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

 

[radomir]

Mr. Taylor,

I am inclined to believe that “wk” stands for “wicked” and I will definitely draw such conclusion if you don’t satisfy our curiosity and give us the promised explanation, tomorrow.

Sir William this time I am teasing a little bit but you have been teasing us for more than a month and I’ve become really curious. My personal opinion is that the major reason for all those features is a reducion of stress concentration, namely fatigue resistance improvement.

Although I have designated myself as “aeronautics” the more correct choice would be “propulsion” (unfortunately not offered as an option) in other words I am rather familiar with turbojet engines. The mentioned practices are a must in the manufacturing of turbojet parts. In particular concerning dynamic parts which suffer from L(ow) C(yclic) F(atigue). Even major casings (eg: Combustion Chamber Outer Casing) could be “victims” of LCF failure. I do remember a failure of a CCOC because of a poor surface finish. Since it is a kind of “pressure vessel” (modern engine compressor pressure ratio is above 20:1) a few machining notches initiated cracks and finally a LCF failure of the casing occurred.

Mr Taylor it is your turn now.

 

[Foreman]

Bump

 

[sancat]

Hi
I don’t know if I am too late or I am repetitious, but here is my cent:
I think that one other reason to decrease roughness, have rounded edges in holes, deburr, etc, is to Increase Friction. As far as I remember, a bolt or a rivet should fasten through means of an increased normal force, and therefore increased friction between surfaces to be joined.
If you have had any experience with very polished surfaces you will have noted the adhesive effect present. In fact, when the surfaces are not rough, there are less “peaks” to be cracked and valleys to be filled, everything resembles more like low dunes, and there are more contact points between the surfaces.
While the absence of roughness points to microscopically flatness, the absence of burrs, points to a macroscopically flatness, much in the same direction. These burrs will flatten in a non controlled way over time loosen the connection, and increasing all the factors above mentioned.
Sancat

 

[bruv]

Please excuse my total ignorance on things aeronautical, but am I right in thinking that a smoother surface finish will lead to better aerodynamic properties?

Bruv

 

[235zzzo]

Hello Guys

One thing is for sure, it has been a long interesting discussion! I feel sorry to get in so late.
Mr. Taylor you are asked to take the "micro" and address to the forum the conclusions in a clear way, some of which you knew since the very beginning, of course enriched with some find answers, you got here.

Let me give an humble contribuition, following BRUV's last reply, just a few closing general thougths:

i)Speaking about aerodynamic characteristics, sometimes you need some roughness level, not necessary and not always quasi-smooth surfaces, all over the aircraft.

ii)The aerodynamics characteristics distribuition, on the aircraft surfaces, must be in balance with coherent and optimized surface treatment(s), that applies for all components.

iii) When you design, build an aircraft, you buid an "entity" with initial physical characteristics, with initial forces, effects inside it, with an history, which must deal correctly along its life-time, with all random involving working conditions concerning.

Best Regards
zzzo

 

[sprintcar]

From the more 'real world' side, I would guess that when the aircraft industry moved from wood to metal part construction, the first guy drawing the parts selected the finish based on the economical capability of his own shop, added chamfers and removed all edges and corners because it was good shop practice at the time (and still is), then added this to the standard practice section of the drawing title block. In strict accordance with "The Dolly Theorum I" (cloning) this first part became many and proliferated into different companies. Following the "The Dolly Theorum II" (most people blindly follow like sheep) the title block was simply used for additional parts and here we are today. When was the last time many of us actually cloned a part and went thru ALL the little details like that? "If it works, don't fix it" or "Always done that way" sound familiar? Most of us can't even get into the the standard section on CAD title blocks now anyway, and you know what happens when you have to ask the MIS or IT person to change anything in this "protected" area!
Before somebody starts looking for a tree to hang me from, let me add that I've worked with a couple aircraft design folks but not had the pleasure of actually working on a project. I have done designs where a few 'A' size pencil drawings could get a $50,000 dollar order out the door, to the complex CAD drawings required to build a motorcycle. The aircraft industry is unique in that if a part fails, there are many lives at risk. Further, the history of this set of finish criteria provides a solid legal footing when one of those parasites (lawyers) decides to pad his pockets at our expense.
This may be the Best place for "If it works, don't fix it!" Keep the wheels on the ground
Bob
showshine@aol.com

 

[knobhead]

I've scanned through this thread, so this might have already been said, but when it comes to shot peening, are you talking about roll-over?

If you have a sharp corner, and you blast it, you 'roll' the corner over, creating a sort of burr which is bad for most of the reasons already mentioned up above.
Other reasons would be wear on sharp edges, or burrs breaking off through usage and contaminating whatever they are part of. (Not good for pipework)

But of course, there are times when a sharp edge is needed. Labyrinth seals in gas turbines spring to mind, as do squealer tips on compressor blades.... Excessive accuaracy is a sign of poor breeding. -Socrates.

 

[aircraftengr]

Where did Wil go?

OK, my 2 cents worth. Those three aeronautical practices are to minimize stress concentration and intensity and instead spread them evenly so as to not create any easy point for failure.

How's that for a summary? Are you gonna take us up on the answer, Taylor?

 

[CoryPad]

For everyone's info, wktaylor last logged in on Thu, Nov 21, 2002.

 

[WKTaylor]

Folks…

My regrets for not communicating sooner. Most of November and December have been terribly unnerving and distracting for me.

