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

 

[WKTaylor]

vonlueke...

You've defined more facets of the question/problem... although vonlueke stretched a couple of aero points beyond where I would have imagined [more of a coatings quality issue... although I've seen atrocious/rough paint jobs that affected acft performance].

I'll throw-out a hint that should help expand the conversation: "stress analysis and gapping".

Regards, Wil Taylor

 

[jimmyw]

A higher surface roughness will lead to higher friction forces to overcome when torquing a bolt. This means that less preload (Fi) will be developed, with a corresponding decrease in load at which gapping occurs (Fi/(1-C)), which increases chances for leaks (stuff coming out, or stuff going in), and also leads to worse fatigue performance (higher alternating tensile stresses). A higher surface roughness may also lead to preload relaxation - exacerbating all of the above.

 

[WKTaylor]

jimmyw...

You nailed a problem I have seen repeatedly in the field: poor "workmanship habits" affecting parts fit-up and load transfer. This is the classic "shanking and sheet gapping” syndrome, caused by burrs and "liberated burrs" [chips].

Again, though, I have to ask... What else????


Regards, Wil Taylor

 

[drawoh]

What about documentation?

You send a piece out to be fabricated. You write a requisition specifying that parts be fabricated as per your drawing, and you list the drawing and its revision number. Your drawing has a note on it stating that it has been done to the ASME Y14.5-1994 standard.

All of this works out as a legal contract that defines what you will accept when the vendor leaves something on your loading dock.

If you don't specify a surface finish, then you must be prepared to accept any surface finish the vendor sends you, like, say, 500RMS? Actually, I am inclined to specify 63 as a standard finish. You should look at a surface comparator sometime. 125 is not that good.

Your drawings show sharp inside corners, and you know full well that machine tools cannot cut these. You must know what the maximum radius is going to me if you have closely fitted parts. The worst thing that can happen when you specify them is that the machinist will call you and tell you he can't do it. Now, you will have to fix your mating chamfers and radii.

Burrs and sharp edges are fairly obvious, and have been discussed, above.

JHG

 

[WKTaylor]

OPTECH...

You indirectly raised a couple of great points...

a) that mechanical-finishes cost big Buck$$$ and should be thoughtfully specified so that they MATCH the engineering requirements. Otherwise You probably will get what you asked for... but not what you wanted... or You can pay-too much for no value-added.

and

b) mechanical parts fit-up [nesting] can be grossly affected by these factors (including the allowable tolerances).

OH yeah... I am very concerned about verifying the quality of mechanical finishes! I've come close to a couple of fist-fights on the shop floor regarding &quot;wild-assed&quot; thumb-in-the-air estimates of finish quality!!! I NOW own (3) high quality surface roughness comparators from GAR Electroforming <<

http://www.garelectroforming.com/

>>
SH-6 [shot-blasted surface roughness]
S-22 [general-machined surface roughness]
G-63 [ID & OD (hole & lathe) turned finish surface roughness]

BUT, thats another war-story...

All points about burrs, sharp corners and surface roughness have NOT been raised Yet... Time for another hint:

&quot;shot-peening&quot;
Regards, Wil Taylor

 

[TVP]

Aha, the light bulb just came on (thanks for the hint, Wil ). Excessive surface roughness can be an indication of over-peening, which negates the beneficial aspects of compressive residual stress. Aluminum and magnesium are especially prone to over-peening, which results in many localized areas of increased stress. Problems with fracture (stress intensity) and fatigue (crack nucleation sites) are then possible/probable.

As others have already mentioned, joint problems can arise from excessive surface roughness, and over-peening is yet another method for creating surface roughness.

 

[Asanga]

Okay probably waaaaaaaay redundant by now but I will add another of my two cents. Shot peening, surface polishing, deburring and rounding off adds a sustained compressive stress into the material. This stress will counteract the tensile stress caused by a crack and help to contain its propagation.
Isn't it November yet Will???
Cheers
Asanga

 

[WKTaylor]

TVP & Asanga...

Not exactly the answers I was fishing for.. but good added info!

At the end of this week I'll put my money where my mouth is and give my answers to this &quot;question&quot;.

OK, this is going to be my last hint. It is a simple quote from and &quot;old&quot; M&P mentor...

&quot;You can't paint a sharp edge&quot; Regards, Wil Taylor

 

[butelja]

Here's a shot in the dark. Poor surface finish could be an indication of heavy finish cuts used when the part was milled. This could have higher residual stresses that would have a negative impact on the part's fatigue properties as well as dimensional stability.

