Dodecyl Sulfate as a Soluble Metal Cutting Fluid for Micromachining with Electroless-Plated Micropencil
2. Materials and Methods 1. Micropencil Grinding Tools
The substrates used for the MPGTs contain a tungsten carbide content of 92%, a cobalt content of 8% with a grain size of 0.2μm. The shaft of the substrates has a diameter of 3.175 mm, a bending strength of 4800 N/mm2and a Vickers hardness of 1920±50 HV30 (ISO 3878) [16]. Figure1depicts the geometry of a substrate and the two main steps in the manufacturing of MPGTs. A 40◦cone is machined on a conventional tool grinding machine onto the substrate to decrease the material removal in the following precision grinding steps. Using a thin grinding wheel, the cylindrical tip of the substrate is machined to have a diameter of 44±2μm at a length of 140μm. For this paper, a grit size of 2–4μm is used. The tool tip diameter must be readjusted when using different grit sizes with different coating thicknesses to allow the coated tool to reach a diameter of ~50μm.
Figure 1.Manufacturing process for micropencil grinding tools (MPGTs): (a) geometry of machined substrate; (b) microgrinding process for MPGT substrates; (c) electroless plating process for MPGT and (d) finished MPGT with 2–4μm grit size.
Following the machining process, the substrate is degreased in an alkaline degreasing solution, which is then neutralized in a hydrochloric acid solution. Then a thin nickel layer is electroplated onto the substrate (Figure2) to provide an active, chemically affine nickel surface. Shrink tubes are applied to the substrates as a resist to limit the nickel coating to a defined area [17]. Finally, the electroless-plating process is performed.
Figure 2.Abrasive layer. cBN: cubic boron nitride.
Electroless plating is a process that is based on the principle of ionic reduction. In the solution, a metal salt, in this case nickel sulfate, provides the solution with Ni2+free nickel ions. A reducing agent like sodium hypophosphite can provide the nickel ions with the two missing electrons, slowly forming a phosphorous nickel coating onto an active surface; the components of the plating solution are listed in Table1. The process is suited to manufacture small quantities of MPGTs, with flexible, custom design choices in regards to its form, diameter, coating thickness, grit size, grit concentration and grit protrusion [17].
Using the ingredients listed in Table1, a quantitative energy dispersive X-ray (EDX) analysis shows that a phosphorous content of 6.01%±0.55% can be achieved. A phosphorus content of less than 7% results in a face-centered cubic crystal structure, while an amorphous structure is produced at higher phosphorus contents [18]. A low phosphorous content generally produces a harder nickel layer [19].
Table 1.Electroless-plating solution composition [20].
Component Concentration in g/L
Nickel sulfate (NiSO4·6H2O) 30
Sodium hypophosphite (NaH2PO2) 20
Sodium acetate (C2H3NaO2) 20
Thiourea (CH4N2S) 0.0004
Hydrochloric acid (HCl) Adapted to a pH value of 5.2–5.4
cBN grits 4
The abrasive grits are whirled up in the coating solution via a magnetic stirrer. The main coating time for a monolayered MPGT is 150 s for a grit size of 2–4μm. After the main coating time, the magnetic stirrer stops and the grits fall to the bottom of the beaker, allowing to embed the grits on the MPGT with an additional nickel layer for 90 s (see Figure2). The final product for a single layered MPGT can be seen in Figure1d. Using scanning electron microscopy (SEM) images and an image processing software, a quantitative analysis was conducted to determine the grit concentration on the tool. A grit concentration of 35%±7% was found.
2.2. Experimental Setup
The results presented in the following chapters were produced on a high precision three-axis machine tool (Figure3) mounted on top of a vibration isolated granite plate. The tool spindle is mounted vertically onto thez-axis on a cross-roller bearing stage. Rotation speeds in the range of 5000–54,000 rpm can be achieved. The X–Y table is guided by air bearings and can move with a positioning accuracy of <1μm [5]. A Kistler 3-component dynamometer (9119AA1) dynamometer for measuring cutting forces is mounted on top of the X–Y table; the workpieces are clamped onto the dynamometer.
Figure 3.Machine tool for microgrinding and milling. MQL: minimum quantity lubrication.
A Venturi minimum quantity lubrication (MQL) system is used to spray the machining process with an air/liquid mixture. By narrowing the cross-section at the nozzle head, a pressure difference is created through which the liquid is suctioned. The air acts as a transport medium for the liquid.
Flow rates of <100 mL/h are defined as MQL in macro machining. However, considering the small sizes of tools and structures and the comparably low material removal rates in microgrinding, 100 mL/h can be defined as flood cooling in microgrinding.
Images of the tools and their respective structures were captured using a scanning electron microscope (SEM). The structures were analyzed using a confocal microscope (Nanofocusμsurf) with a 60×magnification lens and a numerical aperture (NA) of 0.9.
2.3. Experimental Procedure
To test the influence of SDS as a metal cutting fluid in the microgrinding process, MPGTs with diameters ~50μm were used to machine 500μm long grooves into hardened 16MnCr5 (SAE5115;
665 HV30±15 HV30 (according to ISO 6507 [21])). The workpiece was face-machined with a larger pencil grinding tool (diameter = 3.175 mm) to compensate for assembly-related influences and to gain a flat surface. To test the effect of the soluble lubricant, a small amount of 0.2 g/L was added to a distilled water medium and was used for the experiments. For comparison, the microgrinding process was also conducted dry and with distilled water as metal cutting fluid. Both the SDS and distilled water experiments were conducted with a volume flow rate of 60±10 mL/h and a positive air pressure of 0.65 bar. Both form a fluid film around the tool during the process (Figure4b); experiments showed that if the fluid film is interrupted, immediate damage to the abrasive layer occurs.
Based on preliminary studies; a rotation speed of 30,000 rpm (cutting speed of 4.71 m/min) was applied. Feed rates of 0.05 mm/min and 0.1 mm/min were used at a depth of cut of 5μm;
Figure4a shows the parameter combinations studied in this paper with each combination being repeated three times. The tools were maneuvered to the starting position optically using the camera.
The MPGT is used to scratch the surface of the workpiece to determine the zero position between tool and workpiece. This results into a positioning accuracy of±50μm and hence in an according deviation of the groove length. The tool rotates in clockwise direction, while the workpiece was given a feed rate towards the tool.
Figure 4.(a) Microgrinding test series; (b) microgrinding process.
3. Results