System Development of Tools and Core Bits for ...

System Development of Tools and Core Bits for Improved Drilling Performance in Reinforced Concrete S. G. Moseley and D. A. Akyüz Hilti Corporation, P.O. Box 333, Feldkircherstrasse 100, LI-9494 Schaan, Liechtenstein

The paper describes the concept and parameter design phases in the development of two new 1000 Hz high frequency, asynchronous brushless motor, electronically geared diamond core drilling tools together with their specific core bit lines. The development process from two simple numbers – the power output ratings, 3kW and 4.5kW – to the final coring systems with 10 electronic gears and tailored core bit specifications is described. The integrated development of drive parameters and diamond-impregnated segment specifications to maximise drilling performance (speed and lifetime) was facilitated by a semi-empirical physical drilling model based on indentation theory. The model allowed parameter combinations (Torque, Rotation Frequency RPM, Diamond Size and Concentration) to be “virtually tested” before going to actual hardware tests. By modelling these drilling parameters, predictions of drilling speed to within +/- 10% accuracy in reinforced concrete are possible. An indication of segment wear rate is also provided by the model, but with greater uncertainty. Drilling performance predictions from modelling are compared with actual drilling test results during the product development phase, and the performance is compared to tools of similar power ratings with various core bit specifications.

1. Introduction In 2004 Hilti’s diamond drilling product line was rejuvenated with three new drilling tools, covering the power range from 2.5kW to 4.5kW and utilising a single, common drill stand. They incorporate Hilti’s Theft Protection System and benefit from Lifetime Service [1]. One tool, the DD 200, is based on a conventional 50-60Hz / 100-230V single phase 2.6kW universal motor with carbon brushes and three mechanical gears supported by electronic power regulation with LED indicators. The power regulation ensures that the user drills at the optimum pressing force and torque for the core bit, thereby maximising drilling performance. Even a first-time user can achieve a drilling speed within 10% that of an experienced operator by simply observing the LED indicators. The DD 200 replaced two older models, combining the light weight of the DD160 and the performance of the DD250, and provides optimum performance in the core bit diameter range 50-250mm. A second generation model of this tool has recently been launched with more torque and refined power regulation providing even better power delivery to enable enhanced drilling performance for core bit diameters up to 400mm. The respective “H-series” diamond impregnated segments were modified to increase lifetime without compromising the high drilling speed of the previous “P-series” segments. The other two tools, the DD 300 and DD 500, are based on fully encapsulated, water cooled 1000Hz high frequency, brushless asynchronous motors. Both share a common housing and have IP55-class water protection rating. The DD 300 is a single phase tool, running on 110-240V. The DD 500 is a three-phase tool, running on 380-415V. These high frequency motors provide a higher power-to-weight ratio than common universal tools and allow the implementation of many new innovative features that contribute to raising the productivity and competitiveness of coring for the main target customers, namely diamond service contractors, who specialize in providing sub-contract diamond cutting services. Features include: • E-Gears and specific segment specifications: Gear selection is not mechanically coupled to a gearbox, but instead controlled by the on-board electronics. This allows finer resolution (10 gears instead of 3 or 4) and better matching of coring parameters to the core bit diameter and segment specifications (“H-” and “HX-Series”) that were specifically developed for these tools.

•

“Iron Boost”, power optimisation and power regulation: Extra torque can be provided at the touch of a button when drilling in heavy reinforcement to improve drilling performance (coring speed). Power is reduced in the lower gears for smaller core bit diameters where use of the full power of the tools is unnecessary and would significantly increase segment wear without providing the benefit of higher drilling performance. Power reduction maintains the useful maximum RPM of the core bit while reducing applied torque. The power delivery is monitored by on-board power regulation, visually presented to the operator by coloured LEDs, ensuring that the user drills at the optimum pressing force and torque for the core bit.

The three tools mounted on the common HD30 drilling rig are shown in Figure 1.

Fig 1. Hilti core drilling tools and HD 30 rig. From left to right DD 200, DD 300 and DD 500.

