Standard for dimensioning sheet metal flat patterns

[pdybeck]

Is there a standard for proper dimensioning of features in a flat pattern sheet metal drawing view? Is there a standard way to list bend allowance/K factor, or is this something that isn't defined to well and varies a bit more from company to company and sheet metal vendor to sheet metal vendor? Does anyone typicall dimension features across the bend lines in a flat pattern view? Are these dimensions listed as reference dimensions or noted somehow as to being related to the K factor or bend allowance? What is proper or accepted practice? TIA.

 

[MadMango]

I've been searching for something like this as well, so I have the same questions. But I think I can answer one of them. Bend Allowance will vary depending on the condition and type of tooling used, so I don't think you could standarize this.

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[pdybeck]

MadMango,

Sheet metal drawing standards seem to hard to come by as I have been searching a little bit lately as well. I understand that bend allowances vary greatly by material, tooling, and process. What I am looking for is standard accepted language or a format for describing information on a flat pattern view. For example, one of our designers has shown a flat pattern view and dimensioned a feature across a bend on that view. I said to him, we can't really define that as that is vendor specific according to their tooling they use. I desired to put a REF. on that dimension, but my boss then questioned if that is correct to do as this particular dimension was not one that could be deduced by dimensions elsewhere on the drawing. I mentioned that we probably should list the K factor that we used to show the flat pattern view on our drawing since the drawing was labeled with a 1:1 scale. My thoughts are to list the K factor and create some generic langauge that ties the K factor to the appropriate dimensions across bends on the flat pattern view ot let the vendor(s) know how the flat pattern was generated. What I can't find is standard practice to do this, so I might be left forging my own path and speaking to several vendors to see what their experience/preference with this is.

 

[looslib]

We put a note on our flat patterns saying that they are only valid for our shop. If we send the part out, the vendor assumes all responsibilty for the finished part being correct if they use our flat pattern data.

When you buy the part, are you buying the flat state or the formed part? Let the vendor be responsible for producing the finished part and you don't care how he does it, as long as the finished part is geometrically correcty.


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Ben Loosli
CAD/CAM System Analyst
Ingersoll-Rand

 

[pdybeck]

Ben,

That is in fact our goal; to dimension the bent/formed part to what we need. The dimensions on the flat pattern were intended to be a quick reference/help to the vendor to get the overall size of the flat part. I'm not sure if thats even a good idea to dimension that, but if we decide to I was looking for accepted language practice for defining what we are saying and not saying. Right now I am thinking a note would best suite our desire as I can't find anything defined in a standard.

Pete Yodis

 

[drawoh]

pdybeck,

Why are you dimensioning that flat views on your sheet metal? This is like calling up the drill size for tapped holes. The fabricator is supposed to know how to do this.

I show flat views if there is a feature that can only be displayed on the flat view. Otherwise, I place all my tolerances on the bent parts. I do not care what the thing looked like when it was flat. I will inspect the final, bent part.

I do supply the SolidWorks drawing and model to our sheet metal shop when they ask. They decide what K_factor to punch in.

JHG

 

[pdybeck]

drawoh,

The flat pattern view was used to show dimensions to features that couldn't be seen easily on other views- just like you proposed. A designer here put a dimension acorss bends on the same flat pattern view. This is what kicked off my quest to find some standards. I said if we really, really wanted the dimension across bends, we need to give some additional information to explain how that dimension was calculated (by SolidWorks) and how it should or should not be used. I can find no standard that does this.

 

[ewh]

I agree with drawoh. If you are going to dimension the flat pattern as well as the final product, make all of the flat pattern dimensions reference.

 

[pdybeck]

ewh,

I don't know that slapping a "ref" on the dimension is correct either. I am under the impression that a "ref" dimension is used when there are other dimensions on the drawing from which the "ref" dimension can be deduced. This is not the case with our example. Please anyone, correct me if I am wrong.

 

[pdybeck]

Typically we only dimension features on a flat pattern view that don't cross a bend line, such as holes on a face/tab that need to be dimensioned to each other. We would place dimensions from those holes to an outside bend on another view on the drawing where the part is shown in the bent state.

 

[ewh]

If the finished part is fully dimensioned, then the flat pattern dimensions could be viewed as reference (since the part is already fully defined).

 

[pdybeck]

ewh,

I don't think thats correct. There are a few factors that affect bend allowance: the material itself and the tooling and process used to create the bends. One vendor's flat pattern may very well be different from another, but they could both end up with the same bent state due to the differences in tooling they use. I think MadMango was saying this a little earlier in the post. With this information in light, then I don't think you can look at a fully dimensioned bent part and say for certainty what the flat pattern will be and therefore be able to put a "ref" on those dimensions.

 

[ewh]

If you aren't specifying the factors which affect the fabrication (which does mean that any reference dimensions may be wrong), then you probably shouldn't dimension the flat pattern at all.
If the drawing had all of the fabrication info and a fully dimensioned finished part, then any flat pattern dimensions should be reference. They may not be able to be deduced from other dimensions alone, but should be able to be deduced from the other dimensions and the fabrication info.
I think Ben gave the best solution: "Let the vendor be responsible for producing the finished part and you don't care how he does it, as long as the finished part is geometrically correcty."

 

[pdybeck]

ewh,

I think I will have the dimension(s) across bends removed from the flat pattern view on our drawing, as it just seems to open a can of worms. I still wonder about the 1:1 scale on the drawing. I think we should put a note under the flat pattern view that indicates this flat pattern shouldn't necessarily be construed as 1:1. Thanks for all the input everyone. I still would like to see if their was a standard that shed some light on common/accepted practices, even if what we discussed already seems to be what is generally practiced.

 

[drawoh]

pdybeck,

So far, all the features I have dimensioned on flat views have been the same thing. I do not want sharp inside corners, so I specify a hole to be punched in the corner in the flat state. This works out to an undimensionable feature after bending. All I need is a radius.

If you need some functional features in the as-bent state, then your tolerances should be applied in the as-bent state, allowing for +/-.015" variation per bend. Sheet metal is a job for a skilled worker who knows what K_factor to apply to your part. If he does not know stuff like this, the K_factors are the least of your worries. Start looking for cracks in the bends, botched welds and missed tolerances.

Ask yourself how you are going to inspect this thing when it comes back from the fabricator.

JHG

 

[pdybeck]

drawoh,

We use similar tolerances in the as bent state; usually +/-.015 or +/-.020 depending on the size/length of the flange. Even with that, I hear that our QC department still flags parts from time to time, but that is usually what the vendor tells us is reasonable.

 

[Tunalover]

Guys-
Unless you're a sheetmetal fabricator, showing the flat pattern, in my experienced opinion, is a waste of time at best. By imposing your flat pattern on the fabricator, you are stifling his creativity and tying his hands. Just show the end-item and let him "do his thing."


Tunalover

 

[ctopher]

I agree with the others. Dimension the part after bent, not the flettened part. You can create the sheetmetal part in SolidWorks (or whatever CAD you use) and flatten it and send file to shop for ref.

Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP2.0 / PDMWorks 05
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[Tunalover]

ctopher-
Do you mean that you agree with the others or that you agree with me?


Tunalover

 

[ctopher]

tunalover,
others ... as in other than myself, including you.

Chris
Sr. Mechanical Designer, CAD
SolidWorks 05 SP2.0 / PDMWorks 05
ctopher's home site

FAQ559-1100
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