So I’ve got a corner cabinet with a compound curve I need to lay a 1/8th skin over, I’ll need to cnc cut a pattern (living hinge?) in the material to give it the flexibility needed to bend in both directions. I don’t suppose there is a plugin for Rhino that can develop a cut pattern from a curved surface is there? I don’t want to turn the entire sheet into a hinge, just cut the areas I need to relive tension and then conceal it with bondo.
Fitting a flat skin to compound curvature requires some amount of stretching or compression. What type of pattern do you envision as helping with this?
Squish command flattens a compound curvature surface with options for how the stretching/compression will be accommodated.
Wow, squish is a fun command. I’m using semi-flexible materials (bender board and mdf) that can easily bend into a single curve direction, I’d just like to cut a pattern to help it bend in the second. As to the stretching and compression, I can get the skin larger than necessary and trim route the edges to account fluctuations.
It’s for a museum exhibit fyi, I’m not just doing this for my kitchen.
Sounds like you might want to look into the difference between bending a sheet material into what is known as a developable surface which is only curved in one direction at every location, and forming a flat sheet into a surface with compound curvature.
Pick up a piece of paper and experiment. You can easily bend it into shapes which only have single curvature. Then try forming it over something like a basketball. You will have major difficulties.
The reason is bending the sheet to a developable surface only requires a very small amount of stretching of the side of the paper on the outside of the bend, and compression on the side on the inside of the bend. For thin sheet like a piece of paper the amount of stretching and compression is very, very small. But fitting the compound curvature surface can require significant stretching and/or compression through the thickness.
One way to form a surface with compound curvature is to split the surface into several surface, each of which is close enough to simple surface for the material to bend close enough to the compound surface.
1 Like
Yes, an army of 2" strips is my alternative, and a load of sanding to keep the curvature. The living hinge though, isn’t the point of that to stretch and compress for you?
Like this example http://nathanjohnthompson.com/projects/living-hinge/
I feel like there are a number of cuts I could make and everything would slip and flex into place perfectly with some nails and glue. I’m just trying to get a better handle of what that will look like, was hoping for a plugin to do it for me.
The bending I see in the linked video is simple bending, not bending in two directions at once.
You could make a number of cuts so that a large sheet acted more like an array of smaller pieces. I don’t know of any “standard” ways of doing that though. It would probably depend on the shape of the compound curvature surface.
How about doing something like this to a sheet of plywood:
Then packing the gaps with polyester (car body) filler while the sheet is held to shape? I’m not sure what your finished surface needs to look like, but if it’s to be sprayed, the above could work. If you know the profile, the same method could be customised to give relief slots in exactly the right areas.
Without cutting through-slots in a sheet, I don’t see how you’re going to get the live hinge approach to work, as the sheet material has nowhere to go in the areas where it needs to shrink rather than stretch.
1 Like
[quote=“MattE, post:7, topic:33581, full:true”]
How about doing something like this to a sheet of plywood:
Then packing the gaps with polyester (car body) filler while the sheet is held to shape? I’m not sure what your finished surface needs to look like, but if it’s to be sprayed, the above could work. If you know the profile, the same method could be customised to give relief slots in exactly the right areas.[/quote]
Looks like a good concepts. Should be easy to create the repeated pattern in Rhino by drawing one set of lines and then using array to repeat it.
There’s no real benefit to cutting a sheet of MDF like that and then filling it and sanding it, you may as well use other materials which are better suited. The resulting surface will basically be carbody filler and open ended MDF which will need a lot of finishing.
Using the last example would require too much structure for it to hold its shape, it’d flop all over (I guess).
Do you have any examples of what it is that you’d like to do? I’ve recently been looking into something similar for a sculptor over in Ireland. I found some great software for tensile fabric structures but it utilises the weave and stretchiness of the fabric so couldn’t be applied to stiffer materials with no natural weft.
Without seeing your design, and knowing the extreme difficulty of bending a planar surface in multiple axes, here’s something to consider (and I think you’re already thinking something along these lines with your living hinge comment).
If you can trace the main axes of your compound curves on to the sheet(s) of MDF, then draw a series of lines at right angles to these main axes. The greater the curvature deformation on the final surface, the closer together the transverse lines need to be drawn. These lines at right angles to the main axes are what you should cut/mill partially through your MDF. In areas where your main axes cross, the transverse lines will be fairly heavily cross-hatched. Extend your transverse lines a bit beyond where the deformation ends. This process should allow the surface to deform in multiple axes, in spite of the rigidity of the “skin”. Obviously the width of each saw kerf/mill slot and the spacing of the cuts will determine how much curvature you’ll need to generate for your final surface.
You could also use the Contour tool. Run it twice, at right angles on the original surface, and you can use the contour lines to cut an underlying grid upon which you can tack your MDF with the relief cuts. The saw kerfs at multiple angles will allow the compound curve bending of the surface, and the underlying contour grids will give appropriate support for your upcoming bondo/spackle/sanding-fest.
Sounds like an interesting project; love to see a photo of the final install.
Doug
1 Like
P.S. I’d probably dampen or steam (if possible) the MDF before bending, just to induce a bit more flexibility in the bending.
Doug
what’s the actual shape? (and basic dimensions? radii etc)
idk, i do a lot of weird bends with 1/2" plywood and 1/4" phenolic sheets but still, hard to advise without a little more info.
…
that said, i’m not really backing this ‘living hinge’ idea because with that many cuts, you’re going to have to bondo the whole thing anyway… so might as well just use the resin for the surface… or fabric form it then 50/50 mix of bondo & polyester resin… (basically, paintable bondo)…
but if it’s not too complex of curvature, a few well cut pieces of sheet goods should do the trick then just fill the seams.
edit… wait, 1/8" mdf? have you tried just coaxing it through the bends? that stuff is pretty flexy.
or are these some really tight curves?
Here is the corner cabinet I’m referring to,it connects flush with two long rounded cabinets on either side, which were easy enough to fabricate but this compound corner is whats got me stuck.
I think I might just take the Squish advice and laminae two 1/8 MDF sheets together, one at a time over the form to skin the shape. There will be a waffle structure inside for support.
I’m better at the CNC than this software, looking at what your attempting you could slice that up horizontally and generate some Z contouring tool paths with a ball bite, glue up and sand out quickly. I bend wood frequently and that’s a tough one. I’m assume your using a 3 axis machine,otherwise bang that out of a solid glue up if your using a 4 or 5 axis machine. Best…
_Split that up using those longer/vertical isocurves then Squish the resulting surfaces for cutting on the mill…
maybe.
your funnel shape is not too dissimilar from parts of this which was done the same way :
…albeit, these are larger radii… but less bendy material (phenolic generally only likes bending one way)…
1 Like
Cu an extra set of kerfs (following the fan pattern that your model is showing) across the line of kerf core and you’ll be part of the way there.
I doubt whether you’d find a more efficient, practical & cheap method than what jeff_hammond has presented above. That is exactly how I’d tackle it too.
Hot damn Jeff, that looks great. A lot of material though, think I could get away with 5 fans and using horizontal strips (say 2" tall?) as opposed to vertical?
How many do you have to make and what is the finish? I’d probably make a wooden former with a zinc edge and plaster sledge that as a quadrant with a long flange for moulding. Knock out a quick GRP mould with wooden formers and lay them up if there are more than a couple.
I can’t find any photos of old scenic stuff I did a few years ago, we made quite a few substantial sledgings using a bearing and a vertical scaff pole in the centre for some stage sets.