Datum Target Areas

[ewh]

When detailing the drawings of lofted parts for a customer, I am accustomed to using datum target points to establish the GD&T framework. I have use datum target areas in the past on casting drawings, so am somewhat familiar with them.
We recently received some SCD drawing from our customer specifying datum target areas instead of points, with holes located in the center of these areas and controlled to the datums defined by the areas. Is this a legitimate use of target areas? How is such a part to be inspected? These are lofted parts, and no two areas are going to be similar.
TIA!

 

[dingy2]

This is a tough one especially with some of the terms that you used.

What is a lofted part?? What is a SCD drawing?

Drawings should specify datums on features that reflect the fit and function of the part. The primary datum is usually the mounting surface on a non-cylindrical part while the secondary and tertiary datums are usually holes that are used for assembly rather than surfaces that may not have any meaning.

Datum targets are not usually reflected on the drawing by the Designer but rather by Manufacturing Engineering & Quality on how the part is processed and/or inspected.

Datum targets can be points but they can also be round flats and it depends upon the processing of the part.

If your customer gave a large surface as a datum target for a primary datum, it is not practical but not illegal either.
There is nothing in the standard that states that that one MUST apply 3 points to set up primary surface but it is practical.

One Designer out of the Detroit area always placed one (1) datum target for a primary surface and when I was in their plant, I asked about it. The people is Quality disregarded it and used a normal 3 point set up.

In a nut shell, datum target areas are not illegal but sometimes impractical.

Dave D.

http://www.qmsi.ca

 

[Heckler]

SCD = Source COntrol Drawing. Can you post an example of this print.

IAW ASME Y14.5M-1994 section 4.6 Datum Targets - Datum targets designate specific points, lines, or ares of contact on a part that are used in establishing a datum reference frame.

I think what you are looking at is a datum target area that has a defined area. This doesn't define a hole but an area for a tooling pin to locate the part.

Best Regards,

Heckler
Sr. Mechanical Engineer
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[ringman]

EWH,

As I read your thread, I understand that the holes are smaller than the target areas. If this is the case, and the datum precedence is in the proper order, there should not be a problem.

 

[ewh]

dingy2,
A lofted surface is a non-planar, non-cylindrical surface, such as those used in automotive bodies.
We don't have the resources to run this drawing through quality to determine where they would desire the datum locations, so we set them up ourselves. Since there are no regular features for inspection setup anyway, this has not been a problem. They get the datum points from the model file and set up accordingly.

I guess my question boils down to what should a target area be, a flat area of a specified size. or a zone of a specified size on an irregular surface? How do you determine pin orientation relative to an irregular surface?

 

[ringman]

ewh,

If you have a choice in selecting the datum features, I would definitely fo for the point/s rather than an area on a lofted surface.

The question about the pin orientation is confusing.

 

[ewh]

Thanks, ringman. We notified the customer yesterday that we were going to manufacture and deliver to our drawings (using points). If they want to inspect them to their drawings, that's fine with us.
The pin question was in regard to setting the part up for inspection.

 

[ringman]

ewh,

This is interesting. If you manufacture to a diff set of datums than are used for inspection, someone may be in trouble.

Not stated as to the Standard being applied for interpretation. What is the Standard?

I am unclear as to the concern for using pins. A spherical tipped pin could be used if points are specified. 6 pins should stablilze the part for inspection.

 

[ewh]

ASME Y14.5-1994
My concern regarding the pins is how they would work using target areas as opposed to points, which is our current practice.
Fortunately, the customer gave us permission to continue to use our own drawings. I'm curious as to how they would inspect the parts per their drawings.

 

[dingy2]

Thanks ewh

Surfaces such as what you are describing can only be developed using datum targets such as 3 points for the plane, etc. There will not be a datum shown on the drawing since one does not have a feature.

Dave D.

http://www.qmsi.ca

 

[ewh]

The datums points define the datum planes, so even though there are no datum features (usually), there are datums to inspect from.

Here is a sample of the type of dimensioning in question:

http://putstuff.putfile.com/33766/5924546

Scary, aint it?

 

[ewh]

I've got to find another picture host
This one will work, but it has become more of a hassle than it used to be.

 

[thundair]

I work a lot with lofted surfaces and we specify the datum targets as a diameter, with basic dimension and zero tolerance for location.

The manufacture of a die or mold uses the model to build.

The inspection is done with a laser tracker or CMM and is zeroed off the part with a program that incorporates the Datum Points from the model.

If the part fails at zero tolerance for the datum targets then we best fit using the diameter of the datum targets as our limit. It is similar to a datum having a feature control w/MMC.

Cheers

I don't know anything but the people that do.

 

[ewh]

That is what confuses me. You specify the datum targets as a diameter, but the inspection is zeroed off datum points? If the part fails, you can use any point within the target area? Even though the target area is far from planar? The curvature of some of our parts would throw hole locations way off if we followed that proceedure. Is that really more efficient than using datum points only?
Patience is appreciated, as I would really like to understand this subject better!

 

[Heckler]

I must admit, I have never seen a part using holes within the datum targets. I've done hydroforms, die castings, and injection moulded parts. We have always used target ares and fixtured off of those locations. Is this a hydroformed part?

Best Regards,

Heckler
Sr. Mechanical Engineer
SWx 2007 SP 2.0 & Pro/E 2001
Dell Precision 370
P4 3.6 GHz, 1GB RAM
XP Pro SP2.0
NVIDIA Quadro FX 1400
o
_`\(,_
(_)/ (_)

(In reference to David Beckham) "He can't kick with his left foot, he can't tackle, he can't head the ball and he doesn't score many goals. Apart from that, he'

 

[ewh]

No, it is a molded composite.

 

[ringman]

ewh,

I do not believe the sample that you posted on puffstuff complies with Y14.5. If it was intended to do so, the person generating the sample needs an EXTREME REFRESHER course.

For starters: How is it possible to locate a hole relative to only a dautm plane? And four targets areas to locate plane A?

 

[ewh]

Not to mention controlling to a single axis of two features which are no where near coaxial.

 

[thundair]

ewh

I can control the tolerance of the datum point location by the diameter of the datum target. I have seen datum targets as small as .040" dia. on datum A.

Cheers

I don't know anything but the people that do.

 

[CheckerRon]

For what it is worth, the correct abbreviation is SOCD, not SCD for Source Control Drawings (ASME Y14.38). But then, the people at my place refer to them as SCD's too. I'm just happy that they know what they are and actually are using them.