Another GD&T Question

[brandnew1]

Hi all,

i have another question regarding a hole location call out based on the feature control frame.

Based on the attached pdf i would like to confirm what is being asked for in terms of the hole in question.

It has a FCF of .005 M to datum D (area to the left when looking at the part from side), Datum G (regardless of feature size, which is the hole and Datum E (the line).
Datum G has a callout to be within .016 of datum D, E and F. i see the BASIC dimensions related to this location.
However for the hole in question, i see the BASIC dimensions off of Datum G but not Datum E.

So the question i have is, do i incorporate the values from Datum E (calculated values) or do i just focus on the location of the hole in reference to Datum G (since datum G already is based off Datum E)?

Greatly appreciate the help, thank you

 

[ctopher]

Actually that callout is for the other hole.
It's 'assumed' this info is for both holes?

Chris
SolidWorks 10 SP4.0
ctopher's home
SolidWorks Legion

 

[powerhound]

This is an incorrect callout for exactly the reason you stated, the basic dimensions from datum G and E are redundant in that axis. It's obviously a very old print so fixing it isn't an option. You'll just have to do your best and hope the parts work. There's not even a "right" way to check this part..."IMO"

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X4
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[brandnew1]

im not at work right now, but i'll add another pdf to clarify the FCF for datum G hole which is the part that confuses me

 

[pmarc]

I see nothing wrong in a position callout for hole in question. Dimensioning scheme looks alright as well.

Imagine how datum reference frame is constructed in this case:
- datum feature A establishes primary datum plane A and 3 degrees of freedom (DOF) are constrained,
- axis of datum feature G is associated with two datum planes which constrain 2 DOF's. However these datum planes can still rotate around the axis of G, so...
- third datum is needed to somehow orient these two planes. That's why E is referenced as tertiary datum feature - to say that one of the planes must be parallel to E and second perpendicular to E. Without E remaining DOF would not be taken away, so the hole in question could lie anywhere around G at the distance of [1.950] and the part would still meet the print.

But coming back to OP's question... Only location relative to G (basic [1.950]) should be considered when checking position of the hole. Datum E is referenced in FCF just for defining orientation of datum planes associated with datum axis G.

 

[Belanger]

Pmarc nailed it -- there's nothing wrong with this print.

Datum E's job is merely anti-rotation. Use only the 1.950 when assessing the feature control frame of the hole in question.

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

 

[powerhound]

Yeah, I see it now. I guess if the datum precedence would have been DEG it would have been clearer, quicker to me. I guess it just looks weird to me. Sorry about interjecting a BS response without checking myself first.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X4
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[brandnew1]

i attempted to clarify the blue print spec with the attached pdf.

i agree regarding datum E being the rotational constraint. As i see the call out for Datum G, Datum D is the plane in which the part lies (3 degrees), Datum E (2ndary call out) limits the rotation (2 degrees) and F would constrain the 6th degree.

But as i looked at the True Positions for the holes they call out to datum D, Datum G and Datum E. i just want to make sure that when i have the hole locations calculated they would be based on Datum G only and not to where Datum E is located. i apologize if this sounds dumb, but i thought when i have basic dimensions, theoretically they are to be perfect. But when connected to a Feature Control Frame that’s where the tolerance comes into play. But with Datum E called out for the hole locations and no specific Basic Dimensions, this threw me off.

So for the first hole, is it safe to measure the location off of Datum G to be 1.950BSC and 0?

 

[brandnew1]

updated drawing

 

[Belanger]

The distance from datum E to the first hole ("G") is simply the 1.04 dim with the .016 position tol (and any bonus).

The distance between the two holes is simply the 1.95 dim with the .005 position tol (and any bonus).

But if you want to find the distance from datum E to the hole on the left, it would be 1.04 + 1.95 as the basic distance, but then you would have to factor in both position tolerances.

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

 

[brandnew1]

Thanks everybody for the help =)

 

[fsincox]

should be: .382-.387 or .387/.382.
Frank

 

[Belanger]

Sorry -- one other thought before this one is put to bed...

While the GD&T scheme given is legal, I can't help wondering if this might be a candidate for composite position tolerancing. Since the holes are all the same size, can I assume that the holes are all of equal importance? If so, then composite would have been better. But if there is indeed something special about that first hole, maybe keep everything as is.

(It looks like it this an old design, given the use of the S modifier and also the datum feature symbols, so this might be a useless suggestion anyway.)

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

 

[brandnew1]

Sorry for the late response...but yes this is an old drawing. Actually have a fixture from the old company that had this part.

This may be a side question but are fixtures made to be at nominal or do they take into account the FCF called out for a hole?

This fixture i see the datums (D, E, F) for Hole Datum G. But when i see that the FCF for datum G calls out .016 this tells me that the hole can be located off the nominal 1.040BSC by a few thousands. But it doesn't look like the fixture accommodates this change...

Does anyone know if the fixtures are designed to accommodate the .016 tolerance for a particular datum? (i'm just asking if fixtures were made as such, since this was made in 1994) =)

 

[Belanger]

Yes, the position error of the hole needs to be accounted for in the fixture's pin. So rather than nominal, that pin would be made at .366. This is because we need to account for the worst case: the smallest hole that is also out of position (.382 - .016).

There is also a separate question of how accurate that fixture's pin must be. But that's a different discussion -- some shops use a 10% rule, so that any gage or fixture is allowed its own tolerance based on one-tenth of the tolerance on the product drawing. But we'll keep that can of worms closed for now and just say that the fixture pin has a diameter of .366.



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

 

[pmarc]

brandnew1,
If you are interested more in fixtures topic, there is ASME standard which defines dimensioning and tolerancing priniciples for gages and fixtures - it is Y14.43. But if you just need simple answer to your question, John-Paul got it right.

 

[brandnew1]

Thanks again all...you guys are definitely an valuable resource especially to someone such as myself who continually learns how much i don't know =)

 

[dtmbiz]

(Attached dwg mark-up for clarification of below.)
I have seen this drawing a few weeks ago I believe and it really never made sense to me because it appears as if hole (Datum G) is important in an attempt to relate the 4 holes total to a mating part. These FCF’s don’t work for me.
The problem is that the Datum Reference Frame for the 3x holes DGE doesn’t accomplish what the callouts attempt to do, namely establish a relationship to G and create a pattern relationship of all 4 holes.

All DRF’s consist of 3 mutually perpendicular planes. The origin where these planes intersect is where the inspection measurements are taken from. In the drawing the defined DRF origins are not at the center of Datum G’s axis, as I speculate was hoped for. The DGE and DEF origins are actually only .500 apart along plane E.

I agree with John-Paul that composite FCF would satisfy the hole(s) callout and clearly establish a pattern relationship; whereas the DGE really doesn’t accomplish much IMO.

 

[pmarc]

Origin for datum reference frame DGE is not where you put it. It is exactly in the center of hole G.

 

[dtmbiz]

I disagree Pmarc. The DRF origin according to the standard is the "intersection" of the 3 mutually perpendicular planes identified in a FCF.

Datum E is identified as the tertiary datum and is one of the 3 planes which establish the DRF for "DGE". A datum simulator must touch this datum feature in order to setup the part for inspection. It must be considered to identify the DRF orgin.

Ref: ASME 1994 Y14.5M
pg 51 4.2.2.1
pg 52 fig 4-1

If you say that it is exactly at the axis of Datum G, then how would account for Datum E in the callout considering at least one contact point in plane E must intersect G and D?