I was testing a Smena lens, and it was unexpectedly susceptible to veiling glare, compared with other Smena lenses I've used. So I took it apart to make sure the elements were positioned correctly. The aperture unexpectedly popped apart.
I have no problem fitting the first several blades back into the housing (@ 11 o'clock in the photo), but when I get to the blade that has to slide under the first one to reach the corresponding slot, the previously positioned blades pop out and I have to start over.
Is there a trick that would help with putting the iris back together?
While you may be able to reassemble the aperture using the method you describe, the problems you face are:
The previously positioned blades will pop out of place when you try to slide a new blade under them.
The housing wall makes it difficult to hold the blades in place during the process.
As you guessed, there is a trick to reassembling the contraption. Use the ring with the tab sticking out of it (located at 3 o'clock in your photo).
There is no housing wall to prevent you from holding the blades in place.
Instead of having to slide any blades under others, each blades is placed on top of another.
Since blades are placed on top of each other, you can hold all blades in position by applying pressure to the blade that was last placed.
A few more pointers:
Put the blades on the side that the tab is bent toward. If you put them on the wrong side, the tab won't engage the aperture control ring, and you'll have to either start over or bend the tab in the right direction.
For the last few blades, the first several blades will block the holes you'll need to access. Just rotate the blades outward to move them out of the way.
When all blades are in place, rotate them so they are aligned with the ring. Then gently place the housing on top so that the slots line up. When the pieces are together, flip it over and press the retainer over the ring.
The following photo shows the elements in the correct order, facing the correct direction. The aperture goes in before the rear element. When the rear element is positioned correctly, the threads should be facing the back of the lens.