Microstructure of a steel fastener with a propriety double-layered anti-corrosion coating polished with the aid of Springlam, a novel sample-moving accessory, on a manual polishing machine. The microstructure serves as a proof that, many times, high quality results can be obtained with simple machines and tools even without compensating on the degree of automation.
Metallographic sample grinding/polishing is critical in metallographic sample preparation because of the necessity to remove any deformation in the microstructure from sectioning and to remove the scratches on the surface. This phase of sample preparation generally starts with a coarse-grinding step that removes enough material from the surface – 1. To get rid of the deformation layer from sectioning. 2. To render the samples flat for pre-/fine-polishing. Grinding operations can also induce microstructural deformation which has to be removed by the following polishing step. To minimize the damage in the microstructure to the best extent possible, both grinding and subsequent polishing are carried out with refining abrasive sizes. This means that the amount of material removed regresses as polishing progresses. This is why the grinding step has to render the samples flat so that the objective of polishing will be limited to removing the grinding scratches and deformation.
The figure below depicts the ideal grinding plane to have flat ground samples during preparation compared against a convex grinding plane that results in edge damage and non-flat surfaces with extensive scratching on the edges. A non-ideal grinding plane often results from differential material removal arising from, for example, huge difference in hardness between the resin and the sample, the kinetics of the machine, presence of multiple materials of varying hardness in the material, poor choice of consumables, etc.
Difference between polishing a sample with a flat ground surface and a convex ground surface: When the surface is convex (on the left), polishing leaves out the grinding scratches on the edges of the specimen, whereas (on the right), the grinding scratches are removed uniformly in the flat-ground specimen resulting in better edge retention.
Individual pressure mode
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Pressure applied through pistons dedicated for each sample
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Slower transition between steps owing to the placement of samples one by one at the start of every step
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Higher possibility of samples oscillating in the sample-holder while rotating, resulting in edge rounding, differential material removal across the surface of the sample, or damage in fragile coatings
Central pressure mode
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Pressure distributed through a central stem
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Faster transition between polishing steps since the samples are tightened to the sample holder throughout the polishing cycle
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Flat polishing of samples
LAM PLAN’s unconventional central pressure polishing system
By eliminating the complexity of pneumatics and various electronic components, Springlam achieves similar results as a conventional central pressure polishing system at a reduced cost and increased simplicity.
In lapping the loaded pieces turn freely on a lapping plate with abrasives, at a speed determined by the speed of the plate. Lapping principally takes advantage of this kinetic phenomenon to produce the desired flatness with the desired surface finish. The solution is inspired from lapping in a way that, the specimen holder is allowed to rotate freely as dominated by the rotation of the plate.
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