Doubt in deriving datum

[Vimalmechs]

Hi PFA.
Is it a right approach to derive DATUM C from TWO HOLES (2X) ?
Regards
Vimal.

 

[greenimi]

If it *IS* functional I would say *IS* the right approach.

 

[Vimalmechs]

hI greenimi,As per The FCF attached with 4X DIA.281 holes,

1) primary "planar datum A" is from three points.
2) Secondary planar datum is from axis.
3*) Tertiary planar datum (Datum C) from three axis. (Datum B and other 2X dia .XXX).

Is it possible to derive a plane from three axis practically as mentoned in 3*) ?.

 

[3DDave]

The primary method of determining compliance is the surface method. (Preparing to hear objections to using hard fixtures)

In this method the part is checked against ideal surfaces representing A, B, and the two holes C. These surfaces will take into account the variation allowed. I cannot be more precise as there is no tolerance or geometric characteristic associated with B and no tolerance associated with C.

One means of creating these surfaces for checking the part is to create a fixture with a flat surface to mate with A, a pin with the same size as the virtual size of B and a pin of virtual size for each hole in C. It will also have pins that match the four holes, also of virtual size.

The first part check is to make certain that none of the holes is larger or smaller than allowed, then placing the part on the fixture will allow the pins at B and C to limit the part shift and orientation relative to the A plane. If the part can be shifted to allow the four other holes to also fit over the pins the part is good.

It isn't required to determine the locations of the axes of any of the holes to use such an inspection fixture.

You can work through the math required to simulate the same thing mathematically, where the axes are allowed to shift by the amount the datum features exceed their virtual sizes, but it is complicated as there are an infinite number of solutions that might allow the part to meet the requirements and is even more complicated it the datum features are not perfectly shaped. I think the minimum representation is an X and Y offset and a Theta offset, so that's three equations for 7 holes, or 21 inequalities ( 0 < calculated value < tolerance limit) involving trigonometric operators, so not linear equations.

The only time the axis method is of greater value is if the virtual size of a feature is zero or negative; for example, a hole that is .250 in diameter with a positional tolerance equal to or larger than .250. While it is possible to create a fixture with moving parts to simulate this, using mathematics to validate the part is a useful option.

 

[greenimi]

3DDave,

Are the datum feature simulators for “C” diamond shape or regular pins (cylinders)?

Or you might say, the OP datum structure might need translation modifier (per Y14.5-2009)on datum feature C and THEN the regular cylindrical pins for C can be used. What are your thoughts?

 

[3DDave]

Regular round pins as there is no refinement on their locations or orientations and they have the same basis for datums as the holes that depend on them: [A|B(m)|...

 

[Vimalmechs]

Greenimi and Dave, thank u for the detail explanation.

 

[greenimi]

I didn't do anything 3D Dave did!!!
He is the one you should be thankful to.

 

[axym]

Hi All,

Dave's explanation is quite complete and correct. I can't really add anything.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca