Fully threaded socket head cap screws - are they supposed to screw flush?

[WobblyHand]

I don't know if I have some bad SHCS or that's just how they come. Threaded a M5 x 0.8 thread in some steel and deburred the raised edge. Then screwed in a SHCS into the hole. I was expecting the screw to bottom on surface. They don't. They are 0.88mm to 1.14mm from flush. Is this expected? It seems the threads stop about 0.9 - 1.2 mm from the head. That makes sense. But shouldn't the diameter at that point be near the minor diameter instead of the major diameter?

I mean I can fix it with a countersink, or a washer, but is this your experience? These were purchased from McMaster, so they shouldn't be bottom of the barrel stuff. It could be I've never noticed before, or I'm using the screws differently? I feel kind of silly asking this question...

 

[sdelivery]

Not silly but that's normal

 

[ddillman]

I think typically you would be bolting something down that would space the head away from the threaded hole.

 

[WobblyHand]

Probably so.

I was making a fixture to hold a screw flush and was surprised that it wasn't! I want to turn a smaller thread on part of it to make a differential screw for fine adjustment. Guess I'll just drill out a thread and a half.

 

[kb58]

Add a washer. There's no way to have full threading all the way up flush against the bottom face of the head.

 

[WobblyHand]

I know it's practically impossible to thread to the head. But was wondering why the section near the head was at major diameter instead of minor. If it was minor, then it could go flush. In my case, using a washer was awkward, since I planned to counterbore the head. Simplest thing to do (if a washer won't work) is to drill out 1.5 threads so it will seat. (Have more drill sizes than washer sizes)

 

[darkzero]

It's cause of the way mass produced screws are made. The threads aren't cut, they are roll formed.

 

[markba633csi]

Indeed. I think it's assumed the cap screw will be passing thru one piece and threading into another, hence the major diameter situation is not considered an issue most of the time
-Mark

 

[BGHansen]

I've had the same problem with flat-head cap screws when screwing thinner material to a threaded block. I carefully use the screw-size drill bit at the top of the threaded hole to dust off the first couple of threads.

Bruce

 

[main_cogg]

The shank is full diameter and the radius transition to the underside of the head are made that way to strengthen the screw.