How Do I Dimension This?

[SteveFrey]

How would I dimension this elliptical or variable arc?  I can't seem to dimension it in a sketch, measure it as a part or dimension in an .IDW.  I understand it changes as it gets towards the top or the arch but don't understand how to dimension it.  Thanks.

[blandb]

I have never done anything like that, but I'd think as long as you have reference marks as you show, then they should be able to get it close...Is it going to be burned out? How accurate does the curve need to be?

[SteveFrey]

I was trying to find the radius as it changes.  I need to dimension it for a fabricator.  Maybe a better question is how would you dimension the attached?  Keep in mind this was created by projection the geometry of the bent shape onto a new sketch hence the missing dimensions in the sketch.

[kacper.suchomski]

Hi

This is not a arc. This is a spline.

You can create a workpoints and set visiblity workpoints on drawing.

[SteveFrey]

What do you mean by "set visibility workpoints" on the drawing?  How do I do that?

[kacper.suchomski]

Right click on workpoint in browser in drawing.

[SteveFrey]

I set the work points in the sketch and turned them on in the drawing.  How do I dimension the spline?  

[kacper.suchomski]

No. A sketch point is not a workpoint. 

[SteveFrey]

so these aren't wok points?  See my SK

[kacper.suchomski]

Yes

[SteveFrey]

Thanks but I still don't know how to dimension it.  Any idea?

[kacper.suchomski]

It's not a matter of idea. You need to understand the nature and technology of the product. Based on this information, you need to decide how to describe the item.

There is no guessing or fortune telling here. Step away from the computer and learn about the product/process.

[SteveFrey]

I'm trying to describe the shape to a fabricator.  They need to enter it manually into their system.  Pretty simple concept and.  Here's a better question:  How would you dimension it?  You seem to know this stuff very well.

[kacper.suchomski]

I don't know how, because I don't know the process and the cooperating elements. The key is technological. You will find this information in every dimensioning manual - the description should be adapted to the characteristics of the item, not to the idea of an Internet user.

[Pineapple2024]

Most CNC have no idea of spline.

They only know arcs.

Most fabricator will print it 1:1 and cut, grind it as close as possible.

Another way is export to dxf with short segments.  Estimating the shape with short straight lines.

[pcrawley · Accepted solution]

That is a tough one to answer because there isn't a simple dimension (linear/angular/radial) that will fully describe a curve that bends from convex to concave and back again.  (The ends of your curve bend outwards slightly.)

If your fabricator has to "enter it manually" then they are probably asking for the center-point and the radius (assuming it was a simple arc).  Since it's not simple, you may have to dimension to each of those points on the curve - something like this:

You asked the question "How would you dimension it?"  My answer would be as above - but more likely, I would provide a DXF profile.  It would all depend on how technical the fabricator really is.  You may find they're just being a bit lazy because they'll cut it with a CNC profile/glass cutter anyway - for which they will have to create the profile digitally - and you could hand it to them as a DXF.

[WP_Australind]

Back in the day, we would have done something like this, then used French Curves, or even  a thing that I've forgotten the name for, but it was a flexible lead rule covered in rubber that you could bend into the spline shape.

The more sketch lines, the more accurate it would be.

But not knowing the fabrication / techniques of your industry, this may not be acceptable.

[SteveFrey]

That makes sense.  Thank you!

[SteveFrey]

Makes sense to me.  I wasn't sure if the spline could be defined but this should be fine.  Thanks!