trades noted. The advantages of such a system development for diamond impregnated segment bits and tools have been published elsewhere [1] and this paper quantifies the productivity and cost savings achieved, as measured by holes per day (influenced by speed and vibrations) and drilling costs per hole (influenced by bit-life and costs), compared to the previous tool plus consumable system. Model predictions from a semi-empirical physical drilling model based on indentation theory are compared with actual drilling test results and the production technology used in the manufacture of the PCM New bits is also briefly described 1. Introduction A history of the use of PCD for hand-held core drilling, The early 199s marked the appearance of the first PCD-tipped core bits and socket-cutting bits for hand-held, dry core drilling in masonry materials, used primarily by mechanical and electrical installers. It was a niche market in a few selected European countries, with only Hilti and one other company providing the expensive and relatively fragile bits. These bits can be seen in Figure 1. They were developed to address the introduction of increasingly difficult-to-drill non-concrete construction materials, a sample of which is shown in Figure 2. Some application examples are shown in Figure 3, and some of the drilling tools used in these applications are shown in Figure 4. 5mm 4mm

2 Figure 1 (above). The first commercially available PCD-tipped core bits for hand-held drilling in masonry (ca ). Left: German Manufacturer, right: Hilti AG, Liechtenstein.!" Figure 3 (right). A sample of applications for hand-held core drilling using the new Hilti DD 11-M diamond drill. (a) and (b) Socket cutting for the installation of electrical outlets and switches. Figure 2 (left). A sample of masonry materials used in residential and commercial buildings in Germany. Around 75% of electrical sockets are drilled in conventional fired clay bricks and limestone bricks, with the remainder being in concrete, cement board and plasterboard. The limestone bricks are manufactured by cold pressing followed by pressure steam hardening [2]. Compressive strengths of 25-35MPa are usual (up to 75MPa) with a density between 1. and 2.2 g/cm3, which is up to 9% that of a conventional concrete. a b c d (c) Through holes for cables, pipes and ventilation (d) Stitch drilling for cable trays and pipe ducts a b Figure 4 (left). A sample of Hilti tools used for socket cutting. (a) and (b) are the previous generation of rotary diamond drilling tools (Hilti DD1 and DD13) c d (c) and (d) are rotary percussive tools (Hilti TE 56 and TE 7) which can operate in multiple modes, both with and without percussion or rotation

3 By the year 2, masonry construction materials had developed so far that conventional core drill bits were struggling to perform to the levels experienced in previously encountered materials, so Hilti worked closely with one of the German trade associations for brick materials to develop a better solution, leading to the introduction of the PCM bits in 23. This need for the development of alternatives to the conventional diamond-impregnated segment dry coring bits and socket cutters is categorically shown in Figure 5, from tests made in 22. Amberg 2. ("hard") Drilling speed (cm/min) PCM-old, pre-series (>24.5m : 2.9cm/min) Comp. E (22.5m : 11.3 cm/min : 1# resharpened) Hilti P4 (18.8m : 1.3 cm/min : 12# resharpened) Comp. G (16.2m : 1.9 cm/min : 8# resharpened) Hilti HDM2 -old (7.7m : 13. cm/min : 2# resharpened) Hilti HDM1-old (ca. 18m : 8.4 cm/min : resharpen every 1.5m) Comp. H (24.4m : 9.5cm/min : 14# resharpened) Comp. C (11m : 9.4cm/min : #14 resharpen) Comp. D (13m : 8.8cm/min : #22 resharpen) Comp. W (23m : 9.7cm/min : #17 resharpen) Comp. B (12.8m : 6.7cm/min : 24# resharpen) PCM-old, series (>3m : 25.3cm/min) Drilling depth (cm) Figure 5. Hand-held drilling tests of both Hilti and competitor socket cutting bits, using a Hilti DD 13 tool and VCD 5 industrial vacuum cleaner (for non-slotted bits). Tool parameters were 11 min -1 RPM at 15 Nm torque for impregnated bits and 65 min -1 RPM at 24 Nm for PCDtipped bits. Pressing force ranged from 2-25N. Tests with the impregnated bits were made to between 4 and 5cm drilled depth. The results were extrapolated to an estimated end-of-life (based on full segment height wear) and the speed-depth test data duplicated in the chart to the end-of-life depth. The brick material used was the 2. density material shown in Figure 2. Both speed and lifetime of the PC M bits are higher than with the impregnated bits. Following the core bit tests, a number of tools were also tested using the (then) newly developed PC M core bits (Figure 6). These used a tough, high metal content, finemedium grain size PCD normally used in percussive rock drilling. The PCD cutters were brazed onto an investment cast holder using a Cu-Ag braze. A sample of the test results obtained is shown in Figures 7 and 8. Figure 6 (right). The Hilti PC M bits introduced 23, including the starting aid. (Herein referred to as PCM-old ).

