EXPERIMENTAL TESTING OF LOW-POWER HAMMER-DRILLING TECHNIQUES IN A VARIETY OF ROCK MATERIALS

In Proceedings of the 9th ESA Workshop on Advanced Space Technologies for Robotics and Automation 'ASTRA 2006' ESTEC, Noordwijk, The Netherlands, November 28-30, 2006 EXPERIMENTAL TESTING OF LOW-POWER HAMMER-DRILLING TECHNIQUES IN A VARIETY OF ROCK MATERIALS Ylikorpi, T (1), Re, E (2), Magnani, P-G (2), Scolamiero, L (3) (1) Helsinki University of Technology Automation Technology Laboratory Otaniementie 17, 02150, Espoo, Finland Email: Tomi.Ylikorpi@tkk.fi (2) Galileo Avionica S.p.A., Space and Electro-Optics Business Unit Via Montefeltro 8, 20156, Milano, Italy Email: edoardo.re@galileoavionica.it Email: piergiovanni.magnani@galileoavionica.it (3) ESA/ESTEC, Mechanical Engineering Department Keplerlaan 1, PO Box 299, 2200 AG Noordwijk ZH, The Netherlands Email: Lucio.Scolamiero@esa.int ABSTRACT This article presents early results from a technology development program Down-hole Hammering Mechanism (DHM) for a Planetary Drill (ESA ITT AO/1-4687/04/NL/CP). The activities are being carried out by Galileo Avionica Space & Electro-Optics B.U. (Italy), acting as main contractor, in cooperation with the Helsinki University of Technology (Finland) acting as a subcontractor. In the project is developed a hammering system that would be located in the lower end of a 29 mm diameter drill string when drilling roughly 2 m deep in Martian soil. Purpose of the hammering system is to assist in penetration into hard materials that may be encountered under the softer surface layer. A particular challenge is set by the limited envelope for the mechanism that is completely located inside the small-diameter drill pipe and a limited power reserve that falls between 50-100W which is quite low compared to conventional hammer drill tools that range from 800W to several kilowatts. Considering the limited envelope and power resource an early test campaign was carried out to identify possible technologies and their drilling performance in different rock specimens. Such mechanisms as pure low-power rotary drilling, low-power percussive drilling adopted from a commercial drilling machine, high-speed commercial percussive drilling, and low-power high-energy impact drilling were tested. Operational range in the tests has been 20-50 W electrical power, 25 mm drill diameter and rock materials varying from soft calcite to hard granite and mafurite. It has been verified that percussive drilling adopted from commercial drilling machines is functional only at high rotational velocity and at high power level. The results of rotary drilling and high energy (1.88 J) impact drilling tests show that for soft rocks -like calcite- hammering does not provide any significant benefit, but hard and fragile materials, like granite, benefit from hammering the most. Hard and ductile mafurite is the most difficult material of the tested to drill into. Rotary drilling into hard materials is practically impossible with given parameters and in addition rotary drilling would cause strong wear of the tool and severe heating of the rock sample enough to endanger its scientific value. Expected rate of penetration at 30-50 W power using a 25-mm drill tool is around 0.2 mm/min in soft rock with or without hammering, 0.1 mm/min in hard but fragile materials when utilizing hammering and around 0.05 mm/min in hard and ductile materials like mafurite. Rotary drilling can be recommended up to materials similar to marble (100 MPa class compressive strength). INTRODUCTION In search for life the ESA s Aurora / Exploration programmes aim at deep surface sampling - typically a range of 1 to 2 m depth- utilizing extendible drills carried onboard lander or an autonomous roving vehicle. Hammering or percussion is a well-known technique to assist drilling in hard and brittle materials that might be met below the surface. In terrestrial applications percussive drilling is used to drill for instance stone or concrete. Down-hole hammering is used in deep drilling when the bit is actuated through a long drill string: a hammer implemented in the lower end of the drill string close to the drill bit improves the shock transmitted to the bit. Such technique is currently used for example in oil drilling. During years 2000-2001 a Finnish consortium developed for ESA a functional prototype of a small and compact drill mounted on a tracked roving vehicle called MicroRosa 2 [RD 1], while first drilling prototypes were built in Finland for ESA already in 1996 together with an Italian partner Tecnospazio S.p.A. (currently Galileo Avionica Space & Electro-Optics B.U.). The same Italian company has also developed DeeDri tools for Mars drilling [RD 2] and SD2-

