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hp tooling 2021 #2

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  • High precision
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  • Milling
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  • Cfrp
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The journal of hp tooling is an english, global publication on all aspects of high precision tools, accessories and their applications.

processes Analysis of

processes Analysis of contact temperature in grinding CFRP written by Dr. Marco Schneider and Philipp Esch (Fraunhofer Institute for Manufacturing Engineering and Automation IPA) Yevgeny Babenko and Marcel Racs Abstract. This study reports on measurements of the cutting temperature in abrasive cutting CFRP with thermocouple elements. A connection strategy for coupling both thermal wiring components in process via heat sealing is proposed and evaluated. The gained temperature signal and response indicate temperature load up to 740° C. It is also evident, that temperature load is vastly dissipating due to high thermal conductivity of CFRP and short contact time. No thermal degradation or pyrolysis was registered in edge inspection after abrasive cutting and the process was assessed feasible for edge trimming in CFRP. 1. Goal Investigating contact temperature in grinding CFRP applying abrasive tools and evaluation of edge quality. 2. Introduction Fiber reinforced plastics offer high potential for lightweight applications in all production branches. Parts of carbon fiber reinforced plastics (CFRP) save up to 80 % of weight compared to conventional steel providing similar stiffness, tensile strength and chemical resistance (Babenko et. al. [2017]; VDI Zentrum [2015]). Current issues concerning CFRP relate to a material adequate recycling method and an optimization in costs regarding machine finishing jobs of CFRP parts. Since carbon fibers reach up to 5000 MPa in strength, CFRP is difficult to cut (Jäger and Hauke [2010]). Moreover, the fibers have an abrasive effect on the tool, which results in severe tool wear in conventional machining. Propagating tool wear is detrimental to workpiece quality causing delamination, fiber fraying and break outs (Feldhoff [2012]; Che et. al. [2014]). In order to avoid swelling of polymer matrix, CFRP parts are mainly cut under dry conditions, which is normally associated as an aggravating factor for cutting processes (Weinert and Kempmann [2004]). On the other hand, dry machining ensures process stability of subsequent production steps (coating, joining, painting) (Babenko et. al. [2017]). As an alternative to conventional machining, which here means cutting processes with defined cutting edges as in milling tools, of CFRP, tools with undefined cutting edge geometry can be applied. This technology is well established in processing ceramics and cemented carbides. Abrasive machining is characterized by high quality performance without delaminating the material and greater running smoothness (Sheikh-Ahmad [2009]). With first applications of edge trimming CFRP being limited to special cases, the use of abrasive tools is increasing. Driven by low costs (Klingelhöller, C. [2016]), abrasive milling tools are not only exclusively used in finishing of composites (Soo et. al. [2012]), but also for edge trimming (Colligan and Ramulu [1999]; Sheikh-Ahmad [2009]). Despite having many advantages, the use of abrasive tools is limited in its application due to critical disadvantages. Friction as well as multiple abrasive interactions between tool and workpiece are held accountable for high cutting temperatures which might degrade the polymer matrix (Sheikh-Ahmad [2009]; Biermann and Feldhoff [2012]). It is assumed that matrix degradation happens at temperatures exceeding glass transition temperature (120...200° C according to the polymer), but in contrast to this assumption, SEM images proof that there is no pyrolysis occurring at the CFRP edge despite drilling CFRP at high temperature in cutting (Merino-Pérez et. al. [2015]). Another disadvantage associated with abrasive tools is the phenomena of clogging up chip space (Arisawa et. al. [2012]), especially at fine grained tools. By means of optimized tool macro geometry, bond properties and corn protrusion these issues can be addressed. The effect of temperate in machining CFRP with abrasive milling tools has been investigated by Boudelier et. al. [2013], Sultana et. al. [2016] and Biermann and Feldhoff [2012]. Acquired temperature data from studies (Boudelier et. al. [2013] and Biermann and Feldhoff [2012]) refer to wet cutting and are therefore not applicable for dry cutting. Moreover, the mentioned studies acquired data by means of thermography. This method does not allow direct measuring at the contact interface (Liu et. al. [2016]) and valid data is only ensured when using the appropriate emission coefficient of the material. The state of the art suggests that further studies have to be carried out in order to determine the effect of temperature on abrasive machining of CFRP and gaining profound knowledge on contact temperature. This study is about contact temperature in edge trimming of CFRP with diamond grinding points. Measurements were performed under industry related performance conditions. In variating cutting parameters, effects are identified and explained. Edge quality has been assessed using optical mi croscopy and a machined surface was analyzed with SEM. 32 no. 2, April 2021

