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Alignment pegs GD&T

Alignment pegs GD&T

RE: Alignment pegs GD&T

Datum feature symbols are to be attached to the geometry that are used as the datum features - not to center lines; this is because on the actual part there isn't a center line to use. You can construct where it might be from the part geometry, so that's where the datum feature symbol goes.

RE: Alignment pegs GD&T

(OP)
SO both the drawings are wrong? Or is it just the right side one?

RE: Alignment pegs GD&T

Both, I think. What on the part is used as datum feature C and for datum feature F? And datum feature E might be the tangent point, and that's where a datum target should be placed.

RE: Alignment pegs GD&T

(OP)

This is how the part looks for reference.

RE: Alignment pegs GD&T

Yes - that would work to do what I think you want.

RE: Alignment pegs GD&T

(OP)
Why is the datum E wrong. Its at a distance of 14 from the top and passes through the center of the two alignment pins.

RE: Alignment pegs GD&T

It could be either pin or both pins so that's unclear. Also, the [14.00] doesn't seem intended to locate the part relative to the pins, but the pins relative to the end of the part. Also, it's a problem to locate pins from themselves. Where they go they are right where they should be, but not in relation to the rest of the part.

RE: Alignment pegs GD&T

Sharudu,

All your Datum Feature symbols should be attached to physical features specified as Datum Features. They should never be applied to theoretical planes, lines, or points. Theoretical datums (planes/lines/points) are derived from the physical Datum Features (or more accurately, their inverse Datum Feature Simulators). Notice the slot Datum Feature symbols in your attachment are applied to the width NOT the center line/plane.

RE: Alignment pegs GD&T

(OP)
Okay, 3DDave and chez311. I think I understand now. I have attached an updated file and I still have two ways to define the alignment pins. Are both right? Am I right to think that in the left drawing the alignment pins would be derived from datum B and would be biased to that surface and on the right side drawing the alignment pins would be derived from the center of the part making the part more symmetric along the centerline?

RE: Alignment pegs GD&T

That is it - and correctly noting the difference between choosing one side ([B]) and choosing the width ([J]). Which to choose will depend on what gives the more desirable result. [B} is useful if having one side line up is important and [J} is useful to even out any differences.

RE: Alignment pegs GD&T

(OP)
Finally I understand it.. Thank you very much guys..
So on the female part, (which is similar to the above part but instead of pins it has holes where the pins can go) do I dimension it similar to what i did above? And also do I need to provide an mmc on the holes?
Also the part is injection molded..

RE: Alignment pegs GD&T

MMC offers more tolerance for variation, but it also allows more variation by, usually, including clearance. If the pins are to be an interference fit then RFS is the more usual choice.

Is it a plastic or a metal part?

RE: Alignment pegs GD&T

(OP)
It's a plastic part, polycarbonate

RE: Alignment pegs GD&T

In addition to what was pointed out about datum feature symbol placement, consider carefully which features you assign as datum features. Datum features and their precedence order should be chosen according to how the part is intended to be mounted in an assembly and the corresponding constraints of degrees of freedom. You say that the pins are used for alignment, but it isn't reflected in your datum references specification in the drawing on the left. The drawing on the right attempts to use two of the pins as datum references (if that is what E is...), and it results in all 6 degrees of freedom locked by datum feature D primary and E secondary, then F is redundant.

Edit: the above was addressing the first file you posted.

RE: Alignment pegs GD&T

(OP)
Thank you Burunduk. So i am assuming this is what you meant?


And on the right side drawing, I think the datum E is wrong from what I learned above.

RE: Alignment pegs GD&T

Datum feature A should probably be the primary datum reference, I guess that it is expected to make the most contact with the mating part, providing initial stabilization at assembly. Is that correct? I am just assuming. Regarding B and C, it depends on how the part is mounted. I was thinking that you might want to use the alignment pins. If the pins are the locating features (all 3 of them?) consider doing it, but not the way you showed in the first file. I will try to prepare you a sketch later if you approve the idea that the pins are used to locate the part.

RE: Alignment pegs GD&T

(OP)
Yes, you are right about the datum A, it does provide stability and makes the most contact. I am open to pins being used to locate the part. Looking forward to your sketch. Thank you Burunduk. Here's how the assembly is if that helps.

RE: Alignment pegs GD&T

ASME or ISO ?
Which is the governing standards?

RE: Alignment pegs GD&T



Primary datum reference A locks 3 degrees of freedom: 2 rorations (initial stabilization), and 1 translation. Seconday datum reference B locks the other 3 degrees of freedom: translations in 2 directions, and the remaining rotation.
MMC, MMB as shown appropriate if the main concern is about the parts fitting together and less about alignement.

RE: Alignment pegs GD&T

(OP)
Thank you Burunduk. The datum A i guess is flatness but you have profile mentioned and also the profile feature 0.2 A B, is that necessary? I don't really care much about the inside face.
Also the drawing follows iso standard if I am not wrong?

RE: Alignment pegs GD&T

What I have shown is valid according to the ASME Y14.5 standard. If you follow the ISO standards, I think you won't be able to use the circled M following the datum reference in the "tolerance frame" to provide the additional variation which is called in ASME "datum shift". The MMR following the position tolerance is valid.
It makes the most sense to control the rest of the part features with profile referencing A primary, B secondary. It will control the form and size, as well as location and orientation of the surfaces with respect to the "datum reference frame"/"datum system". The all-around profile on the internal geometry was just an example. You can choose the tolerance values. If you have any features critical for function, control them with a tighter tolerance. If nothing is particularly important, you could specify a loose general profile tolerance on all surfaces, and direct to the CAD model for query for all the basic dimensions that are not specified.

RE: Alignment pegs GD&T

(OP)
Thank you Burunduk. Things make more sense now.

RE: Alignment pegs GD&T

You're welcome.
BTW, please ignore the basic dim. 17.4 in my suggestion sketch. It is redundant.

RE: Alignment pegs GD&T

(OP)
Okay👍. Btw this might be dumb question but you mentioned that you used ASME Y14.5 in the drawing but I see that you used 3X instead of 3PLACES for specifying the pins but isn't 3X iso? Is it okay to mix standards that way?

RE: Alignment pegs GD&T

It is the same in ASME.

"Repetitive features or dimensions may be specified by the use of an X in conjunction with a numeral to indicate the “number of places” required"

It is para. 1.9.5 in the 2009 edition and 4.6.5 in the new (2018) edition.

In ISO lowercase "x" is used.

RE: Alignment pegs GD&T

(OP)
Ah didn't know that. Thank you. I guess it's time to switch from iso to asme.

RE: Alignment pegs GD&T

Good idea. You will have all the rules and practices documented in one place (Y14.5) rather than scattered between many documents (1101, 5458, 5459 etc.) that do not get updated simultaneously and you need to keep track of them all.

RE: Alignment pegs GD&T

(OP)
Hello Burunduk, is it correct for me to understand that in your drawing where the pins are used as locating features, it is independent of the outer surface of the device? So in an assembly, it will always fit but the outer surfaces may not align and one way to control that is to put an outer profile feature frame as you did with the internal profile feature.

RE: Alignment pegs GD&T

Hi Sharudu,
Yes, you are correct.
My example is not a complete drawing. All other surfaces of the part, including the external geometry, can be controlled with reference to datum features A and B.

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