A Mental Challenge for Aero Engineers 13

[WKTaylor]

Folks...

I'd like to try something different with this thread... and ask a loaded question as a test of YOUR practical engineering/manufacturing/maintenance knowledge.

I been thinking about, and asking others about, this question for almost 20-yrs now. This is a question that is designed to stimulate an awareness and understanding of FAILURE mechanisms, and other practical issues, for ALL engineers. I believe You will find this question looks simple... but don't think that it is: this question has MANY critical aspects.

What is YOUR response to the following question(s)?

It is standard aeronautical practice to: (a) attain 125-microinches** Ra machined finish [or better] on cut and machined edges/surfaces; (2) deburr holes and chamfer/radius edges; and (3) round-off [rasius] sharp [square-ish] exterior and interior corners.

WHY??? What engineering and practical benefits are derived from these standards practices???

[** Sorry...I am not sure what equivalent SI units for 125-Ra surface roughness are. 125-Ra is a typical U.S. Aerospace industry standard requirement.]

NOTE: I promise to provide MY answers to this question in about (2--3) weeks [29 Oct to 8 Nov 02], depending on rate of responses.
Regards, Wil Taylor

 

[CoryPad]

(a) Fatigue life increases when decreasing surface roughness, and smoother surfaces have less preload loss when they are part of a mechanically fastened joint

(2) Burrs increase stress concentration at hole edges, which already have three times the net section stress at the edge. Therefore, removing burrs decreases stress concentration, which increases fracture resistance and fatigue life. Lastly, burrs can interfere with proper seating of mechanical fasteners, so removing them reduces damage to fasteners and clamped components during assembly.

(3) Sharp corners increase stress concentration, so increasing radii decreases stress concentration, which increases fracture resistance and fatigue life.

Some practical benefits of (2) and (3) are reduced injury to humans contacting the objects.

 

[WKTaylor]

CoryPad...

Great start... very descriptive of one aspect!

Aero-Guys...

What are other good reasons exist???? Regards, Wil Taylor

 

[Asanga]

Mr. Taylor,
Wow,I thought Corypad covered a lot of it. But I would also guess that since you have wires, water and oxygen lines, you would need to have the least amount of sharp edges as possible, to prevent something dangerous happening due to chafing.Also probably rounded off and polished surfaces look a lot better too, appearance wise.
Cheers,
Asanga

 

[butelja]

On radiusing the external corners, Asanga brings up good points, as does Corypad. I would add that if the part is to be heat treated, leaving ANY sharp external corners can lead to quench cracking because of the much greater local cooling rate.

 

[WKTaylor]

guys...

All good points... but there are still MANY more important aspects about this issue.

Hint: this question/issue/problem cuts across a wide span of engineering, fabrication and maintenance disciplines! Regards, Wil Taylor

 

[i278]

I'll just add to the great points presented already; I concede that some of these are pretty minor issues. (But I think that's part of Wil's intent

a) After fatigue life, one of the biggest issues I have with poor surface finish is that it introduces a billion new points for crevice corrosion on the surface. Also, a rough surface can make it difficult to get good results with non-destructive testing methods like die penetrants--especially when the roughness is in a pattern (like flycutting) that will most likely be causing the surface defects in the first place.
Any sort of surface treatment (plating, chemical conversion coat, etc.) will require more material on a rough surface to achieve an equivalent film thickness. Enough to make a difference over a production run.
A rough surface is harder to clean. Nitpicky, yes, but in the words of an old instructor, "Dirt is Weight"

2) A fastener hole with a good, sharp, burred corner will have obvious problems with seating when met with a fastener that has a radiused junction between head and shank.
Also, with riveted structure, friction (due to the clamping force of the fasteners) between faying surfaces in a joint serves a couple important functions. First, the friction provides a bit of 'shear preload'--the joint can take a certain amount of shear without loading the fasteners or sheet in bearing. The greater the friction, the more resistant the joint will be to working loose and smoking rivets. This ties in nicely to the second function: high frequency (engine) vibrations throughout the structure are damped or dissipated through joint friction. The greater the friction, the greater the high frequency fatigue resistance of a mechanically-fastened joint.
The upshot is: if a burr is sitting between the fastened sheets preventing good contact of the faying surfaces, much of this friction is lost.

