268 



HANDBOOK OF MECHANICAL DESIGN 



Undercuts: 



Omit wherever possible, but, if unavoidable, they can be produced through use of 

 collapsible metal core or, if that is impractical, by use of baked sand core. 



Section thickness, minimum . 085 in. 



Cored holes, minimum diameter . 25 in. 



Draft angle, minimum ' 0.5 deg. per side 



Inserts, steel or cast-iron inserts practical 

 Cast threads, not practical 



Aluminum Die Casting 



Size limitations : 



Maximum practical over-all dimensions 36 X 12 X 9 in. 



(Few aluminum die castings exceed 25 in. in length) 

 Largest produced: 



In size 84 X 12 X 1 in. (14 lb.) 



(Die-cast dog used as trade-mark on Greyhound buses) 



By weight 19 lb. (12 X 19 X 20 in.) 



(Die-cast cases for Leeds and Northrup instruments) 

 Section thickness: 



Large castings, minimum 0.085 to 0.10 in. 



Small castings less than 6 in. length or width 0.050 to 0.065 in. 



Avoid abrupt changes in wall thickness. 

 Dimensional tolerances : 



In any one dimension ±0.0015 in. per in. 



(Frequently less by trial and error) 

 Perpendicular to parting line: 



Large castings —0.00 to -HO.OIO in. 



Small castings —0.00 to -1-0.003 in. 



Between points formed by removable part and die, tolerances are the same as those across parting line. 

 Cored holes: 



Minimum diameter of cored holes 0.090 in. approx. 



Length limit for through holes and blind holes: 



Diameter 



Under ^e rn. 

 To Min 



Length limit 



3 diameters 

 6 diameters 



Diameter 



Over }^ in. . , 

 Larger holes. 



Length limit 



10 diameters 

 No limit 



Tolerances on through holes — small holes 0.001 in. 



Closer tolerances on through holes must be obtained by reaming, the hole being cast 0.005 to 0.010 in. undersize. 

 Cores for internal threads — preferable to use threaded insert. 

 Draft allowances: 



Outside walls, recommended 0.010 in. /in. 



Rectangular core, on each side 0.008 in. /in. 



Internal cores, in general, as in table below: 



Approximate Diameter 



OF Hole Amount of Draft 



Less than J^ in 0.015-0.020 in. on diameter 



3^ to 1 in 0.010-0.015 in. on diameter 



More than 1 in 0.010-0.030 in. on diameter 



(depending on size and design) 



If no draft is permissible, ream or, unless hole is shallow, use insert. 



Small holes for tapping usually cast to root diameter of thread plus 15 per cent with standard draft added. 

 Inserts: 



If strained, should be knurled on surfaces within casting. 



Small pins, if subject to pull, should be both knurled and grooved. 



