Hi alll
Are hex socket (Bristol?) cap screws suppossed to need either washers or, holes for them must always be chamfered? They have this tiny conicity below the head. OTOH, is this conicity meant to push against the hole edge sort of as a rotating wedge in order to align the top piece with the threaded hole?
Regards,
Camilo Ramos
Well today I threaded a number of holes in 1020. For some reason they came out close enough that the screw heads did not seat on the top piece. Next task, countersink the holes, or if feeling too lazy put washers on the screws ;)
Regards,
Camilo Ramos
The radius on the inside corner is there to distribute the stress.
A simple counter sink to break the inside edge of the hole is all that is needed, but usually there is enough clearance so this is not usually an issue.
According to Camilo Ramos :
First off --- hex sockets are Allen, not Bristol. Bristol are also called "spline", and are sort of like Torx, except that they have sharp steps instead of rounded edges.
It has been decades since I have seen Bristol socket screws, except in headless setscrews for electronics control knobs for aircraft use.
My opinion is that the rounded edge there is to minimize stress risers which would otherwise be the starting point for failure. This is also why the rolled threads have rounded bottoms to the grooves instead of sharp V grooves.
The holes are typically a bit oversized, and are usually chamfered as well, so there is no likelyhood of that being used for alignment.
If you want to control alignment, you don't do it with the screws, but rather use dowel pins or taper pins, while the screws are used to hold it together.
Enjoy, DoN.
Recommended practice, if the hardness of the material aproaches or exceeds that of the cap screw, is to chamfer the hole if its diameter is small enough to interfere with the fillet under the screw's head. In other words, chamfer close fitting holes in hard materials.
==================== While many people are not aware of this, many washers have a top and bottom in that edge of the center hole is sharp on the "bottom" and rounded on the "top." To a degree this is an artifact of how most washers are produced by stamping, with a rounded edge at the top where the punch enters and a sharp corner on the bottom where the slug is ejected.
It is considered good practice to orient a washer so the rounded edge [top] is against the bolt or screw head regardless of type.
Fortunately in the vast majority of cases, the fasteners are greatly oversized, but in weight [ e.g. aerospace] or fatigue critical [e.g. engine cranks/rods] applications the tiny nicks that the sharp edge of the washer may make in the head/shank transition may act as stress risers or concentrators where failure cracks can start. (A sharp corner between the head and shank would act the same way.)
Many washers are now vibratory finished to remove burrs and sharp edges and this will also generally provide an adequate radius on both sides of the hole.
I have found that an easy way to break the corners of holes even in a socket cap screw recess is to chuck a rotary file in a drill press or mill and run at slow speed. Assuming you can handle the parts, you can get good results freehanding the part against the rotary file. Just a touch is all that is required. If the part is too big to manhandle, use a variable speed drill motor and again just lightly "buzz" the hole.
Be sure and blow the swarf/filings out.
If you are cash strapped, as many of us are, you can also chuck up a drill [center or regular] or chamfer tool in a spare drill chuck and manually break the corner. The cheapest/ worn out drill chuck will work for this as long as it will grip the tool shank.
Just remember that sharp corners and edges are a machine's (and machinist's) enemy.....
Unka' George [George McDuffee] ============ Merchants have no country. The mere spot they stand on does not constitute so strong an attachment as that from which they draw their gains.
Thomas Jefferson (1743-1826), U.S. president. Letter, 17 March 1814.
O.K. I have to ask -- "Why are you bottoming a headed screw in a threaded hole"? Normally, I would expect the screw to pass through a clearance hole in one object and enter a threaded hole in the second object to hold them together. The clearance hole is usually large enough so the fillet under the head would not be a problem.
A screw in only a threaded hole would normally be used to clamp something in place via its other end, and would either be a setscrew ("grub screw" in the UK -- not sure what the term would be in Colombia, and I can't find "setscrew" in my Spanish-English dictionary), or the screw would not go in far enough to bottom the head against the threaded hole.
And if you thread *both* holes on two pieces being clamped together, you normally don't get much clamping force.
Enjoy, DoN.
No the upper pieces have the clearance holes; the bottom ones are the threaded ones. I should have explained better. Anyway some of the clearance holes came out so close that the fillet could not enter. That or some of the screws are a bit funny. I duly deburr every hole I drill or tap.
Regards,
Camilo Ramos
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