Saville
Well Known Member
Hello all,
I've run into a situation which can easily place the beginner (me) into a situation where the builder does not know what to do and does not know what matters and what does not.:
With regard to machine screws, when is it ok for the clearance hole to be enlarged by countersinking, and when is it not?
The plans call for AN509-10R9's (MS24694-S51), 10-32 machine screws to be used to fasten a 0.072" thick plate to a flange using plate nuts. According to the Mil Spec for 10-32 the head height is 0.080". The screws essentially fasten a flat plate to a flat plate.
The screws have to be flush with the plate surface and therefore countersunk.
So if you countersink the match drilled holes the resultant clearance hole will be enlarged such that the screw grip is no longer in contact (or even close to) with the edge of the material.
From Van's Section 5.5:
"Machine countersinking removes metal and is done with a stop countersink
tool and a drill motor . Machine countersinking can only be used in
areas where the skin thickness is sufficient.
Where the skin thickness is insufficient, the countersunk hole for the
rivet head enlarges the original rivet hole and no longer
supports the shaft of the rivet. Only a portion of the rivet head is now
contacting the skin, so it cannot achieve its design strength
which is based on full head contact. See Figure 2, Example 1."
Admittedly this section is talking about rivets and not screws. But I did not see a section on screws, clearance holes and countersinking.
The section also says:
"There are a few instances on RV's where it is considered acceptable to enlarge holes when machine countersinking."
Ok so the beginner is presented with a dilemma:
Is this one of those instances? Can the builder assume that Van's knows the holes will be enlarged. As this application is for a battery, there is considerable weight, and therefore force, being applied to the joint.
More generally:
What is it about countersunk screws that give the joint it's strength in shear?
Is it the grip resisting the sliding of the top plate as it tries to move under a side force?
Is it the friction generated because you are squeezing the two pieces of metal together tightly?
Is it the resistance generated by the inclined plane of the underside of the head?
Is it all three? (Probably)
Is there another factor I've failed to list?
Which of those three is the primary means of making the joint strong?
Which can be discarded under what circumstances?
Yes I know these are detailed questions and that the novice builder can simply follow the plans and be ok. But I have a curiosity about these things and so I'd like to know. If anyone knows the answers and would care to share I'd appreciate it.
I put "Beginner" in the heading to alert the reader as to the level of the questions.
Thanks
I've run into a situation which can easily place the beginner (me) into a situation where the builder does not know what to do and does not know what matters and what does not.:
With regard to machine screws, when is it ok for the clearance hole to be enlarged by countersinking, and when is it not?
The plans call for AN509-10R9's (MS24694-S51), 10-32 machine screws to be used to fasten a 0.072" thick plate to a flange using plate nuts. According to the Mil Spec for 10-32 the head height is 0.080". The screws essentially fasten a flat plate to a flat plate.
The screws have to be flush with the plate surface and therefore countersunk.
So if you countersink the match drilled holes the resultant clearance hole will be enlarged such that the screw grip is no longer in contact (or even close to) with the edge of the material.
From Van's Section 5.5:
"Machine countersinking removes metal and is done with a stop countersink
tool and a drill motor . Machine countersinking can only be used in
areas where the skin thickness is sufficient.
Where the skin thickness is insufficient, the countersunk hole for the
rivet head enlarges the original rivet hole and no longer
supports the shaft of the rivet. Only a portion of the rivet head is now
contacting the skin, so it cannot achieve its design strength
which is based on full head contact. See Figure 2, Example 1."
Admittedly this section is talking about rivets and not screws. But I did not see a section on screws, clearance holes and countersinking.
The section also says:
"There are a few instances on RV's where it is considered acceptable to enlarge holes when machine countersinking."
Ok so the beginner is presented with a dilemma:
Is this one of those instances? Can the builder assume that Van's knows the holes will be enlarged. As this application is for a battery, there is considerable weight, and therefore force, being applied to the joint.
More generally:
What is it about countersunk screws that give the joint it's strength in shear?
Is it the grip resisting the sliding of the top plate as it tries to move under a side force?
Is it the friction generated because you are squeezing the two pieces of metal together tightly?
Is it the resistance generated by the inclined plane of the underside of the head?
Is it all three? (Probably)
Is there another factor I've failed to list?
Which of those three is the primary means of making the joint strong?
Which can be discarded under what circumstances?
Yes I know these are detailed questions and that the novice builder can simply follow the plans and be ok. But I have a curiosity about these things and so I'd like to know. If anyone knows the answers and would care to share I'd appreciate it.
I put "Beginner" in the heading to alert the reader as to the level of the questions.
Thanks
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