Chemical Milling ppt

It's a presentation prepared by me on Chemical milling a type of non traditional machining process to help the students to know the key concept about it.

1. Chemical milling (chm) By – Aditya sharma Mech /67/15

2. Introduction Chemical Milling (CHM) is the controlled chemical dissolution(CD) of the work piece material by contact with a strong reagent Used to produce pockets & contours CHM consists of following steps- @ Preparing and precleaning the work piece surface @ Masking using readily strippable mask @Scribing of the mask, guided by templates @The work piece Is then etched and rinsed

3. Chm Setup

4. Etch Factor Etch factor is the ratio of the undercut (d) to the depth of etch (t). It must be considered when scribing the mask using templates. A typical etch factor of 1:1 occurs at a cut depth of 1.27mm

5. Tooling for Chm Four different types of tools are required- Maskants Etchants Scribing Templates Accessories

6. Maskants Generally used to protect parts of work piece where CD action is not needed. Properties; @ be tough enough to withstand handling @Adhere well to the work piece surface @scribe Easily @Be inert to the chemical reagent used

7. Etchants Etchants are acid or alkaline solutions maintained within a controlled range of chemical composition and temperature. Main technical goals of etchants are- @ Good surface finish @Uniformity of metal removal @ Maintenance of air quality and avoid the environmental problems @ ability to regenerate the etchant solution

8. Scribing TEmplAtEs Scribing templates are used to define the areas for exposure to the chemical machining action. The most common work piece scribing method is to cut the mask with a sharp knife followed by carefully peeling of the mask from the selected areas.

9. Accessories Accessories are used for single or multiple pieces handling into and out of the etchants and rinses.

10. Process Parameters CHM process parameters include the reagent solution type, concentration, properties, mixing, operating temperature and circulation. These parameters will have the direct impacts on the work piece as follows- @ Etch factor (d/t) @ etching and machining rate @ production tolerance @ surface finish

11. Material removal rate ( mrr ) Material removal or etch rate depends upon the following : Chemical and metallurgical composition of the workpiece . Solution temperature. Corrosive power of the etchant. Grain size Metal type

12. CHM average roughness of some alloys after removing 0.25 to 0.4 mm

13. Rolled metal sheets have the highest machining rate accompanied by the best surface quality. Etching rates were high for hard metals and were low for softer ones . Castings having the largest grain size, show the roughest surface together with the lowest machining rate . Generally, the high etch rate is accompanied by a low surface roughness and, hence, narrow machining tolerances.

14. Accuracy – surface finish of component achieved by machining process In CHM, the metal is dissolved by CD action. The machining phase takes place both at individual grain surface as well as grain boundaries. Fine grain size and homogeneous metallurgical structures are necessary for fine surface quality of uniform appearance. Surface finished by CHM do not have regular lay pattern. Based on grain size, orientation, heat treatment and previously induced stresses, every material has basic surface finish that results from CHM for certain period of time.

15. Surface imperfections will not be eliminated by CHM. Any prior surface irregularities, waviness, dents or scratches will be slightly altered and reproduced in machined surface. The depth of cut tolerance increases when machining larger depths at high rates. Aluminium and Magnesium alloys can be controlled more closely than steel, Nikel or Titanium alloys. An etching rate of 0.025mm/min with tolerance of +10 or -10 percent of cut width can be achieved depending on workpiece material and depth of cut.

16. Figure 3.7 : Surface roughness and etch rate of some alloys after removi ng 0.25 t o 0.4 m m .

17. FACTORS AFFECTING SURFACE FINISHING OF A COMPONENT

18. Basically grain size and homogeneous metallurgical structure are necessary for fine surface quality. While surface imperfections will not be eliminated by CHM but any prior surface irregularities, waviness, dents or scratches will be slightly altered .

19. FACTORS AFFECTING SURFACE FINISHING ARE INITIAL WORKPIECE ROUGNESS PREVIOUS COLD WORKING PROCESS MACHINING RATE AND MACHINING TEMPERATURE ETCHANT SOLUTION AND COCENTRATION

20. # INITIAL WORKPIECE ROUGNESS It increases with the increase in the metal ion concentration in the etchant. # PREVIOUS COLD WORKING PROCESS surface roughness decreases with the previous cold working process and thus , we get good surface finish

21. # MACHINING RATE AND MACHINING TEMPERATURE B y increasing the machining rate, machining temperature increases and thus we get surface roughness. # ETCHANT SOLUTION AND CONCENTRATION To get good surface finish we can decrease the temperature by adding high pressure coolant to the echants . For example: Triethanolamine . etchant's specific gravity , oxidation reduction potential can also effect surface finish.

22. Advantages, Limitations and Application of CHM

23. Advantages Weight reduction is possible on complex contours that are difficult to machine using conventional methods. No burrs are found. Design changes can be implemented quickly. A less skilled operator is needed. Simultaneous material removal, from all surfaces, improves productivity and reduces wrapping.

24. Limitations Handling and disposal of chemicals can be troublesome. Surface imperfections are reproduced in the machined parts. Deep narrow cuts are difficult to produce. Porous castings yield uneven etched surfaces. Material removal from one side of residually stressed material can result in a considerable distortion. Welded areas frequently etch at rates that differ from base metal.

25. Applications CHM applications range from large aluminum alloy airplane wing parts to minute integrate chips. CHM is used to thin out walls, webs, and ribs of parts that have been produced by forging, casting, or sheet metal forming Shallow cuts in large thin sheets are the most popular application especially for weight reduction of aerospace components Removal of sharp burrs from conventionally machined parts of complex shapes. Elimination of the decarburized layer from low alloy steel forgings.