EVERYDAY ENGINEERING FOR ABSOLUTE BEGINNERS

摘要

I will mention at this stage the nuisance of 'Shrinkage', because all molten metal shrinks when it cools (yes I know there are some that don't much!) it follows that the casting is going to be smaller than the pattern and mould. This means that you will have to make your pattern slightly bigger than the drawings to allow for this. Reference to the books mentioned shortly will supply information as to what percentage to use. If you are making thousands of identical castings then a metal pattern becomes necessary because a wooden one will wear out. For this the original wooden pattern will have to have two shrinkage allowances, one for the pattern metal used and another for the castings. There is some information on this in the Model Engineer's handbook. I will deal with the problems one by one. If there is too great a mass in one area bounded by much smaller masses and with perhaps a large core adjacent, the metal will set around those smaller masses and the core. This means there may be no, or a restricted flow to the greater mass as it shrinks and cools, something has to give and the metal tears apart in the middle of the greater mass where it is hottest creating a typically ragged cavity. This is of course discovered when you machine into it and of course renders the casting useless, 'Sodds law' ensures that it is in the worst possible place.