"Best" way to drill and coutnerbore for a 1/2-20 bolt?

projectnut said:
If you decide to go with a counterbore keep in mind, there are 2 different sizes for SAE socket head bolts. The "close fit" size is .015" over the size of the head, and the "standard fit" size is .032" oversize. Back in the 1980's I needed a set for a shop I was supervising. At the time I'd never heard of "close fit" counterbores. I don't know if anyone sells them today, but they do come up on eBay every once in a while.
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Does the close fit refer to the size of the pilot on the counterbore & hence the size of the hole. Having a closer fit on the head serves no mechanical advantage whereas a closer fit on the bolt diameter gives less movement and more head engagement.
 
Both the pilot and the cutting section of the counterbore are smaller diameter on a "close fit" counterbore. If I remember correctly the "close fit" pilot requires a hole .015" larger than the bolt, and the cutter bores a hole .015" larger than the socket head. The standard fit has a pilot .032" over the nominal size of the bolt and the cutter bores a hole .032" larger than the socket head.
 
For a cap screw the counterbore also needs to be big enough to accommodate a socket so my piloted counterbores never seem to be big enough. I usually wind up using an end mill for the counterbore. I like a generous counterbore so I don't need a thin wall socket (if there's room, of course).
 
epanzella said:
For a cap screw the counterbore also needs to be big enough to accommodate a socket so my piloted counterbores never seem to be big enough. I usually wind up using an end mill for the counterbore. I like a generous counterbore so I don't need a thin wall socket (if there's room, of course).
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Counterbores are sized for Socket Head Cap Screws, not Hex Head Cap Screws. Counterboring for HHCS is not that common and generally avoided because of wrench access problems.

I have a set of the "loose" counterbores. They frequently frustrate me because I like tighter fits. I end up using endmills a lot.
 
epanzella said:
For a cap screw the counterbore also needs to be big enough to accommodate a socket so my piloted counterbores never seem to be big enough. I usually wind up using an end mill for the counterbore. I like a generous counterbore so I don't need a thin wall socket (if there's room, of course).
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That's the advantage of having a set of counterbores that use interchangeable pilots. You can put a 1/4" pilot in a 3/4" counterbore.

 
To me the difference between a screw and a bolt is the head. A screw has a flat head or Phillips drive. A bolt has a hex. That’s my logic?
You can get fully threaded bolts all day.
 
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