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AlinNS

Well Known Member
I had asked in a previous thread about deburring rivet holes using just Scotchbrite and got some great answers. The thread asked about a few other things but I'd like to concentrate just on deburring, hence a new thread.

It took me a while to figure out why there seems to be so many different ways/opinions on how to do what seems to me an incredibly simple thing. Then it struck me - deburring (holes) means two different things depending on who you are talking to: To me it is simply the act of knocking off extra metal pieces that will cut skin or interfere with a washer or bolt being torqued down or a rivet sitting well in its hole; to others it includes chamferring (not countersinking) the hole to relieve a stress point at the edge of the hole.

So I'm interested in the why here not how. Others have supported the need to chamfer by referring to "best practices", or its been this way for many years and is established procedure, etc. Now I'm not arguing any of that. I don't have the technical background to do so, but I have found references to just deburring the surface in some surprising places.

1. Is there a reason to deburr (not chamfer but deburr) inside the hole, ie immediately below the edge?

2. On a skin as thin as .025", does anybody get concerned with removing more than 10% of both sides when they chamfer to deburr? Can you really just do a twist or two on both sides with say a 100* countersink or larger bit, and be confident you haven't removed too much?

3. So here is my main question: Can someone provide me with a reference that chamfering to relieve stress on a rivet hole is required or proven to reduce cracking? I can't for the life of me find anywhere that this is indeed an established industry standard or best practice or proven to alleviate stresses. I would like to read about it. Stress issues with 90* edges and deep scratches I get. Is it correct to look at that rivet edge as the same 90* problem so needs to be removed?

Sorry for the long winded questions. Mercifully, I removed the how I got so smart with just a high school education part:D

Personally, I don't see this as over thinking. I see it as a desire to learn. I'm particularly enjoying the learning new things part of this.

Oh, and Happy Thanksgiving (Canada).
 
On a thin skin for a control surface, I usually do one twist with a hand-held deburring bit, and then check the surface with my finger to make sure it's smooth. If I feel anything snag, then maybe another half twist, then recheck.
 
Deburring is done to relieve stress.
Chamfering is deeper and is done to accommodate the shoulder of fasteners such as a button head hi-lok.
 
paul mosher said:
Deburring is done to relieve stress.
Chamfering is deeper and is done to accommodate the shoulder of fasteners such as a button head hi-lok.
Click to expand...

When you deburr with a bit of some sort you will cause a slight chamfer on the edges. Some folks do this and say that the chamfer (edge relief) is required for stress reductions. Hence the glove, scotchbrite, sandpaper method is not appropriate.

Some folks, the glove, scotchbrite, sandpapper folks would argue that the chamfer is not required. Hence its better not remove any material if you don't have to and can quickly wipe away the burrs.

That's the question I have. I'm looking for a reference indicating that the small chamfer does indeed alleviate stressors.
 
Deburr, Chamfer, Countersink

Hi, Al!

1. Deburring the inside diameter of the hole makes it easier to get the rivet in. It eliminates a loose chip of aluminum that could interfere with a tight fit of the rivet to the riveted pieces. Deburring the face of the hole makes it easy to get two pieces to fit tightly against each other.

2. Yes, chamfers will be smaller on thinner sheets. How big?? ... I know one when I see one. Perhaps the depth of the chamfer is 10% of the plate thickness.

3. Mentally draw a cross section of the riveted joint, or carefully saw one in half on a bandsaw. The rivet has been shortened by axial compression (from driving or squeezing) which caused the diameter of the rivet to expand and fill the entire void volume of the hole, assuming the hole was appropriately sized and the rivet properly driven. If the holes were chamfered there will be a chamfer under the factory head and the at the shop tail of the rivet. Without chamfers these transitions will be very square, perhaps sharp corners, which are well known to be fatigue initiation sites. If there is any misalignment between the two parts, the same sharp corner geometry will exist at the interface between the two (or more) parts.

Most aircraft structural joints have some factor of overdesign in them. Perhaps an un-chamfered hole will provide adequate life. But every designer of aluminum aircraft structures is assuming the builder will chamfer the hole in order to produce a joint that has design strength.

I have not seen the documents that quantify the degree that an un-chamfered hole compromises the strength of a rivet, but it is logical to think that someone in the aircraft industry has studied this, and concluded that the additional effort of chamfering both sides of every hole was justifiable.

Al, I admire your quest to understand and do things right for the right reasons. Keep up the good work! Does any of this make sense?

- Roger
 
also...

The deburring operation should leave a small chamfer. Creating the chamfer is not an additional step.
 
One item not mentioned yet is the fact that the factory head of a 470 rivet has a 0.010 radius where it meets the body of the rivet.