processes 3. Evaluation, selection and construction of temperature measuring technique With respect to grinding temperature measuring, two methods can be distinguished: in-contact and non-contact methods. In-contact methods are based on the thermodynamic balance between the measuring device and the thermodynamic system. Thermal capacity and conductivity of the contact measuring device may influence the accuracy and must be considered. Non-contact methods utilize thermal radiation characteristics of the material in order to deduce the temperature of the object. Table 1 shows the advantages and disadvantages of available temperature measuring methods (Bernhard [2014]; Czichos and Hennecke [2008]; Klocke and König [2008] and Herwig and Moschallski [2014]). 4. Experimental setup 4.1 Diamond grinding points The tools used in this study were two diamond grinding points provided by DIT Diamanttechnik GmbH. The geometrical features are: diameter of 10 mm, a diamond grain size of 355/425 μm (tool: D427) and 150/180 μm (tool: D181) respectively. The CNC machining center was 5-axis HOMAG BMG 611 fitted with a hydraulic tool holder. tool D427 tool D181 figure 1: Analyzed tools with different diamond grain size In accordance to the outlined advantages and in consideration of the constraints, thermocouple elements are selected for measurement of grinding temperature. The possibility of engaging the thermocouple elements in the direct contact area predestines this method for investigating the process temperature directly in the contact area. advantages + small design + long-term stability and resistance against most chemicals + measuring range 0 °C - 850 °C è resistance thermometer 4.2 Composite system The investigated workpiece material is a laminate system consisting of nine layers of uni-directional carbon fiber pre-pregs stacked in 0°/90° order and epoxy “E022” resin. The plate is covered by a thin woven top layer. Material designation is named “PR-DU CS 600/1250 FT 109 35” and was fabricated by the company disadvantages Weißgerber GmbH. − measuring current leads to self-warming effect and hence measuring inaccuracy − no direct application in contact area no direct temperature measuring in the contact area is possible thermocouple + small design + high sensitivity + fast response time + low inaccuracy + wide measuring range, -200° C to 1,200° C possible + applicable in contact area (loss of couples in machining) è − signal amplification necessary − preparation of specimen required − limited measuring range due to reduced heatresistance of isolation temperature measuring in contact area is possible and exact process temperatures can be identified thermocolor method + no primer necessary + light- and chemical resistant, non-erosive + feasible for large areas è not suitable for direct measuring + reactionless + large measuring range up to 3000° C + high responsive on changing temperatures è pyrometer/thermography − inaccuracy in color-temperature assignment − no direct application in contact area − ±5 % tolerance of activation temperature − elaborate process of calibration and set-up − sensitive to background radiation not suitable for direct measuring; only applicable on back or front side table 1: Overview of temperature measuring methods and evaluation of pros and cons The laminate features a carbon fiber volume content of 60 %. Testing specimen dimensions are set to 290 mm x 80 mm x 5 mm by milling, creating a non-damaged edge surface. 4.3 Thermocouples Temperature measuring was performed with a set of thermocouples “IEC-GG-K-SLE-JU” from the company OMEGA Engineering GmbH. It is specified as type K (NiCr- Ni) providing highest measuring accuracy. The elements are isolated by a glass silk wrapping, which is heat resistant up to 480° C. Both elements measure 0.5 mm in diameter. Full crosssection dimension is 1.3 x 2.0 mm. In order to amplify the low voltage signals of the thermo- no. 2, April 2021 33

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