Stress risers in countersink holes?

[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! :-)

[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).

[kevinh]

also - using a little bit of cutting fluid helps, and go slowish at the beginning.

[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!

[kevinh]

woot!

[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

[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?

[samiam]

I'm no engineer or expert on the matter, but I see two obvious potential problems:

1. Stress risers
2. 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.

[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.

[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.