Does this callout also keep -c- parallel to pattern?

[ronj100]

I am not sure how to do this.
This is a fixture to hold a part for machining.
A locating spud in the center and a dowel will locate and orientate the part being held.
My intent is to have the .250 dia dowel hole kept relative to datum -b- very close.
The distance of datum -c- to the pattern is not critical, but I want the parallelism of
datum -c- to the pattern be very tight for indicating purposes.
I have looked at many examples in the standard, but I'm not sure If my example has achieved my intent.
attached is a sketch.

Thanks

 

[M4C7]

Ronj100,

A couple of points of clarification as to your requirements:

1) My intent is to have the .250 dia dowel hole kept relative to datum -b- very close.

The upper segment of your composite FCF controls location of your .250 dowel hole (tolerance .030). The lower segment (which is not legal per ASME because your referenced datums must be a subset of the datums used in the upper segment, so pos|Ø.0004|A|B would be legal) would control only orientation with respect to the referenced datums.

2) The distance of datum -c- to the pattern is not critical, but I want the parallelism of
datum -c- to the pattern be very tight for indicating purposes

By pattern, are you referring to the pattern of 4 counterbored holes at the four corners of the part, because your current sketch does not have any dimension or tolerances controlling them. Also, you do not have a tolerance locating datum C. You have only specified an orientation tolerance to the primary datum (datum A)

3) As a point of curiosity, is there a reason you call out a straightness tolerance on datum feature B, as opposed to say, a perpendicularity tolerance wrt datum A? This would allow a part such as the one shown attached to be deemed perfectly acceptable according to your drawing spec.

 

[ronj100]

with the fixture bolted (4x) to a machine bed, datum -c- is indicated square (to the machine).

(So the 4x bolt pattern is not relevant to my intent at this point.)

when -c- is indicated square on a machine, the dowel hole needs to be in proper orientation (position) to datum -b-.

Perp of -c- to -a- would not be good for indicating the fixture square. You could hold -c- perp to -a- very close and still have a wavy surface that would make a poor indicating surface.

Hope that helps

 

[ronj100]

IGNORE LAST POST!!!

with the fixture bolted (4x) to a machine bed, datum -B- is indicated square (to the machine).

(So the 4x bolt pattern is not relevant to my intent at this point.)

when -B- is indicated square on a machine, the dowel hole needs to be in proper orientation (position) to datum -C-.

Perp of -B- to -A- would not be good for indicating the fixture square. You could hold -c- perp to -a- very close and still have a wavy surface that would make a poor indicating surface.

Hope that helps

(HOW DO YOU EDIT A POST?)

 

[ewh]

Red flag and ask an administrator to edit it. They're pretty good about that.

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[M4C7]

I don't really have a manufacturing/inspection background so I feel like I might be misunderstanding either your constraints or your setup, particularly when you say

with the fixture bolted (4x) to a machine bed, datum -B- is indicated square

I had assumed you were using surface B to edge find for your machining setup, using a method such as this:

http://www.sherline.com/images/millfig9.gif

To help me understand, please reference the attached image (I’ve again shown Datum feature B at an exaggerated angle). Once you bolt your fixture to a machine bed (green in the image), what exactly do you mean when you say that datum -B- is indicated square?

For the second bit,

the dowel hole needs to be in proper orientation (position) to datum -C-

If you want a closer control of orientation to datum -C- than provided by your chosen position tolerance, 2 options are
1) Specify a parallelism tolerance referencing datum -C- (see fig 6-32 in Y14.5-1994)
2) Specify a perpendicularity tolerance referencing datum -A- to which datum feature -C- has already been controlled

What I stated previously about the composite FCF is not quite correct. pos|Ø.0004|A|B in the lower segment is not correct since B does not constrain any additional rotational degrees of freedom for a single hole. If you were to instead use pos|Ø.0004|A that would simply be a perpendicularity refinement as mentioned in option 2 above.

 

[3DDave]

Flatness should be applied for datum reference B; or even better, datum feature targets.

Straightness applies to elements of a surface, so the surface could be twisted and still meet the requirement. A perpendicularity control with a similar small tolerance value is less wavy than straightness allows because perpendicularity covers the entire surface simultaneously. The disadvantage to perpendicularity is that there is no value in ensuring the surface is perpendicular when two target spots would suffice.

The pin position should be controlled with two independent FCFs to meet the requirements you have - one to A|B|C and the other to A|C.

The perpendicularity for datum C should have a diameter symbol in it.

There is no location tolerance for C.

 

[fsincox]

Along with the comments expressed already, remember the orientation of the "whole world" is really defined by the primary datum choice. the axis of datum "C" itself, will be the axis derived from the true mating envelope of the datum feature C oriented to the primary datum. If you really want axis to axis orientation you would have to orient to C as primary.
Frank