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| Machining Distortion of Monolithic Parts |  | ||
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| Posted by: Matt133229 ® 09/10/2009, 22:12:02 Author Profile eMail author Edit |
I have been interviewing w/ a small machine shop who does parts for the Aerospace and Defense Industry. Anyway, the owner of this shop has been testing me, asking me to figure out how they would make parts in certain situations to tight aerospace tolerances even though he knows that I am straight out of school w/ no real machining experience.
So anyhow, he showed me a monolithic type thin-walled part that they make for aerospace, and asked me how they would machine this part, while controlling the distortion and ultimately the flatness of the part to tight aerospace tolerances. The part is roughly a 4' x 2' part, maybe 3" or 4" thick, and to be machined to 0.060" thin walls. So I take the problem home w/ me and do quite a bit of research and come up w/ some heat treatment processes and an example of a vacuum fixture to hold the piece during machining. The basis of the heat treatment processes that I decided to go w/ were from this post here: https://www.eng-tips.com/viewthread.cfm?qid=31718&page=199 describing stress relief annealing of the part and a cold stabilization process which I also found other sources on. Also, I found some journal articles supporting the use of material which had already been stress relieved prior to machining and the success they had w/ it. Anyway, he didn't like my answer. 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. He also said that they wouldn't spend the time or money in trying to make predictions on the distortion of the part or anything like that. So he said, obviously the part will be distorted again when released from the vacuum fixture, what would we Mechanically do to the part to make the flatness w/in tolerances again? I have been researching this topic for a week or longer now it seems, and 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, Matt |
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| : Machining Distortion of Monolithic Parts -- Matt133229 | Post Reply | Top of thread | Engineering Forum |
| Posted by: Kelly Bramble ® 09/10/2009, 22:38:25 Author Profile eMail author Edit |
My first question is what tolerances? Flatness to what? .060 wall thickness can be done. Also, what exactly is the aluminum material? You're on the right track regarding relieving the internal material stresses; however most heat treating operations change the base material strength characteristics. Rarely is engineering willing to allow changing the material. As you're aware, thin walls in Aluminum or any material tends to allow movement after material is machined off. This is because of the internal stresses relieve and cause the material to try to normalize or equalize the stresses. Additionally distortions will happen after removing the clamping forces required to secure the material during machining as the clamps induce stresses by straightening and otherwise changing the material. What is the surface roughness required? If rough (16 Ra or greater) enough you could EDM the critical thin walls. A final EDM machining process will typically produce very accurate features, but the roughness is an EDM roughness. The accuracy is achievable because most of the material and stress relieving has already occurred and the post EDM just produces the accuracy feature. EDM does not induce any surface loading during machining and the part does not require large clamping loads while holding. |
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| : : Machining Distortion of Monolithic Parts -- Kelly Bramble | Post Reply | Top of thread | Engineering Forum |
| Posted by: Matt133229 ® 09/11/2009, 12:59:42 Author Profile eMail author Edit |
Kelly, Thanks for the reply! To be honest, I am not sure on the flatness tolerance, be I am mainly talking about the flatness of the bottom of this "monolithic part." I have attached a picture of a simple monolithic part that could be used for this example. The reason I am not so sure on the exact tolerances is that it is kind of more a "what if" question from my interviewer to see if I can figure it out and get back to him about it or not, than an actual part to figure out. I hope that makes sense. Anyway, about the EDM process, you mentioned to use it for final machining. But wouldn't the part be distorted just from the rough machining? Is the EDM a process that most smaller machine shops such as this would have the capabilities of doing? I think what he is looking for, is some sort of post process, to correct the distortion that they have created into the part. Although, I don't know that this is completely the correct method, because anything I have learned about problem solving is that you want to get to the root cause of the problem... not to mask it. But I guess thats another issue, and I don't think confrontational is the approach I want to take w/ an interviewer. With that being said, what about other post processes such as shot peening or leaving extra material on the bottom of this part to be lapped? I am not very fimiliar w/ these processes, could they produce a tolerable part? My concern in shot peening would be rather its a controllable method or not, and rather it would also destroy the surface roughness? Thanks for your insight! Matt
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| Posted by: Kelly Bramble ® 09/11/2009, 13:31:27 Author Profile eMail author Edit |
