question on restrained part figure

[AndrewTT]

Attached is a figure from a GD&T textbook. I was wondering what people's thoughts are on the FCF associated with datum feature C?

FYI - this figure is incomplete by intent.

 

[CheckerHater]

I don't see anything wrong with the picture if that is what you are looking for.
The (F) modifier doesn't (simply) mean that feature is checked in free state - after all, everything is checked in free state by default.
So, the modifier means that some other criterias must be met:
1. The part is subject to free-state variation or is, in other words, non-rigid. We can agree on that.
2. The part is subject to special measuring procedure, described somewhere on the face of the drawing - restraining, calculation of the average, etc. No problem with that either.
3. Even though the rest of the part is checked in restrained state, this particular feature is still checked in free state. To me it means that hole must fit over the fixture pin before the restraining clamps are applied. Looks like reasonable requirement to me.
This is how I see FCF in question.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[drawoh]

AndrewTT,

Datum feature[ ]B is inspected in free state. Datum feature[ ]C is not. Is that what you noticed?

--
JHG

 

[pmarc]

AndrewTT,
Are you concerned that the perpendicularity callout applied to datum feature C - width of the slot - does not prevent the feature from being manufactured for example 90 degrees from the shown orientation?

 

[3DDave]

Datum feature C is established in the retrained state while its reference is established in the free state. B & C are defined in the [D] DRF, but then referred to in the [A} DRF making a perfect estimate of the virtual condition impossible since there's no solid conversion from the one DRF to the other.

Play stupid games, win stupid prizes.

Also, KG is not a unit of force.

 

[powerhound]

I think what he's getting at is that C should be controlled with position, not perpendicularity. This perpendicularity callout will not ensure that the slot width is pointing at B. Is that what you were getting at Andrew?

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[AndrewTT]

I asked for feedback because my first though when viewing this figure was that there was something about free state and restrained condition that I did not understand.

Per the U.O.S. note, without the Free State Modifier all FCF are measured in the restrained condition to DRF [A][Bm][Cm]. Because of this, I was thinking that the datum feature C FCF should be modified with the Free State modifier. Per the figure, datum feature C is required to be perpendicular to datum D. Datum D seems, to me, to only be of interest before the part is restrained. Once the part is restrained we are using DRF [A][Bm][Cm]. So if you are measuring datum feature C in the restrained condition then require it be perpendicular to [A] and not [D]. Since datum feature C is used to create the restrained DRF I believe that it should be verified in the free state (similar to datum feature B).

What next confused me about the datum feature C FCF was the use of perpendicularity. You should be concerned with the location of this feature before you are concerned with its orientation. If there is no location requirement between datum feature C and datum feature B then the restrained DRF should have the datum translation modifier added to [Cm]. Otherwise how would you make the gage?

I know the figure was meant to instruct about restraints so I don’t want to make too much out of this one FCF. However, I was going to use this as an instructional figure at my work and these are the types of questions I will get after showing this figure.

I should add that the figure that this book came from was written against the 1994 standard, so the datum translation modifier did not exist yet.

 

[3DDave]

No one has ever shown me an example of a real part/component/assembly that exhibits 'datum translation.' That would be an interesting item. Not a gage that simulates it, or software, but a functioning item that can be done no other way.

 

[AndrewTT]

For the purpose of showing an example of the use of a restraint note and free state requirements in the same drawing I am going to make the attached modifications to the figure in an attempt to not be bombarded with questions that distract from the teaching point.

 

[powerhound]

Attached is a figure from a GD&T textbook. I was wondering what people's thoughts are on the FCF associated with datum feature C?

This is a wide open question and the fact that you got a lot of answers that weren't what you thought they would be shouldn't surprise you, but obviously it did. Maybe next time get to the point right away so we don't all waste our time trying to guess what your point is.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[semiond]

Before i even got the chance to get confused by the more complicated things you described, i got confused by a much more basic uncertainty. Since there is no way to know for sure from that page if this is a third angle projection or first angle projection, i don't know if A and D are on the same side or on opposite sides

 

[AndrewTT]

It is a third angle projection drawing. Also, those are solid lines on the datum targets not dashed lines so the targets are on the near side. D and A are on the same side of the part.