Can someone help me understand how a true position callout box on two dowel holes have the follow:

The dowel holes have one box that relate to both bores.
The top box is a Tp 0.6 m back to A M-N
The bottom box has a Tp 0.025 back to A

The datum A is the Surface that these bore are perp. to.
The datums M & N are raw bores.

The question I have is what does the bottom tp box apply to? (perp. back to datum A, or the span bettwen the two dowel bores)

Thanks to anyone for their help!!

Any way to attach the actual drawing or a sketch of it?

Perp. back to -A- only! The first note covers the relationship between the two bores.

Jim:

I tried to attach the file to this reply.

Not seeing it, My Guess is first or top call out is for the tp of the hole dimentioned from datums and the lower or 2nd callout tollerance is for the hole to hole dim distance from the first hole. so what i am saying is first one is for the absolute postion from datum and second is for relative distance from first thats why its a smaller tolerance given.

Ginn68

Since both dowel holes are controlled by one GD&T callout they are a pattern and the bottom callout controls not only the perpendicularity to A but also the spread between the holes. There are two cylindrical tolerance zones for the axis of each hole spaced the basic dimension between them and those zones are perpendicular to datum feature A. It controls both the spread and the perpendicularity simultaneously.

Paul

Paul:

Thanks for your reply! If the 2nd callout (0.025 back to Datum A) is for the span or the position back to Datum B then wouldn't the callout be label like 0.025 -A- -B-??? I have tried to ref. this back to the Y14.5M standard with no success??


Michael

Tried to attach file again!! It is zipped, and in Autocad Lt format (.dwg)

Ginn68

Ginn68,

Your original note had no mention of datum feature B in it. The two dowels (or two hole pattern) are controlled for location and orientation from from |A|M-N| with the larger tolerance.

If the dowel hole axis are perpendicular to A and the holes are considered a pattern (by being preceeded by a 2X prefix or by having two leader lines extending to both holes or by having a note below the FCF declaring such) then the bottom note controls the orientation of the pattern tolerance zones to A and the pattern itself is controlled by its illustrated basic definition.

Assumptions:
Pattern designation.
Two single segment FCF's
Dowel axis are perpendicular to surface A.

Where did B come from?

Paul:

Datum B is one of the machined dowel's that have the tol. block. The M&N bore's are two raw bores. My understanding is the two machined bores ( B&C ) are called my to the two raw bores ( M&N ) for stock dist. ?

Ginn68

Is the other dowel referenced in the dual specification to |A|M-N| and |A| by (a "2X" preceeding the size) or by (two leader lines extending from the FCFs to both holes) or by (a note including both hole under the FCF) or by (some other non-standard method)?

If not then the bottom note is only a refinement of perpendicularity of diameter "B" to surface A as others have stated.

If the size is preceeded by 2X and the datum feature identifier "B" is underneath the FCF or on the leader line to that FCF then both dowels simultaneously are Datum feature "B" and all six degrees of freedom would be constrained for related features by designating just |A|B|.

If the designations "B" and "C" are applied individually to each dowel hole away from the FCF and the two holes are controlled as a pattern as questioned above then they are unique datum features. If other feature control frames reference them as |A|B|C| they are secondary and tertiary respectively but their spread is still controlled by the bottom note relative to A

It is difficult to comment without a drawing especially if it is non-standard and I don't have AutoCad. You would have to provide a sketch with all the relevant designations, leaderlines, identifiers, etc...

Paul,
I'm trying to formulate an answer for Ginn68 also, but in the meantime here is the attachment in .pdf if it will help you out.

Plain and simple....(Perpendicularity back to -A-)...Thats what the second FCF means........I know, I know....If they wanted to see Perpendicularity, why didn't they just put Perpendicularity.......I know, I know........It took me forever to finally figure that out.

Frikkin Engineers......

ZeissUser

I respectfully disagree Zeissuser since the FCFs are under the 2X size callout, that makes both holes a pattern. Since the lower spec is RFS, the spread is limited to 19.8884 +/- 0.025 and the 0.025 zones at each feature B and C are oriented perpendicular to datum feature A. It is a stretch to say that this sketch is definitive of the design because it lacks much of the definition. The two notes Axis of B-C and Axis of M-N are not Axis' because the pairs are shown parallel to each other.

Paul

I ran into difficulty with an outdated version of auto cad lt and could not open file. My thanks to Bill for supplying the .pdf

I am not an expert but my old version of ANSI Y14.5m-1982 references the following.

Please look up "FEATURE PATTERN LOCATION" and "COMPOSITE POSITIONAL TOLERANCING", then look up where a datum is established by two datum features-for example, an axis established by two datum diameters- "both datum reference letters, seperated by a dash", (A-B).

The upper entry is referred to as the "pattern locating control" and the bottom entry is the "feature-relating control".

From the looks of it Paul is on the right track! :applause:

Goodluck!

I think Paul is correct also! I have been reading up in Geo-Metrics III by Lowell W. Foster, and it is starting to make sense. The first callout is for "Pattern Locating - Position Tol. Zone", and the second callout is for "Feature Relating - Position Tol. Zone" . If I understand correctly then the second callout could be considered as a span (Hole to Hole). This info can be located in the ASME Y14.5M 1994 standard starting on page 93. Does this sound correct to everyone else? My only question still is why in the second callout it is not labeled as 0.025 l A l B l???

Ginn68

The acronyms PLTZF (pattern locating tolerance zone framework) "upper segment" and FRTZF (feature relating...) "lower segment" explained in the standard pertain to composite FCFs (One symbol, Two segments) and composite tolerances differ from single segment FCFs. The lower segment in a composite callout constrains only orientation (not location) of the pattern to the datum features declared. The pattern itself is contrained for both orientation and location by the definition of itself no matter whether its tolerance is declared in single segment or composite FCFs.

The dowel hole specification that we have here is not a composite FCF because it consists of two single segment controls but in this case it wouldn't make any difference if it was since the lower segment cannot constrain location of the pattern to the datum feature A.

Think about this: If the lower segment repeated the entire datum reference used in the upper segment "|A|M-N|" it wouldn't make sense for the single segment controls because the lower segment would override the upper but if the FCF was a composite the lower segment would constrain the pattern itself for location and orientation, then that pattern of 0.025 diameter zones would have to be perpendicular to "A" and parallel to M-N. I wouldn't override the upper segments tolerance of 0.6M for the basic displacement of 10, it would just refine the orientation relative to M-N to 0.025.

I suppose that B wasn't used in the lower segment because it wasn't necessary although when dowels are both the same size and fit into holes that are both round and the same size and neither dowel has precedent in locating and aligning the assemblies then we would use a note like the one you have however both dowels would be declared as datum feature B.