Is there a way of selecting certain surfaces to smash?


I dont want or need to smash the surfaces that get stretched when sheet metal is bent, so to be able to pick surfaces that remain ‘neutral’ and are attached to the bend surfaces that dont stretch would give me the result I need. I also dont need the 0.036inch thick edges as the 2D plan again doesnt require them.

With such an option I could pick my way through the object considering which surfaces would make up the 2D unfolded plan.

Smash upper web too spread out.3dm (258.7 KB)

I have now gone for settings explode=no labels=yes keep properties =yes at least that way the result doesnt get strung out for miles !


Steve, I think your approach to these type of bent plates would be less painful if you modeled a single surface to begin with and offset to the neutral axis to unroll ( half-way is probably ok for this example then discard that offset) and offset into a solid when finalised.

Unroll, not smash, is the method to use here . .

Hi Brian,
If I was working to original engineering drawings I would have drawn them as 2D then turned into a solid.
However this item is crash wreckage, put back into shape as best as was possible, and he did an excellent job. I was tasked with recreating it so he could take my 2D plans, lay on whatever sheet thickness I established was used and then bend to the radii I had established had been used.
It was VITAL that I created it in 3D, simple recreating what I held in my hand and had measured many times, especially the bends. One cannot suss bend radii by looking at an object, one cannot see where the straight ends and the arc begins. One can have an initial idea, but you have to 3D model the item, my first 3D models showed my thoughts on radii wouldnt fit together. When it all fits, then I am to 2D it.

I have to take the inner surface of the item posted, as outer surfaces of bends have stretched. On the lower half bends go both ways so my need to pick carefully.

Coupled with sussing radii is establishing sheet thickness. Only getting both right will the item fit. That took 3 goes and 3 redraws, only possible with seeing how a 3D model fitted together, recreating what I saw in my hand. Getting the thickness and radii combination right needed proving they all fitted correctly, one area being the litmus test !

Recreating the bend twists would not work on part of the lower web with OffsetSrf after a 2D plan. With the bend command being unusable I cannot test that statement though. I have a post on this discovery, unanswered when I last looked, showing bent item doesnt end up where it should do, which floored me a bit and I had to drop the idea of using bend to make 3D. As such another reason to make 3D then make 2D. If I had 2D plans to start with I havent a tool to do the bends. Fusion360 will and its free but needs win10.

You show the steps, extractSrf, join, flip, unrollSrf. (though on the wrong surface). Thanks :slight_smile:

I shall try for that on the inner (unstretched) surface.

That is effectively my hope for selecting specific surfaces that together form the original unstretched surfaces.

Thanks for the revelation on this. :slight_smile:



Here is another way to approach this reverse engineering which may be worth considering?? (or maybe not)

Measure the critical points and model without the bends initially.
Then nominate a standard radius as close to measured as poss and unroll to 2D.
When the actual part is folded to that nominated radius - the critical points should match, I think. Still need to offset the single surface to the neutral axis before unrolling.

If you draw a line MidPt to MidPt along the radius and unroll that line with the surface - a fold centre guideline for fabrication.

Sounds like a fun restoration.

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Just curious, Steve - did you ever work in an engineering office designing parts or in a shop fabricating them? I ask because I wonder if you use experience and knowledge of standard material thicknesses and typical fractional inch (mm) bend radiuses as you are trying to recreate parts like this.


Measure the critical points and model without the bends initially.

yes I did do exactly that.

Then nominate a standard radius as close to measured as poss and unroll to 2D.

My surfaces were not joined though as some were simple bends., so I knew how they would be as 2D, others used welds to make up some tricky areas. Others were not for bending. The area you show was a later discovery and by then I was well into 3D modelling and trying to make the parts fit, so introducing 2D folds I felt set me a greater task still, and your images show the loss of the parts that were the critical area in the fitment.

When the actual part is folded to that nominated radius - the critical points should match, I think.

Having had the ‘flow’ command in a test I did,… show it was not matching the maths and theory of where a marker and end of test sheet should end up, I cant see how I could use Rhinos bending method.

Still need to offset the single surface to the neutral axis before unrolling.

I am needing to understand the offsetSrf to the neutral axis, I google this, in V5 help OffsetSrf there is no mention of neutralAxis. I you tube it and all videos are in Hindi, not sure why my Youtube is giving me foreign language videos, though titles are in english and no mention of neutral.

Could do with knowing or seeing what this means. I have used OffsetSrf and dont see such.

I search in help on 'neutral axis, and find nothing except image post processing.

I attach pics of what I have created and the wreckage.

