Tolerance of Features located by a Centreline

[richg1]

Hi,

Hopefully a simple question, but what is the standard tolerance applied to features located on a part centreline.

To explain I have a symmetric part detailed with a centreline. Certain hole features are located on the centreline with vertical position dimensions from the top of the part. But my question relates to what is the interpreted horiztonal positional tolerance for these features. Drawing notes call for "all dimensions +/- 1mm unless otherwise specified".

(My assumption to date is the features are allowed to move off the centreline horizontally +/-1mm)

Thanks in advance..Rich

 

[KENAT]

What country are you in/standards are you working to?

Is the 'center line' dimensioned to a measurable item or is it just the 'theoretical' centerline?

Implying symmetry just by putting in a centerline isn't correct to most drawing standards if memory serves.

 

[richg1]

Hi Kenat,

Drawing is Australian and so I guess is effectively a theoretical centreline as there is no dimension to the centreline itself.

I will assume that if the centreline was dimensioned it, and the relevant features on it, would be dimensioned to the global +/-1mm tol. It's not a great drawing and I am interpreting it so I can carry out a 3d tolerance study of an assembly of which this is one of the critical parts...

Thanks...

 

[KENAT]

Hmm, not familiar with Australian standards I'm afraid, it doesn't sound like the drawing explicitly states what standards it's drawn too. Most posters here seem to be familiar with ASME specs.

I'll have to think about this one as it's late in the day and my brain quit working, plus it's past my bed time...

 

[drawoh]

richg1,

In the absense of a standard, I would assume that tolerances apply to dimensions. If there is no dimension, then there is no control over the feature. This applies to a feature located on a centreline. You are relying on the good nature of your fabricator.

ASME Y14.5M-1994 allows you to apply a positional tolerance that locates the part with reference to the datums you specify. I am sure the ISO standard does too.

What standard is preferred in Australia?

JHG

 

[KENAT]

So you (may) have several problems.

1. The definition of what is the centerline. Is it the midline of the nominal 'width' dimension? Is it the midline of the actual (measured) 'width' dimension? If you have several 'width' dimensions with implied symmetry and without a defined datum which defines the centerline? etc.

2. How this centerline can 'move' in relation to other features. If all features are dimensioned off the centerline this problem is minimized but if some features are dimensioned relative to the edge and others to the centerline you can quickly start increasing the tolerances.

3. "Implied symmetry" as drawoh says this is usually pretty much meaningless and you're relying on the good nature of the manufacturer.

For a tolerance analysis I'd probably assume some kind of qualified worst case stack up but it would have limited validity. It’s difficult to do tol analysis on an item that isn’t properly defined.

 

[richg1]

Hi All,

Thanks for feedback. Austalian Drawing Standard is AS1100.101-1992. From a quick reading of my historical info is fairly similar to the British Drawing Standards.

Cheers for the info...