Datum and Datum Feature Simulator

[jimbod20]

Hello,

The answer to these questions may be obvious. I am new to many details regarding GD&T.

I provide a section drawing from a part to illustrate my questions. The hole is through the part and of fixed size.

1. I believe the datum feature is a cylinder. Is this correct?
2. I believe the datum feature simulator is a "v-block". Is this correct?
2. I believe the datum is an axis. The axis is where the two sides/surfaces of the v-block intersect. The axis is offset from the center bore axis through the part. Is this correct?

 

[MechNorth]

1) the datum feature is a radial surface
2) datum simulator is a v-block at 90-degrees basic
3) datum is actually two planes (the intersection of which is an axis), established by the v-block. Vertical through the intersection of the v-block sides, and horizontal thru the basic center of the radius

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[pmarc]

Jim,
Are you sure that horizontal plane passes thru the basic center of radius and not thru the intersection point of v-block sides?

 

[fsincox]

Sorry, I want it to be true, but I do not know how we know that. I agree with the rest. The radius is not given here as basic, right, so will we always even know what the basic radius is?

 

[pmarc]

Frank,
The radius is givens as basic = 1.220.

 

[Belanger]

Pmarc, see Fig 4-47 of the standard, and notice where the origin of the x y z axes are. This might be the basis for what Jim stated.

If so, that brings up another question: If they didn't show those x y z origins, would say that the true datum passes through the intersection of the V ?

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[pmarc]

J-P,
Yes, if x y z origins were not shown on that figure, I would say that datum planes passed through the intersection of the V.

 

[fsincox]

Sorry, I thought it was not.

 

[DeanD3W]

In Fig 4-47 of Y14.5-2009 the 40 mm basic dimension is the only thing saving the origin location from being incorrect. The origin can be placed anywhere if basic dimensions from its "natural" location are provided. I think this figure should be changed. It would be better to have the origin coincident with datum A, with its location a normal projection down from the intersection of B1 and B2. The "center of a basic radius" is no more useful than the intersection of B1 and B2... This would be an artificial construction that adds no value.

Dean

http://www.d3w-engineering.com

 

[MechNorth]

Sorry for the late response; I had something pop up Monday morning and I had no free time all week.

The datum feature is the radius, not the 'V', so the center of the basic radius would be the origin; the simulator helps you establish that origin, though in this case inconveniently. Of course, this is based on datum E being the primary.

Dean, I agree that alternate datum selection would be preferable.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com