No and No. I have the item, the material thickness is established with a vernier gauge, as are initial apparent radii, knowing the radii used on such parts normally. I have to simply recreate what it is, the radii they actually used etc. In the upper ‘shelf’ area all my radii that were in use elsewhere, went out the window as it was IMPOSSIBLE given the basics as Brian refers to, to get that area to fit together at all, I almost concluded they achieved the impossible with an L bend zero radius ! only by 3D modelling it did I eventually suss how it was, and could see the ‘shelf’ in fact had to miss sitting on the surface below it to be achievable. Measuring with x5 glasses and an angle poise lamp, taking macro photos etc.

2D modelling that part and trying to use flow command would not have worked, had to be created in 3D until it fitted together.

If I had worked in an engineering office designing parts I would have said they got that wrong. I am not however here to show how it should be as then I would have to have access to the area this fits in to see if my changes to history conflicted with something else where this went. I recreate how it actually was. This was fabricated perhaps with mallet and vice, as measuring it I get a feeling for its variations and construction.


The reason to offset the surface before unrolling is that there will be a bit of compression as well as stretching during the folding process.

Here is a nice sketch from a Google search



Well I never !
I have had folk tell me metal only stretches in a bend.
Sheet steel 0.036inch thick 20 gauge, never thought it could do that,

No doubt different compression amounts depending on hardness of metal, e.g. copper v aluminium v steel.,

So the extract Srf for the bend needs the offsetSrf amount of t into the sheet, sheet is 0.036 inch so I need to find out what the K factor is, maybe K is decided on with type of metal… maybe also speed of bend, as the faster one bends metal the warmer the metal gets in the bend and less crackability, so I am told, again better double check all that out as well !


FWIW, Steve, that’s what we’ve been trying to tell you over the course of several weeks and the reason why it’s not in Rhino. This is very material and process dependent and you need to get in touch with a manufacturer to get this right - or get software that claims to get it right.
(Thanks, @BrianM).

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Understood now.
Item I am drawing up is 0.036" sheet mild steel,
K factor is 0.42 internet says for such, so offset all surfaces (as both bends suit that approach (as opposed to just on bend surface) on upper web 0.42 x 0.036" and offset to place surfaces at neutral position is 0.01512"

Gives a difference on overall height of 2D plan of 0.5842mm (0.023inch). Internet talks of punch and so on, this is vice and mallet and wood for the remake of two parts, and half a mm wont be a factor but its now ingrained in my brain. As to who told me metal doesnt compress on the inner bend I need to catch up with if I ever find them.

The flow command I tried, for a bend in a solid sheet, isnt about K factors so that 2% error in its result on my half inch of sheet is another aspect aside., and to be accepted as par for the course.


Yeah, Steve. This is all stuff considered elementary in a sheet metal fab shop. It’s also part of the “art” of sheet metal work.

The funny thing is it really is only significant when communicating from design shop to fab. Also very important when making tooling for quantity production. A part shown with specific outside dimensions and a bend needs to have the IML (inside mold line) shown accurately so that the bend will be in the right place. Material “springback” must be accounted for so the bend ends up at the desired angle. All shop stuff.

For a small quantity of hand-formed pieces it’s not all that important. Also probably not important for making the part in Rhino. The answer is to draw the sheet metal cross section with all the bends by whatever means you like (I like drawing the inside surface with the inside radii and then offsetting for material thickness - when multiple bends go both ways this may or may not be the best method.) Allow plenty of excess at the edges. Extrude and explode/rejoin to get a solid. Then start trimming edges to final size and shape. Same process works well for real pieces: oversize blank, bends first, “cut to fit”, “paint to suit”. :grinning:

After you get good with the bending brake you can cut the blank to net shape and bend afterward. Even then “near net” with some finish filing can be a good approach on critically precise dimensions.

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I need to read that a few times :slight_smile:
I wish they had taught us more ‘hands on’ skills at Grammar school, not latin !
Engineering was Secondary School, as was cooking, we did the sciences, tech drg was 3h and 9h on paper ! Art was teacher in store room, and a few fags and no tasks set us.
Amo Amas Amant could have been The sheet, the bend, the maths, the mallet.
Also hobbies schools should show us a variety, solve a lot of kids being bored and hanging out at street corners, but I wont get the soapbox out !
A K table has values, none say 0.42 , none talk of hand forming, its a can of worms !


Amount of stretching on the outside vs compression on the inside of bends depends on part on how the bend is made. A very talented metal working I knew would typically do some testing when laying out a new part where accuracy was critical.

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