Fastenter Sizing & Selection Basics

[jride200]

As I work out the detail of my teardrop build, I've been thinking that this would be a helpful thread to start... for me, and all others on the forum. I just can't seem to find any analytical information about wood fastener selection, nor do I have much precision woodworking experience.

I'm certain that I will get many different opinions on the subject. The more the better, IMHO. I'm also certain that others will tell me I'm being way more particular than is necessary (this I can't help... I've studied engineering, aircraft mechanics, and I'm of thick German descent!).

Here are some initial questions I have regarding threaded fastener selection. These are just a start though. I welcome any additional thoughts anyone might have on the subject.

1) How does one choose the best screw size? I know that #6, 8 and 10s are the most common screw size, but which one, when? With aircraft rivets, we choose the diameter based upon the thickness of the materials being riveted. Is there any similar relationship with wood?

2) Obviously, pilot holes and countersinking are important. But does anyone drill a slightly larger pilot through the first piece of material so as to provide a grip length for the fastener?

3) Material? Stainless obviously provides the best corrosion resistance, but is it really necessary? How much more expensive are stainless fasteners?

4) Screw orientation... Is it acceptable to thread screws into the end grain of wood? Or the edge of a plywood sheet? Or, should I avoid these situations? I learned long ago, in engineering school, that it is best to avoid using threaded fasteners in shear...

5) Has anyone worked with the threaded inserts (coarse male threads on exterior to thread into wood with female machine screw threads on the inside)? I'm really liking these and was thinking of using these to attach my fenders.

Thats it for now. Lets start some discussion. More questions likely to come.

J

[RichAFix]

I am no engineer but below are some of my thoughts from my woodworking experience.

2) Obviously, pilot holes and countersinking are important. But does anyone drill a slightly larger pilot through the first piece of material so as to provide a grip length for the fastener?

Yes, when working with a very hardwood and a very soft (brass) screws. But in this type of construction I doubt it is necessary.

3) Material? Stainless obviously provides the best corrosion resistance, but is it really necessary? How much more expensive are stainless fasteners?

I think if you use a good glue the screw isn't going to do as much as the glue after it is dry. There are places on my build where I have no screws (in an effort to avoid having screw heads visible). I still like the comfort of a good stainless screw in a critical point though. If not stainless, I would use galvanized.

4) Screw orientation... Is it acceptable to thread screws into the end grain of wood? Or the edge of a plywood sheet? Or, should I avoid these situations? I learned long ago, in engineering school, that it is best to avoid using threaded fasteners in shear...

A screw into the end grain of plywood will pull out very easily, I would try to avoid it. In plywood a course thread screw will give more holding power than a fine thread.

5) Has anyone worked with the threaded inserts (coarse male threads on exterior to thread into wood with female machine screw threads on the inside)? I'm really liking these and was thinking of using these to attach my fenders.

Yes, I have used them in places that I may want to screw and unscrew. They can be a struggle to get in straight but there are a few tricks/tools to accomplish this without too much stress.

I think most would agree that in most cases on this type of build that a good screw/brad/staple is really doing it's job by holding the wood until the glue is dry. I use plenty of glue, my choice in Titebond III. I still use plenty of screws though as I don't think a pound or two of metal is going to hurt the weight of my trailer. I always use galvanized or stainless even if I am painting over it as I have seen the screws weather and ruin a nice finish.

These are just my thoughts from my experience. I am sure you will find people that have other recommendations. I am sure with your background you will sort through all the info and make an intelligent decision. My other thought is, don't spend too much time researching. That isn't to say don't do your homework but don't think you have to reinvent the wheel. Trust me as a guy that spent about 3 years researching, the build is way more fun than the research.

[afreegreek]

#4 - 3/8" or less.
#6 - 1/2" or less
#8 - 5/8 and up
#10- not used much

most screws now are rolled thread and have shanks smaller than the thread so no second pilot hole is needed. real wood screws have cut threads and shanks as big as the threads so a second pilot is needed or a taper point drill bit.

driving screws into ply is just fine. it's done all the time. usually I angle the drill about 15 degrees so the screw skews through several plies when I can.

[jride200]

#4 - 3/8" or less.
#6 - 1/2" or less
#8 - 5/8 and up
#10- not used much

Assume you mean the thickness of the first, or countersunk, layer?

most screws now are rolled thread and have shanks smaller than the thread so no second pilot hole is needed. real wood screws have cut threads and shanks as big as the threads so a second pilot is needed or a taper point drill bit.

I've seen, and am somewhat familiar, with the traditional wood screws you mention. Even with rolled threads you have a major and a minor diameter and the threading exists on the whole length of the shank. In order for the "countersunk" layer to be pulled tight, wouldn't it need to be drilled equal or greater to the major diameter?

I'm a bit of a fastener snob. I just want them to work as designed. If you work in aviation, you understand. If you ever travel by air, I hope you appreciate it!

[afreegreek]

#4 - 3/8" or less.
#6 - 1/2" or less
#8 - 5/8 and up
#10- not used much

Assume you mean the thickness of the first, or countersunk, layer?

most screws now are rolled thread and have shanks smaller than the thread so no second pilot hole is needed. real wood screws have cut threads and shanks as big as the threads so a second pilot is needed or a taper point drill bit.

I've seen, and am somewhat familiar, with the traditional wood screws you mention. Even with rolled threads you have a major and a minor diameter and the threading exists on the whole length of the shank. In order for the "countersunk" layer to be pulled tight, wouldn't it need to be drilled equal or greater to the major diameter?

I'm a bit of a fastener snob. I just want them to work as designed. If you work in aviation, you understand. If you ever travel by air, I hope you appreciate it!

ya, the sizes are for the thickness of the wood you're screwing into the edge of. in other words #6 screws for 1/2" ply wood.

anyway, a traditional wood screw is formed and then the threads are cut into it. this makes the part of the shank closest to the head the thickest part. you need to drill a tap hole the length of the screw and then a clearance hole into the piece you're screwing through if you use a straight bit but if you use a taper point bit (designed for wood screws) you don't have to because the correct size is the root size of the screw and the holding power comes from the tapered section at the end.

most off the shelf screws now are rolled thread which means the thread is raised out of the shank and the thread are bigger than the shank. the bit you choose is the size of the shank also and because the majority of screws do not have threads all the way the screw does not "bridge" the two pieces. with rolled thread the tap hole is the clearance hole too.

[Miriam C.]

My kiss principle:
I used decking screws three times the depth of the plywood on the sides (3/8")......... Seems to work.

Used #6 stainless on the roof because I was leaving them in and couldn't find a decking screw that small.

Don't remember the size of those used for the spars but I really over did it.

REally good glue!