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Standard for dimensioning sheet metal flat patterns

Standard for dimensioning sheet metal flat patterns

Standard for dimensioning sheet metal flat patterns

(OP)
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.

RE: Standard for dimensioning sheet metal flat patterns

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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RE: Standard for dimensioning sheet metal flat patterns

(OP)
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.

RE: Standard for dimensioning sheet metal flat patterns

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

RE: Standard for dimensioning sheet metal flat patterns

(OP)
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

RE: Standard for dimensioning sheet metal flat patterns

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

RE: Standard for dimensioning sheet metal flat patterns

(OP)
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.

RE: Standard for dimensioning sheet metal flat patterns

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.

RE: Standard for dimensioning sheet metal flat patterns

(OP)
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.

RE: Standard for dimensioning sheet metal flat patterns

(OP)
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.

RE: Standard for dimensioning sheet metal flat patterns

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

RE: Standard for dimensioning sheet metal flat patterns

(OP)
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.

RE: Standard for dimensioning sheet metal flat patterns

  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."

RE: Standard for dimensioning sheet metal flat patterns

(OP)
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.

RE: Standard for dimensioning sheet metal flat patterns

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

RE: Standard for dimensioning sheet metal flat patterns

(OP)
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.

RE: Standard for dimensioning sheet metal flat patterns

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

RE: Standard for dimensioning sheet metal flat patterns

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
ctopher's home site
FAQ371-376
FAQ559-1100
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RE: Standard for dimensioning sheet metal flat patterns

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

Tunalover

RE: Standard for dimensioning sheet metal flat patterns

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
FAQ559-716

RE: Standard for dimensioning sheet metal flat patterns

What I meant was, do you agree that taking the trouble to develope a flat pattern is not worthwhile if you're the end-user and not the fabricator?

Tunalover

RE: Standard for dimensioning sheet metal flat patterns

I leave the flat pattern up to the fabricator. It is no use for anyone else. I use SolidWorks and create sheetmetal parts. I send the sheetmetal company my files and they can "flatten" the model for use.

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

FAQ559-1100
FAQ559-716

RE: Standard for dimensioning sheet metal flat patterns

ctopher-
Sound thinking!

Tunalover

RE: Standard for dimensioning sheet metal flat patterns

Section 18 of Genium's Drafting Manual is "Sheet Metal Practice".  In sub-section 7.0, they show three examples.  None of them include features which cross a bend, but each includes a note:

Quote:


Unless otherwise indicated
Tolerances ± 0.04
Mfg. allowances to 2191K450

Does that number refer to a standard, or is it intended to indicate a shop procedure?  (We don't work with a lot of sheet metal, so I wouldn't necessarily recognize a common standard in that field.)

In sub-section 5.0, Bending Allowances, they state "Forming radius must be specified on drawing."  They do not state that K factors must be specified.

In Section 18.1, Sheet Metal Design Documentation, sub-section 1.4, Blank Development of Parts, states:

Quote:


Although blank developments of parts are shown extensively throughout this text, the developments are intended principally as tutorial. However, there will be instances when the developed configuration of a part is necessary. Because the predictability of the shaped part (final configuration) cannot always be ascertained with confidence (as in, for example, control of springback), a cautionary note should be written beneath the developed configuration for manufacturing guidance. In other words, the development is provided to assist manufacturing in visualization and material planning. It is not the end product that is fully dimensioned elsewhere on the drawing.

And finally, in Section 18.2, Sheet Metal Design Considerations, sub-section 3.0 states, "It is not general practice to show blank developments on production drawings; therefore, the following formulas are presented only for reference."

Hopefully this answers more questions than it raises...

RE: Standard for dimensioning sheet metal flat patterns

This may also be a "Company Standard" issue.  Even though Solidworks creates the flat patterns for us, we put a note on the flat pattern sheet stating "FLAT PATTERN DIMENSIONS FOR REFERENCE PURPOSES ONLY".  This relieves us of any burden in case our flat pattern is wrong, but we also get a fast turn-around for quoting purposes.

Some of our vendors swear our flat patterns are spot-on, but we put the disclaimer above to cover our butts.  As a former sheetmetal fabricator, I am very familiar with triangulation and parallel line developement, but there is no competition between the time it would take me to lay a part out, and the automatic flat pattern that Solidworks creates.  This also depends on whether you are talking about laying out simple channels with features, or laying out square-to-rounds, offset-cones, or helical-chutes.

Some shops primarily use air-bending, and use 8 x the material thickness as a standard for the press die while others use 6 x the material thickness for the brake press die.  Then you can throw in coining, spring-back, and other factors that change the bend allowance/bend deduction.  

Finally, when you throw into the mix that some shops do not use k-factors (like Solidworks uses), but use the Emperical Formula (such as aviation engineering bend allowance charts), you might see why it would be a good idea to have a disclaimer on flat patterns.  

Flores

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