Stress risers in countersink holes?
Posted: Wed Mar 30, 2016 4:36 pm
by 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.
Re: Stress risers in countersink holes?
Posted: Wed Mar 30, 2016 4:40 pm
by 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.
Re: Stress risers in countersink holes?
Posted: Wed Mar 30, 2016 8:49 pm
by 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.
Re: Stress risers in countersink holes?
Posted: Wed Mar 30, 2016 10:07 pm
by 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)?
Re: Stress risers in countersink holes?
Posted: Fri Apr 01, 2016 10:11 am
by 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.
Re: Stress risers in countersink holes?
Posted: Fri Apr 01, 2016 11:43 am
by 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.
Re: Stress risers in countersink holes?
Posted: Fri Apr 01, 2016 11:46 am
by 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?
Re: Stress risers in countersink holes?
Posted: Fri Apr 01, 2016 2:39 pm
by 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.
Re: Stress risers in countersink holes?
Posted: Thu Apr 07, 2016 9:15 pm
by 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.
Re: Stress risers in countersink holes?
Posted: Thu Apr 07, 2016 9:18 pm
by 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.
Re: Stress risers in countersink holes?
Posted: Thu Apr 07, 2016 9:43 pm
by Bacon8tor
It must be my technique. I understand you get what you pay for. A high quality sharp bit should cut like butter. My uncaged bit was top quality yet cut no better that my $27 micro stop from AS. Your $24 micro stop works great. I don’t get it. Looks like my next purchase will be from the Yardstore! 
Re: Stress risers in countersink holes?
Posted: Thu Apr 07, 2016 9:47 pm
by kevinh
Yep. My old avery tools countersink makes nice repeatable holes - I just countersunk a zillion holes in my wing spar quite happily (went with the countersunk skin rivets).
Re: Stress risers in countersink holes?
Posted: Thu Apr 07, 2016 9:48 pm
by kevinh
also - using a little bit of cutting fluid helps, and go slowish at the beginning.
Re: Stress risers in countersink holes?
Posted: Thu Apr 07, 2016 10:45 pm
by Bacon8tor
Kevinh, using cutting fluid made a HUGE difference! I was reluctant to use because of the mess factor but well worth a few shop towels. I used just a drop or two of 3 in 1 multi-purpose household oil on a hole and it cut like the butter I was hoping for. The residual oil on my bit then worked well for about two or three more holes before adding another drop. Great advice. Thanks to everyone’s input!
Re: Stress risers in countersink holes?
Posted: Thu Apr 07, 2016 10:52 pm
by kevinh
woot!
Re: Stress risers in countersink holes?
Posted: Fri Apr 08, 2016 10:11 am
by 9GT
That’s called “chatter” and isn’t good. You can avoid that by using high quality piloted countersinks in a good ball-bearing supported micro stop cage. Run the countersink a little slower and use a cutting lubricant such as Boelube. http://www.mscdirect.com/browse/tnpla/6 … 0=61232534
Re: Stress risers in countersink holes?
Posted: Fri Apr 08, 2016 1:11 pm
by Bacon8tor
Okay, so I understand what countersink “chatter” is. Also called “knife edge.” And I understand they are “bad.” But what makes them bad? What are the consequences of a “bad” countersink? Is it purely aesthetics? Of course I’m looking for any excuse not to buy new stock and countersink 20 new holes “correctly.” (I love using “quotations”). Will the chattered countersink cause cracks in the aluminum in the future?
Re: Stress risers in countersink holes?
Posted: Fri Apr 08, 2016 1:50 pm
by samiam
I’m no engineer or expert on the matter, but I see two obvious potential problems:
- Stress risers
- The rivet will not seat properly in the countersunk hole, thus leading to a weaker rivet.
Again, as I’m no expert, I can’t speak as to whether or not you should redo them.
Re: Stress risers in countersink holes?
Posted: Fri Apr 08, 2016 3:08 pm
by gcm52
One other thing that helped me: I found that the countersink stop would go out of whack a bit and give me the wrong depth after a few holes and I would have to readjust. I solved this by putting it in a drill press and getting the depth just right on a piece of scrap. Then you can countersink your actual work to the perfect depth over and over on the drill press, as long as you can get the work under the press.
Re: Stress risers in countersink holes?
Posted: Fri Apr 08, 2016 3:28 pm
by 9GT
John,a couple reasons that chatter is not good for countersinks. It creates points of possible future stress cracks in the aluminum and and does not allow for 100% rivet seating and contact. Through the vibration that will happen during flight the joint will move ever so slight wearing the mating surface of the rivet and enlarging the rivet hole. You will eventually see a trail of dark blackish streak behind the rivet as it wears through the aluminum if its in the slipstream of the plane. These loose rivets are commonly called “smoking rivets” and obviously are a bad thing.
Re: Stress risers in countersink holes?
Posted: Fri Apr 08, 2016 4:20 pm
by NWade
Just a note on the “Knife Edge” terminology. This can mean different things to different people!
Commonly, I’ve seen this referred to when talking about running a countersink too deep; or trying to countersink a material that’s too thin. See this picture for an exaggerated example of a “good” countersink and a “knife edged” countersink:
https://goo.gl/photos/sPmGuKMNNKW39R5k8
The problem with the “knife edged” hole in this example is that it might have opened up the diameter to be too big, and the thin material near the hole doesn’t have enough structural strength to prevent the rivet from tearing through (or at least deforming the material) when its put under any kind of load or strain (especially over time).
Enjoy,
–Noel
Re: Stress risers in countersink holes?
Posted: Fri Apr 08, 2016 6:10 pm
by kevinh
hmm - the depth should stay good even if doing it by hand but you might need to tape the little lockring on the countersink to make sure it stays locked. Typically I test on some scrap, find the depth then tighten the locknut hand tight and wrap it with a piece of tape then go at it…