Stress risers in countersink holes?

[Bacon8tor]

Almost every hole I countersink in aluminum has little ridges in them. Are these like stress risers? Is this normal? I'm using a hand drill and have used two different countersinks with the same result. Every once in a while I get a smooth hole but it's rare. Is it poor technique? Is it okay to put flush rivets in there? If I try to smooth I'm afraid I'll only make the holes worse.

[samiam]

Are you using a microstop? If not, it's definitely worth the investment. They make beautiful countersunk holes, uniform, in much less time. Find 'em cheap on ebay or the yard store.

[Bacon8tor]

Thanks, Samiam. I'll look into one of those countersinks.

My question now is can I go with the knife edged holes that I have now or should I start all over. This seems to be a fairly common problem.

[kevinh]

If you get a microstop cage (and go slowish with it) I bet it will fix the 'ridges' you describe (assuming you still have enough depth). Test on a piece of scrap first. Can you post a photo of a bad countersink? Also, I'm assuming you are using a piloted countersink (one sized for #30 or #40)?

[Bacon8tor]

Thanks for the tip. I've tried to post pictures from my iPhone but the forum says they are too big. I don't know how to adjust.

[Bryan Cotton]

John,
All my pictures are from the iphone. I mail them to myself and it lets me reduce the size to medium. Then I save the smaller pics to my phone to post. Clunky, yet it works.

I also think the microstop will cure your chatter. I set mine so I can bear down hard and that last bit of cutting smooths out.

[Bacon8tor]

Problem solved. GO SLOW. Drill speed that is. Everybody says that. I guess I should've learned to countersink before assembling the parts and posting this thread. I'll leave the post on so other newbies can learn from my mistakes.

But my other question still remains. Can I go with the "knife edged" countersinks or should I start over?

[Bryan Cotton]

John,
Knife edges on countersinks are generally bad. If the location was structurally critical I would replace or make a doubler. Check in with Sonex tech support.

[Bacon8tor]

I bought the cheapest caged micro stop countersink from Aircraft Spruce and was disappointed with its performance.

In my whole ten hours of countersinking experience I'll give my takeaways and opinions which are just that. I am certainly not an expert.

The micro stop countersink was no faster than the simple bit in my hand drill. The micro stop also drifted and caused an oblong hole! I'm guessing this was somehow caused by not having a backing plate for the pilot.

I'm resigned to thinking countersinking is a long arduous task unlike drilling a hole. They are different animals. Slower speeds work better for countersinks. I get best production with a speed on my variable speed drill that is just a little faster than my eyes can follow the spinning bit. Drill a little then test with a rivet. Drill more and test again until you get the right size. Firm pressure while drilling and a little wobble until you get close to the correct size then as perpandicular as you can eyeball.

[samiam]

This is a very different experience than the one that I had. I bought the Yardstore's nylon tipped microstop, which was about $24. It made beautiful, easy, fast, and highly symmetrical countersinks. Couldn't have made the job easier.