Module 2 usm
- 1. TSN, JSSATEB
- 2. INTRODUCTION Ultrasonic machining (USM), sometimes called ultrasonic impact grinding, employs ultrasonically vibrating tool to impel the abrasives in a slurry at high velocity against work piece. The tool is fed into the part as it vibrates along an axis parallel to the tool feed at an amplitude on the order of several thousandths of an inch and a frequency of 20 kHz. TSN, JSSATEB
- 3. • USM is mechanical material removal process or an abrasive process used to erode holes or cavities on hard or brittle workpiece by using shaped tools, high frequency mechanical motion and an abrasive slurry. • USM is used to machining brittle materials such as single crystals, glasses and polycrystalline ceramics TSN, JSSATEB
- 4. • USM is used to machining complex and intricate profiles. • USM used extensively in machining hard and brittle materials that are difficult to machine by traditional manufacturing processes. • Ultrasonic Machining abrasives contained in a slurry are driven against the work by a tool oscillating at low amplitude (25-100 μm) and high frequency (15-30 KHz) TSN, JSSATEB
- 5. Principle of USM “When a tool vibrating at a very high frequency is brought closer to the work piece with abrasive particles between them, the vibrating energy of the tool can propel the abrasive particles to strike the work piece with a great velocity. The impact of the abrasive particle fractures the hard work surface resulting in the removal of material from the work piece” Videos TSN, JSSATEB
- 6. TSN, JSSATEB
- 7. The tool is fed into the part as it vibrates along an axis parallel to the tool feed at an amplitude on the order of several thousandths of an inch and a frequency of 20 kHz. As the tool is fed into the work piece, a negative of the tool is machined into the work piece. The cutting action is performed by the abrasives in the slurry which is continuously flooded under the tool. TSN, JSSATEB
- 8. The slurry is loaded up to 60% by weight with abrasive particles. Lighter abrasive loadings are used to facilitate the flow of the slurry for deep drilling (to 5mm deep). Boron carbide, aluminum oxide, and silicon carbide are the most common used abrasives. Videos TSN, JSSATEB
- 9. USM set up TSN, JSSATEB
- 10. USM Process 1. Conversion of low frequency electrical power to high frequency electrical signal and fed into transducer. 2. Transducer convert high frequency electrical signal into high frequency mechanical motion(Vibration) 3. These mechanical vibration will be guide and amplified and supplied to the tool tip through the horn. TSN, JSSATEB
- 11. Cont.. 4. The tool which is having the same shape as the cavity to be machined, vibrates at a very high frequency . 5. During this time abrasive slurry will pumped between the tool work interface. 6. The vibration of the tool transmits a high velocity to the abrasive particles, 7. The abrasive particles strike the workpiece with great force. TSN, JSSATEB
- 12. Cont… 8. This impact fractures the hard and brittle work surface resulting in the removal of material in the form of small wear particles. TSN, JSSATEB
- 13. Cont… 9. The abrasive slurry flowing at the cutting zone carries away the fractured particles. 10. The tool is pressed against the work piece by applying a slight force 11. The abrasive slurry is be pumped in at low pressure till the operation is completed. TSN, JSSATEB
- 14. Equipment 1. Power Supply 2. Transducer 3. Tool holder 4. Tool cone 5. Cutting tool TSN, JSSATEB
- 15. Power Supply • USM used very high power sine wave generator that converts the low frequency electrical power (60Hz) to a high frequency electric power (20KHz) • The electrical signal is then supplied to the transducer TSN, JSSATEB
- 16. Transducer •The high frequency electrical signal is transmitted to traducer which converts it into high frequency low amplitude vibration. •Essentially transducer converts electrical energy to mechanical vibration. There are two types of transducer used 1. Piezo electric transducer 2. Magneto-stricitve transducer. TSN, JSSATEB
- 17. Piezo electric Transducer Piezoelectric transducer generate mechanical motion through the piezoelectric effect of certain materials like Quartz or Lead – Zirconate. When an electric current is applied to one of these materials the materials increases minutely in size and when the current is removed, the material instantly return to its original shape. TSN, JSSATEB
- 18. Magneto-stricitve transducer • In this type, transducer materials are constructed from a laminated stack of nickel or nickel alloy sheets • Their conversion efficiency is about 20-30%. • Such transducers are available up to 2000 Watts. • The maximum change in length can be achieved is about 25 microns. TSN, JSSATEB
