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Hey all,

Just wanted to see if anyone had some tips or tools they could recommend for measuring the curvature of something like a ping pong ball or the length around a cylinder. Unfortunately math was not my strongest subject so i'm hoping there is some awesome tool out there that could help me out with this.

Thanks!

Erm what do you mean by curvature? The actual diameter, or the curve expressed as a fraction of the radius?

If you're looking for circumference, it's C=πd (pi times the diameter).

Math? Pfft. Use a bit of string. Wrap the string around the cylinder, then lay the string against your measuring tape. Does that help?

or wrap with masking tape & use a pen to mark the start & end point, unwrap & measure.

Thanks guys,

For clarification, I'm trying to measure the actual curve rather than the length. I guess that's the circumference?

Using the ping pong ball as an example, I would like to cut a a piece of plasticard to fit perfectly against the curvature of the ball. It seems to get even more complicated when I want to attach it anywhere outside of the circumference, like angled or offset from a different part of the ball. since it's not flat, I can't really trace the curve how I want to cut it, so it's really just guess work at this point.

ugh, I know I am not explaining this well. thanks for bearing with me.

*edit* Ok I looked it up on google and I think i'm looking for the "Arc" but everything seems to indicate some mathematical equation / maybe a compass?

Can't you just place it on something, draw a circle around it, and use that as a template to cut out whatever you need?

maybe a picture of what you're trying to fit the plasticard to would help.

and an idea of what the shape you're trying to achieve on it would also be useful I should think (are you making a wedge/triangular shape? etc)

*edit*
also, alternatively, you could possibly just use something like Greenstuff, rolled out to desired thickness, form it to the surface you want to match, clean edges to shape you want and let harden.

To measure a the circumference or some part thereof of a curved object, there's no need to use math. Just use a fabric tape measure. You can get them at a dollar store, or free at Ikea.

If you're trying to measure the curvature (that is, how much a surface curves, not the distance along a curving surface) it really depends what it is and why you're trying to measure it.

Curvature isn't the easiest thing to measure for irregular shapes. But if it's just a circle/sphere, then you just measure the radius and use a compass to recreate a circle of that radius.

If it's a more irregular shape, for a 1 off it's probably best to trial and error it, cut the shape you think it is and keep trimming it back until it fits perfectly.

If you need to do it multiple times with multiple shapes, there's tools known as "contour gauges" that allow you to measure the curves of an object and then reproduce them.

https://www.google.com.au/search?q=contour+gauge&oq=contour+gauge&aqs=chrome.0.69i59j0l5.1578j0j4&sourceid=chrome&ie=UTF-8

Have you thought about cutting a slot out of the ping pong ball and just shoving the plastcard in there?

Calipers are a measuring tool you can use to measure things you can’t lay a tape over. That will give you the diameter of your ball.

Compasses* will let you draw circles and arcs. You should be able to take one, and set the size to the radius (half the diameter) and then draw on the plastcard. Assuming you measure/draw/cut right, they should be the same arc.

* The ones that look like a “V” with a point on one leg and a pencil on the other. Not the one that points north.

Sometimes I measure a curved to get X and Y distances to figure out how large to make a custom mask, for example, for airbrushing, though almost always I have make the mask a couple of times in different sizes for curved surfaces to get it just right (then again, I'm picky).

Other times, I measure an area to get an idea of how a decal would fit, in a more scientific way than winging it.

Either way, it's worth noting that techniques things like basic geometry are not particularly helpful on a lot of objects, because they aren't perfect spheres or ellipsoids, or slices thereof, but rather sculpted parametric curves. A good example is a shoulderpad -- describing it mathematically would entail describing the curvature like it is described in the CAD, and then you're looking at things like cubic Beziers, and making use of those often requires differential and integral calculus. That's kind of crazy if all you want to see is how big a decal would fit

Thanks a lot guys. These are all super helpful suggestions. Now for the practical application!

Hey all - Figured i'd use this same post on an updated question with pictures this time! As stated before, Math was not my strong point. I've tried googling how to make these measurements but I don't think I'm using the correct verbiage.

In picture below, i'm trying to figure out how I would cut a strip of plasticard to fit as perfect as possible between each outline. I know it could not be a simple straight rectangle as the curves would throw off the way it fits. so how would one determine the shape of the cut to wrap around a frame like this? The goal is to try to create a frame for a ship hull and be able to cover it in panels after the fact. A relatively easy task up until the hull starts to take on more complex shapes. I don't have exact measurements so we could run based on hypothetical data if anyone knows how to calculate it.

Thanks in advance!

-Nova

The easiest way to map a curved surface without using maths is to cut a piece of paper to shape rather than plasticard, then replicate in plasticard. The problem with curves is that if you have curves in more than a single direction you will not be able to map a flat sheet to it; you will have to make a number of cuts within the sheet and then conform it to the shape.

For curves such as the one above, the "flat" shape will be some form of "U". You could nominally work out the shape but to be honest it would be easier to use a cheap paper template and cut it to fit.

Alteratively, if you are trying to, for example, cut a curve on the edge of a plasticard sheet to mount the plasticard perpendicular to the curve surface (ie a wing on a curved jet body), use a flexible ruler.

 Novasetri wrote:
Hey all - Figured i'd use this same post on an updated question with pictures this time! As stated before, Math was not my strong point. I've tried googling how to make these measurements but I don't think I'm using the correct verbiage.

In picture below, i'm trying to figure out how I would cut a strip of plasticard to fit as perfect as possible between each outline. I know it could not be a simple straight rectangle as the curves would throw off the way it fits. so how would one determine the shape of the cut to wrap around a frame like this? The goal is to try to create a frame for a ship hull and be able to cover it in panels after the fact. A relatively easy task up until the hull starts to take on more complex shapes. I don't have exact measurements so we could run based on hypothetical data if anyone knows how to calculate it.

