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| allowing for tolerance buildup with formed sheetmetal parts | |||
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| Posted by: CCR5600Design ® 02/23/2007, 01:01:05 Author Profile eMail author Edit |
Ok, I have another sheetmetal question for the group... This one lends itself to designing weldments (and to a lesser extent, assmblies) that are comprised of individual sheetmetal components and dealing with tolerance buildup. I am learning that no matter how carefully you lay out a project on a CAD system, the real world is not perfect. Seldom, if ever, is a sheetmetal part you designed recreated eactly as it was drawn. The human element of production, as well as differences in production runs of raw materials causes variances in product quality and formability. Recently, I designed a product requiring 6 individual formed sheetmetal parts to be welded together to form a completed weldment. Although all of the parts measured well within tolerance individually, the completed weldment was not within tolerance. Typically, what is a good clearance to allow between parts for welding so as to allow for tolerance buildup, be it from forming or weld deformation (warpage)? I realize material selection will have a bearing on this, but I would like to know what clearance is recommended for adequate penetration for butt welds and flare-bevel welds? As a reference, the parts in question were all fabricated from ASTM A569 11 ga. hot-rolled, pickled and oiled material (measured thickness, 0.117"). Our welders are MIG welders utilizing 0.035" diameter wire. If any other info is needed, please ask. I feel this question goes back to good ol' know-how and I figured you guys have been there and done that enough to give me a very well educated answer. Thanks, Ron "What we need are more people who specialize in the impossible." - Theodore Roethke |
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| Re: allowing for tolerance buildup with formed sheetmetal parts | |||
| Re: allowing for tolerance buildup with formed sheetmetal parts -- CCR5600Design | Post Reply | Top of thread | Forum |
Posted by: Marky ®  02/23/2007, 11:12:05 Author Profile eMail author Edit |
Hey wait a minute!! I use Hooters Precision Sheetmetal....I never care what the parts look like....but I do make sure that I screw up on the drawings ;) I'll kindly pay you Tuesday..for a hamburger today....Whimpy |
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| Posted by: Kelly Bramble ® 02/23/2007, 11:34:34 Author Profile eMail author Edit |
At one time I messed up my time sheet because the girl (babe) that processed and checked the time sheets would call and ask that I come down and fix the error. This was the highlight of my week at that time... |
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| Re: Re: Re: allowing for tolerance buildup with formed sheetmetal parts -- Kelly Bramble | Post Reply | Top of thread | Forum |
| Posted by: CCR5600Design ® 02/23/2007, 13:44:19 Author Profile eMail author Edit |
Kelly, Times is hard when you have to rely on purposeful mistakes to spend time with a cutie. I will not deny I have done it a time or two myself! Ron "What we need are more people who specialize in the impossible." - Theodore Roethke |
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| Re: allowing for tolerance buildup with formed sheetmetal parts -- CCR5600Design | Post Reply | Top of thread | Forum |
| Posted by: Kelly Bramble ® 02/23/2007, 09:47:18 Author Profile eMail author Edit |
The welding process introduces internal stresses due to localized heating which will cause the individual parts to warp, bend and do all sorts of creative and random things. Trying to control end-item weldments to tight tolerances at the piece part level is almost next to impossible. If you really need tighter than typical weld shop tolerances you should consider the following: 1) Define your assembly tolerances (perpendicularity, position, limit tolerances, flatness, ect..) on your weldment drawing. Keep in mind, if the tolerances exceed the natural capabilities of the welding process, special fixtures, holding tools or development of special manufacturing techniques will be required. 2) Design the piece parts and your weldment such that extra material is present on the important surfaces/locations. Design the piece parts such that the important features (holes, slots) are not present. Roll the weldment drawing into a machining drawing that defines the post-weldment dimensional requirements. This drawing will show the form, orientation, location, etc. as well as the requirement that new features are to be machined to a specified tolerance. 3) If you have sufficient amounts of material, design the piece parts such that they are self-fixturing. Add dowel pins or other features that will assist the welding professionals in aligning or fixturing the piece parts. 4) Don't design the piece parts separate, create an end-item weldment (inseparatable assembly) drawing with reasonable tolerances. This will allow manufacturing to make the required adjustments on the piece part lengths form, ect. to achieve the end-item weldment tolerance requirements. 5) Regardless of how you approach the end-item weldment design - CONSULT WITH THE WELDING PROFESSIONALS DURING THE DESIGN PROCESS. This will save you headaches, frustration, hair loss, grey hair, weight gain, general hate for talented manufacturing professionals, engineering change notices (orders), unless you enjoy these..., extra work, excessive drinking, lost time, poor quality assemblies, unseen problems, project (program) managers bothering you, phone calls from manufacturing at 4:45 pm on Friday, too many visits to a stinky dirty weld shop (where there very few cute chicks - or guys), schedule slips, and a bunch of other stuff that will have you wondering why you ever went into engineering and design. Besides, they will share with you cool tricks and knowledge that will get your designs out the door quicker, better, and faster. Modified by Kelly Bramble at Fri, Feb 23, 2007, 10:01:11 |
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| Re: Re: allowing for tolerance buildup with formed sheetmetal parts -- Kelly Bramble | Post Reply | Top of thread | Forum |
| Posted by: CCR5600Design ® 02/23/2007, 10:56:17 Author Profile eMail author Edit |
Kelly, I got a good chuckle out of your reply. It has been proven, over and over, the best comedy is based on real-life experiences. Thankfully, our weld shop is neither stinky or dirty, but alas, no hot chicks. I have utilized the concept of self-fixturing our parts through the use of projections and slots. It works quite well. I was just wondering if you had a good rule of thumb for setting up clearances for proper weld penetration. I will consult with the guys in the weld shop (I won't tell them you don't think they're cute) and determine what will be best for each individual application. Again, thanks for your time. Ron "What we need are more people who specialize in the impossible." - Theodore Roethke |
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| Re: Re: Re: allowing for tolerance buildup with formed sheetmetal parts -- CCR5600Design | Post Reply | Top of thread | Forum |
| Posted by: Kelly Bramble ® 02/23/2007, 11:31:13 Author Profile eMail author Edit |
In my career, I have always had access to a welding engineer. The welding engineer concerns themselves with the weld details. We do have some stuff about welding on Engineers Edge here: /weld_design_menu.shtml, however most deals with strength and definition requirements (engineering).
I am aware that proper weld joint (edge preparation) specification does help with deformations, end-item accuracy, strength and reliability of the weld joint. In general, I always designed at nominal to allow the parts to touch during welding. However I do know that when using fixtures to align parts during welding you often define some clearance between welded parts. This clearance is used to prevent interfearance and out of tolerance condition, as well as provide an adequate weld joint. Best I do here... |
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| Re: Re: Re: Re: allowing for tolerance buildup with formed sheetmetal parts -- Kelly Bramble | Post Reply | Top of thread | Forum |
Posted by: santo1964 ®  05/10/2007, 06:25:25 Author Profile eMail author Edit |
The dimensions & tolerences of sheet metal welded assy sholud be always given pot assy. Every different assy distorts differently but it is always consistent in its distortion. During manufacturing trials at times one has to change the input part dimensions for getting the final assy dimensions. will add further comments later when I get time |
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