Tolerances of a 'Basic' Boxed dimension

[RobLN]

Please could you advise me here. My understanding is that a dimension in a rectangle box is theoretical and without tolerances (other than those defined in the drawing border)

I have, however been approached by another take on this. Please see the attached sketch.

I'm told that the 27mm dimension 'becomes' 27 +/- 0.1 because of the positional control on the hole... Is this correct? (NB Datum G is the left leader line of dimension 27)

In my sketch I've indicated how I read the tolerance for the dimension 27 i.e. +/- 0.23 per the default drawing border control.

Any advice is greatly appreciated here.

Solid Edge; I-Deas 7 to 12; NX4 & currently NX5.

 

[PeterStock]

First what standard are you using ASME Y14.5(year) or ISO? This makes some difference. If your drawings do not show what standard you are using there is no standard definition as to what the symbols mean.

Assuming you are using ASME Y14.5M(1994) the standard used the most in the US, then the default title block tolerance would apply to a dimension without a tolerance that is NOT basic. If a basid dimension is use then there MUST be a geometric control (such as the one you have shown) to state the tolerance. Ths symbol you have shown gives the tolerance realtive to direction dim 11 controls (I assume that this is a vertical dimension and is basic). The 27mm basic dimension is based on your drawing without a tolerance, it could be anywhere on that section of the part and still meet the print. I suggest you study the standard for more information.

Note this analysis assumes that dimension 11 gives a vertical distance from datum G. The feature needs to be dimensioned relative to the datum(s) with basic dims.

Picky checker point, remove the first space from the "2 x Ø..." and use a capital X, the standard shows more that one use for X, with the space it could be argued that you have a 2mm X Ø6.75mm slot as opposed to 2 Ø6.75mm holes. The standard does state that there should be no space between the number "2" and the "X".

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[TheTick]

Basic dimensions have no tolerance EVER. Tolerance is applied by GDT feature control frames.

 

[RobLN]

Thanks for your reply Peter. The drawing does indeed refer to ASME Y14.5M (2009). So, if I understand correctly, in this case the dimension 27 would indeed follow the instruction of the hole position (TOP 0.2 relative to G) and 'acquire' a tolerance of +/- 0.1

If it (the 27mm dim) were not in a box, then I take it the drawing border tolerances would then apply...

NB I thought that the direction of the tolerance was cylindrical i.e. all around not just vertical, I am understanding that correctly?

Solid Edge; I-Deas 7 to 12; NX4 & currently NX5.

 

[prdave00]

The feature control frame for the positional tolerance does not have a diameter symbol in front of the .2 tolerance so as its written it is not a cylindrical tolerance zone. You established a true position a horizontal distance of 27 from the vertical edge, but there is no basic dimension indicating the vertical position. Your drawing, whether intentionally or unintentionally, is too ambiguous.

If there was a diameter symbol in front of the .2 tolerance, as I think there should be along with another basic dimension, then you would have a cylindrical tolerance zone with a diameter of .2 and a height equal to the length of the feature. This tolerance zone would control the both location and attitude (i.e. tilt) of the simulated axis of the hole.

 

[PeterStock]

There is only one datum given. A true Datum Reference Frame needs 3 (or 2 for a part made on a lathe) to properly establish the tolerance zone. There can be uses for a hole with a none cylindrical tolerance zone, but there should be two controls, one for each direction. Also we do not know what the datum G is. If it is the plane the dimension 11 comes from, then the hole is free to rotate away from being perpendicular to the face of the tab it is on. If it is a plane parallel to the sheet, the hole must be perpendicular within .2mm (a diameter symbol should be given in the tolerance section) and the location is free to move anywhere on the tab, there is no control seen.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[prdave00]

The only exception I see to Peter's comment is that if datum G was another hole then only a primary datum would be required, but then the basic dimension to the edge would not be used to locate the hole. That's all.