GD&T issue on a slot located l-bracket 2

[Benshep]

At work I am having to design a L-bracket that will hold a test fixture on a set of rails. The bottom face of the l-bracket locates onto the rail with a slot and that slot should be centered on the part. I am having issues with choosing my datums and if someone could point me in the right direction that would be great. I have attached the drawing in the state where I started to get confused.

Ben

 

[KENAT]

What edition of 14.100 are you using, and more specifically which version of ASME Y14.5 are you working to.

Your Datum identifiers appear to be to Y14.5M-1982 or earlier, is this correct?

Is datum A really only the one side, or should it be both of the nominally coplanar surfaces?

The wording of note 3 might need clarification.

Unless this is a detailed assy or installation drawing or something I'm not sure about note 2.

Also, any chance of a sketch showing it installed or at least some annotation showing what side mates etc.

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[JeffMathena]

Hi Ben,

I think for you to get the best suggestions, it would be good to see how this part interfaces to all the other parts that are of concern with your question.

But here are a few questions/comments that I would have.

1. If the assembly that is shown rests completely on the "rail" in the slot, then it seems like the "top of the slot" would be a good functional Primary datum. If this is the case, your choice for -A- is non-functional and will not control the axis of the hole -D- as well from a functional perspective. The one issue with using the bottom of the slot as primary is that it is not very stable in one axis of rotation. You could leave -A- where it is, and control the bottom of the slot with a profile call-out.

2. Regardless of your choice for primary datum, it might make sense to use datum targets to increase measurement repeatability. You might also want to add a flatness control as well.

3. If you move the -B- datum reference to the left and up a bit and use it to replace the dimension arrow, then the center of the slot will be the datum simulator.

4. Some type of straightness or form control should be used on the slot to ensure that it will fit over the rail. This is only if the +/- 0.001" control over size and form are not enough control.

5. I would propose that datum -C- be defined by a target as well, again to allow for measurement repeatability.

6. A form control, like flatness or even a profile call-out that would include the perpendicularity requirement, might be needed on the surface labeled -C-, but that depends on the mating part requirements.

As with everything associated with GD&T, there is more than one way to skin the cat. It's always a conversation versus an absolute, unless there are rule violations. Please take my suggestions as such, and let's see if we can get some other feedback from other minds as well.

Good luck on your efforts,


Jeff

 

[Benshep]

Yes I am using Y14.5M-1982

datum A is only the bottom surface as that is the locating face in the assembly

The notes dont really factor into the problem.

Also to clarify the issue I am trying to figure out where to datum for the horizontal positioning of the hole and slot. I feel that tying it to the left and right side incorrect but am not sure if putting the dimension of 4.00 on the front view as a datum and using the centerplane between the two parralell sides.

 

[JeffMathena]

Hi Ben,

Thanks for the assembly picture, that helps a bit. If you look at my modified drawing, you will see how to move your -B- so that the center of the slot will be your datum simulator.

Jeff

 

[Belanger]

Yes, if you want the part to be centered on the slot, then you are actually asking for the slot's derived center plane to be the datum. However, the datum symbol should be tagged somehow with the width dimension of the slot.

That said, I don't think you want B referenced at the end of the position callouts for the 8 holes. This is because having D as a higher-order datum will already "lock" the sideways movement. The only degree of freedom that would remain is in the same direction as the slot! I suppose you're trying to control the clocking of the 8 holes, but I think having datum A will already do that. Just some food for thought...

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

 

[DeanD3W]

Hi Ben,
Jean-Paul is giving good advice regarding the need to delete reference to datum feature B from the position call-outs on the holes, [A,D] is all you need, unless you want to constrain the remaining translation by referencing C as well, so [A,D,C] might be a good way to go.

The way you had datum feature B labeled was correct in the first place (as Jean-Paul also pointed out). Per Y14.5-1982 the datum feature symbol is placed by a size dimension in order to specify that the center plane of the datum feature simulator is the datum.

While not an explicit method from Y14.5, I would place a "2X" beside the datum feature A symbol, to help clarify that you want both of the bottom planar surfaces used when datum A is established.

I would consider a datum target for C, but not because it will improved measurement repeatability... Datum targets should be specified when they better capture the functional requirements/design intent of a datum feature. In this case much of datum feature C is not touching anything. While it may be adequate to specify a single datum target point C1, that point may not be the high point within the contact area of the mating part. Since the point of the tolerance specifications is to address functional requirements, you could consider specifying a circular area as datum target C1 over the footprint of the mating part. The highest point within the datum target area C1, would then need to be found when establishing the datum reference frame. The approach for C1 depends upon how imperfect datum feature C is and how tight the tolerances affected by the final translation constraint are... For this part, with the tight perpendicularity, so also flatness, control on datum feature C and nothing much toleranced in the direction normal to C, a single point as datum target C1 placed somewhere within the contact area of C's mating part should work fine (so, my recommendation for C1 is the same as Jeff recommended, but I prefer using different reasoning).

I hope this helps.

