Position tolerance question

[mikeojai]

Please look at the attachment.
This is a cast plate that attaches to a housing. Datum –D- is a bearing bore and -E- & -F- are locating dowels to mate to the housing.
Datums –B- & -C- are casting tooling datums for alignment of plate to housing cast features.
The question I have is how to properly calculate the position tolerance back to the bearing bore Datum
–G- with the E-F in the secondary Datum position?
Also is it implied that the Datum -G- is the X0,Y0 as the basics are struck off the center line of Datum -G-.

 

[KENAT]

Couldn't open your attachment.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[pmarc]

Do not use & or any other special characters in the file name.

 

[mikeojai]

sorry about that i will try again with the attachment

 

[CheckerHater]

Do you have any non-datum features, or everything in your part is a datum?

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

 

[mikeojai]

Approx. 70% of the drawing dimensions are basics with positional tolerances, some relate back to datum -G-,-E- and -F-
But for the question I only included what was pertinent. The dowel holes are called back to cast tooling datums, along with the face of the part datum -D-.
I have provided all of the datum features and relations as needed for the interpretation of the question.
Unless you can point out something that I have missed, the other dimensions are irrelevant to the question.

I just need to know how to treat the secondary datum in the position control frame E-F.

Do you add the error in -E- position to the error in -F- or do you add and divide the error by 2?

 

[mkcski]

mikeeojai:

I am not exactly sure what your question is that refers to "back to" but let me offer the following:

Since the two .653 holes have exactly the same datum sequence they can be considered a pattern of two holes - see Y14.5-2009 para 4.12. So add 2X to the .653 size on the left and delete to both FCF's on the right hole.

Because the there is no MMC applied to the position tolerance for the two .653 holes can be assigned as pattern of holes that are a single datum per Y14.5-2009 para 4.12.4

MMC is also not applied to the position of the 1.936 hole. Given this Rule #2 is in effect and RFS applies to the all FOS features and datums. This also means virtual condition (VC) concepts cannot be applied, no bonus tolerance and no datum shift are in effect. The net of this is all position tolerances are fixed at the specified tolerances and hard gauging is not possible to confirm the position of the 1.936 hole. However a "fixture" can be made to simulate the datum reference frame (DRF).

The "cartoon" fixture to confirm the position of "G" relative to D / E-F would look like this. A flat plate to simulate Datum D (3 motions constrained) - the primary datum plane of the three planes of the DRF . Two expanding mandrels for the two .653 holes that are perpendicular to D and 2.620 apart (3 more motions constrained) would establish the remaining two datum planes of the DRF. One passing through both holes and the other at the half-distance between the holes and perpendicular the one passing through the holes. Both of these planes are perpendicular to primary datum plane D.

A CMM would be aligned to simulated datum plane D and centers of the two mandrels. The probe would move the BASIC coordinates to the true-position (TP)of the 1.936 holes. The part would be positioned on the gauge against D and then two .936 mandrels expanded out pe rpar 4.12.4. The CMM would scan the surface of the hole and confirm that the axis of the unrelated actual mating envelope lies within a .003 cylindrical tolerance zone located at TP.

I hope this helps.

 

[mkcski]

mikeojai:

I must apologize for the incorrect response I posted yesterday. I printed out your drawing yesterday. It was on my desk and this morning and I inadvertently noticed the lower frame on the right had .003 and not .030 position tolerance. This changes my response. Unfortunately I am not CAD literate to create a sketch for you of a revise drawing, so words will have to suffice.

I still consider that since the upper FCFs for the two .653 holes are the same, the two are a considered a pattern. However, because the lower frame has a smaller tolerance I assume this is attempting to tighten the relationship between the holes (features) in the pattern. Given this assumption I consider this an application for Composite Position Tolerance per Y14.5 para 7.5.1. With Composite the FCF shows ONE position symbol followed by an upper and lower FCF. The upper frame called the Pattern Relation Tol Zone or PLTZF, would be the same as that on the drawing. However the lower frame, which by definition controls ONLY relationship between the holes (features) in the pattern - called FRTZF - would only repeat the Datum D and not have secondary Datum F.

Like I communicated before, this pattern of two holes would be become as single datum feature as per Y14.5-2009 para 4.12.4. A single Datum Feature Symbol would be attached directly to the top or bottom horizontal edge of the Composite FCF. In order to not confuse this "new" datum with other datums on the drawing I will call it Datum H for this discussion. Since the pattern of two holes are now Datum H, Datums E and F would not be necessary and deleted. Datums E-F in the FCF for the 1.936 hole would be replaced with a single letter H.

The "cartoon" inspection fixture and the inspection procedure I described earlier would still apply.

Might I ask why the MMC material condition modifier is not specified for this design. RFS (the default by Rule #2) is rarely necessary from a function and fitup design standpoint. And it adds lots of complication and cost to verification?

 

[3DDave]

This particular use of E-F on RFS referenced datums is not well described in the standard. Any variation in the datum features is not added to or divided by anything. Once the datum reference frame is located - it's where it is.

The scheme currently suggests that the mating part will have interference pins in E and F, so where they end up relative to each other is a stress problem. Knowing little about all the mating parts I have little to suggest for a useful answer.