 

[TVP]

Wil,

I'm not sure what else you were looking for with regards to shot peening, but I will comment on the painting a sharp edge part. The physics, electrochemistry, etc. are well documented about applying a coating to a sharp edge. When using any type of electrically catalyzed process (anodizing, electrocoating, electrostatic spray painting, etc.) current density fluctuations prevent the build-up of a uniform coating thickness. Variations in coating thickness have many negative aspects, such as variable friction at joint surfaces, areas for localized corrosion/pitting/galvanic cells, etc. Corrosion fatigue and stress corrosion cracking are obvious concerns.

 

[brucoli]

What about this...

In structural analysis of parts, etc., the calculations do not take account about surface finishings... (at least, I think so... I have not so much experience in advanced structural analysis)... I do not know any equations or mathematical theory or tabulations about exact stress distribution in rough surfaces. Only what we can say is that there will be a high probability of all situations that every one has described in all previous contributions.
So, when we make the calculations, or when the computer programs make F.E.M. calculations (or other method calculation...) they do not take account of roughness, and it is supposed that the surface is perfectly finished. Only what we can do are estimations of roughness effects based in experience and fracture mechanics theory, for example.

So, if the surface is highly finished, the behavior of the surface, part, etc., will match better with the mathematical model, because the surface finishing is &quot;quite perfectly smooth&quot;...

Ok, I do not know if what I just wrote is all true, so I will wait replies to my contribution!

Regards,

Sante

 

[WKTaylor]

Folks...

I have not forgotten my promise to answer this question... by the end of this week. Please be patient... working OT has eating-up my spare [writing] time... I'll have my &quot;comments&quot; by Monday AM. Regards, Wil Taylor

 

[FredGarvin]

<insert Jeopardy theme here>

 

[WKTaylor]

FredGarvin...Jeopardy??? Oh God... the pressure!

And the question is...

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???

Ok Guys.. Just a little-bit longer... Hang-in-there! Regards, Wil Taylor

 

[Hayden]

Sharp External Corners - a very general one:

So you don't rip your trousers, sleeves or skin when walking past or or working in confined spaces.

 

[metman]

TVP made some good points but one them could be expanded to follow up on the EE/EL hint. Key phrase being 'current density' with respect to sharp edges/burrs can cut thru protective coatings on mating surfaces/radii providing a minute (did I spell that correctly not = to a unit of time) area of &quot;clean metal&quot; eletrical path to DRIVE corrosion dramatically worse than if no protective coating were there to begin with because of the extremey high resultant current density.

My penniesworth... Jesus is The TRUTH

 

[metman]

wk,
Come on already. Monday am has come and gone so maybe I should ramble and redundatize. Many good points have been reiterated but usually with a little twist adding some new flavor. In my previous reply, I take it for granted that most everyone understands that the hole punching force of high current density results in stress risers to enhance SCC and corrosion fatigue.

Rough surfaces provide less surface area of contact giving rise to higher and very localized contact stresses. If flavored with a little salt mixed in and throw in some corrosion this could be dynamite. Is this way redundant all over again? Jesus is The TRUTH

 

[gisky]

Dear all,

here is my idea:

Sharp corners, burr holes etc. increase not only the stress but the strain as well.
Looking at the strain we can have three different situations:

1. The strain can be inside the linear behavior. (Under the yield limit)
2. The strain can be between the ultimate and the yield limit.
3. The strain can reach the ultimate limit

If the third situation is going to occur, the cracks can develop because the material failure. In this case, the crack can also reach the its “critical value”.
For this reasons, round the corners, deburr the holes, finish better the surfaces will help to pass from the third to the first situation.

Giuseppe

 

[saxon]

wktaylor, Setting a specification can be a dangerous business, Every fabricated piece should have a finish and machining specification set for its application, but what happpens when this is set too hi or low? If its set high, the unit price goes up, sometimes astronomically, with no increased benefits. If too low, there are the possibilities of fatigue failure, improper fit up, sloppy setup procedures, etc., etc. Furthermore, it compounds the problems of quality control for the part producer and end user (multiple numbers of specifications for multiple numbers of parts and confusion reigns in fabrication, quality control and assembly) and off spec parts end up in production and assembly. Opps! sorry that component failed, but we wern't sure what the &quot;real&quot; specification was supposed to be.

In the design of extreme Hi-pressure equipment we also use a single finish/machining specification on all high pressure (in excess of 10,000 psi)components. This simplifies and makes it easy and understandable for all parties concerned what is required, when and where. From the fabrication specialist thru quality control to the millwrights, welders, pipefitters, and other assembly specialists, including the &quot;woodpeckers&quot; putting the equipment together.

Have a nice day!
saxon

 

[longeron]

To counter- sometimes a surface that is finshed too well can hinder sealing. O-rings need something to hold on to- if your surface finish is too fine and the compression on the O-ring is too light- the O-ring is likely to fail. In my industry, we specify 63ra for most surfaces that will contact a secondary sealing element. (we do however require flatness and surface finish to an extreme on other parts- millionths of an inch for mechanical seal faces)