13-10-2014, 09:48 AM
Abstracts: CNC vertical milling center machine is used to manufacture extruder screws. Hence, it is a special purpose machine. Fixture is used in the CNC vertical milling center machine for holding cylindrical rotating blank of variable length. Fixture is a device used to hold, locate& support the part during process. It is essential element in any machine .It provide support to the part during machining process. If we don’t use fixture then there is no arrangement which can hold the job. In CNC vertical thread milling center machine the blank is continuously in rotating motion so in order to hold this fixture is required. It makes the job to maintain its position and reduce the vibration in the job. The vibration in the job is due to some void or space remains between the job surface and fixture surface. Due to this space between them when the cutting force is applied on the job it causes vertical movement of job and radial and tangential forces causes vibration in the job if the vibration in the job is not reduce then it cause bending in the job. Once the job is bent then its performance is reduced specially in case of extruder. In extruder screw is rotating in the barrel. Screw is used for uniform mixing of the material in the barrel. It’s also used to move the material through die after the proper mixing is done. If the screw is in bent shape then I can’t rotate properly in the barrel which causes in proper mixing of the material so the product coming out from the die is defective which is not desirable. So, in order to reduce the bent we have to straight it on the hydraulic press which reduces the bent into the safe limit. But to straight it on a hydraulic press leads to increase in the manufacturing time and cost. So, if we can minimize the bent during the process then it’s not require to straight it on a hydraulic press which can save lot of time and money and also improves the performance of the screw in the barrel. This can be achieved by the use of milling fixture which reduces vibration in it during the process which ultimately reduces the bend in the job and shape of the screw is maintained and no need to straight it on a hydraulic press. Fixture in vertical milling center should be such that it reduces the tangential, radial and axial forces which cause the vibration in the cylindrical rotating blank. We can easily accommodate different diameter job by simply adjusting he parts on the fixture which are shown in the figure. So, there is no need to remove fixture from the bed every time when there is a blank of different size. This fixture retains the job in its position and also saves the cost of using fixture for different diameter jobs. Oil hardening the fixture saves the fixture from friction between the contact surfaces. Once the fixture is manufactured we have oil harden it. The material used for fixture is EN31B and the material used for manufacturing the screw is EN41B which is harder than EN31B.The material is composite. So, in order to prevent the wearing of fixture oil hardening process is necessary. It will increase the life of the fixture. There is no wearing between the surfaces exists.