I thought it might be interesting to talk a little bit about dropouts. This is what's called a tab style dropout, and it's pretty cool. Tab style dropouts aren't all that common on today's production bikes, and especially not on higher end bikes, but they used to be pretty common place. Take a look at the dropouts on any steel bike made before about the year 2000 and you'll quite likely see something that looks kind of like this. A production steel bike from the year 2018, a Salsa or Surly for example, is much more likely to have hooded dropouts with a welded joint. They're great in their own right, but to my eye don't quite have the elegance of the good old tab style. They're also a bit heavier I reckon. The really cool thing about tab style dropouts is that they're super versatile, and allow the builder to get a bit creative. This same dropout can fit into tubing of varying diameter and wall thickness, and can adapt to quite a wide range of frame geometries. Lug and socket style dropouts, on the other hand, really only work with a specific tube, if the tube projects at the exact right angle. All this freedom means that the builder has to custom shape each dropout, and that's where the fun starts.
The way these work is the dropout is slotted into the tube. All those square notches you see are custom cut at exactly the right angle. A portion of the dropout tab goes totally inside the tube, and another portion sticks out on the side of the tube. One problem that you can run into is the cassette, chain, and even the brake rotor can collide with the tubing. Some builders will simply shave away material from the tube until the problem goes away or in a production context this area is often dimpled (there's usually more than enough strength in this area that it's not a huge concern). Instead of doing this, I prefer to cut my slots off center. This buys me clearance without sacrificing strength. And I do that on the mill with my trusty slotting saw.
That thing makes the tiniest little slivers. Not quite as bad as a carbon fiber splinter, but close. The next step is to carve in some scallops. This buys yet a little more clearance and reduces the stress riser effect that you get with sharp edges. I do this with files and a Dremel. Here's where the creativity comes in.
Then I braze the joint. Some folks do it with silver, but I like brass here. It's cheaper, and easier to shape into a nice cap. After a little finish work it looks like this.
If you were to look down into the other end of the tube you'd see it plugged solid with brass at the bottom. The goal is for it to be filled all the way down to the bottom of the tab on the dropout but no further. Some builders blend in the square edges of the tab where they stick out of the tab, but I like to leave them sharp. I think it looks cool.