PROBLEMS IN DIMENSIONS AND TOLERANCES 195 



finished product to the realm of tool making tolerances and gauging toler- 

 ances. The problem of how invariable is "invariable" remains but we are 

 obviously then considering differences of an order of magnitude not usually 

 vitally significant in the functioning of product parts. Theoretically, all 

 "invariable" dimensions should be taken to the best accuracy of good 

 gauging methods which means that any differences of opinion will be re- 

 duced at least to one-fifth and probably to one-tenth of the order of magni- 

 tude of those where tolerances themselves are involved. 



It will be necessary to specially identify gauging dimensions on drawings 

 to distinguish them from ordinary unlimited dimensions and to indicate 

 that they are dimensions for gauges to which only gauge tolerances apply. 



Practical Use oj Datum Lines and Planes 



It is not usual to establish datum lines on all drawings but if their use is 

 necessary in the layout and design of the part they need to be permanently 

 identified. This use of datum lines and planes on drawings, where neces- 

 sary, may require somewhat greater drafting effort in the actual production 

 of the drawing but their use results in a simplification of design and of the 

 work of those subsequently using the drawings. It reduces the effort ex- 

 pended in analysis of drawings preparatory to the construction of tools 

 and minimizes the possibility of misunderstandings or errors in tools. In 

 products manufactured only intermittently it is particularly valuable as 

 it minimizes the need for understandings and instructions supplementary 

 to the drawings which may be forgotten between production periods or lost 

 through shifts in personnel. 



The overall economy in engineering effort and the reduction of the numer- 

 ous possibilities of error more than compensate for the increase in the actual 

 work of indicating datum positions, lines or planes upon drawings. In 

 addition the choice of design of punches and dies and similar tools by pro- 

 duction engineers is better guided by the designer's requirements if func- 

 tional datum lines are clearly identified. An obvious example is the use 

 of either the inside or outside of a punched and formed part as the starting 

 point. In brief datum plane dimensioning is a more explicit expression on 

 the drawing, of the designers "end point requirements". 



When establishing datum planes, it is important to consider them in 

 terms of the actual physical part rather than in terms of the drawing. 

 Lines which appear as definite points on a drawing may not be actually part 

 of the product when it is completed or may be on surfaces shown as a line 

 on the drawing but rough or unfinished in the part. It is difficult to es- 

 tablish any set of rules covering what shall or shall not be done because 

 each drawing and each part must be considered practically as an individual 

 case. That this is so will be amply demonstrated by a serious study of even 



