Which threads per inch is better to use when fastening things to aluminum mast, 24 tpi or 32 tpi? Does it depend on screw diameter?
This is for tapping holes to fasten things like T track for spinnaker pole, refastening motoring light and other stuff that ranges from 3/16", 1/4" to 5/16" screw diameter.
Thanks
Standard practice is to use coarse threads in soft materials and fine thread in harder materials. Materials like aluminum and plastic would utilize coarse threaded fasteners. Steel and stainless materials would use fine threads. This is for two reasons. One being the thread depth is smaller on fine thread fasteners making it easier to cut threads in harder materials using a tap or die as the cutting tooth load is lower the smaller the cut. Coarse threads have a much bigger thread depth. This provides more bearing area in soft materials maximizing the ability of the tread to resist pull out.
Another point to remember, the length of the thread. The rule of thumb is 1x the diameter in hard materials and 3x the diameter in soft materials. This means if you had a 1/4" thread diameter in hard materials, anything longer than 1/4" thread engagement would not significantly improve the treads holding ability to resist pull out. If it were the same 1/4" thread in soft materials you would want to use 3/4" of thread engagement. This of course is for pull out. In most threaded applications shear is the loading we are concerned about and that is a factor of the fasteners diameter and material.
There are some logical exceptions such as threading a hole in thin materials. In this case using a coarse thread would only allow a few threads or a partial thread depending on the thickness. Using a fine thread would engage additional threads and hopefully provide additional strength. Avoid installing threads in thin materials for this reason opting for a nut on the back side.
Of course marine fasteners are different. Most 18-8 alloy Stainless fasteners are typically available in their coarse thread form and not so much in fine threads. As an example 10-24 UNC (unified national coarse) are every where but 10-32 UNF are much harder to find. A few exceptions are set screws in stanchion bases. These are typically 1/4"-28 UNF or 10-32 UNF and are difficult to find in the typical marine outlet.
Dale
I agree with Dale. Coarse thread is the way to go in thin gauge aluminum like an extruded mast. All my mast fittings such as chocks and cheek blocks are held on by SS self tapping sheet metal screws. Like a #10 or #12. Threads on these are very deep and coarse. I find you can tourque the heck out of these. If done right the head will twist off before the thread strips out.
Pete
I agree with Dale and Pete on using Corse thread in Aluminum. Most of my experience is in the aerospace world, which has a distinct fine thread bias. This is mostly because a properly machined fine thread is a little stronger in bolt tension failure because less material is removed from the minor diameter of the male thread. It depends on the material you are using and the critical failure mode and whether stripping the female thread or breaking the bolt/screw is the critical failure mode. In your case, I would think stripping the female thread would be more the issue. One advantage of fine threads is that they have more threads in a given length, so if the length of engagement is less than about 4 threads, fine threads have an advantage.
The down side of fine threads is when the real world hits theory. Especially in aluminum, the threads tend to gall and corrode, especially if it's a stainless steel screw. Corroding will take out fine threads more rapidly than corse. Riggers use several compounds to minimize this problem, things like sailkoat, marelube, lanocoat etc. In addition, locktite blue thread locker can help prevent corrosion as well as keeping the treads from loosening with time. Just make sure to use the right product for the application, if you use Red, getting the screws out is really really hard without lots of heat and bad words. Maybe Pete or Dale could recommend what product they feel is best in that application.
Aluminum sacrifices to stainless On the galvanic series, so in this case the female thread would tend to disappear over time.
Dale is also right, usually marine stores stock corse thread fasteners, except one that should remain nameless that mixed 10-24 and 10-32 nuts in the same bin...... Not fun when you are a 6 hour round trip drive from the parts store.
Thanks for the advice on the tpi question and the rules of thumb. I'll use course threads, probably mostly.
Regarding compounds to prevent loosening and corrosion, It seems that one can either choose locktite (blue or red) to prevent loosening or choose marlube/tefGel to prevent mitigate corrosion. Specifically, with fastening fittings to the aluminum mast with SS fasteners, is it better to use TefGel to mitigate corrosion rather than loctite to prevent loosening?
I presume loctite would be the choice with SS fastener in SS fitting (nut or tapped SS) to prevent backing out because there is no corrosion issue.
Thanks