- 20. Tool Holder • The tool holder holds and connects the tool to the transducer. It virtually transmits the energy • In some cases, amplifies the amplitude of vibration. • Material of tool should have good acoustic properties, high resistance to fatigue cracking. • Due measures should be taken to avoid ultrasonic welding between transducer and tool holder. • Commonly used tool holders are Monel, titanium, stainless steel. • Tool holders are more expensive, demand higher operating cost. TSN, JSSATEB
- 21. Tool /Tool cone • Tools are made of relatively ductile materials like Brass, Stainless steel or Mild steel so that Tool wear rate (TWR) can be minimized. • The value of ratio of TWR and MRR depends on kind of abrasive, work material and tool materials. • The size of the tool is slightly smaller than that of the desired shape in the work piece. • The tool is attach to the tool holder by silver brazing, soft soldering, or by means of screws TSN, JSSATEB
- 22. Abrasives/ Cutting tool • Abrasives are usually suspended in liquid and supplied to the cutting point during the operation. • The liquid will help in the – Removal of materials due to cavitations effect. – Uniform distribution of abrasive particles into the gap. • Selection of abrasive is depends on the hardness of the workpiece materials • The abrasive slurry is stored in reservoir and pumped to the tool work interface through the nozzle. TSN, JSSATEB
- 23. Abrasives/ Cutting tool • The slarry apart from fracturing the workpiece, it also carries away the fractured particle of the work materials. • Eg: Boron carbide, silicon carbide and aluminum oxide. TSN, JSSATEB
- 24. Material Removal Models / Mechanism in USM The following are the Material Removal Models used in USM 1. Throwing of abrasive grains. 2. Hammering of abrasive grains. 3. Cavitations in the fluid medium arising out of ultrasonic vibration of tool. 4. Chemical erosion due to micro –agitations. TSN, JSSATEB
- 25. Tool feed Mechanism 1. Counter weight type mechanism 2. Spring loaded feed mechanism system TSN, JSSATEB
- 26. Counter weight type mechanism TSN, JSSATEB
- 27. Cont.. • The feed force being the difference between the weight of transducer + tool holder and counter weight • Counter weight attached through a lever system using pulley • The force is adjusted through weights • This mechanism is insensitive to change in cutting, it is inconvenient to adjust the weights. TSN, JSSATEB
- 28. Spring loaded feed mechanism system TSN, JSSATEB
- 29. Cont… • This system uses piston cylinder arrangement with a suitable liquid or air as working fluid • Through the working fluid required force is applied to the tool • This mechanism is sensitive to changes in cutting conditions. • TSN, JSSATEB
- 30. Process parameters in USM Process parameters affects the metal removal rate(MRR), surface finish and accuracy of the machined surface. a. Amplitude and frequency of vibrations of the tool b. Slurry ( Abrasive water mixture) c. Tool and work material d. Type of abrasive e. Abrasive size f. Effect of applied static load (Feed force) TSN, JSSATEB
- 31. Amplitude and frequency of vibrations of the tool • MRR increases with increase in both Amplitude and Frequency • F and a0 determines is affects on velocity of abrasive particles. • High amplitude tend to increase the surface roughness, however the effect is minimum TSN, JSSATEB
- 32. Slurry ( Abrasive water mixture) • MRR increases with slurry concentration. • Slurry saturation occurs at 30 to 40% abrasive/water mixture. • Material Removal rate drops with increasing viscosity. • The pressure with which the slurry is fed into the cutting zone affects MRR . TSN, JSSATEB
- 33. Cont.. • In some cases MRR can be increased even ten times by supplying the slurry at increased pressure. TSN, JSSATEB
- 34. Tool and work material • The shape of the tool affects the MRR. Narrower rectangular tool gives more MRR compared to square cross section. • Conical tool gives twice MRR compared to cylindrical tool. • The brittle behavior of material is important in determining the MRR. • Brittle material can be cut at higher rates than ductile materials. TSN, JSSATEB
- 35. Cont.. • Tool has to withstand vibration, it should not fail or wear out quickly. • Harder the tool material, the faster the wear rate • Tough malleable materials such as alloy steel and stainless steel are more suitable • Hardness ratio is tool / workpiece hardness TSN, JSSATEB
- 36. Type of abrasive • The abrasive used be should be harder than the workpiece material being machined. • If the abrasive is softer than the workpiece leads to poor surface finish during the subsequent machining • Boron carbide is used for high MRR and hard workpiece like tungsten carbide, tool steel and precious stones. TSN, JSSATEB
- 37. cont.. • Aluminum oxide wears out fast and loses its cutting power • Silicon carbide is best suited TSN, JSSATEB
