Why dimension sheet metal flat patterns? 1

[STeliczan]

I'm looking at an opportunity to save time on drafting parts for my company. Currently we dimension all features in the flat as well as the formed part. To me this can be excruciating when trying to layout a part that has a lot of features in it. I just want thoughts on why dimensioning for flat pattern parts is not necessary maybe other than for secondary operations like tapping, c'sinks, etc. We make all of our own parts and are in charge of the software and laser. To me, we are wasting too much time on something that can be validated in a few simple steps.

 

[BillPSU]

We have parts we buy in the flat and bend in-house. The parts are laser cut and are 1150mm x 1800mm x 10mm. They are about 210kg and are laser cut around the periphery and have about 50 holes. The supplier would like to know the hole sizes and locations and tolerances on the holes. Each of our machines we build has one. We have about 40 different models each with a different plate. We produce more than 12,000 units a year. Who buys the scrap/rework if the part doesn't work?

What about parts with angular bends across a tab what are not bent 90 degrees. I can give many examples where dimensioning of flat patterns is imperative.

Bill

 

[looslib]

If you dimension your parts from the edge, either formed or flat, the dimension only needs to be in the drawing once, preferably the flat for the punch operator/programmer. Dimensioning across a bend(s) is always more tricky.

We used to laser cut or burn our parts directly from the CAD model. Engineering had to always make a flat pattern, even if the part was flat or a simple 90 degree bend. They only had to put the flat pattern on the drawing if the part form could not be readily derived from the finished part. We put a not on the drawing that the flat pattern was for internal use only. If we had the part made outside, they had to develop their own FP to account for their manufacturing methods.



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

Ben Loosli

 

[ctopher]

It depends on who is your customer.
If it's for in-house only, usually not a problem.
For aerospace use, you may have to make the dwg with all dims.
I made a box for space use, of course everything had to be documented.

Chris, CSWA
SolidWorks 14
ctopher's home
SolidWorks Legion

 

[JNieman]

Because it would suck if you find out the flat pattern was wrong after you've already formed up a few dozen parts. You could have caught it when it was fresh of the laser table with a simple QA check to verify location, and saved the handling and labor hours that sent it to the brake press and had a sample of parts made before checking them.

Checking parts between operations is a money saver. Discretion is required to determine if the risk factor is enough to require it between every operation, some operations, or if it's ok to just run it and let it go. Maybe checking only the first one of the run is acceptable...

I don't think it's wise to let parts get to the 'end of the line' without checking them after each operation in some manner, for new designs/parts. It will save you a large investment if you find out the part was bad from the very first op, but didn't find out until it got to assembly/last op/shipping/whatever.

_________________________________________
Engineer, Precision Manufacturing Job Shop
Tool & Die, Aerospace, Defense, Medical, Agricultural, Firearms

NX8.0, Solidworks 2014, AutoCAD LT, Autocad Plant 3D 2013, Enovia DMUv5

 

[STeliczan]

We are the OEM and also do the assembly of our machines. Like I said all holes requiring post laser operations would be called out. We also call out the overall blank dimensions because we use SI as out UoM for costing purposes. We are not selling any of our parts so I would assume that as long as we can validate that our dxf files are generating the correct geometry off of the laser we could forgo dimensioing flats...Just looking for perspective.

 

[JNieman]

There is a lot of things that can happen between the DXF and the time the operator takes the part of. 95% of the time it's ok. It's just those oddball problems. Also, what if the guy who made the DXF is wrong? What if there was a revision change to the original CAD file (ipt, sldpt, prt, dwg, etc) and he forgot to save a new flat pattern DXF for the programming? What if, in saving the flat pattern, he forgot to put minor-diameter holes in there, and put 1/4" holes in there that are going to be tapped 1/4-20? What if the DXF generated an odd polyline segment that the program didn't recognize and the programmer didn't catch it? Or maybe the table gets bumped without realizing it, just a slight bit, before the part was done and no one noticed.

It's sometimes not about actual process checks to make sure everything is going right when everything is normal - it catches the SNAFUs and 'ghosts in the machine' elements before they cost more money than necessary, as well. It's risk mitigation.

So anyways, that's where I'm coming from. I've been (and am) on both sides of manufacturing supply - both in-house usage and customer-hired manufacturing. It does alleviate a lot of things when you are your own immediate customer for a single part which won't result in a problem for the final machine/product being delivered (Because what customer cares if there's an extra sheet metal screw in the motor housing cover, or if one was welded up and painted over, and relocated to the correct position?) I've also gotten far down the line only to find that there were problems you can't fix (you can't always stretch metal the way you need) and you have to start over. It's also much cheaper to build machines when things bolt up easily and not have to rework holes, redrill holes, or create workarounds like drilling clearance holes over-size because it's misaligned.

_________________________________________
Engineer, Precision Manufacturing Job Shop
Tool & Die, Aerospace, Defense, Medical, Agricultural, Firearms

NX8.0, Solidworks 2014, AutoCAD LT, Autocad Plant 3D 2013, Enovia DMUv5

 

[drawoh]

STeliczan,

I do not show or dimension my flat layouts unless there is a feature that can be shown no other way. When I order parts, they are delivered bent. The flat layout is our fabricator's problem.

Some vendors request our SolidWorks files. They convert the drawings from millimetres to inches, set K[ ]factor, then flatten them. We have a new and very good vendor to requests full sized paper prints.

