In this case, do fine threads provide a better grip in an unthreaded hole or are corse threads better at holding?
1. Many times we don't now the TPI of a sheet metal screw. Like on your package. Some are 15 TPI and some may be 18 TPI. That is a difference of only .011 per thread if you want to relate it to cab sheet metal thickness.
2. The smaller the screw the finer the TPI.
3. In general, you get only one TPI for a certain type of sheet metal screw.
4. We can't equate TPI in Sheet metal to regular machine screws.
5. When we buy wood screws, many of use "drywall screws". That's what I use. They have coarse and fine. Course is for drywall to wood studs and fine is for drywall to metal studs. I use them for wood to wood, with one piece having a clearance hole and the other no hole. Many of the threads are engaged into the wood.
6. With a pointed sheet metal (self tapping) screw like you show, you need a pilot hole. The clamping, in thin metal, is by one thread, approx. It cuts only one partial "thread" into thin sheet metal, with proper pilot hole. But being spiral, the thread clamps by being a one thread ramp wedge. That is why several sizes work on the same thin sheet metal. It is why the pitch is not the same kind of factor as machine screws and wood screw deep engagement. Hard to equate to other type screws.
7. With a self drilling sheet metal screw you don't need a hole.
8. I realize this is way too much blogging about nothing.
It's a perspective answer to your questions. Sheet metal screws are different. Thought of for light weight or low tensile/pull out applications. It would be much better to use machine screws with nuts, but I know that is not possible. I don't think a visor is low weight and I think a headwind will apply great pullout force. That must be wrong because I don't know of any bad stories.