- 38. Abrasive Size • The size of the abrasive particle varies between 240 – 800 grit • Coarse grades are suitable for high metal removal rates, but result in rough surface finish • Coarse grades are used for roughing operation • finer grades 750-800 grit are used for fine surface finish btr MRR is less TSN, JSSATEB
- 39. Cont. Note: A higher grit number indicates a smaller abrasive grain and a finer abrasive product. TSN, JSSATEB
- 40. Effect of applied static load (Feed force) • Initially with increase in static load on the tool, the depth of penetration of the abrasive particle on the work surface is more lading to increase in MRR • However there is a limit to the applied static load, beyond this limit the depth of penetration is found to decrease leading to low MRR TSN, JSSATEB
- 42. Process capabilities ( Characteristics) of USM Following are the process capabilities of USM process – Metal Removal Rate (MRR) – Surface Finish – Accuracy – Drilling hole capacity – The corner radius obtained is limited to 0.025mm TSN, JSSATEB
- 43. Metal Removal Rate (MRR) • Brittle non metallic materials can be cut at higher rates than ductile materials • USM work satisfactorily only when workpiece hardness greater than HRC40 • It gives excellent results for workpiece with hardness greater than HRC 60. • USM is more suitable for hard and brittle materials TSN, JSSATEB
- 44. Sl No Work Material MRR(mm3/Min) 1 Glass 425 2 Ceramic 185 3 Mica (glass Bonded) 390 4 Tungsten Carbide 40 5 Tool Steel (Hardened) 30 TSN, JSSATEB
- 45. Cont… Stainless steel, cobalt-base heat resistant steels, germanium, glass, ceramic, carbide, quartz and semiconductors are machined. Material removal rate per unit time is inversely proportional to the cutting area of the tool, provided the circulation of the slurry and other operating conditions are held constant. TSN, JSSATEB
- 46. Cont… Tool vibrations also affect the removal rate. The type of abrasive, its size and concentration of the slurry also directly affect the material removal rate. Boron carbide is the hardest and has the highest material removing capability. TSN, JSSATEB
- 47. Surface Finish The size of the abrasive grit has a major influence of the surface finish Larger the grit size the finer will be the surface finished Surface finish may rang from 0.2 to 0.6 micron Finer abrasive results in slower metal removal rate. TSN, JSSATEB
- 48. Accuracy The process accuracy is measured through the overcut (over size) produced during drilling of holes The term over size measures the difference between the hole diameter, measured at the top of the surface , and tool diameter. TSN, JSSATEB
- 49. Cont.. The clearance between the tool and machined hole is necessary to enable the abrasive to flow in to machining zone under oscillating tool. Grain size of the abrasive represents the main factor, which affects the overcut produced. The over cut is about two to four times greater than the mean grain size in case of glas and tungsten carbide The accuracy level in USM is of about ±25micron TSN, JSSATEB
- 50. Drilling Hole Capacity • Hole as small as 76 Micrometer can be drill in USM • Hole depth up to 51 mm have been easily drill • 152 mm deep hole has also been drilled using special flushing technique. • Aspect ratio of 40:1 has been achieved TSN, JSSATEB
- 51. Advantages of USM • Ability to machine non conductive materials Like Glass, ceramics etc. • It is capable to machine intricate cavities in single pass in fragile or /and hard materials. • In USM, there is no direct contact between the tool and workpiece hence it is a good process for machining very thin and fragile components. • A brittle material can be machined more easily than a ductile one. TSN, JSSATEB
- 52. Cont.. • It is considered as a very safe process because it does not involve high voltage, chemicals, mechanical forces and heat. • It is non thermal, non chemical, creates no change in the microstructure, chemical or physical properties of workpiece material. TSN, JSSATEB
- 53. Limitations of USM • Low metal removal rates. • Depth of holes and cavities produced are small. Usually the depth of hole is limited to 2.5 times the diameter of the tool. • Tool wear is more. • Not suitable for soft work piece materials. • Abrasive slurry has to be periodically replaced for efficient machining • It is costlier for complex shapes TSN, JSSATEB
- 54. Applications Of USM • Drilling and machining cavities or holes in conductive and non-conductive materials like glass and ceramics etc. • Threading of various glass and ceramic materials. • Hard materials and precious stones such as synthetic ruby for the preparation of jewels to watch and timer movements are successfully machined by this method. TSN, JSSATEB
- 55. Cont.. • Ultrasonic machining is useful in micro-drilling hole up to 0.1mm. • Enabling a dentist to drill a hole of any shape without creating any pain. • Casting and welding of metals. • Measurement of hardness and grain size determination of metals. TSN, JSSATEB