Machining Distortion of Monolithic Parts 1

[Matt133229]

I have been presented with the question of how to manufacture a thin-walled Monolithic aerospace part and to control distortion to within the tight aerospace tolerances by a possible employer. The part is roughly a 4' x 2' part, maybe 3" or 4" thick, and to be machined to 0.060" thin walls.
After doing much research, all I have been able to find on this is to perform stress-relief annealing and certain cold stabilization methods of heat treatments, or to reduce the residual stresses in the finished part by using materials that have already been stress reduced.
However, being a small-job machine shop for the aerospace industry, these processes are neither feasible or allowable by time constraints. Thus, my question is: obviously the part will be distorted again when released from the vacuum fixture, what could we Mechanically do to the part to make the flatness w/in tolerances again?
I have been unable to find anything like this, other than the stress relief annealing and on how to pridict the distortions using FEA and such, If anyone would please have any insight into this at all, it is greatly appreciated!

Thanks in advance,
Matt

 

[TVP]

Matt,

Why is stress relief annealing not "feasible or allowable by time constraints"? Heating parts for 1 hour at a low temperature is sufficient for stress relief and can be done in a small furnace right next to a lathe/turning center/machining center.

 

[KENAT]

deja vu

I think I previously suggested rough machining, re fixturing, finish machining. May not work on a part like yours but we've done similar.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[rb1957]

i don't think we can predict torsion from machining.

i'd talk to people who do this machining already and see how they do it. control the heat, slow speed maching, shallow cuts, etc. try and keep the flat webs in the middle of the billet.

when you do get distorsion, consider shot peening to restore the correct shape.

maybe you'll need more stiffeners on the webs ...

 

[Matt133229]

TVP,
This is a question which has been asked of me by an interviewer. I suggested to him that a) they start w/ materials that have already been stress relieved and b) that they perform stress relief annealing after machining the part and/or a cold stabilization method. His response was basically that being a small job-shop, they wouldn't waste time and efforts in getting these specialized materials and processes, and that the engineers who designed the parts wouldn't allow these extra heat treating processes.

Kenat,
Sorry for the repost, I didn't get a chance to read your last reply before my post was deleted by the moderator last night. I was asked to repost w/out the "interviewing drama."

rb1957,
Shot peening may be the post operation he is looking for... my only concern is, is it possible to use this in any kind of controllable manner? Would this not destroy the surface quality of the part?

I am not very familiar w/ shot peening myself, but will start looking into it.

Thanks to everyone for your suggestions!

 

[rb1957]

shot peening will affect the surface finish, so you want to peen non-interfacing surfaces. and it is difficult to control; you're relying on an intelligent operator (as opposed to an intelligent designer ?). but i have seen it work well with singificant pieces of structure, like wing spars, heavily machined billets with thin webs.

start with a very slow process, working on both sides of the billet, and then see how much you can speed it up.

 

[Matt133229]

Thanks for the info! Now say you can't avoid peening an interfacing surface where the roughness is critical, would lapping help in this instance? I am not real familiar with lapping either, but is seems to me like that might be useful in that instance.

 

[rb1957]

i said don't peen interfacing surfaces, 'cause it messes the surface finish. if you have too then you might use something like a copper skim to accomodate the surface roughness (not that i've ever seen that done. don't, repeat don't, lap the surface smooth 'cause that would remove the correcting stress field (created by the peening).

a slow process should get you there ... control the heat release, and balance the machining.

 

[Matt133229]

Ok, thanks very much! I am just asking about the surface incase he is to bring that up to me after suggesting shot peening when I go back in there for the next interview. He really likes testing me.

 

[rb1957]

hope you get the job ! (it's tough out there)

 

[Compositepro]

If you do get the job, do not expect "the grilling" to stop. This seems to be the way he teaches. The tone of your new post is much better than the original. Keep this attitude when you work there. Remember that in any professional position you are expected to learn whatever is required to get the job done. From your perspective that maybe to get a part machined. From a small business owner's perspective that is to get the part made as profitably as possible. Good luck.

 

[TVP]

Roller burnishing is another possibility similar to shot peening. Take a look at the following sites for more information on both processes:

http://www.shotpeener.com/library/index.php

http://www.shotpeening.org/ICSP/confrns.htm

http://www.cogsdill.com/

http://www.ecoroll.com/

 

[Matt133229]

Thanks for everyones help, it is greatly appreciated! I will be sure and keep you all updated on rather or not I end up getting the job. Like I said in my previous post, this has been going on for close to a month now, but I do really want to get my foot in the door in the aerospace and defense industry.

Now another quick question on shot peening, from what I have looked up on it so far, it does change the materials mechanical properties a bit... now my question is, would this not change the mechanical properties from the design intent of the part? Say we are working w/ a part that is to be 7075 T6, would shot peening not change the material properties to not match design?

