Dimensioning Lofts and splines in a 2D drawing 4

[Triniprince]

Hello,

Long time reader and reference searcher, first time thread poster. I am currently in the process of placing dimensions on a 2d drawing of a CAD designed enclosure that has lofted surfaces. (providing a 3d model as a deliverable is not an option...) So far I have been taking cues from the aeronautical and maritime documentation method of using section cuts with X-Y / Y-Z dimension points from various views to give an origination of individual radii then superimpose a series of arcs that best fit the spline of the respected section cut.

(see link for example:

http://aviationarchives.blogspot.com/2015/04/xf-85-loft-sheet-and-3-view-drawing.html)

My question is if anyone knows of a simpler way or better yet a spec that I can reference to accommodate this effort. My intuition tells me that there must be a way to convey this information due to there being lofted surfaces long before CAD, and that there had to have been a way to document and/or technical depictions of such features for products in the past. If it helps this is a part that is to be machined by a hand milling machine from the technical 2d drawing information.

Thanks.

 

[KENAT]

Even on old Linen & Vellum drawings I've always seen the approach of taking multiple sections along the length of the item and dimension each cross section.

I think I've seen 2 ways of indicating the blend of section to section.

One would be side and top views giving the dimensions of the outside profile.

The other was a note saying something like 'smooth blend' from section to section.

This really is one of the areas where going to Model based definition is really useful - shame it's not really an option for you.

I once had to create a 3D CAD model from a drawing like this - it was a real pain.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Triniprince]

@KENAT

Thank you for a prompt response. So I am going down the correct path using this as a method of feature identification. Thank you for confirming. Also, any idea if there is a standard somewhere out that that can be used as a reference or is this something that I will need to use the "industry / fabricator / as needed qualification statement" to justify doing the 2D drawing in this manner?

 

[KENAT]

Done properly it will comply with ASME Y14.100 series - but I'm not familiar with a standard in that series that actually shows how to do it.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[MintJulep]

Article of interest, if not help.

http://www.leithshipyards.com/mould-loft/lofting.html

When I built my kayak I was provided with a "table of offsets" for each station. Simply X-Y cordinate pairs.

There is nothing wrong with what you are doing, relative to arcs. But how is the guy in front of the milling machine going to be able to use that information to turn knobs to produce the desired curve?

"Fairing" a surface is not an inherently repeatable process.

 

[Triniprince]

@MintJulep, Thanks for the reference website. I actually found that site as well in my search for preceding situations that fit our needs and it was a great resource. And to answer your other question....honestly that was a concern of mine as well hence the suggestion to just use the CAD source data to have the part CNCd....which is how we do it "in house" but beyond my professional suggestion, the powers at be chose this "exciting" route...

 

[MintJulep]

Well, carry on then I guess.

Draw your sections. Plot them at full scale on stiff paper and cut them out to use as templates.

Give it all to the machinest and have him start cutting until everything "looks about right".

 

[BUGGAR]

If it will be hand milled, then giving the operator the X,Y,Z's like you mention should fit with the way it will be machined.
What I've found useful for doing stuff like this (in 2d): Scripts in ACAD can be used to easily plot curves of any equation, even fictitious ones. You can also draw a fancy curve and back engineer the coordinates to replicate and modify as desired (I'm working on shock absorber energy curves right now).
Can a 3d graph be printed from your coordinates in Excel? I haven't tried this yet.

 

[BUGGAR]

I forgot to mention the way they carved the statues on Mt. Rushmore.

 

[berkshire]

One of the methods used on the shop floor to machine spline curves, was to take a grid of squares of a known dimension, and lay it over the spline curve. then using radius gauges, determine the nearest true arc in the square, if a true rad could not be determined, a smaller set of squares was used, this was repeated until the spline curve was reduced to a series of true arcs.
This information enabled the milling machine operator/ machinist to set his radiuses for a rotary table to cut the required curves. with a minimum amount of blending on the finished part.
B.E.

You are judged not by what you know, but by what you can do.

 

[Triniprince]

Hello all,

Just wanted to share that I found the ASME Y14.5-2009 case of "Irregular Outlines" in section 1.8.7 that references Fig. 1-33 to Fig 1.35. Using this Standard in conjunction with ASME Y14.3-2003 Section 3.2.4 Fig. 29 gives the information needed to capture these types of features. Just wanted to leave the reference here for anyone that comes into this case.