drill onboard Rosetta-spacecraft for comet 67P/Churyumov-Gerasimenko [RD 3]. As the Italian-Finnish consortium - Galileo Avionica Space & Electro-Optics B.U. (Italy), acting as main contractor, in cooperation with the Helsinki University of Technology (Finland) acting as a subcontractor- took over the technology development program Downhole Hammering Mechanism (DHM) for a Planetary Drill (ESA ITT AO/1-4687/04/NL/CP) the partners join their drilling experience collected over a decade and continue deepening their knowledge and understanding of planetary sub-surface drilling. Under the ESA contract participants initiated a series of preliminary tests with different drilling methods. Purpose of the preliminary tests was to identify possible technologies considering the limited envelope and power resources- and verify their drilling performance in different rock specimens. Such mechanisms as low-power rotary drilling, low-power percussive drilling adopted from a commercial drilling machine, high-speed commercial percussive drilling, and lowpower high-energy impact drilling were tested. Actual prototype is to be built in 29 mm diameter and it is anticipated to have 50 W continuous power source. The preliminary tests, however, were conducted with existing and off-the-shelf hardware and simple demonstration models from our own workshop. Therefore the test item dimensions differ a bit from final prototype dimensions which must be taken into account when predicting prototype performance based on measured demonstration model performance. The testes were conducted at Automation Technology Laboratory of Helsinki University of Technology with existing Miranda drilling test rig illustrated in Fig. 1. Originally the test system was designed and built by students for ESA student competition. It constitutes of a vertical roller guide equipped with a ball nut and a feed motor. Drill rotation motor and drill mandrel mounted on a platform travel along the vertical guide. Coupling between the ball nut and mandrel platform is realized with the aid of springs so that approximately constant axial drilling force can be adjusted with the ball screw (instead of providing constant axial feed velocity that is often the case). Total drilling force is then the mass load of the drill added with the tension from the springs. When the drill is resting against a rock to be drilled, drilling force can be increased by driving the lead nut downwards which extends the springs. A drill string exceeding 1 m in length can be mounted on the mandrel and it penetrates into a deep sand-box placed just below the drill rig. For these tests the drilling depth data was collected visually with the aid of rulers; drill rotation speed, input voltage and current were measured with external metering devices. Also an additional sample holder was built on top of the sandbox to hold rock samples being used in tests. TEST ROCKS AND TOOLS The drilling tests were conducted into four different rocks, described in Table 1. and shown in Fig. 2. The figure also shows the tip of a commercial 25 mm rock drilling tool (40 cm long) with a hard-alloy tip that was used in the tests. Calcite specimen can be considered as a soft rock comparable to marble, a specimen of diopside is a harder target. Granite is hard but containing quarz it is fragile and very abrasive. Volcanic mafurite combines hardness and ductility into quite challenging sample to be drilled into. Table 1. The test rocks properties. Rock Hardness MPa / Moh Density kg/m3 Calcite ~50 / 3 2710 Diopside ~120 / 6 3260 Granite ~180 / 6.3 2580 Mafurite >250/>6 2545 Fig. 1. Miranda test rig. 2

