Chemical Machining- PCM , ECM , ECH, ECG | Interview Question and answers

Chemical Machining- PCM , ECM , ECH, ECG – Interview Mechanical Engineering question and answer , viva ,oral examination question and answers

1. What is the principle of Chemical Machining (CHM)?

Chemical attacks metals and etch them by removing small amounts of material from the surface using reagents or etchants.

2. What are the processing steps in CHM?

I. Preparing: precleaning
II. Masking : application of chemically resistant material (if selective etching is desired)
III. Etch: dip or spray exposure to the etchant
IV. Remove Mask: strip remaining mask and clean
V. Finish: inspection and post processing

3. What is etch factor?

Ratio of undercut to depth of cut

4. What are the two major processes in CHM?

• Two major CHM processes are:
I. Chemical milling – eroding material to produce blind details – pockets, channels etc.,
II. Chemical blanking – for producing details that penetrate the material entirely (holes, slots, etc.)

5. What is the purpose of etchant used in CHM? Give some examples.

• Purpose: to dissolve a metal by turning it into a metallic salt, which then goes into solution
• Many chemical are available as etchants: FeCl3, Chromic acid, FeNO3, HF, HNO3

6. What are the criteria used for selection of etchant?

Required surface finish
Removal rate
Material type
depth
Type of resist
Cost

7. What is the purpose of maskant and how is it classified?

Maskants (chemically resistant coatings) are used to cover the surfaces which are not to be machined – does not allow the etchant to react reach and react with work piece to dissolve it.

8. Based on the techniques of applying maskants, classify them

• Cut and peel – Involve the use of relatively thick material which is scribed and removed to create a selective exposure to the etchant – Neoprene, butyl or vinyl-based material is used as maskants
• Screen printing – A fine mesh silk or stainless steel screen, which has areas blocked-off to allow selective passage of the maskant is used.

• Photoresist masks – materials that produce etchant resistant images by means of photographic techniques.

9. How are maskants selected?

Selection of proper maskant for a particular application is accomplished by evaluation of the job with respect to six factors – chemical resistance, part configuration, quantity of parts, cost, ease of removal and required resolution

10. What are the applications for CHM and photochemical machining (PCM)?

CHM is used extensively to etch preformed aerospace parts to obtain maximum strength to weight ratios:
• Integrally stiffened Titanium engine ducts
• Spray etching a rotating tube for cruise missile launch tubes
• Thinning and sizing of a delta booster tank bulkhead
• Chemical sizing of engine cowl inlet duct skins
• Undercut on clad aluminium

11. What are the advantages of CHM and PCM?

• Metal removal is completely stress free
• Complex shapes and deeply recessed areas can be uniformly chemically milled
• Extremely thin sections can be chemically milled
• Metal hardness or brittleness is not a factor

12. What are the limitations of CHM & PCM?

• Fillet radius is approximately equal to depth of cut
• Extremely deep cuts are usually not cost effective
• A homogenous metal structure is normally required for good results
• Suitable photographic facilities are not always available

13. Please identify the principle of ECM. How does it differ from electroplating?

Principle of ECM – electrolysis.

When a D.C potential is applied across two electrodes separated by a small gap and an electrolyte is pumped through the small gap, the
constituents of the anode work piece material goes into the solution and not plate on the cathode tool. Electroplating is the reverse of ECM where the cathode is plated by the depleted metal from the anode.

14. What is Faraday’s first law?

The amount of chemical changes produced by an electric current (or the amount of substance deposited or dissolved) is proportional to the quantity of electricity passed.

15. What is Faraday’s second law?

The amount of different substances deposited or dissolved by the same quantity of electricity are proportional to their chemical equivalent weights.

16. Explain the parameters controlling the MRR in ECM. (Ref. P.N.Rao and Benedict)

MRR in ECM is primarily controlled by current density. When all other variables are held constant, the tool penetration rate is directly proportional to the current density. Current density is not only controlled by the amount of current but also by the size of the gap between the tool and the workpiece.

Feed rate: a high feed rate results in a higher MRR. It also decreases the equilibrium machining gap resulting in improvement of the surface finish and tolerance control
The velocity and the electrolyte flow through the gap is also an important parameter affecting the surface finish and removal rate

17. What are the various process parameters of ECM?

The process parameters that have a control on the performance of the ECM process is: Feed rate, current density, voltage, electrolyte concentration (low concentration of the electrolyte decreases the machining gap and results in a better surface finish and finer tolerance control) and electrolyte temperature (low temperature of the electrolyte is conducive to a better surface finish and tolerances), velocity and flow of electrolyte.

Electro Chemical Machining Diagram -Parameter, Advantages and Disadvantages

18. What are the various process characteristics of the ECM?

Material Removal Rate – the MRRs with ECM are sufficiently large and comparable with that of the conventional methods. MRR of 16m3 /min for 10,000 A is generally obtained in ECM .
Surface finish – under certain conditions, ECM can produce surface finishes of the order of 0.4 m
Accuracy – under ideal conditions and with properly designed tooling, ECM is capable of holding tolerance of the order of  0.02mm and less.

