Some basic woodworking questions

Hello all,

I'm about to start cutting spruce for a wooden aircraft project. I've done some super basic woodworking projects around the house, so I'm familiar with the tools and general processes, but I've got a few base-level questions that I don't see addressed in most literature I've consumed. Maybe I'm just supposed to know this but I don't yet, so I thought I'd see what you all say. I want to make sure I do things properly to have good strong parts.

When cutting wood for an assembly, if you cut a piece just right and it fits together with everything else, great. But if it needs tweaking, can you sand it to fit? I’ve heard that can make the joint weak. Should you not sand at all and just get new material and do better next cut? Or if you have to sand a piece to fit in a joint or assembly should you do something to clean it before you glue it?

When you use the plywood for gussets, I’ve heard you should sand the sheen off. Is that literally all the more it needs, just a little bit to dull the surface? Doesn’t that fill the plywood with dust? Do you need clean out the grooves in the plywood before you glue it on?

Speaking of gusseted joints like a rib intersection: if some of the capstrip pieces are maybe a tiny bit different thickness than the others, do you sand the area where the gusset goes so they will all be at the same level and the gusset lays down flat? Like above, if you do, do you clean the sanded wood afterward before you glue the gusset on?

Thanks for the help, WIll

Quote:

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Originally Posted by willmartin

When cutting wood for an assembly, if you cut a piece just right and it fits together with everything else, great. But if it needs tweaking, can you sand it to fit? I’ve heard that can make the joint weak. Should you not sand at all and just get new material and do better next cut? Or if you have to sand a piece to fit in a joint or assembly should you do something to clean it before you glue it?

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Ideally, you shouldn't sand it. The strength of the glue joint depends on the glue soaking into the pores of the pieces of wood, and if there is sand in the pores, the glue sticks to the dust instead of the wood fiber itself.

FAA Advisory Circular AC-43.13-1B "Acceptable Practices" says:

"The mating surfaces should be machined smooth and true with planers, joiners, or special miter saws. Planer marks, chipped or loosened grain, and other surface irregularities are not permitted. Sandpaper must never be used to smooth softwood [spruce is a softwood - RJW] surfaces that are to be bonded." (Page 1-5)

However, the section continues to say:

"When surfaces cannot be freshly machined before bonding, such as plywood or inaccessible members, very slight sanding of the surface with a fine grit such as 220, greatly improves penetration by the adhesive of aged or polished surfaces. Sanding should never be continued to the extent that it alters the flatness of the surface. Very light sanding may also improve the wetting of the adhesive to very hard or resinous materials."

Note it says, "very slight" sanding, basically to improve glue penetration, not shaping.

You can download the complete AC-43.13-1B in chapters at:

http://www.wanttaja.com/avlinks/AC43-13-1B.zip

In addition, the Fly Baby community has a workmanship guide that addresses woodworking as well as other issues with building wood aircraft.

http://www.bowersflybaby.com/PB100/workmanship.pdf

Ron Wanttaja

You now have an excuse to own more tools! Aircraft woodworking is not cabinet or instrument making but many of the tools from those worlds are very useful when making wood aircraft parts. Use small planes or scrapers to do the final shaping that makes parts fit up to each other. Woodcraft, Rockler, and Lee Valley will be happy to take your $$ and sell you cool tools. And when you have enough tools to do great work on your airplane, expect your spouse to start talking about custom cabinets in the bath and kitchen.... Resist until your airplane flys.

Best of luck,

Wes

Quote:

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Originally Posted by willmartin

When cutting wood for an assembly, if you cut a piece just right and it fits together with everything else, great. But if it needs tweaking, can you sand it to fit? I’ve heard that can make the joint weak. Should you not sand at all and just get new material and do better next cut? Or if you have to sand a piece to fit in a joint or assembly should you do something to clean it before you glue it?

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Making the joint fit is probably a good thing to do. Sanding to fit with a course sandpaper will create a rough surface that the glue can adhere to. I usually just knock the dust off by tapping the piece on edge or brush it away with a soft brush.

Quote:

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Originally Posted by willmartin

When you use the plywood for gussets, I’ve heard you should sand the sheen off. Is that literally all the more it needs, just a little bit to dull the surface? Doesn’t that fill the plywood with dust? Do you need clean out the grooves in the plywood before you glue it on?

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Yes. definitely sand the sheen off. See above.

Quote:

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Originally Posted by willmartin

Speaking of gusseted joints like a rib intersection: if some of the capstrip pieces are maybe a tiny bit different thickness than the others, do you sand the area where the gusset goes so they will all be at the same level and the gusset lays down flat? Like above, if you do, do you clean the sanded wood afterward before you glue the gusset on?

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Good quality capstrips will be very regular. You shouldn't notice any difference in thickness. If there are slight differences, T-88 epoxy glue is very forgiving. It will fill small spaces. I bought some Finnish Birch plywood. It's 6mm as opposed to American 1/4" Birch that is 6.35mm. There was noticeable difference between the 1/4" capstrip and the Finnish Plywood. I thought 0.35mm was too much of a difference. I used the Finnish plywood for other stuff.

And congrats on starting an airplane project. My project has wood wings. I kept a blog. It may be some help for your project.

http://myhatz.blogspot.com/2016/04/april-2016.html

I picked up a Lion(?) miter trimmer to shave things like cap strips and the like for my projects. Essentially a lever operated edge shear. It’s not designed to make cuts, but shave like a plane.

As far as sanding to make it fit goes I think a lot depends upon whether you're talking end grain or the face of the wood. If you're scarfing and you sand the scarfs, you may drive sanding dust into the pores of the wood. You can tell you've done that in furniture or boat building the moment you apply a little stain to the end grain. If that happens then I don't think you get the same glue penetration. Unless you have grain run-out you won't have that problem sanding a face.

But also, if you sand to make it fit, you run the risk of rendering your surface "un-flat". Especially with regard to (again) scarfing. It's very easy to round over and/or make depressions in flat surfaces if you do much hand sanding, or even sanding with a random orbital sander.

I suspect that's why the AC recommends:

"The mating surfaces should be machined smooth and true with planers, joiners, or special miter saws. Planer marks, chipped or loosened grain, and other surface irregularities are not permitted."

Even in boatbuilding and furniture making (two areas I have a lot of experience with) where the fit isn't as critical as on an airplane, I tend to avoid sanding whenever I can.