Fig. 2. The test rocks Calcite, Diopside (top row), Mafurite and Granite (bottom row), and the drill tool. LOW POWER PERCUSSION DRILL TEST As the aim of the project is to develop a reliable and durable hammering system for a planetary subsurface drill, the very first approach was to look for working and efficient solutions from existing markets. One such example is an electric hand-held percussive drill that usually comes in 500-800 W range. It was of interest to study whether this operation principle would be applicable also for our small-scale and low-power application. An electric percussion drill was purchased from the closest hardware store and was disassembled in order to transfer the functional parts for our testing system. Fig. 3. shows the dismantled ratchet mechanism parts. The mandrel bearing assembly of the test rig was modified to allow a small axial motion that would be needed for operation of the ratchet mechanism. Fig 4. shows the modified mandrel with ratchet mechanism mounted. Now upon mandrel rotation the percussion device gives 16 impacts per revolution (that is 8 Hz @ 30 rpm or 32 Hz @ 120 rpm) which equals to 22.5 deg. indexing between impacts. The drilling force is generated by drill mass load (5.5 kg) and spring force. Drilling force in these tests was adjusted to 170 N ± 7 N. Variation of the drill thrust is due to advance of the drill into rock material which reduces the spring load. The vertical feed motor was not used to maintain spring load as constant during these tests. Fig. 3. The ratchet mechanism parts adopted from a percussive hand drill. Fig. 4. The ratchet mechanism on the test rig. 3

Percussion Drill Test Results Four drilling tests were performed in calcite with and without percussion. Between the tests drill rotation speed was changed from 115 RPM to 180 RPM, and also the drilling force was increased from 170 N to 190 N; none of the actions are, however, visible in the test results. Power intake was in the beginning in the range of 30 W while increase in thrust force increased it by 10 W, increase in drill rotation velocity by another 10 W, and adding percussion during the last test increased it finally close to 50 W. The drill penetration depth graph in Fig. 5. (left) reveals that in these tests the benefit from percussion was negligible, if not existing at all. Similar results were obtained in similar tests in diopside. The lowest curve in Fig. 5. (left) indicates a poor performance of the last rotary drilling test with respect to the three others. Reason for this remains unknown; it might be due to hard inclusion in the rock sample. Result was a bit surprising; everyone who has drilled a hole in a brick or concrete wall with a hand-held percussive drill knows that it works. But this drill rotates ten times faster than our testing system. Therefore it was necessary to conduct a comparative test with higher and adjustable rotation velocity. Another piece of similar percussion drill was purchased from a store and it was equipped with a similar drill tool as the test set-up. Several drilling tests with different velocities were conducted into the same piece of calcite rock with this 500 W drilling machine. A vertical jig and a weight was used to provide constant vertical thrust of roughly 31 N. The test results are shown as a graph in Fig.5. (right). The graph shows the increasing effect of the percussion in drill penetration rate as a function of drill rotation speed. Drill performance appears to be exponential to the rotation velocity, but without percussion the penetration rate is dependent on rotation speed in a much less extent. Below 1000 RPM percussion does not provide much advantage over the rotary drilling. This indicates that pure kinematic study of the ratchet mechanism is not sufficient to describe the action of the percussion mechanism but there must be other dynamic effects that take place inside the ratchet mechanism and/or between the drill tool and hole bottom. Also at high velocities the dust removal from the hole may be much more effective which would play also a significant role. Since the penetration rate curve has an increasing slope as the drill rotation increases, this means that the material removal rate is not only increasing, but also drilling efficiency increases so that during each rotation (and during each impact) the tool separates more material at high velocity than at low velocity. After the tests a conclusion was made that this approach would not be suitable for a down-hole hammering mechanism that would operate at a moderate velocity and with low power intake. A decision was made to make a survey on technologies using high-energy impacts at low frequency and low drill rotation velocity, such as for example in handheld electro-pneumatic hammer drills. CAM-HAMMER TESTS Since the percussion drill turned to be ineffective with available resources it was decided to experiment a low-frequency high-energy impacting system familiar from hammer drills. Impact energy in these electrical hand-held devices starts from 1 J per impact up to 10 J. Heavy pneumatic or hydraulic devices provide even stronger impacts. Impact