Quote History Quoted:
There appears to be differences in the mil. drawing screw and a 1936 or 1960 series standard socket screw. The knurling requirement, the finish, the type steel is spec'd instead of all of types the ASTM allows to be used, at least one dimension appears to be slightly different, and the thread class appear to be different. It is not just the socket size being different than a 1960 series screw.
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Quite true.
ASTM A574-13 and NAS 1352 standards allows for steels with the following composition:
Carbon - 0.31%
Phosphorus - 0.035%
Sulfur - 0.040% max
one or more of the following to ensure strength requirements are met - chromium, nickel, molybdenum, or vanadium (That covers a lot of steel alloys.)
The mechanical requirements are:
Tensile strength - 180 ksi
Proof load - 140 ksi
Hardness (Rc) - 39-45
Yield at 2% - 153 ksi
Elongation - 10% min (Most 41XX and 43XX series steel this is 15% to 20%)
Reduction in area - 35% min (Most 41XX and 43XX series steel this is 40% to 50%)
DWG 8448508 calls for AISI 4037 heat treated and tempered to 15N 78.5 to 83 (HRc 36 to 45). At this hardness 4037 will have the same ultimate and yield strength, but the elongation will be only 5% and the reduction in area of 36%
So, what the difference? It's all steel with the same yield strength, right?
Well, the strength and hardness are the same but the other properties have a bearing on the behavior of the screws and explain why Armalite/Colt's was so specific in material choice. The elongation at break is 5%, and 36% reduction in area for 4037, the other alloys have values in the 15% to 20% elongation and 40% to 50% reduction in area. This means that 4037 is a very "stiff" alloy, in takes a lot of force to stretch the material a small amount
Also, the required torque on the 8448508 screws is 58 in-lbs, which means the clamping force is between 1,793 and 2,080 lbs. Clamping force is the result of the steel of the bolt stretching like a rubber band and holding tension.
The proof load for 8-32 UNC ASTM A574 screws is 1,960 lbs. Proof load means the bolt stretch at this load remains inside the elastic region of the stress-strain curve. Loads above the proof load can result in permanent bolt stretch and loss of torque and clamping load. So, some ASTM A574 screws can stretch under this torque. 4037 screws, being stiffer, do not stretch as much and therefore retain their clamping load. This is why BCM and Colt's use the 4037 screw.
(You can mitigate the bolt elongation problem by simply reducing the torque to about 50 - 55 in-lbs and making sure the screws are installed dry. You still maintain a 1,500-1,700 lbs of clamp.)
Also, there is that one dimensional difference, ASTM A574 and NASM1352 screws specify a 9/64" (0.140") key and the DWG 8448508 screws specify a 1/8" (0.125") key. Since the head diameter is the same, the drawing screw has a thicker web.
Does it matter? I don't know, that's up to you. Personally, I can live with the minor differences, but just be aware that there are differences.