One Million Datum Targets? 4

[ikrayg]

I have a plastic part that attaches to another part by using several hoop snaps. The coworker I am working with is insisting on using a datum target at each hoop snap location to establish datum A since the snaps are functional features. The resulting drawing ends up having 12 datum A targets. Has anyone heard of doing it that way instead of using three target and profile of a surface control frames on each hoop snap?

 

[MechNorth]

Pls post a simplified graphic of your layout so we can be sure of what you're asking.

Jim Sykes, P.Eng, GDTP-S
Profile Services

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[drawoh]

MechNorth,

On a cast, planar surface with flatness that I do not trust, I would define three datum targets. This means that the fabricator and inspector for the casting, and the fabricator and inspector for the subsequent machining, all are working from the same nominal plane.

Twelve datum targets are a solution for a non-rigid structure in which the datum shape ought to be defined by the snaps.

JHG

 

[MechNorth]

Sure, drawoh, but typically for a cast surface I'd use area targets rather than point or line contact (at least for the primary datum). At each of those areas, how many points of contact will you make? Minimum one, but potentially many more subject to the coarseness of the cast surface. You are treating the net effect at each target area as a single "reaction point" as opposed to numerous contacts.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[ikrayg]

Here is a simplified sketch of the datum A set up. The datum scheme has changed a little since last I saw it; now the hoop snaps have their own profile callout. The mating part contacts at each area where there is datum target.

 

[ringster]

Not to be offensive, but, has the one who generated this had basic training in the application of Y14.5? I believe that to be the Standard applicable.

 

[MechNorth]

A couple of observations. First, the datum targets are points where they likely should be area targets...is the mating component contacting only at the center point on those surfaces...likely not. Datum area targets covering the entire surface should be considered at each location. The indication of Datum-A as origin of measurement is not clear; are you intending for that one small surface to be the origin of measurement, or is it to be at a location in space located at a basic distance from one of those targets?
Next, the surface profile only applies to the top surface in each of those cutouts...is that the intent, or is it to be the entire profile of the cutout?

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[ringster]

What about using the 2 rectangular cutouts as a datum paattern feature and relating the points to them by means of a profile of surface or such? It appears to be a Model Data set definition part.

 

[ikrayg]

MechNorth,
That's a good point, the entire top surfaces of the two tabs at targets A1 and A10 are in contact with the other part and should be target areas. The rest of the points contact small ribs so maybe axis targets should be used.
Regarding the origin of measurement, Datum A is the entire back edge of the part. In further discussions with the engineer for that project I found out there are more contact ribs that touch the edge of the wall where Datum A is where additional targets should be added.
The surface profile is intended only for the top surface. The rest of the square hole has sufficient clearance to the receiver ramp feature on the mating part.

Thanks for all your help everybody!