Countersinking the rear spar using a 100 degree cutting bit in a countersink cage.
BUILDER TIP:Based on prior experience I have found that countersink bits used with hand drills tend to make holes in thin metal larger. A good way to help minimize this is to first drill the proper sized hole into a piece of hardwood and place the wood under the piece of metal being countersunk. Align the wood piece so the tip of the countersink bit goes into the hole drilled in the wood. This really helps ... a lot. Give it a try on some scrap aluminum. Unfortunately, in the photo one can’t see the wood piece under the spar I was countersinking.
From left to right - countersink cage with a cutting bit installed, a cutting bit, a cage without a cutting bit installed.
Avery makes a nice countersink kit consisting of cutting bits and a high quality cage made with bearings as opposed to bushings. Most builders (myself included) will want an extra countersink cage (or two) because it takes time to get it adjusted correctly to cut at the proper depth. Once it is adjusted, you tend to want to leave it dedicated for that size cutting bit.
For those not familiar with how the countersink cage works, I’ll try to explain by making the simple sound complicated. The round shaft protruding from the cage assembly connects to a hand drill or drill press which rotates the shaft. The end of the shaft inside the cage has a threaded hole which the desired sized cutting bit screws into. The shaft is spring loaded and when downward pressure is applied, the shaft slides deeper into the cage allowing the cutting bit to go deeper into the hole to be countersunk. While the inner shaft is spinning, the outer portion of the cage does not spin as long as you hold onto it.
Now the time consuming adjustment I mentioned earlier ... there is a depth stop adjustment because one needs to limit the depth of the cut. Without a depth limit stop, it is very very very easy to countersink much deeper than desired … thus there is a cutting depth adjustment. Unfortunately, the depth adjustment is all trial and error so one must make sure they start shallow and slowly adjust to the proper depth. The bits cut metal fast so it takes time to slowly get to the proper cutting depth. Once the tool is adjusted you are off to the races for it only takes a few seconds per hole.
The cage is on a threaded barrel and adjustment is made by loosening a locking ring at the top of the assembly then pulling back on the lock and screwing the cage in or out of the barrel. This essentially makes the cage longer or shorter thus allowing the cutting bit to go deeper or shallower. If you double click on the above photo it will come up larger. Looking closely at the photo, you will see I have loosened the locking ring and pulled back the lock then wedged the mandrel of a rivet between the cage and the lock so one can see the locking teeth and also how small of an adjustment that can be made.