2. Methodology Technology research and development on the drives started with the definition of the boundary 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, etc. Naturally, a detailed marketing analysis of customer needs and a full evaluation of the product portfolio offering at the time were also made. Being a fully integrated system development (i.e. tool, rig and consumable), many aspects needed to be considered. Product positioning of the new tools and core bit lines; backwards compatibility of all three major system components with existing products; certain compatibility with competitor tools, etc, were all taken into consideration. A detailed benchmarking and characterisation exercise of Hilti and competitor tools and core bits was also made. An example of one such exercise evaluating the torque levels recommended for coring (18 competitor and Hilti tools in the power range 1 to 7kW) is shown in Figure 2. The difficulty in starting such a complex development project with a completely blank piece of paper is clearly indicated by this simple graph. Taking the example of a 202mm diameter core bit, the tools investigated recommend gears which provide torque in the range 41 to 255Nm and rotational frequencies in the range 530 to 160 RPM (Note: RPM data not included in graphical form in this paper). How does one decide which combination provides the best value proposition for the customer, or, formulated in a simplified way, what provides the

optimum in terms of drilling speed and segment lifetime to reduce costs per metre? This was the challenge facing the project team defining the E-gears and segment specifications. 400

350

300

Torque [Nm]

250

200

150

100

50

0 0

50

100

150

200

250

300

350

400

450

500

Diameter [mm]

Fig 2. Available torque [Nm] for core bits of diameter 8mm to 500mm provided by the most popular diamond drilling tools in the power classes 1kW to 7kW (data represents 9 Hilti tools and 9 competitor tools). Minimum, average and maximum torque levels in the recommended gears (from manufacturer’s data) are presented. The diagram shows that, depending on the tool used, the torque available for drilling with a given core bit diameter can vary by a factor by up to a factor of 10. .

In developing a complete core drilling system, each component – including the operator must be considered. Although the HD30 drilling rig is designed to provide stability at a normal pressing force (weight on bit) of up to 10,000N, a comfortable optimum pressing force for the operator using the gearing in the hand wheel is 2500N in concrete and 1500N in reinforcement steel bars. Therefore, based on measured coefficients of friction, the required torque to achieve these pressing forces can be calculated for each core bit diameter. With a fixed power available to each tool, the 3kW rated DD 300 and the 4.5kW rated DD 500, the RPM is then pre-determined for the selected torques, subject to the upper limits of the tools. These values can then be used as a basic starting point for further “fine tuning”. This complex activity included testing drilling parameters at, above and below the initially proposed torque levels, first with the drilling model described below and also in real tests with suitable segment specifications predicted to provide good performance. The decision whether to focus on speed or lifetime of the core bits was naturally taken together with the marketing department. Figure 3 shows the selective results of some of the performed drilling tests with 202mm diameter drillbits in a limestone based concrete with two 26mm diameter reinforcement steel bars per hole.

Diameter 202mm, Limestone aggregate concrete, 26mm rebar 4.5

DD 300 Tool 1

4.0

Tool 2

Vm [cm/min]

3.5

RED

HX2

BLUE

Bit A

YELLOW

Bit B

9

10

3.0 2.5 (Polishes)

2.0 1.5 1.0 0

1

2

3

4

5

6

7

8

Lifetime [m/mm]

Fig 3. Influence of tool drive parameters (RPM and torque) on drilling speed (Vm, measured in cm/min) and segment lifetime (measured in metres drilled per millimetre segment wear). Tool 1 delivers 2.4kW power to the spindle, translated into 100Nm torque and 230 RPM. Tool 2 delivers 2.6kW at 59 Nm and 430 RPM. The DD 300 parameters lie between these two tools. Core bit A used segments with 40/50 mesh diamonds at a concentration C30, while core bit B used a 50:50 mixture of 30/40 and 40/50 diamonds at C32.

From Figure 3 it can clearly be seen that in the case of Tool 1, all three segment specifications achieve approximately the same drilling speed (around 3 cm/min) although the lifetime varies by a factor of 4, which is more greatly influenced by the matrix (bond) wear resistance than the diamond type and concentration. Tool 2, with much lower torque, only really works with core bit B, which used a soft matrix. Core bit A polishes, while this HX2 specification is also in a borderline polishing regime. However, by combining the DD 300 parameters with the initial prototype HX2 specification, the highest drilling speed was achieved, being over 40% higher than the average of all the other tests and nearly 20% faster than the best other combination. Excluding the combination of Core Bit A with Tool 2, the lifetime is less than 15% lower than the average of the other tests. These test results were very close to the predicted values from the model and also achieved the initial target values for the system (which are given in Figure 8 below). All comparisons between the actual drilling tests and the drilling model demonstrated good correlation which meant that much of the “fine tuning” of the tool parameters and segment specification could be made without the need for extensive drilling testing at first. Verification was, however, done by actual testing. 3. Drilling Model The semi-empirical model used for specifying the drive parameters (the “E-Gears”) and respective segment specifications is based on indentation theory. In this case, the model is essentially a “reversed hardness model”, predicting diamond indentation depth and calculating penetration per revolution in a concrete of known “hardness”. This can then be transferred into drilling speed values. Additionally, the polishing tendency of the segments (for both types of polishing mechanisms, namely diamond wear flatting and diamond pull-out, also known as pop-out) can be predicted and the expected magnitude of matrix wear can be estimated, based on matrix wear resistance, the gap between matrix and concrete and the size, amount, morphology and abrasivity of the drilling detritus generated. A number of input parameters are necessary to achieve useful output from such a model. Many parameters can

be measured directly, while others must be estimated using available literature or determined in the validation and calibration stages of modelling, by comparing model outputs with actual values and adapting the input values to achieve closer correlation where necessary. The model assumes steady-state conditions, whereby the numbers of active diamonds, diamond shape, protrusion, distribution, etc, are assumed to be constant and uniform during the drilling process. Hence, the model provides an indication of performance under ideal conditions with the maximum drilling speed for the exact conditions defined. The primary model inputs are given in Table 1. Not all inputs will be discussed within this paper. Component Segment Diamond Drive parameters Concrete properties Other inputs

Necessary Input Diameter Geometry No. of segments Bond type Concentration Size PPC Shape Strength Active sites Protrusion Torque RPM Martens Hardness Drillability Abrasivity Rebar Friction coefficient Table 1. Primary drilling model inputs.

Typically, models of indentation theory have been used to predict indentation hardness in terms of uniaxial material (substrate) properties, but they do not adequately cover the material properties and indenter geometries of interest in the modelling of diamond drilling in concrete. The most accepted models are based on the elastic indentation model, the rigid perfectly-plastic indentation model, and the so-called spherical cavity expansion model. Yu and Blanchard [2] developed a general model for a wide range of elastic and plastic materials and for several indenter geometries, which was used in this work along with the more well known simple models for 3-body wear with conical indenters from Achard [3] and the established formulae for Vickers Pyramid Hardness and Brinell Hardness. An example of the work of Yu and Blanchard is shown in Figure 4.

Fig 4. The geometry of the indentation of a semi-infinite elastic perfectly-plastic solid by a rigid indenter of arbitrary surface profile is shown on the left. On the right, the pressure distributions predicted by various models in frictionless contact are shown. (a) elastic theory, (b) slip-line-field theory, (c) axisymmetric (plane strain) indention of Yu and Blanchard [2]

In the case of a near cubo-octahedral diamond with a diamond shape factor (Tau-factor) of 0.25 to 0.33, as defined by Diamond Innovations [4] and schematically shown in Figure 5, the geometry of the diamond indenter is essentially a combination of cone, wedge and flat-ended cylinder and is therefore difficult to approximate using indenter geometries described in conventional models, thus requiring the application of the work of Yu and Blanchard.

Fig 5. The image analysis system of Diamond Innovations uses a diamond modelling parameter called “Tau” to measure crystal shapes. Tau measures an octahedron as 0.0 and a cube as 1.0, as shown on the left. Diamond shapes in between have a measurement within this range. For the indentation modelling described in this work a Tau factor of 0.25 to 0.33, approximately equating to a cubo-octahedral diamond, was used. This diamond shape (here for Tau=0.33) is shown schematically in the middle, with an actual photograph of representative diamonds shown on the right.

