Dimensioning of Button Head Screw

[BHunter20]

Hi, I'm hoping someone can point me in the right direction. I'm working a tolerance stack to make sure at worst-case that a button head screw cannot protrude past the sloped countersink of a part. The problem is, I cannot find the specs/dimensions of the radius'd portion of the button head of the screw or where the radius stops at the top of the head. Spec ASME B18.3-2012 shows the min/max size of certain features of the screw, but nothing the radius.

 

[Burunduk]

Maybe you can contact the screw manufacturer you order from and ask them, or if you have stock of such screws, take a large enough sample to measure the radius and make a statistical estimate.

 

[Brian Malone]

Isn't this a case where excessive effort is used for a non-functional feature? Is your design so constrained for size that you cannot choose to make a size envelope for the fastener head that you know it will not exceed - treat it as a cylindrical cap head instead? Then even with the unknown variable of the actual button head profile, you are assured the head will never protrude. Certainly, there are methods to dial this in but ensure the effort used to achieve the gnat's-ass detail is commiserate to the functional requirements.

 

[BHunter20]

Thats the first thing I did - I treated it as a cylindrical cap hoping at that at the extreme, the "corner" would fall within the boundary. Using that assumption, there was a significant protrusion outside the boundary. This design is highly constrained due to the funtional requirements. Tolerances as small as +/-.001.

 

[jassco]

As Burunduk suggested, contact the manufacturer. Alternatively, you can do a capability study based on some screws, say 16 or 32 pcs.

Best regards,

Alex

 

[ctopher]

I have never had to look at the radius. The head height and diameter is what's important. Give yourself enough space and tolerance for clearance.

Chris, CSWP
SolidWorks
ctophers home

 

[Brian Malone]

Ensure your design cannot be changed to accept the cylindrical corner is below the exterior surface. Are you not able at this time to make the cbore deeper or translate the screw position inward slightly to accommodate the cylindrical head envelope at MMC? Are your design constraints truly limiting your options? Tolerances to +/- .001 just indicate a level of precision, this is not the same as saying the functional requirements cannot be modified to accommodate a slightly deeper cbore or hole location. Make sure you are not overdesigning something that is a cosmetic issue rather than a physical functional feature.

 

[MintJulep]

I've said this here before, and I'll repeat it now.

It's often a sign of a poor design when you find yourself saying "This will work fine, if we can just hold the tolerances tight enough."

Having said that, here's the button head page from my vintage Unbrako catalog. It has radius "R" of interest shown as a ref dimension.