Dovetail problems

I'm in the middle of making a simple open toolbox similar to the one described in the dimensional woodworking pep course.

I decided to use the project as an opportunity to try dovetails for the first time. It has been educational, but I realized there are some complicated forces going on and I still don't entirely understand where I went wrong.

I know my measurements and cuts were not perfect, but they were very close, very much within a mm. I tried to undercut the inside of the joints by angling my chisel in a degree or two under 90 degrees towards the center of the joint where there was nothing showing.

When I put the ends on, the wedging force of the pins split some of the fibers in the ends of my toolbox. It's as if the wood is being pried apart by the dovetails.

I do 99.5% hand tool woodworking with almost no power tools. This project has no power tools involved. So all the error is mine.

I'm upcycling old wood that used to be a shelf in our pantry. Some of it does look like it has had some water damage or humidity damage in the past, so that might be part of the problem, but I think it's a little unlikely. It is certainly very old wood and not wet now.

I was thinking maybe the inside of the joint needed some relief space to give the wedging space more room to pull, but the issue might be more basic. Any feedback or pointers from people who have been dovetailing for ages?

I realized I showed something else I didn't mention, so I'll talk about that in the second post.

I made a different mistake on the bottom tails showing here. I didn't transfer the lines to the opposite side and make cuts from both sides, so I got major blow-out when I cut them out. I didn't have that problem with subsequent tails, because I transferred all the measurements and cut lines to both sides of the piece and started the cuts with a chisel strikes.

How snug were they when you tapped them together?  I think dovetails should almost slide together by hand and maybe need a tap or two with a mallet to persuade them once in a while.

Purely judging from the photos it looks like the pins may have been too wide and split apart the tail stock.

Also, the geometry didn't work in your favor.  The dovetail from the side was almost as deep as the chunk of tail under it (tiny distance between the white arrows).  So the end piece of your box was really fragile on the lower left corner.  Any splitting forces would easily crack it due to the lack of support.  

Having thinner wood on the long sides would have allowed for a shallower dovetail (Yellow side dovetail for example).  Or using the same sides but thinning it down where the dovetail pins are would allow for a shallower dovetail hole in the end piece.

Regarding my post above, if the pins are too fat or wedge themselves in too tightly, you get side forces that cause the splits (Green arrows).

Mike Haasl wrote:How snug were they when you tapped them together?  I think dovetails should almost slide together by hand and maybe need a tap or two with a mallet to persuade them once in a while.

Purely judging from the photos it looks like the pins may have been too wide and split apart the tail stock.

Yes, this. They were definitely too snug. I gently tapped them together with a mallet instead of using hand power. I thought I would be okay if I checked the joint as it came together for obvious tight spots. It did come together, but not easily.

Mike Haasl wrote:Having thinner wood on the long sides would have allowed for a shallower dovetail (Yellow side dovetail for example).  Or using the same sides but thinning it down where the dovetail pins are would allow for a shallower dovetail hole in the end piece.

I see. This seems like an important point going forward. I obviously didn't have the experience to tell me this was going to cause problems when I laid it out. So as a rule of thumb I could think of needing more than double the size of the pin in supporting wood.

As it is, the split wood isn't going to prevent the box from being useful since the bottom corner tails are what hold the weight, but I will need to reinforce the side so it doesn't fall off when it's loaded.

Cool, I'm glad that all made sense.  Newer wood that might not be as brittle/dry may also have helped but I'm just guessing there.

Luckily that's why you start with a simple box before you make cabinet drawers.  After a few dozen drawers you can do a blanket chest and have it look pretty good.

Yeah Mike called it.The pins are still a little wide.
As for the lay out of the Dove tails another option would have been to cut a rabbit in to the edge of your side pieces effectively making the side thinner as Mike suggested.  Then cut the dovetails into the shoulder of the rabbit.
As you  said the wood is old and very dry that combined with the wide tails would make splitting easy.
They are lots of fun if you have sharp chisels and the patience to keep shaving small amounts until the fit is perfect. 

I took them apart and shaved the pins enough so I could get it back together with hand power alone. Much less pressure on the splits. Thanks for the feedback!

It all came together and with the handle on with tusked tenons it's fully usable without fasteners!

I'm probably going to do a little more to enforce the joints, but I'm not sure exactly what yet. I'm considering and have enjoyed using 木釘 (kikugi) or wooden nails on other projects. It's a Japanese joinery technique where you drill a small hole and hammer in a square edged wooden "nail", basically just a shaved down wedge just bigger than the hole size. The wood grains compress and it doesn't come out.

Yep, use those wooden pins! The specific technique sounds very good. I think you need to figure out how big the pins should be relative to the dovetail material. Just big enough to work without shearing under load would be best. The larger the pins, the less surrounding wood there is to hold them without splitting.

One technique from timber framing is to drill the housing member and go just deep enough into the second member to mark the location, then disassemble the joint and offset and drill the second member by a fraction so that when the pin is hammered in, it draws the joint more snug than you can get by pushing the parts together.

Finished.

I used scrap wood from the same shelves and cut them a bit longer than the holes I drilled. I split the pins with a chisel to use the strongest grain. Then I pared down the pins with a minor taper. The pins do not fit the drill holes until encouraged with a mallet.