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

 

[DELTA98]

Hi Will
I hope all is well on the home front and your doing well.
After seeing your challenge, I thought what the heck why not??

Almost without exception fatigue cracks start @ the surface of a part rather than internally. One possible reason may be that the highest stresses are usually found at the surface (e.g. bending and torsion) and the surface is vulnerable to stress raisers such as maching notches, scratches and pits. Surface finish affects the strength of a part subjected to fatigue loading because most machining operations leave a notch pattern and fatigue cracks usually originate in a notch. 125 is a a difficult machine finish on steel except on open flat or cylnd surfaces.

This finish I was thought is acceptable on
a) AL ALLOYS
B) STEEL HEAT TREATED OVER 200,000 PSI.
C) STEEL SURFACES IN CONTACT WITH AL ALLOYS EXCEPT WHEN IN CONTACT WITH HIGHLY STRESSED AREAS OF A 7075 PART.

A sharp edge can be compared to turbulence that exists around a rock in a stream (fluid flow analogy) therfore sharp edges are stress risers and usually the place where cracks begin. So if we use a radius, chamfer, and deburr holes we lower stress concentrations.

Well I tried!! lol

 

[Emuman]

It makes the holes look nice.

 

[awparran]

I would like to add my 2 cents worth here. I too am not an engineer, far from it, but as a computer/network/WAN/LAN manager person, we found that communications cable with smooth surfaces was the worse to pull through fixtures, whereas cable with dimples/rough surfaces presented less of a problem - less surface contact. In regards to metals the surfaces must be smooth as taught to me by my great-great grand-daddy. All holes and edges had to be deburred or made smooth when using metal aircraft components to reduce the possiblity of concentrated points of failure.

When I was taking history in high school, the teacher would walk around with a $20 bill in his hands and snap it. To this day I can still see this image and I can rememebr how 25 students watched in facination. He would then take a pin knife and knick the center edge and snap it while again circulating through the class room. As you can imagine, the bill eventually snapped in half. He had made his point. He had a lot of scotch tape too. Why a History teacher? The Industrial Revolution was our topic.

 

[gciriani]

To all who have found this thread interesting, there is a Deburring Technical Group at SME who found the discussion very stimulating. Many of its members deal with aerospace deburring issues.

There will also be a deburring-surface finishing technical session at the WESTEC MANUFACTURING CONFERENCE, in Los Angeles, CA on Apr. 6. During this session, a paper will probably be presented, which deals with additional issues that have not been brought up yet by this thread.

 

[WhiteRabbit]

My one cent:

On the mating surfaces fretting will be an issue with rough surfaces. The fretting will expose the grain inherently causing premature stress corrosion.

Also, a rough surface finish may cause problems with rejection indications during final NDI such as Dye Pen or Eddy Current.

 

[Boathouse]

Has anyone mentioned that anti-corrosion coatings do not like sharp corners? Very hard to plate to a sharp edge.

Just under 2 cents worth!

 

[rikman]

Oddly enough, many of the UK standard practices regarding surface finish, deburring, etc. were - up until quite recently - related to working practices developed during World War II.

During the war, aircraft manufacturing work was farmed out to many companies with no aerospace background and many practices were introduced to account for what these companies were actually capable of. By the time the war was over, these had magically become "standard" and we were then stuck with them until we started going metric (which means that a lot of us are still using them!)

If Beethoven had been killed in a plane crash at the age of 22, it would have changed the history of music...
and of aviation.

 

[dryfinish]

I have been following this thread dealing with deburring/surface finishing issues for aerospace applications with considerable interest. Many of the observations have been extremely perceptive. Anyone following this thread who has continued interest in deburring and surface finish issues as related to part performance, longevity, fatigue failure etc., etc. might want to continue the discussion in a live forum. The Society of Manufacturing Engineers is putting on a technical conference that includes an extensive session on deburr/finish technologies as related to aerospace structures and components. This is normally a fee based tech conference. However, SME is handing out FREE VIP passes to folks in the deburring and aerospace communities. Event is at WESTEC in Los Angeles, CA April 5-7 concurrent to the Westec Machine Tool Show, anyone who would like to attend in order to discuss some of the issues taylor and others have probed here are welcome to do so. For more details you could contact myself or Society of Manufacturing Engineers directly: attn: Rodney Grover, direct line = (313) 425-3222, or rgrover@sme.org

David A. Davidson
[DESC] Deburring Technical Group
SOCIETY OF MANUFACTURING ENGINEERS

http://www.deburringsolutions.com