A Paper Topic on
HIGH SPEED MACHINING USING
CRYOGENICS AND IT’S FEASIBILITY
ABSTRACT:
In conventional machining processes when we go for high speed excessive heat is generated during the chip formation process, which increases the temperature of cutting tool and accelerates tool wear. Conventionally cutting fluid is used to cool and lubricate cutting process. However conventional cooling process has inherent health and environmental problems .In addition to this a process should facilitate longer tool life ,higher cutting speed ,better work surface, less built up edge, easier chip breaking and lower production cost.
Cryogenic machining is one such process in which above mentioned features are present. Cryogenic machining uses liquid nitrogen as a coolant .Liquid Nitrogen has a boiling point of -199 degree centigrade. It has been observed that at cryogenic temperatures tool materials get harder and stronger.
A micro nozzle is formed between tool face and chip breaker and LN2 is passed through it. The chip breaker helps to lift chip so that the jet reaches the hottest spot i.e. tool chip interface. At this point LN2 absorbs heat and evaporates producing gas cushion causing lubrication effect. This localized cryogenic cooling reduces the tool face temperature, enhances its hardness, so less wear rate .The lubrication effect reduces friction and tool wear.
This new cryogenic machining eliminates the built up edge (BUE) problem on tools because cold temperature reduces possibility of chips welding to tool. so the process will allow high speed cutting with out built up edge.
In addition to varies advantages of the cryogenic machining, the paper discloses the comparison of cryogenic machining with conventional process in the aspects of tool life, productivity, and production cost.
INTORDUCTION :
In conventional machining processes when we go for high speed excessive heat is generated during the chip formation process, which increases the temperature of cutting tool and accelerates tool wear. Conventionally, cutting fluid is used to cool and lubricate cutting process, there by reducing tool wear and increasing tool life. However conventional cooling process has inherent health and environmental problems. Dumping of cutting fluid waste causes environmental pollution, even if we resort to recycling of cutting fluid waste it would cost double the purchase price. Conventional coolant poses health problems to worker, like Dermatitis and skin cancer.
Industry continues to investigate ways to achieve longer tool life, higher cutting speed, better work surface, less built up edge, easier chip breaking and lower production cost and which considers above mentioned health and environmental problems.
Cryogenic machining is one such process in which above mentioned features are present. Cryogenic machining uses liquid nitrogen as a coolant. Liquid Nitrogen has a boiling point of -199oC.
CRYOGENIC MACHINING :
Cryogenic machining is one such process in which above mentioned features are present. Cryogenic machining uses liquid nitrogen as a coolant .Liquid Nitrogen has a boiling point of -199 degree centigrade. It has been observed that at cryogenic temperatures tool materials get harder and stronger.
A micro nozzle is formed between tool face and chip breaker and LN2 is passed through it. The chip breaker helps to lift chip so that the jet reaches the hottest spot i.e. tool chip interface. At this point LN2 absorbs heat and evaporates producing gas cushion causing lubrication effect. This localized cryogenic cooling reduces the tool face temperature, enhances its hardness, so less wear rate .The lubrication effect reduces friction and tool wear.
This new cryogenic machining eliminates the built up edge (BUE) problem on tools because cold temperature reduces possibility of chips welding to tool. so the process will allow high speed cutting with out built up edge.
CRYOGENIC MACHING APPROACH :
The cryogenic properties of the both workpiece materials and tool materials were studied. Those studies concluded that it is not desirable to cool the workpiece for three reasons
1. Metals tend to increase in hardness and strength at cryogenic temperature
2. Exposing the workpiece to low temperatures may increase the abrasion to the cutting tool.
3. Precooling the workpiece will increase the cutting force. In addition for dimensional accuracy and distortion reduction.
The study of cryogenic properties of common tool materials (E.g. high speed steel and various grades of carbides) shows the advantages of cooling the tool. At cryogenic temperatures, the tool will get harder and stronger. Impact testing shows that the tool materials do not noticeably change in brittleness, nor do they fracture more easily. As a result of material studies, it is recommended that the cutting tools, but not the work piece materials, be cooled.
The cryogenic machining approach has following concepts.
1. Cryogenic fluid is applied directly to, and only to, the very tip of the cutting tool, where the material is being cut and heat is being generated. The cooling power is not wasted on any unnecessary areas.
2. Using the energy balance concept, the flowrate of the cryogenic fluid is in proportion to the heat generated in the cutting process. The workpiece will maintain constant temperature and not subjected to dimensional inaccuracy and geometrical distortion.
3. The micro nozzle is formed between the tool face and chip breaker as a commercial new cutting tool assembly, a design that is economical and convenient for users.
Based on the above concepts, Fig. 1 illustrates the cryogenic machining cooling principles using a flat cutting insert with an obstruction chip breaker as example LN2 is released through a nozzle between chip breaker and the face of the tool insert .The chip breaker helps to lift up the chip so that LN2 can reach the tool-chip interface. This important feature allows LN2 to cool the hottest spot closely and the flow not blocked by the chip. At this point, LN2 absorbs the heat, evaporates quickly, and forms a fluid/gas cushion between the chip and tool face thereby providing
“ Lubrication effect ”.
