Dan spent a lot of time figuring out the proper countersink depth to allow the dimple to properly form over it. See this post Dan used the countersinking cage and bit where we could. We purchased it from Pan American Tools.
On the spar cap the cage would not fit so we used the drill press and measured the outside diameter of the countersink to get the right size.
We had purchased a 5/32 dimple die for our C Frame dimpler but they did not seat as well as wanted. In the end we aligned the sheetmetal over the countersunk layer and placed the bolt or rivet in the top flat sheetmetal hole. We used a arbor press to compress the rivet into the countersink. This created a really tight and crisp dimple. It got tricky in some spots so a little collar was used.
Here is our countersunk and dimpled spar layers:
