Sheet development on drawing

[sandruz]

Dear all,
I've read many times this forum, but this is the first time for a thread. This is a wonderful forum.

I have a little question for you.
In the drawings relevant to parts made by bended metal sheet I always represent the development.

Some suppliers have a problem if they follow this development because the final product doesen't guarantee the final dimension.
Some other suppliers haven't any problems and the final piece is correct although they'are using the same drawing.

This happens because the machinery/equipment are different.

At this point I've decided that the future drawing won't have the development representation.
What do you thik about?

Thanking you in advance,
regards

 

[CorBlimeyLimey]

Unless you are fully aware of the tooling and practices used by the fabrication shop, the flat layout will rarely match the parts needs.

The formed part is what you require and are paying for. Let the fab shop create their own flat.

On the rare occasions the shop requests a DXF flat, I ensure they understand it is for "lookalike" purposes only. If the part turns out to be out of spec, they own it.

 

[TVP]

I agree with CBL. It is up to the manufacturer to make the part, you just need to provide them with the finished part drawing with the relevant dimensions, tolerances, etc.

 

[11echo]

I worked in a sheet metal fabrication shop a number of years ago (don't ask!)...and I was told you always generated a flat presentation! Especially if you wanted the assembly constructed a certain way. "IF" you just generated a formed view the fabricators could "fabricate" the shape in any way they would deem "easiest", and structurally may not be suitable for your use.

 

[hendersdc]

I worked in a sheet metal shop and if the drawing had a flat pattern on it we disregarded it. The bend deductions/bend allowances never matched up with what was used by our shop (if they were used at all in the solid model instead of solidworks k-factor). A fully dimensioned flat along with a hole table was always generated by us for each part.

Different shops have different methods so a flat pattern generated by the customer may or may not be accurate. I would say always dimension the finished part and make it the fab shops responsibility to make sure that is what you get.

11echo, I have never seen a shop move bends or welds without as least asking for customer approval first.

 

[drawoh]

sandruz,

Specify what your inspector will accept. Don't tell them how to make it.

JHG

 

[looslib]

When working for a manufacturing company, we put the flat patterns in all of our CAD files. If the part was bent with more than a single 90 degree bend, we put the flat pattern on the drawing and labelled it "For Internal Use Only". We knew what the k-factor should be for our tooling and process, so that is what we generated. We also used a nesting program that would take many different parts and cut them from a single large sheet. When requested to provide DXF files for subbed out work, we worked with the vendor to be sure that they could use our flat patterns and would end up with the final bent shape before agreeing to let them use our files.


"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

 

[rb1957]

we've recently started doing flat patterns of s/m parts, mostly 'cause of a loss of experience of people downstream of engineering (in planning "how big of a sheet do i need for this detail ?" and in the shop "i started with a sheet that was too small") ... sigh

different vendors making the parts with different processes, making work for engineering ... say it isn't so ! we've recently had to redo done a bunch of pipe details (with all sorts of bends) ... initially the fab people worked from 3D drawings (no surprise there) but then along came a cheaper guy who worked from a bend table ... sigh^2

 

[KENAT]

ASME Y14.5M-1994 1.4c & e pretty much addresses this, if your drawings comply with that standard. "... dimensions of an end product shall be shown." etc.

If you do feel the need to put the flat pattern on the drawing, I'd make all the dimensions on it reference, and if necessary add an explicit note explaining that the 'flat' view is for reference only etc.

Given that many CAD packages can 'unfold' the finished item to get the pattern it always seems a shame not to use it, but a few of the stories above pretty well demonstrate the issues.

If you share a common/interchangeable CAD format, there may be merit in supplying the model though this has it's own configuration control issues.

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[drawoh]

rb1957,

We send our sheet metal out to be fabricated. Our vendors routinely ask for our SolidWorks files so that they can flatten them and apply their favourite K[ ]factor. They also convert our drawings from millimeter units to inches.

This all is fine with me, as long as our inspector accepts the results.

JHG

 

[rb1957]

quite agree, don't care how it's formed so long as the final detail meets the design spec (and hopefully the design intention).