A novel approach for machining of cylindrical ultra-hard materials with a highly deﬁned contour is presented. Diamond grinding tools with complex geometry are manufactured with picosecond orthogonal and quasi-tangential laser ablation. Hitherto, laser manufacturing required a special axis conﬁguration and often optical beam deﬂection devices are utilized. Here, strategies and processes on a scanhead-free conﬁguration are discussed enabling straight-forward implementation in industry. This rapid and ﬂex- ible approach for small-scale production of master tools for industrial grinding processes reveals beneﬁts compared to conventional approaches. The manufacturing time is comparable to standard processes, however increased grain protrusion is attained with the presented laser strategy. A combined laser manufacturing strategy enables an ablation rate of 35 mm3min1and a maximal geo- metric deviation of 3 µm after ﬁnishing. The ﬁnal grinding tools are sharpened by a radial laser process removing the metal-based binding material. Following, this isolates the stochastically distributed diamond grains from the binder. Hence, high-precision diamond grinding wheels with a mean error of smaller 1 µm over the complete contour can be manufactured. The meta-stable diamond structure persists and is assessed via Raman spectroscopy. Keywords: diamond grinding tool, laser conditioning, laser manufacturing, ultra-short pulses, Raman spectroscopy, precision machining 2018 MSC: 08-08, 18-19 1. Introduction The demand for high precision gears to reduce wear and acoustic emission points to the necessity of tight manufacturing tool tolerances. Super-abrasives, like diamond and cubic boron nitrites (cBN) master tools, are therefore required to produce hard hobs with long endurance and precision. These master tools are conventionally manufactured by wire electric discharge machining (EDM) or single-grain diamond turning. One main drawback of the wire-EDM concerns the non- conductive super-abrasives. The grains are not cut by the wire, but the conductive binder is removed and grains are washed out from the surface leaving a void. Nevertheless, a deﬁned outer geometry can be produced reaching high precision with a lack of abrasives on the surface. Hence, the grinding tool must be used for some time to set the undeﬁned cutting edges free allowing a high-speed grinding process. Laser conditioning has proven to be a viable alternative to the conventional processes, however the manufacturing time is for most applications not competitive. Timmer (2001) investigated the applicability of laser manu- facturing to proﬁle rotary tools. Subsequently, Zhang and Shin (2002) reported in the early 2000s a technique for laser-assisted truing and dressing. A vitriﬁed cBN grinding wheel was pro- duced with a diamond dressing tool in conjunction with a laser. Corresponding author. Tel.: +41 44 633 73 50 Email address: ackerln@ethz.ch (Norbert Ackerl)This should increase the life time of the expensive dresser and enable a homogeneous condition during the production of the grinding wheel. Subsequently, the next step of dropping the dia- mond dresser was reported using directly the laser radiation for truing and dressing. Jackson et al. (2003) introduced an orthog- onal dressing process for chromium-doped alumina with a high- power Nd:YAG laser. Explicitly, it is stated to use proper laser parameters in terms of power and scan strategy to avoid massive damage and phase transformations. Following, chemical anal- ysis pointed to a re-solidiﬁed surface with alumina dissolution and multi-valent chromium oxides. Xie et al. (2004) reported dressing of resin-bonded diamond wheels, where the ablation threshold di erence enables an e ective sharpening process. However, high precision was not reached due to evaporation of the resin and following loss of the diamond grains. Bronze bonded diamond tools were studied by Hosokawa et al. (2006) and a sharpening process with a ms Q-switched laser investi- gated. They used an air jet nozzle to blow away the metallic binder before re-solidiﬁcation and convection cooling, which leads to high ablation rates. In case of low power no damage of the diamond grains was detected. However, the established pro- cesses were not fast enough to be economical and subsequent increase of average power lead to high thermal impact. Chen et al. (2010) reported laser assisted truing and dressing of bronze-bonded diamond grinding wheels and measured the process forces of such tools. The study revealed a strong depen- dence on laser parameters, surface quality and following grind- ing performance. A numerical model revealed the inﬂuence of Preprint submitted to TBD September 