4 speed cm/min PCM-old performance in various limestone bricks KSVb soft (>39m : 29.6cm/min) KS 2. hard (>24.5m : 2.9cm/min) KS 2.2extreme (>2m : 11.4cm/min) cm drilled Figure 7 (above). Hand-held drilling tests with the PCM-old socket cutting bits in three different limestone brick materials, ranging in density from 1.4 to 2.2 g/cm3 and with various aggregates, from <15% basalt of 2mm ( soft ) to >15% gravel of 8-15mm size ( extreme ). Tests were made using a Hilti DD 13 tool (65 min -1 RPM at 24 Nm) and VCD 5 industrial vacuum cleaner. The blue and pink rectangles on the X and Y axes indicate the speed and lifetime achieved using impregnated bits. They were unable to drill in the KS 2.2 material, and it is clear that this KS 2.2 material is extremely difficult to drill even with PCM-old bits. Speed (cm/min) PCM-old, 82mm, KS 2. medium, various tools Hilti DD 13 Hilti TE 56 Comp. B Comp. W Comp. C cm drilled Figure 8 (above). Hand-held drilling tests with the PCM-old socket cutting bits in five different tools (four of which are dedicated hand-held diamond drilling tools, and one of which the Hilti TE 56 is a combi-hammer with a setting where the percussion mechanism can be switched off). It is clear that the tool parameters (torque and/or RPM) of most of the tools, with the exception of the DD 13, are not fully suitable for frequent drilling with PCD-tipped bits. Data curves have been smoothed for ease of differentiation between the five tools.

5 For a number of years, the PCM-old bits, in combination with the Hilti DD 13, were the performance benchmark in the applications noted previously. The technology gained a foothold in the market with a loyal customer base, but it only achieved a relatively small market share and widespread substitution of impregnated bits with PCD bits did not take place. There are a number of reasons for this, but to simplify the situation it can be put down to three basic reasons: I. Although the overall cost per hole was lower with PCM-old than conventional impregnated bits in most difficult-to-drill masonry materials, the high initial cost of the bits was a barrier to many potential customers. II. In order to get the best performance (speed and life) out of the bits a suitable tool is needed, as clearly shown in Figure 8. Many customers simply did not have such a tool and therefore failed to achieve the cost savings possible. III. As a direct consequence of point II, the use of the bits in tools with extremely high RPM (ca min -1 or higher) sometimes caused the Cu-Ag braze joint to overheat in certain brick materials resulting in the loss of the cutters and the failure of the bit. Although the incidence of de-brazing was low (single digit percentage failure levels, many of which were for various other reasons), for many customers the risk was not worth taking so they remained with their known, impregnated segment bits and the market for PCM-old remained a niche. 2. Market changes The period 24 to 27 Despite the relatively low uptake of the PCD-tipped PCM-old bits, in the years following the market introduction an increasing number (around 6 at the last count) of other manufacturers also started to offer PCD-tipped bits. This was a sure sign that the technology was gaining greater customer acceptance, and it also coincided with a general upturn in the construction market in Europe, and particularly in Germany which is the main market for these products. Some examples of these PCD-tipped socket cutting bits can be seen in Figure 9. Although the designs of the bits may at first seem varied, in most cases the general design principle is the same: a precision machined steel tube carrier with 3 or 4 small (3.5 to 5mm wide) PCD cutters joined to it. In all but one case the cutters are brazed. One uses laser welding to fasten the patented steelbacked PCD cutters [3] to the steel carrier. 4mm cm 4mm a b c 4mm Figure 9 (above). Three examples of PCD-tipped 82mm diameter socket cutting bits from German manufacturers. During this period, and within a year of the introduction of the PCM-old bits, Hilti began to develop a completely new hand-held coring system, comprising tool and consumables. Whereas