Thanks again,
Matt

 

[KENAT]

Matt, I said something similar before, I wouldnt necessarily see getting a job in a machine shop that services the aerospace/defense industry as getting a foot in the door of the aerospace/defense industry. If it doesn't require a security clearance, then it's not really in. I'm not saying it's not worth taking, and having experience of how parts are made etc. is valuable just don't get too hung up on the foot in the door issue.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[rb1957]

i don't think it changes material properties (not at least the way i think of them). shot peening works by effectively expanding (plastically) the outer surface of the part, setting up a compression stress in the outer layer, and a balancing tension stress within the core of the material. the surface compression stress is the main reason that shot peening is used to improve fatigue performance. in your application expanding one surface will bend the part back to flat (hopefully). caution, it can be over done, particularly on thin gauge webs, so that you yield the entire thickness.

as an interview question, there are many answers ... what experience the shop has with this ? (do they already know the dangers? or are they looking to learn ??) what do you Know, as opposed to what you've heard/read ? (always be careful about appearing smarter/more knowledgable than you really are !) rather than focusing on fixing a warped part, i'd focus on making a good part ... use a slow process, control the heat generated, balance the machining (on both sides of the billet) ... flipping the billet is slow and expensive and could be a step to try to remove later.

 

[Matt133229]

Thanks, that is a good thing to know that it won't change the material properties... That is why he was opposed to my other ideas of thermal stress relieving the part, etc.
I do agree w/ your comment about focusing on making a good part to begin w/ though... as everything I have learned about problem solving has taught me to go after the root cause as opposed to using the band-aid approach and only masking the problem. But, as an interviewee, of course I have to be looking for the answer he is looking for. But hey, maybe this is why they need to be looking for an Engineer huh?

Yeah, this machine shop already knows how to do these things, they are not trying to learn this from me. Like I mentioned before, he is really trying to test me... I believe trying to test me if I am going to stick w/ it more than anything. He does realize that I am straight out of school w/ my b.s in ME, w/out any real machining experience, so I don't think I have to worry about him thinking I know more than I do.

Kenat, That is a good point, I do see what you are saying there. Though I haven't had much luck w/ the aerospace companies themselves so far (Graduated back in May) and I figured that at least potentially working with them would be a step in the right direction.

 

[MikeHalloran]

For introducing a counter- stress, you can probably get better control (and certainly less mess) with a peening gun than with shot.

{ A peening gun looks just like a needle scaler, except the needles are ball-ended instead of pointed. }

For inducing less stress in the first place, in the case of aluminum (not specified in the OP), sharp tools and high speeds put more heat in the chips and less in the workpiece. Slow machining is not always the answer... and is often part of the problem.



Mike Halloran
Pembroke Pines, FL, USA

 

[WKTaylor]

New PostMatt133229

You have not clearly specified what material/temper [metal?] and what configuration part you have.

In my experience, details are everything. Clearly defining the problem, usually points to solutions.

So, in-detail, what is Your problem, IE:

Raw stock size?
Configuration [if not explicit, then approximate... The part is roughly a 4' x 2' part, maybe 3" or 4" thick, and to be machined to 0.060" thin walls" means nothing to me...]? Alloy and required tempers?
Critical tolerances?
Aero or space rated? Structural, mechanical, electrical function?
Fatigue critical?
Environment? corrosive, thermal, sonic, conductive, etc...
Conventional or Alternative manufacturing processes allowed???
how many will be built? 1? 10? 100,000?
Every manufacturing process has consequences

Regards, Wil Taylor

 

[rb1957]

"won't change the material properties... That is why he was opposed to my other ideas of thermal stress relieving the part,"

IMHO thermal stress relief is a much more controlled process and produces more desirable results. yes it changes the material properties, but to obtain the properties you want ! to oppose thermal stress relief because it affects the material properties shows (again, IMHO) a basic misunderstanding of how it works. it does tend to be pretty expensive, in dollars and time; that is a reasonable reason to exclude it.

 

[mfgenggear]

if you do not know the answer. never guess
I would reply, "how have you been doing it".
or what do you recomend.
This is what I know.

never try to second guess an answer.

secondly wtaylor is on the target.
What are the attributes.

machining I have found is an imperfect world.
even tho Mike is correct about the process I find
machinist don't allways do it correctly.
and there will be stresses induced into the parts.

generally "from the mill or forging", & the machining
it will induce stresses into the part. time comes at heat treat, you have a distorted part.
if an employer said to me stress relieving prior to semi finish machining & heat treat is a waste of time. I would refuse to work there. work closely with the heat treat department, or subcontractor make them your #1 friend.

an other very important aspect is the tooling used to hold & restrain the part during heat treat. & process used.
some distortion is very unavoidable the if the process permits tooling is required. only experience is the guide.
very thin wall parts do require support.


shot peening can also stress relieve your parts, I have seen this first hand, parts must be stress relieved after heat treat and hogging out operations prior to any very close tolerancing machining & shot peening.
on the other hand peen straighten works very well.

seeking advice here on the boards is very wise.
never try to attack a problem like this with out
someone who is experience helping you. allways seek advice.

as a newbie you will make mistakes, the odds are against you.