3) Sharp edges and corners make any sort of forging or casting of a part difficult if not impossible.
Sharp outside corners on structure act as electrical charge concentrators, and can be a static discharge hazard. For the same reason, sharp corners can cause undesirable results in electroplating operations.
Sharp inside corners, in addition to being stress concentrators, are hard to clean (Dirt is Weight), and can collect pools of salty bilge water or alkaline cleaners and such.

I hope this came across well, I'm not having a very lucid day. Whatever the case, more material for discussion.

Regards

 

[TVP]

i278,

Your post was very good. I would even say lucid. I think Wil is getting some great stuff here.

 

[WKTaylor]

i278 ... Excellent contribution, many good points!

Guys, Keep it up... there are still more facets to this problem just hiding in the wings to be uncovered!

Regards, Wil Taylor

 

[brucoli]

First at all, the above contributions covers all principal reasons for filleting, chamfering edges and finishing faces.
However, also I think that in a high finished surface will be more easy to find cracks with visual inspections, than in a rough face...

Also, it will take less primer (i.e. alluminium parts) if the surface is not rough, and again we are talking about WEIGHT.

There is a special case, when the parts are subjected to relative movement. Obviously, the mating faces must be very fine machined to:
a) avoid friction
b) avoid heat due to friction. Excessive heat may change the properties of the material surface, with unpredictable consequences.
c) better lubrication. The active film in a fine machined surface will be more efficient because there will be more surface in contact with the lubricant. This will permit better heat transfer from the part to the lubricant
d) Excessive roughness may develop high material wear, leading to high play, and high replace frequencies of the parts.

Let me know what think you about the above...

Regards,

Sante

 

[WKTaylor]

brucoli... good points!!!

Guys, there are still many more reasons awaiting comment!

Hint: some of the other reasons have yet to be identified; while others are significant refinements/variations of existing comments. Regards, Wil Taylor

 

[PreAeroStudent]

Like the other guy who posted above, I am a high school student, and really I have no practical experience in engineering, but would like to take a crack at this one. Please tell me if I'm way off the mark, as this is really a guess. a) smooth surfaces create less friction regarding anything that may be rubbing up against it, which equals less wear and tear. b) What does it mean to deburr a hole? if it just means to chamfer it, then I believe that chamfering the edges would help when inserting any objects into the hole, because it would reduce the chances of damages if the object is inserted at an incorrect angle due to the reduced stress levels at the edges. c) not only are rounded edges safer, less damaging, and better to look at than sharp edges, they also require less material, which equals lighter weight. Also if this edge is a moving part, it will cause less friction on anything it is rubbing up against, which will result in less wear and tear. Furthermore, if the edge is moving, the rounded edge should reduce the amount of drag the part produces, which is generally a good thing.
Please excuse me if I am totally incorrect, as this is without any knowledge of physics, flow dynamics or any other of the staples of aero/astro engineering. If someone pleases, could they help me in my selection process of a good engineering school?

 

[WKTaylor]

PreAeroStudent...

You've made a couple of worthy contributions! Great!

Everyone... there's still more to be brought to light! Keep it up! Regards, Wil Taylor

 

[Asanga]

Wil,
If the fastener holes that are deburred are inspected with the use of HFEC (High Frequency Eddy Current) prior to installation into the aircraft, could those qualify as being zero timed?? I was looking through an older Boeing Structural Repair Document (D6-81987) and there was a statement about increasing the inspection threshold of these fastners (if the holes were zero timed) from 60,000 flight cycles to 100,000 flight cycles (only the Boeing 737-200 aircraft was mentioned). I bet that is a significant reduction of cost for a maintenance shop. Here of course they had specified an increase in '1/16 inch in diameter' which might not be practical in the case of smaller holes.
Anyway thanks to your question, I am starting to look at these maintenance books with a little more motivation.
Cheers.
Asanga

 

[WKTaylor]

Asanga...

