Critique on application of GD&T please.

[M_Nicholson]

Hi

I am fairly new to using GD&T, and I'm looking for some advice as to whether or not I have applied GD&T correctly on this drawing to ensure the function of the part.

The part is a simple bus bar, however it needs to go into an assembly where the position of the hole and feet are critical and subject to tight tolerances within the assembly. To sum up the assembly requirements, the midplane of the center foot sits on the central YZ plane of the assembly, and the hole is offset by 0.90mm in the negative X-axis. The axis for this hole also needs to be 7.15mm from the top edge of the bus bar, and sit on the midplane of the tab. The midplanes of the outer feet need to sit 18.30mm each side of the midplane of the center foot, and the center of each foot needs to lie within a tolerance zone of 0.5mm x 0.2mm (exaggerated in the image below for clarity) in the XY plane. The last requirement is that the feet need to extend below the tab by 3.5mm ±0.1

I've drawn this up as best as I can based on what I've read and some example drawings, however my inexperience is making me question whether or not I've defined the datums correctly and then referenced them correctly in the feature control frames etc. This is especially true for the composite position tolerance I've included for the feet.

If anyone can provide any feedback on this drawing, and/or any advice then I would greatly appreciate it.

Thanks in advance

Mike

P.S. The drawing (attached) is not complete yet, so there are some things missing.

 

[Burunduk]

I think before anything else, you should determine what standard you are working to.
The drawing frame states ASME Y14.5, but the drawing uses COMMON ZONE and position applied to planar surfaces, which are only ISO concepts.

 

[powerhound]

IF IN DOUBT ASK? Is that an attempt at ensuring you can reject a part that meets the print but doesn't fit/work?

John Acosta, GDTP Senior Level

 

[Belanger]

IF IN DOUBT ASK?

That's still better than the drawing I saw where a dimension was tagged with the note "MUST BE HELD AS CLOSE AS POSSIBLE."


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

 

[M_Nicholson]

Hi all

Thanks for the comments and apologies for the delayed response, been away all weekend.

@Burunduk - thank you for this. We normally state BS8888 as the standard we draw to, however this has not been enforced for many years and there is now a big push to get people trained to a standard. I say that because we received some advice recently stating that ASME Y14.5 is a better standard, so there is now a debate as to which standard we should train people to. For this example drawing though, because of the ongoing debate and because a lot of the free resources I've been using to learn GD&T reference ASME Y14.5, I put this in as I thought it would be more relevant to what I'd done on this drawing. However as you've pointed out, it appears I should have kept BS8888 in there.

I appreciate your comments, and you've highlighted to me the importance of selecting a standard and getting the training done to this standard asap before going any further.

@powerhound - this is just a standard thing we have on our template that was probably inserted many years ago. I assumed it was there to instruct the manufacturer to contact us if something on the drawing wasn't clear, however I would prefer the drawing be done in such a way that it is unambiguous in the first place, thus negating the need for this text to appear on the drawing. Hence why I'm reaching out here, I want to do things the right way. It's becoming increasingly apparent to me that investing in proper training to a standard is the only way though, so I will push harder for this.

Thanks again for your comments, much appreciated.

Mike Nicholson

 

[3DDave]

Y14.5 is a less complex standard with less specificity, but has a lower price, than the ISO series. The lower complexity and specificity means there is less to memorize. Both groups are hard at work to price small companies and individual users out of the market.

The main problem with training is that there is a major lack of discussions of how tolerances need to be assigned for anything except zero force assembly fit. Characteristics such as flatness are generally limited by the amount of force or contact pressure necessary for the joint to work correctly, but one is unlikely to find anything about that. Instead the tolerance is most often given as some throw-away value that typical machining operations can meet.