My Dad was crying when he called to inform me that my Mother was on the edge of dying in early November. She was helo evacuated from their AZ desert home to Phoenix for a number of exotic tests and treatments for her heart disease. She was finally allowed to go home in mid-late November with an “ICD” implant... then, I had a terrible cold so I was unable to visit her for another (2) weeks ( early Dec). I am afraid that her “full” recovery at 78-yrs is not going to happen… and my 84-yr+ Dad is not “up” to the challenge of caring for her. My sister and I are now faced with the many challenges of helping with our folks deal with their new/stark realities, at long distance.

And on top of all this I still had to work some OT.

I am trying to reconstruct my original answer, which was corrupted in early Nov when I saved the wrong file and “blew-away the good file”. I am sure this has never happened to anyone else. I HOPE have my “long-awaited answer“ by the end of this week… before the Christmas holidays.

Regards, Wil

PS: I still haven't reviewed all the latest imputs... plz be patient. Regards, Wil Taylor

 

[ornerynorsk]

Best regards to your folks. That's always a tough stage in life. Well, here's my input, from a machinist's and toolmaker's point of view. While it's true that burrs and sharp edges raise stress concentration, and poor surface finish raises all kinds of problems, the plain fact-of-the-matter is . . . GOOD WORKMANSHIP !!! You can tell a lot about a company and an individual by the APPEARANCE of the work that has been done. This one factor digs deep into the psychology of the craziest species. IE: If you have taken the time to do a good job, it shows. Also: If you care about the small details and the appearance, you have probably also cared about the important details. It's the same reason that us dads (and moms) tell our teenage boys to keep their hair short, treat a lady like a lady, and to quit listening to that damned rap music. Quality shows, and so does the lack of it. Well, after having opened that can of worms, I believe it is time for me to exit, stage left, to avoid being trampled by the onslaught of message posting to come. LOL Have a good day, everyone!

 

[mickryding]

Poor bonding of structures, in lightning strikes can cause catastrosphic local structural failure.

 

[benno]

Hi all
I hope that you have had some good fortune with your folks wktaylor
Now my contribution to the "puzzle" If an overly rough surface causes corrosion could this joint develop a static charge? The two conductive metal surfaces separated by a dielectric (oxide). Add some movement/vibration and hey presto....static charge. I am not in the aircraft biz but I do get to enjoy the outcome of big static discharges on occasion Ben Roberts

Broadcast Engineer

Sentient being.

 

[yates]

Since this thread is obviously expanding as fast as the universe, I don't feel bad about coming in late in the day. Ther have been an awful lot of good reasons posted by everyone as to why it is good practice to deburr holes, edges etc. the only one left I do not see is : sharp edges become sparkover points whenever voltage is applied (static, lightning strikes...)

yates

 

[dooron]

My belated thoughts on your question. 125Ra equivalent to 6 micrometers is considered a reasonable machining finish for a lathe or milling machine without undue cost implications. A better surface finish, now available from high speed machining was not available when all the aero book were around. The alternatives then were grinding, which is very expensive. So, I think, the driver was cost and available machining. The designs for fatigue, stress concentrations and static discharge came as a result of the machining capability. Deburring is always good practise, especially once you managed to cut your hand on an un deburred pin so that is an occupational safety requirement.

 

[davida3d]

What ever happened to this chap ? Has anyone any idea ?
This was one nice exercise he started.

Regards, David

 

[graemew]

Standard practices, or "customs and practices" occur in many facets of life and are all the distilled essence of many years of experience by many people. They are the assumed baseline for further actions upon a subject and are a great aid to communication, since one only needs to deal with the exceptions in order to proceed. It would be extremely tiresome to have to detail all of the considerations covered by standard practices each and every time work was to be done.
It is WISE to understand that one is dealing with a set of well founded assumptions with STANDARD PRACTICES. Whilst they are often a good starting point for design, it is important to recognise that PARTICULAR circumstances may dictate the selection of different practices for particular applications.
Just by-the by, with damage toleranct design now the accepted standard for new aircraft (the new standard practice as it were):
1. don't forget that safe-life and fail-safe approaches still have perfectly valid applications, just not everywhere, and,
2. that each manufacturer has their own suite of standard practices (usually detailed in the SRM ) for hole preparation and fastener installation that result in a presumed joint condition for the purposes of test conduct and the setting of inspection thresholds and intervals. eg. HLT type fasteners which burnish the hole during installation which should not be replaced by HL types without specific clearance, since the HL's do not have the same effect upon the hole.

It has been noted by many people over many years that paint will not provide good cover on a sharp corner, and will fail first on a sharp corner. I have no doubt that some of the esoteric explanations above for why this occurs are accurate, but try explaining the reasons to a painter or a sheet-metal worker and watch their eyes glaze over. The use of a standard practice is easily understood, and readily complied with.

Mr Tayor appears to have some significant personal issues on his plate at present. I wish him well and hope that his parents are not still distressed. I am in Australia, but if there is anything that I can do to assist I would be pleased to do so.

 

[235zzzo]

Hi Taylor!

Come to meet us again, just come up and say something!
All the best to you, and this very peculiar and really interesting forum,
zzzo

 

[Miper]

Wil,
Been away a while myself, and was sorry to read your post from last December about your mother's illness. Speaking for all of us who've enjoyed your helpful and professional comments, we send our best hopes for you and your family. I will include you all in my prayers.

 

[ScotCan]

There is an actual field problem (which has been solved) where an Exacto knife had been used to trim away excess adhesive film in an adhesively bonded wing structure and the resulting superficial scratches on the wing skin (.002" and under) eventually opened up as cracks leading to fuel leaks from the wet wing. The aircraft concerned is characteristic of an extraordinarily long lived type and the field fix solves the problem BUT the example demonstrates the extent of care required where surface imperfections are concerned. The problem, by the way, turned up 18 years after the aircraft was built.