After work has been undertaken on a drawing for a reasonable amount of time, then that drawing will possess some financial value. The draughtsman responsible for the drawings must be concerned with the reproducible quality of his work as prints or photographic copies are always taken from the originals. Revisions and modifications are regularly made to update a product, due for example, to changes in materials, individual components, manufacturing techniques, operating experience and other causes outside the draughtsman's control.
When a drawing is modified its content changes and it is vital that a note is given on the drawing describing briefly the reason for change and the date that modifications were made. Updated drawings are then reissued to interested parties. Current users must all read from a current copy. Near the title block, on a drawing will be placed a box giving the date and Issue No., i.e. XXXA, XXXB, etc. These changes would usually be of a minimal nature.
If a component drawing is substantially altered, it would be completely redrawn and given an entirely new number.
Drawings on a computer, of course, leave no trace when parts are deleted but this is not necessarily the case if the work is undertaken manually on tracing film or paper. The point to remember is that on the area covered by the erasure, part of a new drawing will be added and the quality of this drawing must be identical in standard with the original. Obviously, if the surface of the drawing sheet has been damaged in any way during erasure, then the draughtsman performing the work starts with a serious disadvantage.
The following suggestions are offered to assist in the preservation of drawings when erasures have to be made.
1 Use soft erasers with much care. Line removal without damaging the drawing surface is essential.
2 An erasing shield will protect areas adjacent to modifications.
3 Thoroughly erase the lines, as a ghost effect may be observed with incomplete erasures when prints are made. If in any doubt, a little time spent performing experimental trial erasures on a sample of a similar drawing medium will pay dividends, far better than experimenting on a valuable original.
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