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Cored Holes for Cut Threads
The standard tolerance recommendation is based on allowing 85% of full thread depth at the bottom (small end) of the cored hole and 55% at the top (large end) of the cored hole. A countersink or radius is also recommended at the top of the cored hole. This provides relief for any displaced material and can also serve to strengthen the core.
Cored Holes for Formed Threads
Cold form taps displace material in an extrusion or swinging process. As a result, threads are stronger because the material is not broken and is work hardened. Because material is displaced, a countersink is recommended at the end of through holes and at the entry of blind holes. The standard tolerance recommendation is based on allowing 75% of full thread depth at the bottom (small end) of the cored hole and 50% at the top (large end) of the cored hole. The 65% of theoretical hole size should be at the depth that is one-half of the required engagement length of the thread in the hole. Blind holes should be cored deep enough to allow a four thread lead at the bottom of the hole.
NADCA Product Specification Standards for Die Castings.
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