Dean

http://www.d3w-engineering.com

 

[DeanD3W]

Ben,
I forgot to mention one other thing... Y14.100 does not state that reference to Y14.100 invokes Y14.5, so you should change to a note something like "DIMENSIONING AND TOLERANCING PER ANSI Y14.5M-1982".

ASME Y14.5-2009 is a much improved standard. It would be worthwhile to consider using it instead of the 1982 version.

Dean

http://www.d3w-engineering.com

 

[powerhound]

Per the 1982 standard, you need to specify either RFS or MMC in your callouts. There is no default.

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[KENAT]

Not sure on the 82 version, but to use the joint surfaces of for datum A, should coplanarity of those surfaces be established first using surface profile control?

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[Belanger]

Good point, Kenat. I would say that a profile tolerance for coplanarity is not required, but the datum A flag should be supplemented with "2 SURFACES."

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

 

[DeanD3W]

I wouldn't use "2 SURFACES". Like "TYP", "n SURFACES" is no longer used. "2X" is best, for this case.

This is based upon Y14.5-2009... It could be argued that this point isn't relevant for a Y14.5M-1982 drawing, but if the drawing is being newly created it is probably best to use current practices.

If this seems odd, then I would ask the what the difference is between "2X" and "2 SURFACES". There is no difference, but I've run into those who have invented a difference in their meaning (invented, since no standard defines a difference). This is why I'm happy to see "n SURFACES" no longer being used.

Dean

http://www.d3w-engineering.com

 

[KENAT]

I'm not sure this is a situation where 2X makes sense. At least at 14.5-94 it explicitly uses '2 SURFACES' in the relevant section 6.5.6 where it talks about co planarity.

Alternatively, in section 4.5.7.1 it shows giving each surface a separate datum and then saying 'A-B' in the relevant fcf.

It also talks about just extending and extension line across the gap between surfaces.

Either way, I have my doubts about just saying '2 SURFACES' I definitely think saying 2X in this situation would be very unclear and likely to be misinterpreted.

However, likely none of this is directly relevant to the 82 standard.

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[Belanger]

Ah, I dusted off my copy of the 1982 standard. They show two surfaces as the hyphenated A-B, and give no example of an interrupted surface labeled with a single letter. So I guess "2X" and "2 SURFACES" are both out (if we elect to refrain from adding a profile control).

Of course, datum targets would be another alternative, as Jeff suggested above.

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

 

[DeanD3W]

In what way could a "2X" beside the datum feature A symbol be misinterpreted?

ANSI Y14.5M-1982 section 1.9.5, titled "Repetitive Features or Dimensions", states "Repetitive features or dimensions may be specified by the use of an "X" in conjunction with a numeral to indicate the "number of times" or "places" required." This looks pretty clear to me... If there are two coplanar surfaces with one labeled as datum feature A, and a "2X" appears beside the datum feature A label what else could this mean, except that datum feature A is two "repetitive" features.

It is a really common misconception that you can only do what an example in the standard shows... The concept is there, even in the 1982 standard, which allows using a "2X" for this case.

Regarding section 6.5.6 of the 1994 standard, I agree that KENAT is correct, but then look at how figures 6-20 & 6-21 from the 1994 standard have changed as they appear as figures 8-14 & 8-15 in the 2009 standard. We don't need or want two ways of saying the same thing, in general, so now the way to say this is "2X". I thought this might bring up some controversy, but wanted to point out anyway that "n SURFACES" has been set aside in 2009.

Dean

http://www.d3w-engineering.com

 

[KENAT]

Believe it or not I just don't find pictures in the standard to copy, I actually do read the text, hence my comment about just extending the extension line.

I don't have 1982 version of the standard and have never really worked to it.

However, given the OP has explicitly said he uses 82 version, then telling him to detail something in a way that's new in 2009 seems questionable.

I'm not a fan of just saying '2 SURFACES' or for 2009 I suppose '2X' without some other indication such as the extension line, or one of the other explicit approaches shown in the standard.

Beyond what's in the standard, I believe the principle of making the drawing unambiguous should be followed.

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[powerhound]

Dean,

While the standard does not define a difference between 2X and 2 SURFACES, it is clear in which situation to use what.

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[DeanD3W]

KENAT - I didn't mean to imply that you're just looking at the pictures. I acknowledged that what you're saying is completely consistent with the 1994 standard.

The reason I justified the "2X" per the 1982 standard is due to the fact that the OP is using that standard. I wouldn't have brought this up unless "2 SURFACES" was proposed.

Powerhound - I'm quite sure there is no use of "n SURFACES" in Y14.5-2009. Yes, if dealing with a feature of size "nX" tends to be used and in the figures in both 1982 and 1994 the standard uses "nSURFACES" if dealing with a feature that is not a feature of size, but in every use of "nSURFACES" you could instead use "nX" and it means exactly the same thing. Per the words in section 1.9.5 of the 1982 and 1994 and 2009 standards "nX" has always been fine to use instead of "nSURFACES".

Now that it's 2011, it shouldn't be too much of a problem to bring up a less prominent change in the 2009 standard. I brought this up in the interest of helping us all get on the same page with GD&T.

Dean

http://www.d3w-engineering.com