2, 2019 power density and adjacent heat impact leading to a speciﬁc removal rate. A non-negligible amount of graphite at the di- amond grains points to a thermal process and a heat-a ected zone, if a density of above 100 MW cm2is used. Shortly after, von Witzendor et al. (2012) investigated sharpening and dress- ing of grinding tools with di erent pulsed laser sources. The sources ranged from a pulse duration of 12 ps to 20 ns in the near-infrared (1064 nm) and with green wavelength (532 nm). The ablation rate and selectivity was studied between binder and diamond grains. Short pulses turned out to be beneﬁ- cial and the stochastically distributed diamond grains could be dressed without any detectable phase transition. In similar man- ner (Dold et al., 2011) studied touch dressing of single-layer large diamond grains with ultra-short pulses of 10 ps. Cut dia- mond grains were generated, which are favorable for high sur- face quality grinding with small surface roughness. Further- more, the grinding e ciency could be increased with lower processing forces, if structuring a deﬁned clearance angle on the grains. This angle dependency was investigated in detail by Transchel et al. (2013) on single diamond grains. A deﬁned modiﬁcation of the ﬂank angle proves to be beneﬁcial and al- tered the cutting characteristics of the tool in terms of smaller forces during the process and higher surface quality. Walter et al. (2012) reported laser dressing and truing of cBN grinding tools with high geometric accuracy using tangential irradiation conditions, which enabled an edge radius smaller 20µm. A material removal rate of up to 46 mm min3was reached at 48 W average power with a NIR 200 ns ﬁber laser at 75 % pulse and 25 % line overlap. These parameters pointed to a layer thickness of 50 µm and therefore a high material removal rate leading to short manufacturing time. Subsequently, the tools were tested in grinding experiments up to a speciﬁc ma- terial removal rate of 7000 mm3mm1and showed good dura- bility. Following, Chen et al. (2015) investigated the proﬁling of diamond tools with a pulsed ﬁber laser and tangential pro- cessing. Processing of the grinding wheel was reported with laser power up to 40 W and 210 ns of pulse duration. However, the graphitization of the diamond grain persisted for all param- eters with the broad characteristic Raman peak at 1582 cm1. A follow-up study from the same group (Zhou et al., 2016) investigated the graphite layer evolving under ambient, argon and grinding ﬂuid condition. Nonetheless, a graphite layer was clearly detectable in the Raman spectra at all ablation condi- tions after laser manufacturing. Walter et al. (2014) presented laser dressing and truing, which is proﬁling and sharpening, of industrially used grind- ing tools. Cleaning and surface structuring techniques were ap- plied to modify the tool behavior. Laser-induced deﬁned pro- ﬁles were studied and point to an increased speciﬁc material removal rate of the grinding process. However, the wear of the sharpened edges was higher and lead to shorter lifetime and, therefore, higher tool change costs. A similar study was re- cently presented by Zhang et al. (2018), where reduced cutting forces for structured tools have been reported. The wear started at the laser induced sharp edges leading to round o , which changes the cutting characteristics and inﬂuences the surface quality.A study by Warhanek et al. (2015) pointed out the appli- cability of ultra-short pulsed laser processing with a laser touch dressing process. Combining fast deﬂection with a galvo scanner and slow rotation on a high-precision XY-table lead to accurate laser machining and positive clearance angles. Hence, the cutting forces during grinding diminished, showing the po- tential of ultra-short pulsed laser ablation. The quasi-tangential process is inherently self limited and enables the production of high precision geometries. Moreover, if an iterative measure- ment step is introduced, the total manufacturing time can be decreases (Warhanek et al., 2017) allowing high precision and fast laser manufacturing. Within this study, combined processes for conditioning of super-abrasive diamond tools are presented. Orthogonal laser ablation is combined with a subsequent quasi-tangential inci- dence manufacturing step. Hence, a high material removal rate strategy is combined with the self-limiting ﬁnishing for high- est precision. Ultimately, a radial sharpening process sets the cutting edges of the super-abrasive grain free from the binding material for an optimized grinding performance. 