processes Double disc
processes Double disc fine grinding: The sharpening process is an important factor regarding the lifespan of the double disc grinding wheel A scientific study by Dr.-Ing. Marijke van der Meer (Krebs & Riedel Schleifscheibenfabrik GmbH & Co. KG) shows that by adjusting sharpening parameters such as pressure, speed and grain size, the lifespan of double disc grinding wheels and sharpening tools can be significantly extended. This makes a standard sharpening process less advantageous and advocates for a targeted adjustment to the specific requirements of the tool. Technological consulting and optimization at the customer’s site can positively influence the cost-effectiveness and efficiency in double disc grinding with planetary kinematics. Conditions and parameters during sharpening: A test machine from Mµtech, model AC500F, was used. This has been specifically adapted for Krebs & Riedel to accommodate smaller tool diameters to keep testing costs low. To ensure reproducible sharpening conditions, the wear of the sharpening ring is recorded, documented, and at least one repetition is performed. The coolant used is the emulsion Syntilo 81 E (5.4 %). Sharpening rings are immersed or wetted in it, while no coolant is supplied during the sharpening process. Subsequently the grinding wheels are cleaned. In the standard sharpening procedure, one side is sharpened concavely and once convexly to keep the wheels straight. Only the rotation direction of the inner pin ring changes. The speeds in revolutions per minute (rpm) are: concave: n top 30 / n bottom -36 / n middle -24 and convex: n top 30 / n bottom -36 / n middle 24. The main load force is 38 daN, and the sharpening time per rotation direction is 25 s, with the first 5 s dedicated to wetting with coolant. Wear from both the sharpening ring and lower grinding wheel is recorded – the latter after both sharpening and grinding. Initially grinding wheels with specifications 4D 76 X 18 V8318-40 and closed layout are examined, which are sharpened with 57C 220 I 5 V59 (figure 1). For dulling, components made from aluminum oxide ceramic (figure 2) are processed. Here, nine components measuring 20 x 20 mm² from ALOTEC92 per carrier with five carriers correspond to a material layout of 22.4 %. The main load speeds are n top 200; n bottom -200; n middle 20 rpm with a starting grinding force in main load of 30 daN. The removal rate is set at 100 µm/min with an infeed per run of 200 µm. The grinding force increases automatically when dulling the grinding wheels to maintain removal rate. Runs continue until a maximum force of 250 daN is reached to ensure uniform dulling of the grinding surface. In each trial process cycle times, forces, actual removal rates and wear on grinding wheels are documented. For aluminum oxide plates, roughness primarily arises from coarse material structure and is independent of sharpness level; thus, evaluation of component roughnesses is omitted here. Even with a dull tool, a Ra > 0.5 µm and Rz > 5 µm are achieved. To capture sharpness levels under various process variations, a method using graphite imprints has been introduced. Holding devices for graphite pieces were created using 3D printing. These graphite pieces represent the produced envelope curve of grinding wheel topography as roughness when processed by upper and lower discs. Results Initially starting from the described standard sharpening process, pressure was varied. Higher pressure does not automatically mean sharper discs based on roughness measurements; this is confirmed by material removal on graphite (figure 3). figure 1: Used grinding & sharpening tools figure 2: Aluminum oxide ceramic workpieces 32 no. 2, May 2025
processes Graphite imprint: Material removal after sharpening/grinding 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 outer inner V1-76daN-Sharpening V1-76daN-Grinding V2-38daN-Sharpening V2-38daN-Grinding V3-19daN-Sharpening V3-19daN-Grinding V4-9daN-Sharpening V4-9daN-Grinding V5-5daN-Sharpening V5-5daN-Grinding figure 3: Material removal on graphite after sharpening and after grinding at various sharpening forces The values in figure 4 were recorded during roughness measurement of graphite; it should be noted that graphite prints correspond to a negative impression of overlaid grinding wheel topography. This means Rvk represents grain tips while Rpk represents topographic valleys (pores/chip space). Rpk and Rvk illustrate that more dulling occurs externally than internally; thus, the center of the cutting tool ring becomes most flattened. The top side remains slightly sharper than the bottom side. After sharpening surfaces are always distinctly rougher than after grinding; Rvk shows less difference between sharpening and grinding compared to Rpk where grains (Rvk) wear less than the bond (Rpk). 3.00 Graphite imprint: Roughness Rpk top after Sharpening/Grinding 5.0 Graphite imprint: Roughness Rvk top after Sharpening/Grinding 2.50 4.5 4.0 2.00 3.5 Rpk µm 1.50 Rvk µm 3.0 2.5 1.00 2.0 1.5 0.50 1.0 0.5 0.00 outer middle inner 0.0 outer middle inner V1-76daN-Sharpening V1-76daN-Grinding V2-38daN-Sharpening V2-38daN-Grinding V3-19daN-Sharpening V1-76-daN-Sharpening V1-76-daN-Grinding V2-38daN-Sharpening V2-38daN-Grinding V3-19daN-Sharpening V3-19daN-Grinding V4-9daN-Sharpening V4-9daN-Grinding V5-5daN-Sharpening V5-5daN-Grinding Graphite imprint: Roughness Rpk bottom after Sharpening/Grinding 3.00 V3-19-daN-Grinding V4-9-daN-Sharpening V4-9daN-Grinding V5-5daN-Sharpening V5-5daN-Grinding Graphite imprint: Roughness Rvk bottom after Sharpening/Grinding 5.0 4.5 2.50 4.0 Rpk µm 2.00 1.50 Rvk µm 3.5 3.0 2.5 2.0 1.00 1.5 0.50 1.0 0.5 0.00 outer middle inner 0.0 outer middle inner V1-76daN-Sharpening V1-76daN-Grinding V2-38daN-Sharpening V2-38daN-Sharpening V3-19daN-Sharpening V3-19daN-Grinding V4-9daN-Sharpening V4-9daN-Grinding V5-5daN-Sharpening V5-5daN-Grinding V1-76daN-Sharpening V1-76daN-Grinding V2-38daN-Sharpening V2-38daN-Grinding V3-19daN-Sharpening V3-19daN-Grinding V4-9daN-Sharpening V4-9daN-Grinding V5-5daN-Sharpening V5-5daN-Grinding figure 4: Rpk and Rvk after sharpening and grinding above and below no. 2, May 2025 33
