Multiple radius call out 1

[rhmeng]

Hi,
I have a part that has several radii in it that basically makes this long multiple radius curve, some convex and some concave. I was wondering what the best way to dimension this is. Do you dimension to the center of each radius (or the circle that makes the radius) and dimension the radius, or do you dimension to the inflection points in the curve along with the radius dimension.

 

[KENAT]

It's probably the classic case of which ever better reflects function taking into account the type of tolerancing you're using.

If using surface profile & basic dims I'd probably go with center and radius - but I haven't really thought about i.

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[M4C7]

For what its worth, Y14.5-1994 states in para 1.8.6

"A curved outline composed of two or more arcs is dimensioned by giving the radii of all arcs and locating the necessary centers with coordinate dimensions. Other radii are located on the basis of their points of tangency. See Fig. l-30."

That supports the point of view given above, but as worded, I don't see the language of the standard being binding (not to mention the fact that you might not follow that particular standard)

 

[rhmeng]

MC47, thats exactly what I was looking for, thank you for the information. FYI, I did this, but when you dimension several radii centers its hard to tell which is for which, so then you add reference circles to the radii so you can see which is for which and it gets very messy. If this is the correct way to do it tho then thanks for the info. Probably easiest to just call out a note that says use CAD file for profile, since everything is done CNC nowadays. Thanks again.

 

[M4C7]

If you're supplying 3D data with the drawing, then calling out the model as basic should work.

If there is some value in fully specifying the profile in 2D, making sure that radii callouts originate from their respective dimensioned center-points and using enlarged detail views might go some way towards making the drawing clearer, but I can't say how useful that might be in your case.

 

[KENAT]

We've been doing more of the model based stuff, but our inspection is struggling with it and for them a conventional print is still very helpful. - for what it's worth.

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[rhmeng]

@KENAT, assuming you are using a scope, wouldnt it be easier to identify the inflection points since they are actual tangible locations, vs trying to find center of an radii in space? And with that said if you know how difficult it would be for someone programming a CNC to be able do it based on inflection points and radii vs centers, I would love to have some insight into that.

 

[M4C7]

If you're specifying a profile tolerance, I don't think the method of definition is too critical, as long as the true profile is defined unambiguously.

Inspection with a CMM looking at the specific inflection points seems to be of limited utility. Admittedly, I don't have a background in inspection techniques, but I feel you'd need to probe a lot more points to say with any degree of certainty that your part meets the criteria laid out in the drawing.

As far as manufacturing is concerned, if your CNC is set up using CAM software, the 3D model is probably the single most useful piece of information. My only experience is with a machine with a DRO like this

http://www.southwesternindustries.com/images/products/ELX_prog.jpeg

where you specify the X,Y coordinates that the tool moves between as well as the connecting radii to specify an outline, but I'm sure there are different ways to set it up

 

[tbuelna]

It depends on what application the curve profile is used for. For some applications using the basic digital curve definition plus a profile tolerance should be adequate. However, there are some situations like the profile of precision gear tooth flanks where the basic digital curve definition and a profile tolerance are further controlled by concavity/convexity/slope limits at points along the inspected curve profile.

 

[rhmeng]

ok thanks for all the help guys, i think that I am just going to have the shop reference the CAD model for the shape and add a profile tolerance as was mentioned.