19. What are the various tool materials that can be used effectively with ECM?

Generally aluminium, copper, brass, titanium, cupro-nickel and stainless steel are used as tool materials.

20. What factors should be considered in selecting the tool materials in ECM?

High thermal and electrical conductivity
Good stiffness- Rigidity of the tool construction and material is important because the high pressure can cause deflection of the tool
Easy machinability- particularly important if complex shaped tools are required
High corrosion resistance- to protect itself from the highly corrosive electrolyte solution

21. What are the different types ECM operations?

Electro Chemical Milling (ECM)
Electro Chemical Grinding (ECG)
Electro Chemical Honing (ECH)
Electro Chemical Deburing (ECD)
Electro chemical turning (ECT)
Electro chemical trepanning (ECTr) etc.,

22. Enumerate some typical examples where ECM process can be effectively adopted.

Deep hole drilling having L/D>20
Contouring operations in the die blocks
Through cutting and through cavities

23. What are the different applications of ECM?

Aerospace industries: machining gas turbine blades, airframe component fabrication, honey-comb aircraft panels, jet engine blade airfoils
Manufacture of general machine parts: thin wall mechanical slotting, difficult to machine hollow shafts, chain pinions, internal profile of internal cams, driving joints, pump glands and impellers, connecting rod, hydraulic spools, gear wheels
Facing and turning complex 3D surfaces
Cutting off

24. What are the advantages of the ECM process?

Long tool life
Machines complicated shapes in single pass
Machines any material hardness – independent of physical and mechanical properties
No burrs
No stress
Good surface finish
Good accuracy

25. What are the limitations of the ECM process?

Work must be electrically conductive
High maintenance
Can cause intergranular attack (IGA)
High tooling and set-up costs

26. For which type of work, ECM is useful?

ECM is suitable for hard and difficult to machine materials and when complex shapes are required which are not machinable by conventional methods.

27. What is ECG? Identify its applications.

ECG is a process that combines the ECM with the mechanical grinding operation to remove material. It uses a grinding wheel with an electrically conductive abrasive bonding agent.

Applications:

Single largest use for ECG is in the manufacturing and remanufacturing of turbine blades and vanes for aircraft turbine engines
Grinding of tungsten carbide tool inserts
Re-profiling worn locomotive traction motor gears
Burr-free sharpening of hypodermic needles
Machining of fragile or very hard and tough material – honey comb, thin walled tubes and skins
High MRR’s when grinding hard, tough, stringy, work-hardenable or heat sensitive materials

28. Identify any two advantages and limitations of electrochemical grinding (ECG)

Advantages
• No thermal damage to workpiece
• Elimination of grinding burn
• Absence of work hardening
• Long-lasting wheels – less truing
• Higher MRR;
• Single pass grinding – reduced cost of grinding;
• Absence of burrs on the finished surface;
• Improved surface finish with no grinding scratches;
• Reduced pressure of work against the wheel – no distortion;
• In ECG, the ECM action is efficient

Limitations

High capital cost / Higher cost of grinding wheel;
Corrosive environment
High preventive maintenance cost
Tolerance achieved are low;
Difficult to optimize due to the complexity of the process;
Non-conductive materials cannot be machined

29. What is ECH? Identify its applications.

ECH is a process in which the metal removal capabilities of ECM are combined with the accuracy capabilities of honing. The process consists of a rotating and reciprocating tool inside a cylindrical component.
Applications: the process is easily adaptable to cylindrical parts for trueing the inside surfaces.

30. What is ECD? Identify its applications.

ECD is a process for the removal of metal burrs by anodic dissolution, the same principle as ECM.

Applications:

Generally employed for far away located as well as inaccessible places where other deburring processes are not effective.
ECD process has been successfully applied in many different industries ranging from consumer appliances and automotive to biomedical and aerospace products
The automotive industry, a heavy user of ECD, employs this process for deburring and radiusing the crossholes in crankshafts.
Not only is ECD applicable to the high-volume environment of the automotive industry, but it is also applicable to the automated factory.

31. How does ECD differ from ECM?

ECD is a special version of ECM used exclusively to deburr or radius workpieces. It differs from ECM in that the electrolyte pressure, electrolyte flow, and current are all relatively low. Another major difference is that the cathode (tool) is usually held stationary in ECD.

32. ECD is same as ECM except much simple to use. Justify the statement.

ECD is same as ECM except much simple to use because, there is no feed mechanism needed for the ECD tool (the tool is held stationary). Moreover in ECD, the electrolyte pressure, electrolyte flow, and current are all relatively low.

33.. What are the functions served by the electrolyte in the ECM process?