The small chamfer will help the rivet seat better in the hole and not cause a stress point if the rivet head radius is forced into the sharp edge of a non-chamfered hole.
 
paul mosher got it right, and here's a more detailed description of why to remove burrs inside holes, even if the fastener fits. A burr there will act as a sort of small spring between the fastener and the hole, reducing the shear stiffness of the joint, for low levels of load. That means that for that load level, the adjacent fasteners are having to work harder. And this slightly but adversely affects their fatigue life.

It's not something the designer can predict or allow for, since it's an uncontrolled artifact that's being left in the hole.

You don't want to chamfer holes unless necessary for the fastener, as paul said, because that reduces the bearing area for the fastener to bear against. That directly reduces the strength of the joint. With driven rivets, you can expect some filling in of the chamfer space with rivet body, but it might not fill the space, and anyway, you have no control over it. So minimize chamfering on holes.

Dave
 
RKellogg said:
3. Mentally draw a cross section of the riveted joint, or carefully saw one in half on a bandsaw. The rivet has been shortened by axial compression (from driving or squeezing) which caused the diameter of the rivet to expand and fill the entire void volume of the hole, assuming the hole was appropriately sized and the rivet properly driven. If the holes were chamfered there will be a chamfer under the factory head and the at the shop tail of the rivet. Without chamfers these transitions will be very square, perhaps sharp corners, which are well known to be fatigue initiation sites. If there is any misalignment between the two parts, the same sharp corner geometry will exist at the interface between the two (or more) parts.
Click to expand...

 
David Paule said:
So minimize chamfering on holes.

Dave
Click to expand...

Yup. The reason this gets confusing is that guys talk about spinning a countersink bit in a hole "a little," or "lightly", but "not too much." The bit, if allowed to bite too much into the metal will chamfer the hole too much, because that's what that bit is designed to do (countersink).

You must excercise judgement to use this "potentially harmful" tool to debur "just enough" which is why it's hard to put into words on this site.

The end goal is to have a smooth, cylindrical hole (not hour-glass-shaped, over-sized, etc.), with no burrs inside or on the top or bottom edges. How you get there is up to you...
 
Not to much

Keep in mind, the riveted joint is carrying almost 100% of its load in shear, so every bit of surface area in the bore is critical. Over de-burring/chamfering creats less surface area for the rivet to transfer load to. The production skins on big pressurized aircraft made in Seattle, require the skin deburing process to be accomplished by a flat vixen file or scotchbrite quickly passed over the hole to remove only the burr left from drilling.Any additional material removed just weakens the joint.
 
I have never worked in a commercial aircraft production facility, but my cousin has. He tells me that the big companies typically do not disassemble anything to deburr the parts. They simply run a chip chaser between the parts to remove anything that would prevent intimate contact of the parts.

Assuming that he is telling the truth, this certainly implies that any chamfer is not necessary at all. Just remove the interfering burrs and rock on.

YMMV.
 
OK. Finally.

I have been struggling to make my point. The two posts above help me with that.

No chamferrng (delete chamferring and insert edge relieving) of the rivet hole. Just a swipe with something to knock off the burr. I suspect that this is more common than I initially thought. I have a number of technical manuals that make no mention of relieving rivet hole edges for stress reasons, part 5 of our plans say nothing about it.

Please, I am in no way trying to start a bun fight. All the comments so far have been helpfull, make sense intuitively, and are much appreciated. I'm looking for a technical reference that says it must be done and why. At the end of the day we can all do this however we want - no disputing that. This is about educating myself.:)

Thanks.
 
AS a Tech Counselor, I think I see way to many cases of OVER-deburring - almost countersinking - than I do UNDER-deburring. Get the burs off so that sheets lay flat - and sometimes, if you have dull drill buts that create more of a lip on the hole - this is hard to do without just a touch of a chamfer. I try for a cylindrical hole - haven't had a crack at a rivet for, well .... can't remember one....
 
Deburring

vfrazier said:
I have never worked in a commercial aircraft production facility, but my cousin has. He tells me that the big companies typically do not disassemble anything to deburr the parts. They simply run a chip chaser between the parts to remove anything that would prevent intimate contact of the parts.

Assuming that he is telling the truth, this certainly implies that any chamfer is not necessary at all. Just remove the interfering burrs and rock on.

YMMV.
Click to expand...
Piper in Vero Beach used an automatic machine to install wing stringers. The holes were punched, dimpled and riveted, one hole at a time with the machine. No deburring. The recollection of work that I did on metal Pipers in the 60's is that there was no deburring. They may have run a chip chaser between the skins but that was it. the workmanship on the Aztecs and Twin Commanches was very poor by my standards.
 
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