frequency is often several hundred, or even several thousand, impacts per minute, which, however, is not a reasonable goal for the small DHM-system. We would aim for 1-2 J impacts at 60-120 blows per minute. Mechanisms for generating the impacts in power tools were studied during the project and there are several. For demonstration purposes was selected a mechanism that could be easily miniaturized for the DHM and that is simple, reliable and durable in operation. Such mechanism is based on high-rising rotating cam. In principle this is quite similar to the percussion mechanism tested earlier, but the 16 small 2 mm high teeth are replaced now with a single 12 mm high cam. In addition a separate spring powered striker is added to provide the impacts instead of using the drill mass itself. Penetration (mm) 10.00 9.00 8.00 7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 Calcite 0 10 20 30 40 50 Time (min.) Calcite w -o percussion 1 Calcite w. percussion 1 Calcite w. percussion 2 Calcite w -o. percussion 2 Penetration (mm/min.) Calcite high speed 3,00 low thrust w percussion 2,50 low thrust w-o percussion Expon. (low thrust w percussion) 2,00 1,50 1,00 0,50 0,00 0 500 1000 1500 2000 RPM Fig. 5. Percussion drill test results in calcite at low-rpm (left) and variable high-rpm (right). 4

CAM-Hammer Mechanical Construction The existing test-equipment was modified to produce high-energy impacts with adjustable indexing. The impacting system consists of a circular cam and a rotating striker. The original drill rotation motor was modified to operate the striker, another motor was added to rotate the drill tool to provide indexing between the impacts. The striker motor rotates the striker that follows the cam and elevates upwards (total rise 12.8 mm) until it suddenly drops into the bottom of the cam and hits the upper end of the drill tool. Mechanical coupling between the motor and striker is realized with a spline shaft that allows vertical motion while transferring the torque. The striker impact energy is provided by two extension springs mounted on the striker. A ball bearing is used to connect to the stationary springs to the rotating striker. As the striker drops from the highest point of the cam it hits the upper end of drill shaft. The drill shaft is mounted on its bearing assembly in such a way that it can move up and down for a couple of millimetres. Vertical drill thrust pushes the drill tool and drill shaft into upper position so that the striker impact is able to kick the drill downwards. Vertical drill thrust is provided with the aid of two additional springs and the weight of complete drilling system adds to the spring force. The drill itself is mounted on a vertical linear guide with a linear rolling bearing. The lower end of the thrust springs are mounted on a lead-nut that is driven with a lead screw mounted in the middle of the linear guide. So the vertical thrust comes from the weight of the drill (supported by linear bearings) added with the tension from springs (driven downwards with the lead screw, when needed). In order to provide indexing (so that the drill tool would not impact all the time the same location on the rock surface) an additional indexing motor was mounted to the side of the bearing assembly. The drill is rotated then with the aid of a short belt-drive. Fig. 6. presents the demonstration assembly and a detail of the striker and the cam profile. Table 2. presents the measured and calculated data describing the cam-hammer performance. Cam-Hammer Performance Analysis In order to predict drilling performance of the system a simple analytical calculation was performed according to common formulae used for terrestrial drilling analyses. Here is assumed that all kinetic energy of the striker is transformed into elastic compression of the drill tool upper end. The tool is assumed to be a solid rod with uniform cross-section. All couplings and changes in cross-section are ignored in this early phase. Fig. 6. The Cam-hammer demonstration model (right) and a detail of the striker (left). 5