2. Materials and Methods 2.1. Experimental Conﬁguration The ablation setup consisted of ﬁve mechanical axes, illus- trated in ﬁgure 1. A Trumpf TruMicro 5070 regenerative- ampliﬁed picosecond laser acted as source and was guided via modifying optics into the pivot point of a swivel B axis. Di er- ent anti-reﬂective coated focusing lenses were used to adapt the focal beam diameter. For this study, a 150 mm lens leading to a focal radii of 23 µm was applied. Furthermore, =4 and=2 wave plates allowed to control the polarization state and direc- tion. The X,Y ,Z and A axes were driven with controllers from Figure 1: Experimental setup for synchronous laser ablation utilizing an ultrashort-pulsed Trumpf laser source. The axes are three linear stages for XYZ movement, a fast rotating spindle axis A and manual swivel axis B. Aerotech Inc. and controlled with Aerotech’s A3200 software. The swivel axis B is manually adjusted and not used for the pro- duction of grinding wheels, but necessary for threaded grinding 2 tools. This approach enabled synchronous movement of the X,Y ,Z and A axes for combined feedback controlled motion. Fast relative surface motion of the laser beam and workpiece was realized with the spindle axis, which operated up to 100 s1 to adjust the surface speed and pulse overlap, accordingly. A suction unit with a two stage particle ﬁlter (G4, H15) was used to collect the ablation debris and generated nano-particles. Ad- ditionally, a pressurized air operated nozzle at 2 bar was used for convection cooling mounted on the opposite side of the suc- tion unit. For the laser path computation on the target geometry a CAM tool (Gysel, 2017) was developed for orthogonal and quasi- tangential laser manufacturing. The details on the calculation routines were recently reported by Ackerl et al. (2019). Here, solely a brief overview of the program ﬂow is discussed. A CAD ﬁle is exported to the stereo lithography (STL) ﬁle for- mat, which can be imported into the CAM software tool. Fol- lowing, the axes movement, laser paths, and hatches are cal- culated for an adjacent set of laser parameters. Therefore, the settings leading to a certain layer thickness and quality have to be established by a parameter study with the used laser source on the target material. Speciﬁcally, the layer thickness, beam distance, and spindle rotational speed have to be determined for orthogonal and quasi-tangential processing strategies. An additionally implemented feature allows to consider hardware limitations for the axes movements, like maximal velocity and acceleration. These constraints lead to idle rotations for po- sitioning and accelerating rotations to consider inertia of the workpiece and axes. 2.2. Process Strategies Laser processes have a trade-o between maximal ablation rate, surface quality and dimensional accuracy. Each character- istic can be optimized with an associated laser parameter set, but in many cases a combination of these characteristics is nec- essary. For this reason, three distinct processes are introduced for the laser manufacturing of high-precision grinding wheels. An orthogonal roughing processing step is followed by quasi- tangential roughing and ﬁnishing. Subsequently, a sharpening step is applied to generate the protrusion of diamond grains and clean the surface. Orthogonal laser processes are utilized for high ablation rate and the concept is depicted in ﬁgure 2a. The workpiece is rotated with the spindle axis and positioned with the X-axis and Z-axis at the focal position for each layer. Start- ing with a blank cylinder, the layers with a depth of lrare ab- lated with a beam distance of lbon helical paths. The surface speed is controlled by the rotational speed vrand constantly accelerated for layered ablation to keep the pulse overlap con- stant at smaller radii. Moreover, the generated ﬂanks project the laser beam, which leads to a decreased power density and a part of the beam is reﬂected. Therefore, a deviation to the designed geometry evolves, which has to be compensated for. In ﬁgure 2b a basic principle of empirical beam path compen- sation is shown. The laser paths are compensated already in the CAM, where a reduced ablation rate at the ﬂanks is consid- ered by the axial shift of xc. Additionally, the ablation at the Figure 2: Radial strategy for high ablation rate roughing (a). The laser beam is steered with constant surface speed vson helical paths with distance lbablating a layer of thickness lr. In (b) an empirical compensation strategy for radial massive laser ablation process is depicted. The ﬂanks are shifted by x cand the section with orthogonal incidence moved by r c. bottom of the contour is shifted radially by rcto consider accu- mulated errors. Both values are attained experimentally. In an iterative manner, a