best way to fabricate "clean" inside corner

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Hi,I'm in the process of designing a part, and I was hoping to get a little bit of early feedback on the best way to approach some of the joints.  The part belongs to a biomedical assembly that will have to be routinely disinfected.  As a consequence any nooks and crannies will act as hiding places for bacteria, and will need to be ground smooth to present a "cleanable" surface.  Modest surface roughness is fine but things like ridges and surface pitting would be issues.So the joint I need to make is actually very simple, it is just a welded 3-sided corner in 14 gauge 304 stainless.  The question is should I do an outside weld on the corner, and then try to grind the inside corner clean (let's call this approach #1, see corner1.jpg).  Or, should I bend the corner, and butt weld the flats together (let's call this approach #2, see corner2.jpg).I'm not actually going to be doing any of the welding myself, I'll leave that to the pros. But I was hoping to get a little insight from some experts on which of these approaches seems like the best way to go, since it will determine how the rest of the part is designed.If I go with approach #1 will the inside corner be smooth?  What if I specify a backing material?  If not, how hard will it be to grind that corner smooth (or how much is it going to cost me)? What if I specify welding the inside corner and then grinding, better or worse?If I go with approach #2, am I going to have trouble with warping?  I'm guessing this will be easier to grind smooth than #1, but is it going to have other issues?Any thoughts you have would be really appreciated.  Whichever joint I go with will be reused many times throughout the design, so I would like to get more info before I get too far down the wrong path.Thanks. Attached Images
Reply:Being no expert I would say go with OPTION 1. Option 2 will stress the outisde corner and this is to be avoided. A proficient welder will provide you with a round smooth outside corner (after welding and sanding).The inside corner might be trickier to sand down thatn the ouside but you will face the same problem with both options.Good luck,Mikel
Reply:You could insert a copper block(shoe or mold, so to speak)on the inside of the joint using option #1. Have the weldor do a full penetration tig weld that will leave a flat surface on the inside. Then grind the outside.If the weldor misses a spot on the inside, that weldor can do an autogeneous tig pass on the inside to blend in make it hermetic.Option #2 looks like too much work.Last edited by shovelon; 10-15-2012 at 01:21 PM.Weld like a "WELDOR", not a wel-"DERR" MillerDynasty700DX,Dynasty350DX4ea,Dynasty200DX,Li  ncolnSW200-2ea.,MillerMatic350P,MillerMatic200w/spoolgun,MKCobraMig260,Lincoln SP-170T,PlasmaCam/Hypertherm1250,HFProTig2ea,MigMax1ea.
Reply:Since you are the designer, you need to take the time and effort to study what other manufacturers have done on similar equipment jointsand joints and what the end results look  like. Medical equipmentjoints may be nicely done and strong or just whacked together tolook good out of the box, then break.You can and should discuss this face-to-face with a fabricator,the pay him to furnish you some sample joints with basic time/costestimate.Blackbird
Reply:ok, what needs to be sanities, the inside of the weld or the inside of the joint? Meaning is the weld on a tube or channel or just flat stock. First and foremost, the best step to take is to talk to the person/s who is going to be doing the welding. Find out what they would do in this situation. and in conjunction how much it would cost.a good weldor should be able to weld a  fairly smooth  joint leaving minimal grinding to clean up the joint (on the outside)If it's a tube, and the inside needs to remain smooth, you'll need the advice of a pipefitter.There are no problems. There are only solutions. It's your duty to determine the right one.Hobart Handler 210Airco 225 Amp MSM Stinger
Reply:Thanks for the feedback, this has been helpful.@Mikel_24I see your point about stress on the corner.  Given how the part is loaded, most of the stress is in the mitered joint, but it is something I'll keep in mind.@shovelonThat sounds like a good idea.  Thanks.@dave powelsonI have studied the design of existing products. Cheap products will do an outside weld, and then do a little cleanup on the interior.  More expensive products will do an outside and inside pass, and then grind both down. I haven't seen anything like option #2, and it isn't clear to me if that means it is a good idea or a bad idea.  Obviously, when I get further in the design I'll be interacting more with a particular fabricator, but I wanted to get some early feedback from a broader community before I spent time drawing up various versions of the design.@ThorsHammerBoth the inside and the outside will need to be disinfected.  The joint is pretty close to as pictured.  It is a 3-sided corner in sheet metal.
Reply:Just tig weld this with a copper block behind. The more I have looked at this the easier it gets. 14 gauge penetrates completely. The idea of the copper block is to eliminate the air and act as a flat mold surface for the melt through. I would fill the bend relief first though and then weld from the open side to the bend relief.I do the same weld profile on galley parts but I use alum backup blocks.Weld like a "WELDOR", not a wel-"DERR" MillerDynasty700DX,Dynasty350DX4ea,Dynasty200DX,Li  ncolnSW200-2ea.,MillerMatic350P,MillerMatic200w/spoolgun,MKCobraMig260,Lincoln SP-170T,PlasmaCam/Hypertherm1250,HFProTig2ea,MigMax1ea.
Reply:Thanks again for all the feedback.
Reply:If it's just sheet metal, there there is open access to the inside of the corner. Tig it with a block on the back side, and and clean up any defects after the fact. this really is something that any reputable fabricator would be able to figure out after you discribe it to them.There are no problems. There are only solutions. It's your duty to determine the right one.Hobart Handler 210Airco 225 Amp MSM Stinger