Table 2. Parameters for the impact drilling test Parameters for the impact drilling test Parameter Striker mass Striker spring load (each of 2 springs) Striker travel length Striker energy Drill body mass Impact frequency Indexing speed Indexing btw. impacts Thrust spring load Total thrust with mass load Drill diameter Value 0.84 kg 63.77 N min 83.39 N max 12.8 mm 1.88 J 7.66 kg 59 1/min 1.277 rev/min 7.44 deg (48.387 1/rev) 122.5 N 197.6 N 25 mm The impact is a dynamic event where stress propagation along the tool is considered. A very quickly introduced impact generates a stress wave that proceeds along the tool with the speed of sound velocity in this particular material. The impact energy in the drill rod is divided between compressive strain energy and kinetic energy. In conventional impact drilling technology stress wave amplitude σ [N/m 2 ] in the rod, produced by a piston may be calculated as: σ = ρcv A p A p + A r, (1) where ρ is density of drill steel [kg/m 3 ], c is the velocity of stress wave in the drill rod [m/s], v is striking velocity of the piston [m/s], Ap is cross sectional area of the piston [m 2 ] and Ar is cross sectional area of the drill rod [m 2 ] [4]. The dynamic force, F, produced by the stress wave may be given as: F = σa r (2) To illustrate effect of impacting velocity, material parameters and geometric properties (2) may be written open as: F Ap Ar = ρcv A + A p r (3) Table 3. presents the required striker velocity and calculated dynamic force that would be achieved with the demonstration assembly with two different impact energy. From these the lower energy was used in tests. The results indicate that with 1.88 J impact energy we can generate 8.8 kn dynamic force. Literature study in rock drilling technology indicates that a rough 7-8 kn force would be sufficient to break granite with 1-inch 60º conical indenter [5]. Thus the gained dynamic force is at lower limit of requirement, also noting that drill tool geometry is closer to a wedge than a cone. Table 3. Analytical results for predicted dynamic force. Parameters for predicted dynamic force Impact energy (J) 1,88 10 Striker mass m kg 0,84 0,84 Striker velocity v m/s 2,12 4,88 Striker area Ap m 2 7,07E-04 7,07E-04 Rod density δ kg/m 3 7800 7800 Sound velocity in steel c m/s 5470 5470 Rod area Ar m 2 1,13E-04 1,13E-04 Dynamic force F kn 8,80 20,29 6

Cam-Hammer Test Results Drilling tests were made in all four sample materials with and without hammering action. During the tests power intake and penetration rate were monitored. During hammer drilling power intake does not depend on material being drilled as the primary power user is the striker motor that does not feel the sample material at all. The indexing motor is rotating very slowly and its power intake is less than 1 W. In all tests impact frequency was 50-70 impacts per minute and used power was between 26 and 33 W where the variation is due to on adjustment of the motor velocity. Also some variation was visible during testing, possibly due to changes in hardware temperature and friction properties. Comparative rotary drilling tests were carried out in same sample materials with the same tool and same equipment. Drill rotation velocity was 144-167 RPM, drill thrust was 166-167 N and used power 30-40 W. Variation for rotation velocity and power intake is for the same reasons as for hammering tests. The test results are collected in graph presented in Fig. 7. The results indicate that for soft materials -like calcitehammering does not provide any significant benefit. Hard and fragile materials, like granite, benefit from hammering the most. Calcite is the easiest material of the ones tested to drill, and it does not show any benefit from hammering. Very dense diopside is clearly more challenging to drill but hammering appears to provide only little benefit. Hard and ductile mafurite is the most difficult material to drill into but it shows already a clear benefit from hammering. A completely different material from the others is granite that with hammering is almost as easy to drill as calcite, but without hammering it is as difficult as for the mafurite. Rotary drilling into hard materials is practically impossible with given parameters. Rotary drilling would cause strong wear of the tool and severe heating of the rock sample enough to endanger its scientific value. DHM DEVELOPMENT The mechanical realization of the hammering system was for demonstration purposes only. The following steps in the project consist of developing a system to give a similar or better- hammering effect within mechanical envelope suitable for down-hole-hammering. Actions have been taken to design a 50 W cam hammer that would fit inside a drill string. Expected length of such mechanism is roughly 500 mm with 29 mm outer diameter including drill pipe auger. Fig. 8. illustrates some interior parts of the DHM cam hammer. It shows the motor, the coupling, the cam itself, camroller and supporting bearings and guiding ball bushing. Rotary drill and Hammer drill tests in Calcite, Diopside, Mafurite & Granite Penetration (mm) 12,00 10,00 8,00 6,00 4,00 2,00 Hammer-Calcite Rotary-Calcite Hammer-Diopside Rotary-Diopside Hammer_Granite Rotary Granite Hammer-Mafurite Rotary Mafurite 0,00 0,0 10,0 20,0 30,0 40,0 50,0 60,0 Time (min.) Fig. 7. Rotary drilling and hammer drilling test results. 7