quasi-optimum can be found to reach prac- tical dimensional accuracy for the subsequent quasi-tangential manufacturing steps. As discussed, the orthogonal roughing process shows a high ablation rate accompanied by low dimensional accuracy. There- fore, a self-limiting quasi-tangential process is subsequently utilized for high precision manufacturing. The laser beam ir- radiates the surfaces under grazing incidence, depicted in ﬁg- ure 3a. Surface speed is held constant by controlling the ro- Figure 3: (a) Sketch of the self-limiting quasi-tangential ablation condition and projected laser spot on the ﬂank and bottom. This projection leads to a dif- ference in ﬂuence and is compensated in the laser path calculation. The Laser beam is steered along the contour with v f, and constant surface speed adjusted by the rotational speed vr. tational speed vrand the laser steered synchronized in X and Y with a feed rate of v f. Similar to the orthogonal process, the projection of the laser beam on the double curved surface is a challenge for stable ablation conditions, shown in ﬁgure 3b. This discrepancy is compensated in the CAM path calculation by lowering the feed rate at the ﬂanks keeping the ablation rate constant. The last step of the production routine involves an orthogo- nal sharpening process. This is attained by a similar motion as the ﬁnal layer of the orthogonal rouging process. However, the process is carried out with a fast rotation to remove only a small portion of the binding material, which has a lower ﬂuence com- pared to the super-abrasive. A combination of these three pro- cesses enables the production of high-precision grinding tools 3 in viable time. The distinct calculation of the laser paths with- out a parameter study is not implemented yet and would lead to computational complex models. Out of that reason a parameter study is carried out to attain the process settings for certain ma- terial removal rate and accompanying surface quality. Prospec- tively, a database will be established for di erent materials and geometries relying on the empirical parameter studies. 2.3. Material and Measurement Methods The grinding tool blanks were supplied by Ultrawheels with 25 µm natural diamond grains statistically distributed in a metallic binding material. During the production a granular mix of binder constituents and diamond grains is heated under pressure to obtain the dense raw material. 30 mm cylindrical blanks mounted on a precision-grinded cemented carbide shaft is used throughout this study. Optical stereo microscopy was utilized for ﬁrst inspection af- ter laser manufacturing. A confocal Leica DCM3D 3 Dmicro- scope served as measurement device to determine the depth, ablation volume and surface roughness. Additionally, an Al- icona G5 enabled the inspection of the whole grinding wheel before and after the grinding tests. Small features and diamond grains were analyzed with a Hitachi SU-70 scanning electron microscope. Raman spectra were attained with a WITec alpha300R spec- troscope and green wavelength of 532 nm. At the used 50x magniﬁcation the focal diameter is about 5 µm. The grating was set to 900 cm1with the spectral center at 1200 nm to mea- sure the Rayleigh peak for calibration. This leads to a measure- ment range of210 cm1to 2450 cm1with reasonable signal- to-noise ratio. Each spectrum was measured ﬁve times at the same spot and integrated over 4 s. The measurement data was ﬁltered to remove cosmic rays and smoothed by the Savitzky- Golay windowing algorithm. The geometric precision was determined with a Zoller Ven- turion 450 measurement device. The specimen is illuminated with collimated light and the shadow of the sample is detected by a large scale CCD detector. Two precise linear axes allow the positioning of the sample and a rotational axis enables the measurement over the whole circumference. The Zoller pilot software was used for evaluation of the measurement and export to a vector ﬁle format. The measurement accuracy speciﬁed by the manufacturer is 1 µm. A Diavite tactile measurement device allowed to measure the geometry and surface roughness of the hob after grinding it with the laser-processed tools. 3. Results and Discussion 3.1. Parameter Study A set of parameter studies for the two processing strategies has been carried out. Due to the inhomogeneous condition of the blank body with metallic binder and distributed diamond grains, single pulse ablation characteristics are not considered. Square pockets with 2 mm side lengths have been ablated on the lateral area of cylinders and on ﬂat specimen of the same mate- rial. Hence, a broad parameter range has been experimentallyinvestigated due to limited