PRINCIPLE OF ECONOMIC CRYOGENIC MACHINING :
This localized cryogenic cooling reduces the tool face temperature, enhances it hardness, and so reduces its wear rate. The lubrication effect further reduces the friction and tool wear. The tool body remains tough, as it is not cooled directly. This is the most effective way for long tool life while the least amount on LN2.
This approach improves chip breaking in three ways: embrittle the chip by cold temperature, improve chip grain structure by reducing secondary deformation and recrystallization, and bend the chip with the chip breaker. This new cryogenic machining approach eliminates the build-up edge problem (BUE) on tools because the cold temperature reduces the possibility of chips welding to tool. The high-pressure cryogenic jet also helps to remove possible BUE. This process will allow high-speed cutting and no BUE, therefore it will produce better surface quality.
TOOL LIFE COMPARISION WITH CONVENTIONAL MACHINING :
Machining titanium with a commercial cutting tool insert called Kenna metal tool does comparative study of tool life between cryogenic machining and conventional emulsion cooling.
When machining titanium, tool life is most sensitive to cutting speed, but it is also sensitive to feed and depth of cut. A fixed depth of cut (1.27mm) and feed (0.254mm) is selected based on the machining data handbook.
The best cooling result and the best tool life is that using both primary rake nozzle and auxiliary flank nozzle. The location of the chip breaker with nozzle affects the effectiveness of LN2 Penetration in to chip tool interface to provide lubrication effect and cooling. Another study shows the tool life can be improved 15% to 50%. The best nozzle /chip breaker location was identified in the test, about 1.7mm from the cutting edge.
The Tool Life data i.e. volume removal verses speed is shown in the figure. The tool life under cryogenic machining is even better than that obtained under high-pressure water assisted machining. The tool life increases were more significant at higher cutting speeds. Longer tool life in cryogenic machining allows cutting material at higher speeds.
Taylor tool life equation for fixed speed and depth of cut is usually expressed as
Where ‘T’ is tool life in minutes, and ‘V’ the cutting speed in surface feet/min. The index ‘n’ depends mainly on the tool material, and ‘c’ is cutting speed for one minute tool life, which is a function of workpiece material strength and tool materials. To fit the test results in to the Taylor tool life equation, the tool life is plotted against the cutting speed in log-log scale as shown in figure. The following constants of equation were obtained
For emulsion coolant n=0.3214,c=507
For cryogenic machining n=0.5164,c=1142.
From the index change the carbide tool behaved like the ceramic tool in cryogenic machining it also showed titanium machinability improves in cryogenic machining as constant C increases from 506 to 1142.
PRODUCTIVITY COMPARISION :
The time needed for making a part includes actual machining time, part loading/unloading time, and tool change time. For a fixed feed, depth of cut, and speed, the actual cutting would be the same for both conventional and cryogenic machining time. The time required to load and unload the part is fixed. Tool change time per part varies depending up on the tool life, as does the frequency of changing the tool. Actual machining time can be reduced if cutting speed is increased, but overall productivity may suffer from shorter tool life and an increase in time spent changing the tool. Therefore, the actual production rate is tied to improved tool life.
The results of productivity analyses are depicted in figure. The productivity of cryogenic machining is higher than conventional machining, and productivity continues to increase as cutting speed increases. Conventional machining productivity dramatically drops at high cutting speed due to rapid tool wear.
PRODUCTION COST COMPARISION :
The production cost, apart from the workpiece material cost, includes the machining cost, tool cost and coolant cost. The production cost comparison between the cryogenic machining process and conventional emulsion cooling process are shown in figure.
The total cost is lower for cryogenic machining because
1.The tool lasts longer and
2.cryogenic coolant cost is lower than conventional emulsion cost.
For all cutting speed, cryogenic machining costs less than conventional machining. For conventional machining, the production cost continues to rise as the cutting speed increases, which explain why titanium is not cut at speeds higher than 200 or 300 feet/min. Yet at higher speeds, the production cost for cryogenic machining continues to drop, showing that cryogenic machining has special advantages over conventional machining at high speeds.
CONCLUSION :
Localized focus jetting only to the cutting point, liquid nitrogen consumption is minimized to a level cheaper than conventional coolant.From the tool life productivity and production cost analyses, the new cryogenic machining approach proves to have the following advantages over conventional emulsion cooling.
1. Longer tool life
2. Better chip breaking and chip handling
3. Higher productivity.
4. Lower production cost
5. Better work surface finish.
6. Environmentally safer.
7. Healthier for worker.
REFERENCES:
- Delaney. R. J., 1957”Sub-Zero Machining and Quenching,”
- Ueharat.K., and Kumagi.S.,1970 “Characteristics of Tool Wear in Cryogenic Machining”.
- Hollins .W., 1961.”The Application and Effect of Controlled Atmosphere in the machining of Metals, ”
- Dillon, O.W., Angelis, R.J.De, Lu, W.Y., Gunasekera,J.s., and Deno,J.a., 1990, “The Effects of Temperature on Machining of Metals,”.
- Hong. S., Marcus, I.,and Ding Center, 1980. “ Machining Data Handbook” 3rd Ed.,Vol.1 Metcut Associates, Cincinnati, Ohio,pp.1-39.
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