cylidrical fearure of size as a secondary datum refrence 2

[waqasmalik]

In the drawing attached the position tolerance of holes has been applied wrt two datum planes i,e Planar Datum "K" and Cylindrical Feature of size Datum "M"(with maximum material condition).My question is that if i dont want a datum shift then is it still necessary to use second Datum M in the datum reference frame.Will not "K" be enough as a reference for position tolerance control.

 

[waqasmalik]

Actually i want to ask that what is the significance of secondary datum in this application if datum shift is not required

 

[Belanger]

The term datum "shift" refers only to the circled M that follows the datum letter M. (So for starters, that is a little confusing because when we say "M," we have to specify if we are talking about the datum letter itself or the modifier.)

Anyhow... if the circled M modifier were removed, then there is no datum shift.

To your main question: The purpose of datum M itself is to ensure that the pattern of four holes is centered on the part; datum K would say nothing about the centering of the hole pattern. Each hole should be pretty close to 24 mm from the axis created by the outside diameter (in techno-speak, datum M is the axis of the outside diameter's related actual mating envelope).

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

 

[SeasonLee]

[/Will not "K" be enough as a reference for position tolerance control.]

Single datum reference on the FCF means to control the spacing and orientation of holes in a pattern only, in this case (without secondary datum M) the position control limits the spacing between the holes and the perpendicularity of the holes wrt datum plane K, and the TOP does not control the location of the hole pattern. Therefore the purpose of secondary datum M is to control the location of the hole pattern.

Season

 

[waqasmalik]

@BELANGER
sir u called Datum axis M to be the axis of the outside diameter's related actual mating envelope but in ASME 14.5M-2009 what i think is it seems to be bit contradicting .See the drawing attached please.Here Datum axis B is the axis of the unrelated actual mating envelope.

 

[pmarc]

waqasmalik,
No, fig. 1-1 shows datum axis B as the axis of related actual mating envelope.

Notice that the description of "axis of related actual mating envelope" on the top left of bottom picture contains "datum axis B" term, while the description of "axis of unrelated actual mating envelope" does not contrain this phrase.

 

[waqasmalik]

what is the difference between related actual mating envelope axis and datum axis B?
To me Datum axis B is the axis of datum feature simulator of datum feature B and it is also the axis of the related actual mating envelope

 

[pmarc]

There is no difference, assuming that by "datum feature simulator" you mean "theoretical datum feature simulator", not "physical datum feature simulator". There is a tiny difference between the two (see 1.3.17 in Y14.5-2009).

 

[waqasmalik]

No, i meant to ask that in top left corner of bottom figure it is written that" Datum axis B and axis of related actual mating envelope are used to control position"

 

[waqasmalik]

It means that these are two things

 

[SeasonLee]

waqasmalik

Suggest to read the link below, it will tell you:
1. What is Related Actual Mating Envelope (RAME)
2. What is Unrelated Actual Mating Envelope (UAME)
3. When to use RAME
4. When to use UAME

http://www.tec-ease.com/gdt-tips-view.php?q=77

Season

 

[waqasmalik]

I understood seasonlee
a)Perpendicularity tolerance is verified from the axis of unrelated actual mating envelope.
b)Position tolerance is also verified from its axis of unrelated actual mating envelope but the origin of its PCD (Pitch circle diameter) is datum axis B which is the axis of Related actual mating envelope.Bonus tolerance would be calculated from the difference between unrelated actual envelope size and MMC.
Correct me if I am wrong.
If i m rite then i have following questions.
1)How to find UAME?
2)Whether the Perpendicularity tolerance is applied to Datum axis B which is the axis of RAME? Is the UAME axis compared with the Datum axis B?

 

[SeasonLee]

Further link for your reference.

http://www.tec-ease.com/gdt-tips-view.php?q=245

Season

 

[waqasmalik]

which method is more reliable for measuring the UAME for positional tolerance either CMM or Gaging

 

[waqasmalik]

concepts cleared,thank u