Allowing for paper thickness

[Plumdragon]

Hello friends To begin, I have tried searching for this in the Forum, but if it's here already, my apologies and put it down to inadequate navigation on my part!
What I'm going to talk about seems to mostly affect models designed using computer programs; at least it does on the last ones I've built, and the one I'm doing now. In the pic you'll see a shape, Fig.1; it could be part of anything but it'll serve.The issue is two-fold really.
Firstly, whether by intent on the designers' part, whether it's part of the program used or if it's done to impart strength, I've noticed that frequently parts like Fig.1 are broken down into several sections, as seen in Fig.2. My maxim is to keep things as simple as possible, so the part would be just one piece of card, or as few as possible on the paper size used, so a child could build it.
Secondly, and this is the bugbear, the program obviously deems it sufficient to 'plan out' the four individual pieces seen in Fig.2 at absolutely the correct outside dimensions. Which is fine and good in the world of electrons on your screen, but by the law of cumulative error, there are now four thicknesses of folded card on the top sections, as well as any tabs that may have been added. Added to this is, the chances of cutting/folding errors on the part of the builder have also been multiplied four times. It is obvious, then, that the three segments on top will be wider than intended when assembled. It also follows that the more bits like this stuck together, the greater the error, to the point where things simply don't fit anymore. At least, that's what happened to the models I was building.
So, please tweak things to accomodate the thickness/es of the cardstock and/or tabs, and it would be very helpful if each designer annotated their models with the weight of cardstock or paper used so I'll know if it needs building from writing paper or old cereal boxes Lastly, I have no idea about programs that 'unfold' or flatten designs (or even what the correct terminology is!) My PC cannot spare the RAM, and I like geometry, maths, rulers, compasses and, above all, trial fitting everything so I know it all fits together.
Plumdragon

[Vermin_King]

I know exactly what you are talking about. Some designers do specify what weight or thickness of paper to use, but most do not.

Often times, the figure 2 type of construction vs. figure 1 type is due to a necessity for extra strength. Lamination helps. But like you say, it adds to the potential for accumulating errors. Paper is somewhat of a forgiving medium and sometimes it can be coaxed to adjust for the errors. Sometimes you just have to make shims.

Don't get too frustrated. I am probably the worst at aircraft models for some reason. At one point I just decided that they weren't for me. Then I had a situation where I really needed to do a model (actually two. one for my mom and one for my aunt), so I did four of them. One to practice, then three to come up with the best two to give as gifts. Since then, I have gotten better at aircraft, but still I do two to get one that is good enough to give away.

It gets better. Honest

[airdave]

I build almost everything with 65lb cardstock
...occasionally 110lb for large flat parts, or anything I want a little more rigidity.
And I never consider anything lighter than 65lb when designing parts.

I don't use 3D design software or any "unfolding" apps.

My design work involves a lot of math.
I have two calculators at the ready all the time.

Anyway...I always involve a tolerance for card thickness when designing
any parts that fit together, contact or interfere with each other.
I calculate .3mm for a single layer, or .5mm for a double.

I also allow tolerance for inside and outside diameters.

I wouldn't design that part (in your diagram) with four parts...
but, yes, I would calculate the dimensional increases caused by the card thickness
and adjust the design as necessary.

[mbauer]

Great questions! Yes, best to do the math to correct for paper thickness, and any bends and radius need compensated for as well. Used to have an engineers radius / length chart showing how much metal thickness will bend and grow length wise around all radii/bends.

Found the radius to be a problem area when doing airfoils with ribs under them.

Another issue that can happen: If you use to much glue, the paper will get wet and can stretch just enough to make your day go bad...

Like Vermin_King says, "it will get better".

Mike

BTW-Just did a quick search found this link and calculator for sheet metal bends. http://sheetmetal.me/bend-allowance/ has downloadable PDF charts.

[airdave]

wow...thats too much math!!

[rickstef]

I ran into this exact problem when I was building a StarCraft building from a Korean designer.

The thickness became a problem when the final pieces were attached and the whole model became unstable, and needed too many fixes, so I tossed the model because it frustrated me.

first model ever that I tossed because it upset me

Rick

[spaceagent-9]

well, I cnat help you much on what paper thickness to use, but yes, you are right about this, and I have heard that some mix paper dust with glue to fill in the gaps.

[elliott]

It is frustrating indeed. There's nothing to indicate paper/card thickness that should be used to build a model in most cases, Dave being a notable exception. And what really steams my cookies is to get to the final assembly, like Rick, only to realize, at the last moment that the designer forgot or didn't care enough to account for material thickness. Toss another model into the burn pile.

Yes I do appreciate all those who go through the incredibly difficult process of designing a paper model. You guys (and gal, Cami) are heroes in my book. Just please, take an extra moment and account for material thickness in your designs. A grateful paper world will thank you, at least I know I will.

[Plumdragon]

Some excellent and well considered replies, thank you. I do hope the info filters down to the young (or otherwise ;-) designers who are maybe just starting out and perhaps regard software as the 'catch-all' answer to their designing needs. Paper is a very 'hands-on' medium, after all.
I design nearly all my models using 80gsm cardstock, and my simple method of allowing for folds is to take off 1 line thickness for each fold/tab (that's 2pixels wide) That's 'cos I do everything on PS7 and it's the only way I know how! After a decade of designing though, it's kind of automatic.... and there's an awful lot of trial and error (some refer to this as 'beta' building!) because my geometry is still rusty :-)
Plumdragon

[kcorbin]

I use calipers to measure the thickness of paper then input that thickness for the material I use in my 3D CAD design program. By undersizing the actual thickness a couple of thousandths of an inch It gives me enough flexibility in the design to allow for bend radius when I choose to do manual unfolding. However the program also has a sheet metal function with automatic unfolding which I sometimes use if I am working with shapes that will be curved such as cones, cylinders or variable lofted shapes. I use the actual measured thickness to create a custom metal I can then select to use in the model.

If I were to begin selling downloadable paper models I would state the weight plus the range of measured thickness of material it was designed for and let the customers know they would need to change thickness according to the percentage of change of scale.