6 the PCM-old bit was designed to work optimally with an existing tool (DD 13), this development would be a true system development where tool and bit would be designed together to achieve even better performance and reliability. Through the use of technology platforms, the basic drive could also be adapted to create a family of tools (wet, dry, hand-held and rig-based) for a range of consumables (wet and dry impregnated plus PCD-tipped). This project, with its many facets, is what led to the Hilti DD 11 and DD 12 tool family and respective core bits and socket cutting bits. The tools, based on a nominal 1.6kW 2-gear motor, can be seen in Figure 9. Figure 9 (above). The Hilti DD 12 rig-based wet coring system (left) and the hand-held dry drilling system Hilti DD 11-D (right). Each comes with a range of accessories, chucks and other features and benefits, such as Hilti s TPS (Theft Protection System) and Lifetime Service including 2 years no costs. The tools weigh only 1kg and 5kg (DD 12 with rig, and DD 11-M, respectively). 3. Development methodology 3.1 From concept to product definition (focus on DD 11-M / PCM New ) Extensive knowledge was gained during the PCM-old project and also following the product launch. This included performance benchmarking and characterisation of Hilti and competitor tools and core bits (such as seen previously); a detailed marketing analysis of customer needs and construction material trends; and a full evaluation of the product portfolio offering at the time. Product positioning of the new tools and consumables; backwards compatibility with existing products; certain compatibility with competitor tools, the variety of applications and operating modes, etc, were also all taken into consideration. This information was a crucial input into the technology research and development, which started with the definition of the boundary

7 conditions, such as the nominal power of the two tools (also known as drives or simply motors within the construction industry); power delivered to the spindle; maximum rotational frequency (RPM); maximum deliverable torque; requirements of the consumables, etc. The development of a common platform for the DD 11-M (hand-held, dry, masonry) and DD 12 (rig-based, wet, concrete) tools was an essential requirement of the project with both tools using the same motor and gearbox (i.e. have the same RPM and Torque values in the two gears). Existing wet consumables were the basis for the DD 12, although a completely new PCM range was to be developed and necessary modifications to the specifications of the dry impregnated segment (HDM) core bits for the DD 11-M were foreseen. The applications, bit types and bit diameters are very different in the two tools, so the major challenge was to achieve a suitable compromise of drilling parameters. 3.2 Drilling model Starting with just the available power at the spindle and the limitation of only having two gears, the first stage of the development was performance modelling. The semi-empirical physical drilling model based on indentation theory used has been described elsewhere [1]. The model was modified to be used in all three drilling cases: wet, rig-based in reinforced concrete with impregnated bits; dry, hand-held in masonry with impregnated bits; and dry, hand-held in masonry with PCD-tipped bits. The specific model for PCD-tipped core bits is covered in further detail elsewhere [4]. Sample calculations can be seen in Figures 1 and 11. Speed in reinforced concrete, [cm/min] DD 12 parameter modelling Diameter [mm] 1 gear, 75/15 (42-122mm) 1 gear, 6/18 (42-132mm) 2 gears, 9/12 (42-12mm) & 45/24 ( ) 2 gears, 1/11 (42-82mm) & 5/22 (87-162mm) DD13-11/15 (to 52mm) & 66/24 (>52mm) Competitor systems Figure 1 (above). Performance predictions for wet, rig-based drilling in reinforced concrete

8 speed, cm/min Speed vs Diameter, PCM-old & HDM2-old KS 2. medium, DD 13 All impregs, actual, DD13 2nd gear Predicted speed HDM2 PCM-old, DD13 1st gear Predicted speed PCM-old bit diameter, mm Figure 11 (above). Performance predictions for dry, hand-held drilling in masonry compared to actual test results. An accuracy of better than ± 1% was demonstrated with PCM-old in the DD 13. With the impregnated bits, predictions were comparable to the best test results but not to all tests, indicating that the ideal sharpness and steady state conditions assumed in the model are not always achieved in reality. When the reliability of the model was verified, further calculations were performed in parallel, contributing to designing both the core bits and the tool parameters together. A small number of possible gearbox layouts providing specific torque and RPM values were defined and compared virtually. An example of the calculations made for the bits is shown in Figures 12 and 13. Then, by designing the shape of the torque-rpm curve, taking into consideration the pressing forces and pseudo-friction coefficients in each application, it was possible to define the two gear settings that would provide excellent drilling performance in all applications. It must be noted, however, that other factors such as the slip clutch and motor characteristics, are just as important as the gear settings. Upon deciding on a specific set of parameters, testing in prototype tools began. 4 4 cutter width, mm Lifetime cutter w idth, mm Speed # cutters Decreasing lifetime 115%-12% 11%-115% 15%-11% 1%-15% 95%-1% 9%-95% # cutters Increasing speed 11%-12% 1%-11% 9%-1% 8%-9% 7%-8% 6%-7% 5%-6%

