Sheet metal box spreadsheet

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Micbyrdev
Posts: 5
Joined: Sun Jun 26, 2022 1:17 am

Sheet metal box spreadsheet

Post by Micbyrdev »

Howdy yall,
I'm trying to make a parametric sheet metal box/lid spreadsheet tool.

Image

I've got almost everything I need, but I'd like to make it easier to use for other guys at work who aren't familiar with CAD. The intention for the finished tool would be to input the box's inner dimensions and flange length, and have output the overall flat pattern width and length, as well as the corners notch dimensions for cutting from a piece of sheet stock. I can get those dimensions easily enough by measuring the part and the flat pattern in the part design work bench, but I'd like the spreadsheet to calculate, and show those numbers in an easy-to-understand way.

Basically, I'd like to know how FreeCAD derives the length of a flat pattern from the bent model using the given K factor, material thickness and bend radius, and how to parametrically express that in the spreadsheet. So far, freeCAD, my own calculations, as well as online calculator tools for flat pattern length like this one

https://www.smlease.com/calculator/shee ... alculator/

have all disagreed with each other.

Any help is much appreciated.
Workshop_Notes
Posts: 590
Joined: Wed Sep 29, 2021 8:35 am

Re: Sheet metal box spreadsheet

Post by Workshop_Notes »

It is not very clear from your post if you are using the FC Sheetmetal tool in your project or rolling your own from scratch.

One way to find the answer is to look at the source code https://github.com/shaise/FreeCAD_SheetMetal and https://github.com/shaise/FreeCAD_Sheet ... -unfold.py

If this does not bear fruit, you could go a long way to determining what FC does by experiments and graphing your results.

Start with a simple (90 degree) L-shape. You have three parameters: K-factor, radius of bend and material thickness. Vary one of these at a time and graph your results.

That being said, practical sheet metal bending is a black art: it is highly dependent on the machine, the tooling and the material (and direction of bend in relation to the material's grain). Your users need to appreciate that what you give them is only a starting point and actual pattern dimensions need to be confirmed and adjusted based on a test item.
Micbyrdev
Posts: 5
Joined: Sun Jun 26, 2022 1:17 am

Re: Sheet metal box spreadsheet

Post by Micbyrdev »

box spreadsheet.JPG
box spreadsheet.JPG (272.83 KiB) Viewed 715 times
Micbyrdev
Posts: 5
Joined: Sun Jun 26, 2022 1:17 am

Re: Sheet metal box spreadsheet

Post by Micbyrdev »

Okay, figured out how to post an image properly.

Thanks for the tip, I'll take a look around the source code to see how their formula differs from the other ones I've used.

Yes, I am using the sheet metal workbench, in conjunction with spreadsheet to make a parametric box generator sort of thing, which works pretty well so far. The last hurdle is figuring out the flat pattern overall length and width, which I should be able to get (I think) with this formula -
90 * pi / 180 * (C2 + 0.4 * C2) where 90 is the degree of bend, 0.4 is the k factor, and C2 is 0.125 inches (the material thickness, and inner bend radius), which gives me the bend allowance. (2 x Bend allowance) + (length of all flanges) gets me ruffly close to the measured length that Freecad shows using the part design workbenches measuring tool, but not close enough, sometimes over an eighth of an inch depending on material thickness.

I'll give your graphing idea a shot too, as well as bending out some real-world test pieces to compare to.

Haha, black art indeed. Luckily, we do have a bending wizard where I work, and I guess that makes me the sorcerer's apprentice. Some of the fellas struggle with it though, and I'm hoping that this calculator, with its nice visual representation to go along with it, will reduce some of our workload.
Micbyrdev
Posts: 5
Joined: Sun Jun 26, 2022 1:17 am

Re: Sheet metal box spreadsheet

Post by Micbyrdev »

Thanks for showing me that sheet metal workbench code, it helped square me up. So, the formula I was using was a different approach to reach the same bend allowance as the formula used by FreeCAD, my screwup was not realizing kfactor can be set differently in the tree under Bend-Parameters Ex- kfactor, than the one set when using the unfold tool to make a true flat pattern. Oops.

So, the things working great now. Only improvement I'd like to see is to have the ID and OD both be driving inputs. right now, only the ID is used to set the output, which is what we go by in the shop most of the time. I think that's just a regular spreadsheet problem to figure out though. Gonna have to dust off my copy of Excel for dummies.
Better box spreadsheet.JPG
Better box spreadsheet.JPG (230.87 KiB) Viewed 634 times
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