Datum target areas or points ?

[SeasonLee]

Attached print is a forged part around 11 inches long, there is a 3° draft angle all around the contour, for an irregular forged surfaces datum reference frame should be established by datum targets which could be points, lines or areas, the print note specified “all datum target areas are Ø.093”.(The original print specified with a X-shaped symbol as datum target area)

For datum A, it’s easy to understand a datum target areas A1, A2 and A3 could be assigned to on the flat surface, my questions are :

1. There is a 3° draft angle on the side wall datum B, shall we made a 3° end face on the B1 and B2 target pins ( Ø.093) to match with the side wall ? Why not to choose datum target points here ?
2. Besides 3° draft angle on Datum C, it’s a curved contour on this end, shall we made a 3° and curved end face on the C1 target pin ( Ø.093) to match with the part contour as well ? Can we use datum target points here ?

Any comment or help is appreciated

SeasonLee

 

[powerhound]

I believe you should absolutely NOT add draft or curvature to the ends of the .093 diameter pins. If that was the intent of the drafter, it should have been made clear. You will basically have what amounts to a datum point, as you correctly point out, but I think inexperience and/or lack of understanding on the part of the drafter is causing the dilemma. These should absolutely be datum points.

Powerhound, GDTP T-0419
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[MechNorth]

It's a small area, so I'm inclined to agree with PowerHound. If the areas were larger, I'd suspect that the drafter was hoping to hit the high (tangent) point in the designated area so as to minimize the impact of a bad surface.

Jim Sykes, P.Eng, GDTP-S
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[drawoh]

SeasonLee,

Can your make your pin faces spherical?

A spherical face can contact the draft angle at the precise point indicated.

JHG

 

[SeasonLee]

Yes, JGH. This is what I wanted, a datum target point means a spherical-tipped pin.

SeasonLee

 

[MartinSr00]

I'm inclined to agree. Let's do a thought experiment. Let's say instead of two datum targets, datum feature B was the entire surface. When you constrained the part on the datum reference frame, you would catch along the edge at the widest point. With the datum target, that line contact would reduce to two points that are perpendicular to the primary datum.

But lets say on that surface you had a draft angle of 3 degrees basic. That of course would change everything. Your datum feature simulator would be inclined 3 degrees to the datum reference frame plane perpendicular to the primary datum plane.

Remembering that the targets simply partition the datum feature, if you want the pins inclined, call the draft angle on that side basic. Of course, then you have to control it with a profile or angular tolerance of some sort. But that's another problem. Maybe a small one.