Motor Coupling Ball bearing Roller Cam/striker Ball bushing Fig. 8. DHM Cam-hammer interior parts. CONCLUSIONS The tests indicate that percussive drilling adopted from commercial drilling machines is functional only at high rotational velocity and at high power level. Therefore this method is not foreseen as suitable for DHM application. Fig. 7. presents the results of rotary drilling and high energy (1.88 J per impact) impact drilling tests. It shows that for soft materials -like calcite- hammering does not provide any significant benefit. Hard and fragile materials, like granite, benefit from hammering the most. Rotary drilling into hard materials is practically impossible with given power and thrust force as it would cause strong wear of the tool and severe heating of the rock sample enough to endanger its scientific value. Fig. 7. shows expected rate of penetration at given power which is around 0.2 mm/min in soft rock with or without hammering, 0.1 mm/min in hard but fragile materials when utilizing hammering and around 0.05 mm/min in hard and ductile materials like mafurite. Rotary drilling can be recommended up to materials similar to marble (100 MPa class compressive strength). SUMMARY This article has described the mechanical systems and test results from drilling tests into several rock materials utilizing pure low-power rotary drilling, low-power percussive drilling adopted from commercial drilling machine, high-speed commercial percussive drilling, and low-power high-energy impact drilling. Aim of the campaign has been to figure out which kind of drilling methods would be beneficial for drilling with low input power and what would be the expected rate of penetration. Operational range in the tests has been 20-50 W electrical power, 25 mm drill diameter and rock materials varying from soft calcite to hard granite and mafurite. Table 1. presents properties of the test rocks. It has been verified that percussive drilling adopted from commercial drilling machines is functional only at high rotational velocity and at high power level. Therefore this method is not foreseen as suitable for DHM application. Fig. 7. presents the results of rotary drilling and high energy (1.88 J per impact) impact drilling tests. It shows that for soft materials -like calcite- hammering does not provide any significant benefit. Hard and fragile materials, like granite, benefit from hammering the most. Hard and ductile mafurite is the most difficult material to drill into. Rotary drilling into hard materials is practically impossible with given parameters as it would cause strong wear of the tool and severe heating of the rock sample enough to endanger its scientific value. Expected rate of penetration at given power can be read from Fig. 7. and could be around 0.2 mm/min in soft rock with or without hammering, 0.1 mm/min in hard but fragile materials when utilizing hammering and around 0.05 mm/min in hard and ductile materials like mafurite. Rotary drilling can be recommended up to materials similar to marble (100 MPa class compressive strength). Manufacturing of a 50 W cam-hammer based down-hole-hammering mechanism prototype is ongoing. REFERENCES [1] T. Ylikorpi, G. Visentin, J. Suomela, A robotic rover-based deep driller for Mars exploration, Proceedings of the 35th Aerospace Mechanisms Symposium, Ames Research Center, USA, May 9-11, 2001. [2] E. Re, P-G. Magnani, T. Ylikorpi, G. Cherubini, A. Olivieri, DeeDri' Drill Tool Prototype and Drilling System Development for Mars Soil Sampling Applications, 7th ESA Workshop on Advanced Space Technologies for Robotics and Automation ASTRA 2002, ESTEC, Noordwijk, The Netherlands, 19-21 November 2002. [3] Magnani PG., Re E., Robotics and technology aspects of the Rosetta Drill, Sample and Distribution System, Proc. ASTRA'98, November 1998. [4] Mänttäri, M, Laboratory scale rock drillability tests, Licentiate thesis, Helsinki university of technology, 1997 [5] Pang, S.S., Goldsmith, W., Hood, M., A Force-Indentation Model for Brittle Rocks, Rock Mech. Rock. Eng. vol. 22, pp. 127-148, 1989. 8