Yes the part is distorted due to the rough machining processes. One would leave an extra .020" more or less and then post machine with EDM ram or plunge for the inside geometries and probably wire edm the bottom to achieve a really good flatness and size. A smaller machine shop may not have an EDM machine, but could send out the roughed part to an EDM job-shopper relatively inexpensive. I'm not an expert on shot peening and the effects on flatness and dimensional conformance, but I don't see how the process would significantly improve dimensional conformance (wish Randy was hanging around). See - /manufacturing/shot-peening-manufacturing.htm
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| Posted by: Matt133229 ® 09/11/2009, 14:41:57 Author Profile eMail author Edit |
Kelly, Thanks again for the information! I will definitely look more into the EDM process, as I am pretty much unfamiliar w/ it. It sounds like it could be a real good idea though! Thanks, Matt |
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| Posted by: jboggs ® 09/10/2009, 22:37:16 Author Profile eMail author Edit |
You didn't mention what equipment he has available, or what kind of budget is justifiable. You should look carefully at waterjets. They are used extensively in high precision thin wall aerospace machining applications. The main advantage: NO heat is created, so no distortion to deal with. I've seen them create honeycombs with .030" walls out of 3" thick glass. I've seen them cut all the way through 8" thick titanium - at a 45 degree angle! And usually the initial cut is accurate enough that a final machining pass is not required. I've also been involved in the design and construction of fixture tables that consist of an array of linear actuators with vacuum cups on each that together form a three dimensional form to support shaped pieces. There are several suppliers, but you could check www.flowcorp.com for some good examples. I'm sure their application folks could send you some good info. |
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| Posted by: Matt133229 ® 09/11/2009, 13:05:09 Author Profile eMail author Edit |
To be honest, I am not completely sure on what kind of equipment he has available, since I am only interview w/ him. It is kind of more a what if question he is seeing if I can figure out and get back to him with.
I am not sure that a water jet would be useful in this situation or not though. The problem would be that this part isn't cut all the way through, along w/ the thin walls, it is also to have thin "floors". Waterjets are only intended to cut through all the way aren't they? Or am I wrong and they are capable of cutting to a certain depth? |
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| Posted by: rwolfejr ® 09/11/2009, 16:27:35 Author Profile eMail author Edit |
I really like this fellows interviewing process. He is indeed testing you. My guess is he's looking for your reactions to what he's asking and how you handle it more than to expose your lack of machining experience. He already knows you don't have experience. He wants to see if you'll try to fake it by tossing out some B.S. blabble or if you'll keep cool. He might be looking to see how much effort you're willing to put into finding the answer? He might even be simply waiting to see how long it will take you to ask him how he'd do it or did it? He might want to know if you're capable of asking someone when you don't know or aren't certain of something? I'm guessing he's looking at you with all that in mind. He's likely wanting to know if, should he happen to bring you on board will you be willing to learn from HIM (or her) what knowledge they've acquired over their ?? years. You can't make a print of a part for a shop floor without any knowledge of how it can be machined. He knows that and knows you still have a lot to learn. Sure anyone can draw a perfect part on CAD but you have to know if what you've drawn is possible with your equipment and what steps will need to be taken to get the desired finished part. If he still has the line of communication open with you then he's giving you serious consideration. If he was looking for someone with machining experience he wouldn't be interviewing you in the first place if you're straight out of school. That's my guess anyway... Far as that part you're trying to cut... The answer will depend greatly on how many and how much. If you're figuring one piece you're approach will most likely be entirely different than a thousand pieces. I'd consider scratching the part out with my teeth for just one part before I'd run out and buy an expensive piece of equipment I don't need in order to machine one part. (figuratively speaking of course... meant to read pad the price and figure several passes and some hand massaging, or sub it out if I didn't have the right machine... but you get my point...) Another thing to look at is how is the finished part used. Will it be restrained in some way? We've produced parts with a tolerance window of .00025" for a company for many years and we have the allowance of measuring the critical features in the restrained condition. Meaning as the part is chucked up in a manner similar to the way it will be used in it's application. Like boring the jugs on an old Harley... although not nearly as critical as some things, you want to torque them down to a jig before boring to put them in the same mode they'll be in when torqued to the block on the bike.
I apologize for my rambling...
Then the biggy... Add a footnote to your detailed suggestion.
My guess is if you say that last part, and mean it... you'll be in like flint. Good luck!! |
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| Posted by: jboggs ® 09/11/2009, 15:53:14 Author Profile eMail author Edit |
You are correct. They are intended to cut all the way through a part. |
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