Your question does not track with the question I posed. Do You have any Comments to add? Based on the nature of Your question, I can sense You have a feeling for the potential seriousness of "burrs" and flaws!!! Add Your 2-Cents worth! There are still some serious issues of regarding affects of "burrs and sharp corners/edges" to be addressed.

Another Hint for all: EE/EL types should take this topic very seriously!

Asanga...

You have raised an interesting and VERY serious question: Please paste Your question [above] into a NEW POST for separate discussion!!!! Note: I used to watch Forrest Service S-2's, C-130s and PBYs getting "zero-timed" at a shop in Ca without removing permanent fasteners is critical locations... then I learned about DADTA... and Rouge Flaws. Yeoooouch. Regards, Wil Taylor

 

[astroclone]

My turn, I guess.
This goes along the EE route mentioned above as well as other posts.
Roughness can equal friction as stated above. Friction can lead to electricity (Tribo-electric effect). Electricity can lead to corrosion.
That's all I have for now.
Tony

To pre-aero, University of Cincinnati (Good co-op program), Georgia Tech, St. Louis University, Purdue are all good schools for aero engineering. There are many many more, but I'm midwestern based so I'm midwestern biased.
Check the web, and look for aero design competitions, look for what schools participate and win, that should be a good indication of a decent school.

 

[WKTaylor]

Astroclone...

Come-on now, try harder!!!! You almost had a good point, except Your logic trail fell apart.

I'll expand the last EE-EL hint just-a-bit: "resistance".

Note for pre-aero: I went to Cal Poly [CPSU] San Luis Obispo [71-76]. Great Aero dept and getting better... but entry qualifications are stiff! Understand Cal Poly Pomona and Embry Riddle Aero departments are class acts also.


Regards, Wil Taylor

 

[i278]

I'm not sure if this is what Wil is gently pointing us toward, but:

Burrs and surface roughness will both interfere with good, uniform surface contact between faying surfaces in a mechanical joint. This, of course, increases the electrical resistance of the joint and, if severe, can cause problems with electrical bonding of structure; interfering with effective grounding of electrical equipment and/or antennae, and become a miniature plasma cutter in the event of a lightning strike.

Regards

 

[WKTaylor]

i278 ... I AM VERY IMPRESSED....

You found a very compact/precise way to describe this important facet of the "burrs and surface roughness" question!!!

NOTE: there is at least one other problem that burrs and roughness induce relative to the bonding/grounding comments within the previous thread. Astroclone alluded to it, but missed the mark, slightly. What is it??

Also there still are a few other issues that haven't been addressed (or haven't been fully addressed), yet!!!

The Famous Aero engineer, John Thorp, often stated that engineers must take calculated [not stupid] risks to "move forward and make thing happen". One of his favorite quotes was: "The tortise only makes progress when it's neck is stuck-out".

Who else willing to "risk" presenting their thoughts/ideas on this topic??


Regards, Wil Taylor

 

[vonlueke]

I'll try a crack at this, no pun intended.

(1) Rougher surfaces or sharper exterior edges can scratch coated or painted surfaces during assembly and might allow hidden corrosion to spread underneath what might temporarily appear as good finishes.

(2) Or, the scratches of item 1 could create an accelerated galvanic-corrosion anode or cathode site, if all (most) other surfaces are coated (insulated).

(3) Higher values of surface roughness (and burrs) increase leakage rate under/around gaskets and seals.

(4) Nipping gaskets, seals, and o-rings on sharp edges during installation, or scouring them on rougher surfaces during operation of rotating equipment, could accelerate leakage of item 3.

(5) i278 already mentioned hard to clean, so I can't mention increased air drag or turbulence over rougher, dirtier surfaces, or something to that effect, without being redundant.

(6) Rougher surfaces could create a better adhesive surface to build up (and hold on to) more ice, overstressing the structure or adversely affecting operation.

(7) If water creeps under interfaces via higher surface roughness and fills up a cavity or interface, then freezes, it could create high stresses and/or accelerate material fracture, not to mention stress corrosion cracking at scores from the hidden, trapped water/chemicals (i278 already alluded to that one, too).