Cylinder surface as a datum
Cylinder surface as a datum
(OP)
I have an old drawing showing a cylinder in profile and specifying the side of the cylinder as a datum. That datum is referenced by another cylinder elsewhere in the weldment, making a side of that cylinder parallel to the datum.
Is this an allowable call out? I'm thinking the datum and feature control should be referencing the axis, not the sides.
Is this an allowable call out? I'm thinking the datum and feature control should be referencing the axis, not the sides.





RE: Cylinder surface as a datum
RE: Cylinder surface as a datum
Your datum must be a real feature that can be fixtured to. With ASME Y14.5M-1994, a datum on an axis is a mistake.
Your drawing probably is correct.
Apparently, an ISO standard somewhere allows you to place datums on axes, but I have not seen it explained. I have no idea what it would mean.
JHG
RE: Cylinder surface as a datum
Can't holes or other ID's be used as a datum and that applies to the axis of the feature?
In this case though the outside is probably more useful for fixturing.
RE: Cylinder surface as a datum
RE: Cylinder surface as a datum
I belive that your Designer wanted the C/L of the cylindrical feature and should have reflected this a bit differently.
Holes and other IDs are used as datums and should be. It depends upon how the part assembles.
If the part assembles with bolts protruding throught some holes, then one of the holes is the secondary datum and all dimensions come from that hole. There would be a 3rd datum or teriary that is used to orient (rotate) the part.
Dave D.
RE: Cylinder surface as a datum
RE: Cylinder surface as a datum
RE: Cylinder surface as a datum
Points, Edges, Axes' and Surfaces...
Any part of a surface of a cylinder would be classed as a profile and would not qualify as a datum.
The axis of the cylinder is the datum and the other cylinder on the weldment should be referenced to it. The form of the cylinders should be controlled by additional tolerances such as cylindricity and run out etc...
Hyd'
RE: Cylinder surface as a datum
Time to get the actual standard. The datum may be the axis of a cylinder, but the datum feature is the cylinder itself. As far a locating the datum callout, that is more a matter of preference, as long as it does not violate Y14.5. It is just as correct to link the callout to a cylindrical dimension in a side view as it is to a circular dimension in an end view (of a cylinder).
RE: Cylinder surface as a datum
Read section 4 of the ASME Y14.5M-1994 standard.
David
RE: Cylinder surface as a datum
Here's some crappy ascii art to show kind of what is happening on the drawing. The datum and the feature control are applied to surfaces of the cylinders. No dimension or axis is involved.
O------O top view
____
|//|A| |-A
|_ _|
| | | |
| |__| | side view
| |__| |
|_| |_|
RE: Cylinder surface as a datum
htt
David
RE: Cylinder surface as a datum
RE: Cylinder surface as a datum
David
RE: Cylinder surface as a datum
RE: Cylinder surface as a datum
I am a little unclear as to the geometric definition of an edge. Where was this course you took, given?
Thanks
RE: Cylinder surface as a datum
Definition of an edge...intersecting point of two or more surfaces.
I aint saying it's right or wrong...it's just what i was told.
H
RE: Cylinder surface as a datum
Hydromech,
Very interesting to say the least.