9 Figure 12 (above). Example outputs from the drilling model, indicating the influence of the number and size of the PCD cutters (of a specific geometry) on lifetime and speed for a given torque-rpm parameter set and bit diameter. The 1% reference is the Hilti DD 13 with PCMold. Similar outputs were calculated for fracture probability, vibration levels, allowed drilling time per day and costs. Holes per day versus core bit costs (based on speed and vibration levels) (based on lifetime and cutter costs) holes per day (speed x mins/day) 24% 22% 5 cutters 1% = DD 13 with PCM-old 2% 18% 4 cutters 16% 14% 12% 1% 3 cutters 8% 6% 6% 7% 8% 9% 1% 11% 12% metres per unit cost (life/cost) Figure 13 (above). Collection of all model outputs into a single diagram. For example, the 3- cutter PCM New plus DD 11-M system should provide over 1% cost savings with almost 2% more holes allowable per day than the DD 13 with PCM-old. 3.3 Production technology of the PCM New bits There are four major differences between the old and new versions of the PCM bits, which are indicated in Figures 14 to 16, respectively. All contribute to an improved overall drilling performance of the new bit, as measured by speed, lifetime, reliability and vibration levels: I. The grade of PCD II. The design of the cutters III. The geometry and method of manufacturing of the steel carriers IV. The joining method!#$ HILTI PCM-new HILTI PCM-old &''% (')*#$ KS 2. extreme % 2µm PCM New Coarse, low Co PCM-old Fine, high Co

10 Figure 14 (above). A significantly more wear resistant grade of PCD is used in the PCMNew bits. It provides a much higher lifetime in abrasive materials than the previously used grade, and has also been proven to be suitable for dry core drilling in reinforced concrete [5]. Probability (%) X5=75 X5 = 135 Min.=3 Min.=8 Figure 15 (left). A simple change to the cutter geometry results in the elimination of early failures and a near doubling of the average number of holes to end-oflife. This is the result of a combination of factors, such as increased drilling speed, better removal of drilling detritus and reduced heat generation. No. of holes in KS 2. extreme (pre-series) Figure 16 (right). The new steel holder is machined and has less surface area in contact with the hole wall, which reduces frictional heat generation near the cutter. The use of intermediate stabilising inserts reduces vibrations and provides for truer drilling. Hardmetal substrate PCM-old PCMNew PCD layer Joining zone Carrier Figure 17 (above). Photographs of the resistance welded PCD cutters on the PCMNew bit. The direct joining of hardmetal to steel by resistance welding has been a standard feature of Hilti rotary-percussive drill bits (diameters 5 to 17mm) since the mid 199 s. Now, this technology has been extended to PCD. Process parameters had to be modified due to the fact that the cutters cannot be heat treated following welding. Static and dynamic strength is equal to or greater than that of a brazed joint and the joint is less susceptible to weakening by overheating. End-of-life is

