MMM LECTURE NOTES FINAL.pdf

Chm will not eliminate surface irregularities dents

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CHM will not eliminate surface irregularities, dents, scratches, or waviness. Successive steps of mask removal and immersion as shown in Fig. 3.3 can achieve stepped cuts. Tapered cuts (Fig. 3.4), can also be produced without masking the work piece by controlling the depth and rate of immersion or withdrawal and the number of immersions. Continuous tapers, as great as 0.060 mm/mm for aluminium and 0.010 mm/mm for steel alloys. Tooling for CHM Tooling for CHM is relatively inexpensive and simple to modify. Four different types of tools are required: maskants, etchants, scribing templates,and accessories.
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Modern Manufacturing Methods Fig., Machining tapers and steps by CHM. Maskants Maskants are generally used to protect parts of the workpiece where CD action is not needed. Synthetic or rubber base materials are frequently used. Table 3.1 shows the different maskants and etchants for several materials together with the etch rate and etch factor. Maskants should, however, possess the following properties: 1. Be tough enough to withstand handling 2. Adhere well to the workpiece surface 3. Scribe easily 4. Be inert to the chemical reagent used 5. Be able to withstand the heat generated by etching 6. Be removed easily and inexpensively after etching Spraying the mask on the workpiece through silk screen, on which
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Modern Manufacturing Methods the desired design is imposed, combines the maskant application with the scribing operation since no peeling is required. The product quality is, therefore, improved as is the ability to generate finer details.However, the thin coating layer applied when using silk screens will not resist etching for a long time as will the cut-and-peel method. Photoresist masks, which are used in photochemical milling (PCM), also combine both the coating and scribing operations. The relatively thin coats applied as dip or spray coats will not withstand rough handling or long exposure times to the etchant. However, photoresist masks ensure high accuracy, ease of repetition for multiple-part etching, and ease of modification. The accuracy obtained for lateral dimensions depends on the complexity of the masking. Typical tolerances for the different masks are as follows: Cut-and-peel masks ±0.179 mm Silk-screen resist ±0.077 mm Photoresist ±0.013 mm Etchants Etchants are acid or alkaline solutions maintained within a controlled range of chemical composition and temperature. Their main technical goals are to achieve the following: 1. Good surface finish 2. Uniformity of metal removal 3. Control of selective and intergranular attack 4. Control of hydrogen absorption in the case of titanium alloys 5. Maintenance of personal safety 6. Best price and reliability for the materials to be used in the construction of the process tank. 7. Maintainance of air quality and avoidance of possible environmental Problems.
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  • Fall '12
  • JeraldBrevick
  • EDM, conventional machining processes, Jet Machining

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