Location of bend line on flat layout?

[daspferd88]

For a flat layout sheet metal part, is the bend line located directly in the middle of the bend allowance section, or is it inset from one side by a distance of the bend radius? So for a U-channel with an opening of approx. 4.56" (distance between inside of legs), would the distance between the bend lines on the flat pattern be 4.56? This is assuming an inside bend radius equal to mat'l thickness (.188).

 

[MikeHalloran]

It depends.

If you're using a press brake by eye, a centerline may be sufficient.

If you're using a box & pan brake, you might want to mark a tangent line and a line offset by the radius.

In either case, you'd get better precision with a back gage, for which you don't need a mark if you're doing it right.


When I am working in primitive style, e.g. clamping a sheet between a couple of rectangular bars and beating a bend with a hammer and a third bar, I'll mark a centerline and two tangent lines so I can track my progress.



Mike Halloran
Pembroke Pines, FL, USA

 

[DiegoLGraves]

On your print the bend line should be dimensioned to the center of the bend from the nearest edge that can be used to gage from, which is the middle of the bend allowance section. This is common practice for press brake work and will work alright for folding machines and hand brakes. I don't do enough stamping die layout to know what the common practice is.

Diego

 

[drawoh]

daspferd88,

Are you trying to fabricate this thing, or prepare a drawing? When I prepare drawings, I include the flat layout only when there is detail on there that cannot be shown anywhere else. If you are using SolidWorks correctly, it flattens your part on command. I assume other 3D[ ]CAD packages are similar.

If you are trying to fabricate this thing, you need help from an experienced sheet metal guy, almost certainly, on-site. Failing that, you can punch out several pieces and practice on them.

--
JHG

 

[daspferd88]

I'm creating a drawing using AutoCAD 2D. We do not have SolidWorks, so I can't rely on the program to decide for me where the bend lines are located.

 

[MikeHalloran]

Providing a flat pattern layout as part of a design is usually an egregious waste of your time, because every fabricator will have his or her own way of processing a part, and his or her own tools, so the mapping between finished part and blank will be different for every shop and probably for every fabricator within a shop, and even for every lot of sheet material.

Just draw and dimension the finished part you want.

The fabricator is guaranteed to be much faster than you are at developing a blank, and if you give them a blank and the part comes out wrong, you have also given them an excuse for supplying a wrong part.
Don't do it.


Mike Halloran
Pembroke Pines, FL, USA

 

[CheckerHater]

I'm creating a drawing using AutoCAD 2D

Let me get this straight, are you:
-Trying to "unfold" drawing of formed part using 2D CAD?
-Trying to create "flat" layout and hope it will form the correct way?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[JNieman]

I've been on both ends of this situation.

MikeHalloran has given the best advice. The best way to do it is "Don't."

If you /are/ the sheet metal shop then you should be talking to your superior or the guy bending the parts. Because, as Mr Halloran says, it'll be specific to your shop.

 

[KENAT]

I wasn't going to reply to this thread as my initial response would have been the same as Mikes' second response and I wasn't sure you'd appreciate it.

Per US drawing standards (ASME Y14.5M-1994 section 1.4e etc.) you don't normally* give details of how to make the part rather define the finished article. So in this case the finished formed sheet metal part.

* There are some exceptions but I don't think the flat pattern for sheet metal normally falls under that.

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[MikeHalloran]

If you are still determined, or have been ordered to, make sheet metal developments, AutoCAD is just about the worst tool for the job.

Maybe you can afford a seat of Rhinoceros. Using Rhino, you can bang out a 3D model in a few minutes, and if's developable, Rhino can unwrap it on command.



Mike Halloran
Pembroke Pines, FL, USA

 

[CheckerHater]

I will back Mike - if you do substantial amount of sheet metal drawings, get the right tool for the job.
It may be SolidWorks. It may be Inventor (which will include AutoCAD licence, so you still have your familiar tools handy).
Even VariCAD has some basic unfolding capability (not like I recommend it, but as a curiosity)

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[drawoh]

I second Mike's original piece of advice. Specify the bent part, and let the fabricator figure it out.

--
JHG

 

[3DDave]

I've only made one bend drawing in 30 years and that was because the bends were not at right angles to each other so it would take 2 auxiliary views to show the relationships. The sheet metal guys were going to make only 1 part, so rather than make them lay it out I did the flat pattern. Did I mention I knew the sheet metal guys?

Otherwise it is endless trouble. For example, if the bender doesn't have the right radius tool in the right length then the flat pattern needs to change. If it is bent on a brake it will come out a little different than a press brake. If the metal is a few thousandths thicker or thinner, it will be different. Too many options for not coming out right to make a pattern ahead of time.

 

[berkshire]

If you absolutely have to produce folded sheet-metal patterns in AutoCAD ( which I do not recommend, and I am a tinbasher by original trade.) then there is an add on unfolding program by a company named Litio that can generate flat patterns.
Remember a flat pattern allowance is only good for a particular metal type , thickness ,and bend radius. Change any of these , and your allowance goes out of the window.
For shop use since you do not know what bending method will be used , add a line at the start of the bend ( bend line in), a line at the end of the bend (Bend line out), and the centerline.
B.E.


You are judged not by what you know, but by what you can do.