Hi MIchael,
In another forum we're discussing unfolding a cone. In a tutorial I'm following I need to create an arc and control its length (the circumference length)

I was trying to figure out if MoI does this and if so, Which input field it is. Otherwise, it becomes a math problem.




The tute may be talking about a tool that MoI doesnt have yet.

Hi Burr - that's kind of an uncommon thing in CAD programs (or just drafting in general as far as I can remember anyway) to define an arc by its length rather than its angle.

So you'll need to use a formula to do it.

For the formula you're going to need to know the desired radius and length and then calculate what angle to use from those.

Let's see - the length of an arc goes like this:

Length = Angle_in_radians * Radius

So then we want to re-arrange that to be like this:

Angle_in_radians = Length / Radius

Then the input to MoI will be in degrees so you have to convert from radians to degrees:

Angle_in_degrees = (Length * 180) / (Radius * PI)

So you type that in for the angle value in MoI's Arc center command, replacing "Length" and "Radius" with your actual length and radius values. You can hit the Tab key to put keyboard focus in that Angle box at that stage of the arc command.

- Michael

Hi Burr, for the next v2 beta I've also added in a couple of helper functions for expressions to make it a bit shorter to do conversions from radians to degrees.

So in the next beta you'll be able to enter in: deg( length/radius ) for this one.

The new functions are deg() and rad() - deg() takes in a value in radians and converts it to degrees, and rad() takes in a value in degrees and converts to radians.

- Michael

Thanks Michael!

Could I see one short input line for reference?

FYI: It's comming up for me with regard to programming 4th axis CNC work. Some of the lessor packages (under 5K) only do whats called "wrapping" which requires manual unfolding in certain aspects. This will help me alot! As you know, I am new to math so when you explain "radians" I have more homework! (57.2958!)

I'v been looking at Lamina which is cool, but for some other specialized work.

I very much appreciate your time in answering these things for me that really have nothing to do with the development of MoI. I'll always try to give back somehow.

Burr

Hi Burr, no problem!

Did that make sense for how to use that formula inside of MoI?

Just in case here is a step-by-step for how to use it.

Say you want to create an arc of radius 5.5 that has a length of 8.1 units.

To begin with, start the Draw curve / Arc / Center command.

Pick the center point somewhere that you want to place the arc at.

For the start point of the arc, type in 5.5 <enter> to activate distance constraint. That will make the next point you pick be constrained to be 5.5 units away from that center point, that is how you set a specific radius value when drawing an arc with that particular arc tool.

Then click to place that start arc point.

Now the Arc command will be at the prompt that has an Angle: value you can enter.

This is where you enter that forumla - to enter it press the Tab key to put keyboard focus into that Angle box (if you enter numerals your focus will go there automatically but since we are entering an expression that starts with a parenthesis character it won't go there automatically so hit tab to do it).

Then you type in the formula: (8.1 * 180)/(5.5 * PI) <enter>

So note there that I used this formula: (Length * 180) / (Radius * PI) but replaced the Length and Radius with the specific values for length and radius that you want to create for this arc.

So those steps above will make an arc that has a length of 8.1 units.

- Michael

Hi Burr,

> Could I see one short input line for reference?

I think I wrote that last message before seeing this, but that one has step-by-step instructions, let me know if you get stuck on any step and I will show you an illustration for that one.

- Michael

That was it and it looks like that is exactly what I need to do my "arc by length" thing.

Thanks again.

I dont think the kids are going to understand when I tell them I'm a "wrapper". (dancing around the house)

Thanks again Michael!

I'm now going to go to the other forum and post like I'm some kind of genius or something. But if questioned, I'll have to say I have a "secret friend" that can answer all my questions. :) (and of course, Danny!)

Hi Burr, that's great that will work for you!

That new formula evaluation for v2 came in handy for this one...

- Michael

Gee thanks Burr, secret friends maybe, as for answering questions, some, not all.

Cheers