To me, this is like specifying tap drills on the drawings. The fabricator is supposed to know all this. If I have to explain it to them, I need a few fabricator. At least, I have actually tapped some holes. I bent a piece of tin-plate once, way back when I was in grade[ ]seven.

If you are doing fabrication in-house, your requirements could be different.

--
JHG

 

[KENAT]

I never include the flat - let alone dimension & tolerance it - on formal Engineering drawings essentially for reasons drawoh gives - my engineering drawing defines the finished part.

I will provide dxf of the flat my CAD software generates 'for reference' if the vendor asks for it, I'd potentially send the native CAD model if any of our vendors wanted/could use it.

However, if I were creating shop drawings for making the part then quite likely would be appropriate unless you do it all CAD/CAM as you suggest.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Tunalover]

I'm with KENAT and drawoh. The engineering drawing should show only the end-item. How to get there is the fabricator's job (fabricator can be a supplier or internal). A secondary drawing showing the flat pattern may be required but that should be a manufacturing drawing and should not be part of the engineering drawing. The purpose of the engineering drawing is to show the end-item with functional dimensioning and, although it is often used directly on the shop floor, it should be an engineering and inspection document only.

Tunalover

 

[CheckerHater]

I am totally for only providing final product drawings, especially if you send them to “outside” vendor who may blame you for providing “bad” flat pattern.
Unfortunately most of my sheet metal drawings are for internal use and you don’t want to know how spoiled some fabricators are.
Sometimes by the end of the day you just do what your boss tells you to do.
Doesn’t mean providing flat pattern information doesn’t have its advantages especially if you made proper disclaimers about assumed K-factor, etc., etc.

 

[berkshire]

This question is double posted on forum559
B.E.

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

 

[JNieman]

I'd 100% agree that you should only provide a flat pattern drawing if it's internal. I think providing it to an OSV is potential disaster. If the OSV demands a flat pattern so that they can just "put in machine, push button, get part" then that raises many flags about whether or not they should be doing the job. If it is some crazy geometry and atypical requirements, I could see the request coming from respectable vendors, but if you're making metal boxes and enclosures and flanges, then no.

For internal use... I'd echo CheckerHater... to me, it's the job of the 'support staff' of a shop to provide enough instruction to the floor personnel to keep them running machines and not doing work. Money is made when spindles are spinning, dies are hitting, etc. If providing a flat pattern allows your OWN guys to "put in machine, push button, get part" then it benefits your company. But it is not your job to be another company's 'support staff'.

I provided my previous answers with the understanding that the OP was speaking of his personal situation regarding internal production.

_________________________________________
Engineer, Precision Manufacturing Job Shop
Tool & Die, Aerospace, Defense, Medical, Agricultural, Firearms

NX8.0, Solidworks 2014, AutoCAD LT, Autocad Plant 3D 2013, Enovia DMUv5

 

[Tunalover]

I agree with JNieman with one exception. I believe the engineering drawing should show the end-item but should not contain the flat pattern. I believe the flat pattern should be shown in a separate manufacturing drawing that does NOT have to come under formal revision control. This gives Manufacturing the freedom to do the flat pattern the way they want (and as many times as they want) without having to do an ECO every time there's a change.


Tunalover

 

[ajack1]

I would have thought it was down to each individual / company to decide how much “additional” information they want to give.

If it has no benefit then it would seem a waste of time and money, however there may be hidden benefits, for example if whoever is quoting on the part has a flat blank there material costs will be clear, they might not have the time to calculate it themselves accurately so would add a margin, so it might indeed save money.

Also the number of times I have seen someone model up a part where when you unfold all the flanges the part is impossible to make from one piece as they cross over makes it worthwhile checking the flat blank, especially for people who have little experience in this field, although that would not require a fully dimensioned part.

 

[drawoh]

ajack1,

Modern 3D[ ]CAD packages like SolidWorks, are intelligent. They understand you are making a sheet metal part. They cannot cope with bad sheet metal modelling.

A lot of problems with parametric 3D[ ]modelling actually are real world problems. If you don't sort them out, your fabricators and end users will have to.

--
JHG

 

[btrueblood]

"They understand you are making a sheet metal part."

Drawoh, my CAD package is smart, but not that smart. I have to tell it that I'm making a sheet metal part, or that I want to convert a model to a sheet metal version.

 

[Tunalover]

drawoh,
I'm with btrueblood. Pro/E requires you to go into a sheet metal mode if you want to do sheet metal operations like flat pattern drafting.


Tunalover

 

[drawoh]

btrueblood and tunalover,

SolidWorks understands what I am doing sheet metal because I turn on sheet metal mode.

What I do in SolidWorks is build the 3D[ ]model that I want, I rip the corners, then I convert it to sheet metal. SolidWorks maintains a flat pattern from then on. Configurations that cannot be flattened out, do not work in SolidWorks. A really determined and stupid CAD operator can work around all of this, but they will break everything else too.

--
JHG

 

[MikeHalloran]

I certainly qualify as both determined and stupid.

I have done it both ways in SW, i.e. begin with a solid and convert it to sheet metal, and begin by starting a sheet metal part in sheet metal mode.

The resulting models looked virtually the same, but behaved a little differently. SW seemed 'happier' with the part that started as sheet metal.




Mike Halloran
Pembroke Pines, FL, USA