For me, it doesn't matter if the hole is #19, #18 and #17, it can crack. That's not to say that going up a size or two on your pilot hole doesn't relieve some pressure, but I found that it wasn't the complete answer.
Just for kicks I drilled some #18 and #17 holes to see how it would go. I deburred the holes and dimpled them. They cracked.
So, for me that kinda solved the hole size issue. I did notice that there was a difference in hole sizes after they were squeezed. Its a bigger pilot hole, and after squeezing, surprise surprise, the hole in the dimple was larger too. Seeing that it did not eliminate cracks...I resolved to focus on the #19 holes. What seemed to be the common denominator in all my cracked holes was how well/clean the holes were drilled and the condition of the deburing. I holes were good, the cracked dimple was 1 in 28. Holes that were not as clean I got 1 in 10 cracked. It's easy to be sloppy with the drill in opening up the holes and its very easy to debur too much. My 3-flute deburring tool does not do a nice job when looking under a magnifying glass. The one hole debur tool is better, but its not perfect.
I carefully drilled all the tank skin holes (39 test holes) up to #19. I started #25, #22, #20, then #19 (This process gave me brain damage the whole way...its not a great way to enlarge holes this way) Once I was done I then used a small piece is 600 grit sandpaper on my Dremel, (Just a small piece wrapped on the mandrel. I carefully polished the holes (It took a long time to prep the holes) but they looked great.
Here is what a marginal hole looks like. This is likely to crack.
This is a #19 hole that was final polished with the 600 grit sandpaper.
At the suggestion of a friend to lube the dies, I sprayed WD40 on the work and the dies and started squeezing. All 39 holes made beautiful dimples and no cracks!
For me this was a bit of a win. I feel confident that I am on the right track, but I still want to perfect the process. I'm still waiting for a #19 reamer to arrive and I may abandon the deburing tool completely. As you can see from the pic its not a perfectly clean edge on the dimple, they are some very tiny jags on the edge. I may use the Dremel felt and polishing compound to debur the edges instead.
I am one step closer to solving this.
