Datum Shift ? 1

[par1]

I have one dwg which has 3 holes on the plate.
In a dwg, they defined a

datum point A1, A2,A3 as the centre point of the hole.

datum B is defined as centre plane of one of the hole
In a feature controlled frame for datum B

block 1 has 0.5 positional tolerance with 0.5 MMC
block 2 has mentioned datum A

Datum C defined as centre plane of one of the hole & there is attached Featured controlled frame
blcok 1 (In feature control frame) has 0.5 MMC of positional tolerance
In block 2 there is only mentioned datum A
block 3 has datume B with MMC condition

Now my question is

1, There is nothing like datum A defined but only the datum points A1, A2, A3 so how could i define the datum B& datum C using reference from datum A?

2. While defining the datum C ; what does the mean of block 3 in feature controlled frame " datume B with MMC condition" ( I think it is a datum shift - but not sure about the understanding or how to calculate tolerance stack)

could you explain me in detail.

Thank you very much in advance,

 

[drawoh]

ewh,

The datum features do not have to be orthogonal to the primary datum plane. The holes have centres which pass through the datum plane, and define the datum points.

If datums_B and_C are specified at MMC, a round and a diamond pin respectively should work fine.

There is one problem here that I did not think of before. My model works fine if the injection molded plastic has the holes cast in place. If the holes have to be machined afterwards, then there is no way to inspect the raw casting.

Ringman,

I cannot see how tooling balls would locate a part reliably. If the holes are drilled, someone takes a larger drill or one of these dental tools, and they remove the burrs and sharp edges as per the note on almost all fabrication drawings. It is difficult to predict the resulting chamfer or radius. This affects how the part sits on a tooling ball. A tooling ball of a given diameter works if it sits in a countersink with a controlled diameter and angle. I cannot see an accurate contact with the edge of a drilled hole.

This is why I indicated that the datum points required a diameter. We have to understand how the fixture is going to make contact with the part.

JHG

 

[type26owner]

Well I can start making assumptions too just like you're not suppose to do. Guessing the three holes are mating and aligning features so they're probably the best choices of datums. Assuming this is a Free State Condition and some type of fasteners either fixed or floating go through these holes then likely this gets fastened to something else. Datum targets A1,A2 &A3 should have had the mating contacting surfaces described as some diameter of a circle, square or whatever the shape which represents the contact are of that something else. Since the pattern of holes are already been brought into play together directly to describe the primary datum then logically it's confusing to suddenly treat them as not being a pattern later on. Datum B should be all three holes as a pattern period. Datum planes B & C are then defined from the centroid of those holes. Deriving the dimensions from that centiod by way of a height gage or CMM is painful but with a functional guage it's not. They tried to skirt around the pattern of holes as a datum centroid complexity issue by the usual method but made a confusing mess of it instead.

 

[ringman]

Let's imagine three marbles in space. Each is a specified distance from a given plane. OK far? The marbles are A1,A2, and A3. The plane they are related to is A. OK so far?

The perpendicular from one of these is labelled B and aother is labelled C. Now we have a plane and two lines from which we are able to derive planes to make measurements. If the part is restrained in a similar manner we could likewise perform inspection on it.

with mmc being appllied to the line of B, a shift as suggested would be permitted.

 

[drawoh]

type26owner,

Most of the figures in ASME Y14.5M-1994 show parts located by three surfaces. The primary datum is the plane located by three points of the first datum. The secondary plane is perpendicular to the first, and is located by two points of the secondary surface. The tertiary plane is perpendicular to the first two planes, and is located by one point of the third datum.

Our case here is less common. The primary plane is located by datum targets_A1, A2 and_A3. The secondary plane is perpendicular to the primary plane, and is located by the centre points where datums_B and_C intersect the primary plane. The tertiary plane is perpendicular to the primary and secondary planes, and is located by the centre point where datum_B, not_C, intersects the primary plane.

Labeling the planes A, B and_C is an awful mess, so I refrained from doing it.

These planes are a theoretical concept, especially if the part's features are orthogonal to one another. We can jig the part and do measurements.

I do not see how you can use a hole pattern as a datum. Each hole is a separate feature. The whole point of a diamond pin is that the distance between hole_B and hole_C is not known for certain. You will need a positional tolerance to define where the holes ought to be.

Ringman,

How are your marbles going to make contact with the part?

JHG

 

[type26owner]

The hole pattern is simply represented by the virtual condition boundarys which are normally just pins on the functional gauge. My way has simple datum target landings (each one must contact the part at least at one point) and pins which are expanding if the holes are at RFS and if at MMC then go/no-go type. Points projected into the intersection of a hole centerline and the surface it penetrates is imaginary only. The theory seems plausible but they are not practical to use without more instructions. Appying marbles of any kind there may or may not meet the criteria to restrain the part okay. Your guess is as good as mine but if there was not a specifc mention of datum target spheres at a certain diameter then it's arbitrary and therefore poorly executed. I'm confused.

 

[ringman]

The marbles example was only for a visualization of what might be achieved by using the tooling balls.