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****************** Recommended Reading: Manufacturing Engineering and Technology Marks' Standard Handbook for Mechanical Engineers ******************
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5.1 BASIC DESIGN CONSIDERATIONS As the need for higher production rates, reduced variation in geometric tolerances and fuller machine utilisation becomes ever more important in today’s increasingly competitive market place, certain aspects of machine design become critical. Higher production rates require faster stock removal, hence higher cutting speeds and feeds: this in turn creates greater dynamic loading on the machine. To be able to perform adequately under these conditions the machine must possess adequate strength and vibration damping characteristics. Reduced variation in geometric tolerances, otherwise known as repeatability, also requires adequate strength and vibration damping as well as rigidity, accurate control of slides and minimal thermal expansion. Full machine utilisation requires that simple housekeeping and maintenance tasks, such as swarf clearance and tool replacement be carried out automatically or with the minimum amount of downtime. Many modern C.N.C. lathes have what is known as a slant bed. Fig 5.1.1.
Fig 5.1.1. shows how a slant bed lathe is effectively a lathe that has been inclined to allow swarf and chips to fall clear of the slideways to prevent clogging and jamming. A swarf conveyor allows chips to be removed from the machine continually without interrupting production. Both lathes and machining centres may employ swarf conveyors. It is also important that table overhangs (unsupported lengths protruding from the slideways) are kept to a minimum to decrease static loading; similarly, spindle overhangs must also be kept to a minimum to prevent deflection and vibration due to dynamic loading caused by cutting forces. It is advantageous for large motors to be thermally isolated from the beds and slides by using belts (where possible); this prevents undue vibration and thermal expansion of critical areas of the machine.
5.2. MATERIALS USED IN MACHINE CONSTRUCTION The choice of the correct material when designing a machine is imperative as this will affect the fundamental performance of the machine. In general a material that has good thermal stability, vibration damping qualities and density is desirable for machine bases. Cast iron has long been popular for it’s reasonable vibration damping and stability (however large hollow sections may resonate like a bell). Many manufacturers are now using fabricated steel. Granite is excellent, it has a very low co-efficient of thermal expansion, is very dense and dampens vibration. Granite is however very difficult to work and is therefore generally limited to bases for inspection equipment. A very good compromise is Concrete. Concrete has good density, thermal stability and vibration damping, it is also versatile in that any shape can be cast, very large machines can be cast in-situ and Cast Iron beds bolted on, along with ancillary components. |
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