11 primarily reached by abrasive wear flatting (sometimes combined with micro-chipping at the cutting edge), although complete loss of a cutter caused by failure in, or near, the weld cannot be ruled out in some of the extreme materials that these bits sometimes drill or by misuse. 3.4 System performance Before entering the final development phase of the project, binding speed and lifetime targets for the PCM New bits in the DD 11-M were defined. Outputs from the indentation drilling model were combined with energetic modelling and results from prototype testing to set realistic performance targets together with the marketing department, also taking customer needs and competitor benchmarking into consideration. Results following the final minor adjustments to the tool parameters and PCD core bit design are shown in Figure 18. Drilling Speed [cm/min] KS 2. medium PCM+TE56-ATC PCM+TE56-ATC DD11 (model) DD13 (model) Target, rotary tools Drilling Speed [cm/min] HDMU+DD Tools Full Brick DD11 model DD13 model Target, rotary tools 1 HDMU+DD Socket cutter / core bit diameter [mm] Socket cutter / core bit diameter [mm] Figure 18 (above). Results of pre-series production bits tested in two brick materials. Measured results with PCM New (symbols) are compared with model predictions (lines). Tests were made with the new Hilti DD 11-M, the DD 13 and TE 56 in rotary drilling mode. For reference, some results with HDMU diamond impregnated segment bits are also presented. Despite the DD 11-M having less power than the DD 13 (nominal power ratings of 16W and 19W, respectively) the performance in all base materials are similar. In some materials the DD 13 performs slightly better while in others the DD 11-M is faster. For prolonged drilling, the lower weight of the DD 11-M (less than 5kg versus almost 8kg for the DD 13), coupled with an integrated hole starting aid, is of significant benefit to the customer. The overall result of this system development is around 15% lower costs per hole than any other system in hand-held, dry drilling in masonry materials with around 2% more holes allowable per day. The PCM New bit, plus some of the marketing logos, is shown in detail in Figure 19.

12 PCD cutting edge (3 pieces) Wear mark (3 pieces) Resistance welded High strength tube Laser-welded joint Steel carrier (3 pieces) Intermediate stabilising parts (3 pieces) Figure 19 (above). The benefits of an integrated system development are marketed as The Hilti Power Effect, and the PCM New PCD-tipped core bits and socket cutters are clearly labelled with the PCD logo. 4. Summary In spring 28, around 15 years after the first such product, the 3 rd generation of Hilti PCD bits was launched, together with a dedicated drilling tool. The acceptance of PCD in the construction industry has been a long time coming, but major new developments in PCD materials and joining techniques, assisted by newer power tools delivering more suitable drive parameters (torque and rotational frequency, RPM) and more manufacturers seeing the potential of the technology, will surely result in the more widespread use of PCD in the future. References [1] Moseley, S.G. and Akyüz, D.A., System Development of Tools and Core Bits for Improved Drilling Performance in Reinforced Concrete, 2 nd International Industrial Diamond Conference Diamond at Work 2, 19-2 April 27, Rome, Italy [2] [3] US patent US 27/34416 A1 [4] Moseley, S.G., Bohn, K-P. and Goedickemeier, M., Core Drilling in Reinforced Concrete Using Polycrystalline Diamond (PCD) Cutters : Performance Modelling, 4 th International Conference on Diamond, Cubic Boron Nitride and their Applications Intertech 28, 5-7 May 28, Orlando, USA [5] Moseley, S.G., Bohn, K-P. and Goedickemeier, M., Core Drilling in Reinforced Concrete Using Polycrystalline Diamond (PCD) Cutters : Wear and Fracture Mechanisms, 9 th International Conference on the Science of Hard Materials ICSHM9, 1-14 March 28, Montego Bay, Jamaica

13 Author Details and Acknowledgements S. G. Moseley (presenting author) has worked in the area of research and development of cutting materials at Hilti in Liechtenstein since 1996, covering cemented tungsten carbide hardmetals, diamond impregnated segments and polycrystalline diamond. His current position is Chief Scientist within the business unit Drilling and Demolition. Prior to this he worked in the steel and hardmetal industries in the UK. L. F. Garcia is a Technical Project Leader in the Business Unit Diamond. He was responsible for the technical aspects of the PCM New project. O. Griessinger is currently Product Manager for diamond drilling consumables in the Business Unit Diamond in Liechtenstein. He was formerly the Project Manager (in the same Business Unit) of a number of core bit development projects, including the PCM-old bits highlighted in this paper. K.-P. Bohn has recently moved into a senior technical position within the Business Unit Drilling and Demolition. He worked for a number of years as Technical Project Manager in the area of research of diamond consumables, covering both diamond impregnated segments and polycrystalline diamond, before assuming his previous position as Head of Projects in Corporate Research and Technology. Thanks are extended to all our colleagues, past and present, from Corporate Research and the Diamond Business Unit (Hilti Corporation, Liechtenstein) and the various Development Departments (Hilti Entwicklungsgesellschaft mbh, Germany) who were involved in the various HDC ( hand-held, dry coring ) projects. The material used in this paper has been compiled from internal data generated during these research and development activities, hence the different layouts and formats of certain figures and tables.