Another basic parameter required by the model is the number of active diamonds on the segment surface. The pressing force (weight on bit) is assumed to be uniformly distributed among all active diamonds, and therefore this model input is critical in determining the accuracy of the model outputs. A number of calculations are available to predict the total number of sites on a given surface area [5-7], and the difference in the numbers generated can be marked. Figure 6 demonstrates differences between the various calculation methods for two examples. The drilling model used an average of all calculations, which correlates very well with measurements of the number of active sites on real segments. Diamonds on segment surface (100mm

2

)

Diamonds on segment surface (100mm

2

)

C30, 30/40 % active diamonds ________ 40 Diamond protrusion Average diamond size Equivalent PPC Geometric model - equivalent spheres (Moseley 2001) Gassmann equation (Hilti 1984) Diamonds per cm2 table (DeBID 1999) Diamonds per cm3 - PPC basis Diamonds per cm2 (Engels, E6, 2003) Diamonds per layer (Calculation 1; Moseley 2001) Diamonds per layer (Calculation 2; Moseley 2002) Average Active diamonds on surface Total diamonds on surface

0.4 0.55 675 13 14 17 25 19 14 17 17 17 (13 - 25) 43 (33 - 63)

C30, 40/50 % active diamonds ________ 40 Diamond protrusion Average diamond size Equivalent PPC Geometric model - equivalent spheres (Moseley 2001) Gassmann equation (Hilti 1984) Diamonds per cm2 table (DeBID 1999) Diamonds per cm3 - PPC basis Diamonds per cm2 (Engels, E6, 2003) Diamonds per layer (Calculation 1; Moseley 2001) Diamonds per layer (Calculation 2; Moseley 2002) Average Active diamonds on surface Total diamonds on surface

0.4 0.39 1850 25 27 33 49 36 22 23 31 31 (22 - 49) 77 (55 - 123)

Fig 6. Sample calculations of number of active diamonds on a segment of 100mm2 surface area. Depending on the calculation used, values can differ by more than a factor of 2. The true percentage of active diamonds differs in different concrete types and is also dependent on the torque of the drilling tool and matrix bond type.

The properties of the rocks used as aggregates in concrete differ greatly based on their type and geographical location. In the development of the DD 300 and DD 500 tools and segment specifications, a number of representative concretes from three main property groups were used for modelling and actual testing purposes. The range covered is indicated in Figure 7, as classified by standardised tests for wear intensity [8] and abrasion resistance [9]. Concretes from all around the world have been classified by Hilti and for the purpose of defining segment specifications these can essentially be split into one group containing primarily carbonates and calcites; a second group with higher proportions of crystalline quartz; and finally a third group with very hard amorphous quartz, such as flint.

Fig 7. Properties and structure of test concretes used.

Many other researchers have also developed rock classification systems that qualitatively describe the “drillability” or “sawability” of natural stones. For example, Diamond Innovations ranks the sawability [10] of hard stones by grouping them into 4 major classes. Rau Karanam and Misra [11] summarised work that defined an “overbreak” function, which is the ratio of actual material removal depth to diamond indentation depth. This is rock dependent but is typically of the order of 1.1 to 1.3:1 and a similar function is used in the model. These drillability terms also depend on (among other parameters) the values for work of deformation and the amount of elastic/plastic deformation of the aggregates, an example of which is given in Table 2 for a typical crystalline quartz aggregate and a typical limestone aggregate. The Martens (Universal) Hardness of the aggregates is also an important input in the model, since the indentation depth of the diamonds is closely linked to this material parameter which is measured by the depth-sensing indentation method. Units

Quartz

Aggregatt e

Limestone aggregate

Work of deformation

Relative, %

70

100

Elastic deformation

%

68

33

Plastic deformation

%

32

67

Fracture

surfac ce

Table 2. Fracture characteristics of typical quartz and limestone aggregates used in the test concretes.

These and other input parameters were then integrated to produce the sequentially calculating semi-empirical model used in the system development of the DD 300 and DD 500 together with specific core bit lines. Because the model calculates everything under steady state and ideal conditions, utilising only around 25 individual calculations and presenting the

data in a basic tabular format with limited graphical representation of the data, it can be simply incorporated in a spreadsheet of minimal size (