Hole locating dimension

[Madhu454]

Hi,

I have a doubt regarding locating the hole from an inclined surface. This may be very elementary question but need to have a better clarity on this.

Please see the attached drawing.
1) Is the hole locating dimension is wrong.? Genarally we dimension the holes from the specified datums.
2) Since the origin of DRF starts at the end of datum C, Do we need to dimension it as shown in the drawing in Red font.

Question : 1)Does the First one is illegal specification ?
2)Does DRF is always a three mutually perpendicular planes? (Evan for inclined parts)

Madhusudhan Veerappa
Mechanical Engineer

 

[flash3780]

The hole locating dimension is not incorrect, assuming the outer shape is fully defined...

The datums are still three perpendicular planes. It's important to differentiate the datum planes from the datum features. The datum feature for "C" is not perpendicular to "B", yet the datum plane is. There is an illustration of this in ASME Y14.5-1994 (though I don't have my copy nearby to tell you what section it's in).

The dimension shown in red would overspecify the location of the hole (assuming that the outer shape is fully defined elsewhere).

It is, in fact, best not to dimension from a corner if it can be avoided. I'd go with your initial dimensioning scheme and consider adding an MMC modifier to the hole position tolerance. Often for things like bolt holes, zero positional tolerance at MMC is the best bet.

 

[SeasonLee]

A datum reference frame may need to be established from datum features that are not mutually perpendicular to each other. All the angles between the datum features must be indicated as basic. True contacting planes are orientated and established at these angles relative to a datum reference frame and all measurements originate from the datum reference frame.

Pls ref to Y14.5M-1994 Section 4.4.1.1 Fig. 4-4 or Y 14.5-2009 Fig. 4-7 for more details, attached is the snap shot from Fig. 4-4 1994 standard.

SeasonLee

 

[pmarc]

Madhusudhan,
1. I think figure presented by Season Lee shows the best approach for your case. I would suggest putting horizontal basic dimension from intersection of datums B and C (not from the 'end' of datum C) and that would pretty much do the thing. DRF origin is exactly at this intersection. Make also sure that you have an angular relationship of datum feature C to A and B defined (angularity callout).
I would not however say your initial approach (inclined basic 30 dimension) is incorrect - it would also work as long as basic angle between B & C was defined.

2. As other already said, although datum features are not perpendicular to each other, DRF always contains 3 mutually perpendicular planes. You can think of DRF as of Cartesian coordinate system for particular measurements. Datum planes of DRF correspond with coordinate system axes. To obtain any measurement data, direction of every axis (x, y, z) has to be perpendicular to each other.