Thanks in advance

I want to clarify a few things because it's not quite clear to me. In your diagram (or at least from what I make sense of it) you have the framing running on the horizontal plane, and would theoretically put the panels vertical to that frame? Is that correct?

If it is, you are literally boat building.

Typically boats are built with the frames and panels opposite to your diagram; your framing should run in the vertical plane and your panels horizontal; it's done this way because its typically easier to build namely because you don't have to worry so much about curves because your panels lie essentially in a straight line.

Why not use a contour gauge and save the aspirin?

https://scontent-iad3-1.xx.fbcdn.net/t31.0-8/13680393_10206652646783577_8944992089931931490_o.jpg

If I'm correctly understanding what you're trying to do (make a frame the same way as you would for a boat or aircraft and then skin it with panels) your best best would be to use paper templates to get the shapes you need for the panels and then transfer those to plasticard.

Be aware, if your vehicle changes radius linearly and only in 1 direction (like a cone) then you can skin it with flat sheets bent to shape. BUT, if the radius changes in 2 directions or non-linearly (which is the case for basically every boat/aircraft in real life) flat sheets won't want to fit over the armature without some significant forming.

It's a difficult process. If it wasn't difficult, scale aircraft manufacturers like Airfix and Revell would be out of a job because people would just scratch build aircraft from original plans

What are you actually trying to make? If it's very large it might be better to carve the shape from polystyrene or something like that and then skin it with fibreglass. Or you could sculpt it from clay or plaster and then create a plaster mould to pour resin in to to make the final piece.

Thanks for taking the time to respond everyone. Looks like i'll be taking the route of mapping out the plating via the paper transfer method. The goal is to create an ork airship that acts as a drone host for a big mek. after some extensive research on ship plating, I have determined that the mathematical route is insanely complicated so I'm thinking i'll try to simplify the shapes and hopefully not lose out on the overall aesthetic.

SilverMK2 wrote:The easiest way to map a curved surface without using maths is to cut a piece of paper to shape rather than plasticard, then replicate in plasticard. The problem with curves is that if you have curves in more than a single direction you will not be able to map a flat sheet to it; you will have to make a number of cuts within the sheet and then conform it to the shape.

For curves such as the one above, the "flat" shape will be some form of "U". You could nominally work out the shape but to be honest it would be easier to use a cheap paper template and cut it to fit.

Alteratively, if you are trying to, for example, cut a curve on the edge of a plasticard sheet to mount the plasticard perpendicular to the curve surface (ie a wing on a curved jet body), use a flexible ruler.

Thanks Silver! I was hoping there would be an easy way to come up with that "U" shape but using paper makes sense. May even try it out with some painters tape and trace large sections at a time - or - reduce the complexity of the frame where I can to use simple rectangular cuts for plating.

Winter wrote:
I want to clarify a few things because it's not quite clear to me. In your diagram (or at least from what I make sense of it) you have the framing running on the horizontal plane, and would theoretically put the panels vertical to that frame? Is that correct?

If it is, you are literally boat building.

Typically boats are built with the frames and panels opposite to your diagram; your framing should run in the vertical plane and your panels horizontal; it's done this way because its typically easier to build namely because you don't have to worry so much about curves because your panels lie essentially in a straight line.

Thanks for the clarification! I was nott thinking much about the orientation, just more on the feasibility of plating something like this. I'm still in the early conceptual stages and not have determined what the actual hull of the airship will look like. Good thing you pointed that out before I got started

SlaveToDorkness wrote:Why not use a contour gauge and save the aspirin?

https://scontent-iad3-1.xx.fbcdn.net/t31.0-8/13680393_10206652646783577_8944992089931931490_o.jpg

umm this is awesome... I did not know such a thing existed! Not sure if it would help with finding the "U" shapes I would need on some of these more difficult angles but there could definitely be some practical application for a tool like this. Thanks man!

AllSeeingSkink wrote:If I'm correctly understanding what you're trying to do (make a frame the same way as you would for a boat or aircraft and then skin it with panels) your best best would be to use paper templates to get the shapes you need for the panels and then transfer those to plasticard.

Be aware, if your vehicle changes radius linearly and only in 1 direction (like a cone) then you can skin it with flat sheets bent to shape. BUT, if the radius changes in 2 directions or non-linearly (which is the case for basically every boat/aircraft in real life) flat sheets won't want to fit over the armature without some significant forming.

It's a difficult process. If it wasn't difficult, scale aircraft manufacturers like Airfix and Revell would be out of a job because people would just scratch build aircraft from original plans

What are you actually trying to make? If it's very large it might be better to carve the shape from polystyrene or something like that and then skin it with fibreglass. Or you could sculpt it from clay or plaster and then create a plaster mould to pour resin in to to make the final piece.

Thanks man, i'm actually trying to make an ork airship of sorts. Luckily this leaves some margin for error, however, I do want everything to look cohesive when all is assembled. Unfortunately I don't think my skill / Cash level is up for the fiber glass or moulding techniques. Hoping to do the whole thing in plasticard but I think I could put something together with simplified forms like a half cylinder.

hmm...could you get some foil(tin/aluminum/aluminium, whatever it's called where you are), like an inch-wide strip, and roll it up into a long, thin straw...(oh, or some kind of wire could work too actually )...that might be thin and flexible enough to be moulded around the contours, but sturdy enough to hold the shape, and be used to trace the contour onto some plasticard?

...not sure if I sound like a gibbering idiot or not there, or if I've even understood the problem...it's late here.

http://www.harborfreight.com/6-inch-contour-gauge-907.html