Inspection Datum Points 2

[ewh]

It has come to my attention that our tooling is being inspected using the same three points to establish the primary, secondary and tertiary datums. I checked Y14.5-1994, and it does not specify that datum points need to be unique.
I have not seen this on a drawing before, and was wondering if it is indeed allowed per the standard, or just our inspection departments own interpretation. We are at the point of revisiting our company standards, and this is becoming a sticking point. Thanks in advance!

Believe it if you need it or leave it if you dare. - [small]Robert Hunter[/small]

 

[MadMango]

Am I missing the point or something? If datum points are defined, they should be used.

"Art without engineering is dreaming; Engineering without art is calculating."

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[ewh]

It's just that I am used to seeing six unique points to define the three datums. The question is, is it acceptable (per Y14.5) to use three points to define the three datums?

Believe it if you need it or leave it if you dare. - [small]Robert Hunter[/small]

 

[fcsuper]

I'm not seeing the issue either from the description. Can you go into more detail about your concern?



Matt Lorono
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[dingy2]

Three (3) points define a plane such as a primary datum. The secondary is defined by 2 points (defferent point from the primary) while the tertiary is 1 point. One cannot define all three (3) datums with the same 3 point set up.

My background is in quality and measuring and I remember so many years ago measuring or setting up a V8 block at GM. I used exactly the same set up as noted above.

Hope this helps.

Dave D.

http://www.qmsi.ca

 

[ewh]

Thanks, Dave. That was my understanding also. However, inspection claims that, as long as all of the points do not lie in a line, they can use all three to establish datum A, two of them to establish datum B (perpendicular to A) and the third for datum C (perpendicular to A and B). I can't find a good argument against this, and am hoping for some logical reasoning as to why it is wrong (or is actually allowable).

Believe it if you need it or leave it if you dare. - [small]Robert Hunter[/small]

 

[dingy2]

Please forward a schematic of the datum points.

Dave D.

http://www.qmsi.ca

 

[ewh]

Picture a square block with two tooling balls on the top and one on the front face. These are the three points being used to establish the datum structure.

Believe it if you need it or leave it if you dare. - [small]Robert Hunter[/small]

 

[dingy2]

The 2 tooling balls on the top face will achieve an axis or straight line but not a plane. The 3rd ball could have been used or anti-rotation of the axis but you do not have an axis but a plane.

There is absolutely nothing here for the tertiary such as another ball at 1 end of the block.

I can only suggest that if you are following ASME Y14.5M-94. section 4.6.4 on page 73, it states:

"A primary datum plane is established by at least three target points or areas not on a straight line. See fig. 4-31. A secondary datum plane is usually established by two targets. A tertiary datum plane is usually established by one target. A combination of target points, lines, and areas may be used."

Good Luck!

Dave D.

http://www.qmsi.ca

 

[ewh]

Again, thanks for your input, Dave.
The argument is that they ARE using three points for datum A, two for B and one for C. They just happen to be non-unique points, and the standard does not specify that they are to be unique.
Perhaps a square block was a poor example. Picture a mold shaped like a potatoe chip with three tooling balls placed around the edges. The three are not in a line. The three together establish the primary plane. Two of them establish the secondary, perpendicular to the primary, and one establishes the tertiary, perpendicular to the other two datum planes.
I apologize for repeating myself, but I am trying to ensure that I am expressing the situation accurately.

Believe it if you need it or leave it if you dare. - [small]Robert Hunter[/small]

 

[HGMorgan]

A tool reference system can consist of 3 (min) or more tool balls.
An alignment (datums) can easily be created via 3 tools balls. It is an established & preferred process.
A 3D contour mold is a good example: with 3 T/B's I can align, then CMS (Laser, CMM, etc.) the tool surface, best fit if need be and re-identify the co-ordinates of the T/B's.

Datum Target points on a production part, forging or casting are different. These almost allways have to be 6 separate points that are used to establish datums.

Y14.5 is more for production parts.

 

[axym]

The situation that ewh is describing is not the same thing as datum target points. This is probably where the confusion is coming from.

Datum targets typically involve specifying a set of 6 points on the part, in a 3-2-1 configuration as dingy2 described. The fixture would have 6 tooling balls (or 6 round-tipped pins). When the part is put into the fixture, it makes contact at the 6 specified points.

The use of 3 tooling balls to define a datum reference frame is different. The tooling balls are not used to hold the part - they're used as datum features to constrain the six degrees of freedom of a coordinate system. I like to look at it this way:

The center point of the primary tooling ball sets the origin (constrains 3 translational DOF's)
The secondary tooling ball sets one axis direction (constrains 2 rotational DOF's)
The tertiary tooling ball sets clocking (constrains 1 rotational DOF)

Another way of looking at it is that the 3 tooling balls define a leveling plane, two of them define an axis direction line and one of them defines an origin point.

Either way, it works. It's just a different concept than datum targets.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[ewh]

Thanks very much, guys! I now feel confident that our proceedures are good. Another new thing I learned today!



Believe it if you need it or leave it if you dare. - [small]Robert Hunter[/small]

 

[powerhound]

Evan,

I'm trying to wrap my brain around what you're saying. Although I think I understand what you are explaining, I don't think it lines up with what the standard says or at least tries to convey. Your explanation of the restriction of 6 degrees of freedom makes sense but only within the context of measuring the fixture itself. Any parts that are checked or machined on the fixture will be touching the tooling balls at a tangent point, not wrapped around the tooling ball, and will not be restricted in all degrees of freedom.

ewh,
If you can physically set a part on this fixture, touch all the datums, and have all the degrees of freedom restricted, then it will work fine but based on the "potato chip" visual, I don't see how it's possible. It seems that somehow there should be some way to orient the "chip" and keep it from sliding right off the balls or from falling through them. I think something is missing here.


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[HGMorgan]

Hi Powerhound,
The T/B's are the tool reference system only, and don't have anything to do with the part. The part will not index to nor use the T/B's in any manner.

A part indexing system (such as rest buttons to 6 Datum Target Points) would be a separate feature of the tool.

The standard in discussion applies to parts, not tools.

Tool Design & Fabrication groups have their own standards for tool build.
Those standards are created by the company making or procurring the tools.
All Tool Design/Fabrication standards will state that the tool must produce product to meet/exceed Engineering requirements.
The Tool Reference System (3 T/B's is just one type of that) has been used for decades, and is a good sound business process.

 

[ewh]

As HGMorgan infers, this is for the tool only. Sorry that I wasn't more clear about that. We need to establish the reference systen for incoming (and periodic) tool inspection, as well as for any future modifications of the tool.
Our production parts generally use the six point system per the standard, and I was trying to reconcile the two methods in my head. Thanks again for the input!

Believe it if you need it or leave it if you dare. - [small]Robert Hunter[/small]