GD&T drawing feedback 4

[panzer32]

Hello, I'm a recent mechanical engineering graduate. I'm currently out of work so in the meantime I decided to study some GD&T skills (since we basically learned nothing about GD&T in school).

For practice I took a part found in my old graphics text and drew it in a trial version of autoCAD using what I know of GD&T principles. The actual tolerances that aren't specified in the original drawing I mostly just made up but hopefully are reasonable.

Here is the drawing from the text -

https://lh5.googleusercontent.com/_...AAAAAAAdk/fdPsNQcSnnU/s912/IMG_0900 small.jpg

Here is my drawing -

https://lh5.googleusercontent.com/_...o/Ork01ZT_3fM/s912/Graphics Txt Fig 11.51.jpg

I was hoping to get some feedback whether I am on the right track or not, especially with setting the B and C datums as the two holes - is this done properly? I would be grateful for whatever feedback you can give me. There are a few niggly bits which I'm not satisfied with but I'm not sure how to change in autocad, like the centrelines not having the short crosses in the centre of the circles.

 

[DrCADD]

KENAT,

I am sorry.

However, if you go back through this blog and look at all the posts, I think I was not the one to take us down this road of disrespect.

I have a passion for what I do and a passion for doing things correctly. I am sorry if this offended anybody here.

I had a very strict father who demanded perfection. I tend to do the same. Things are either right or they are wrong.

Still, I will confess there can be more than one right way to do things.

The important lesson I was working to convey is basic dimensions should only be used when you truly demand that kind of exactness. One could say basic dimensions are the foundation for positional tolerancing.

Regards to All

 

[KENAT]

Trouble is, the 'exactness' comes not from the Basic Dimension but from the GD&T control applied. You could use Basic Dimensions to define a shape that you then apply very loose surface profile to it. Arguably there isn't much 'exactness' involved.

Many of us here have concerns over the idea of assuming that GD&T & basic dims meens 'exactness'/precision/tight tolerances and get's interpreted that way by folks we work with (either directly or at vendors etc.) even when we're actually using GD&T to take advantage of more tolerance etc.

So, we tend to be a bit picky with how some things are phrased.

For whatever part of the apparent conflict is my fault I apologize.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[ewh]

When I was a young punk like ewh

Thanks for that! I actually started out in industry only a few years after you.
I have to confess that I still have problems with some of your terminology...

The studs... are assumed to be perfect

No, the locating basic dimensions are perfect, not the resultant studs. They are toleranced by the geometric control, not the basic dimension.

Thanks for the cool head, KENAT!

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[DrCADD]

Hey ewh, we were all punks at one time.

I would just encourage you to look into the use of basic dimensions more.

The most important thing, to me, is that you not get into it some day with some guy that can make your life miserable.

I once had an old soul lecture me about drawing asking me if a part was an "innie" or an "outtie"? I had never heard the expression or his lesson. Some parts are made from the outside in and some are made from the inside out. The casting of this blog would be an outtie as you would begin by grinding the base and then the two flange ends to establish datums.

Working in industry has offered me as much as being a student and teacher of the classroom.

 

[ewh]

I do admit to still being a punk at times, just not a young one!


I feel secure in my use of basic dimensioning, and think much of the flack on this thread (at least from my viewpoint) has been regarding terminology rather than use. I welcome the old(er) guys trying to make my life miserable. It always results in a learning opportunity for one or both of us.

Regardless, welcome to the fora.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[Belanger]

Wow -- quite the thread. All I will say is that it is perfectly fine to have basic dimensions come from a rough edge. Basic dimensions really come from the datum, and recall that the datum is a theoretical perfect plane formed by the highest points of the rough edge.

For repeatability, this isn't necessarily a wise practice, but it's perfectly legit.

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

 

[fcsuper]

So, has anyone looked at ASME Y14.41 para 7.2 yet? This is the applicable standard in this case because the OP is essentially relying upon it by invoking the general Profile FCF in the general notes.

Matt Lorono, CSWP
Lorono's SolidWorks Resources & SolidWorks Legion
Follow me on Twitter

 

[DeanD3W]

Matt,
The drawing has a general profile, but should have also included a note with that profile something like "WITH BASIC DIMENSIONS FROM 3D CAD MODEL".

Since it is a 2D drawing and especially since there is no explicit reference to Y14.41, I don't think anything from Y14.41 could be invoked. That note about basics from the CAD model may tend to put us into a Y14.41 world a bit, but I think the drawing would live with only a "DIMENSIONING AND TOLERANCING PER ASME Y14.5-2009" note (with some of improvements to bring it into compliance with Y14.5, of course) and the other note regarding where basic dimensions come from for that general profile.

Dean

http://www.d3w-engineering.com

 

[fsincox]

I know this is a repeat, but, One of the things I saw when I started looking at the ISO metric system drawings an how differently they express tolerances was that without a tolerance block on the drawing really all dimensions are basic. We imply the tolerances because of a title block and/or notes on the drawing, so without that you must assume all dimensions basic. Whose tolerances do you apply? mine? yours?
Basic dimensions are not dimensions in a box they are dimensions without a tolerance. We all work in environment where we just automatically think implied tolerances.
Frank

 

[fcsuper]

Dean,

Notwithstanding the explicit mention of a standard (since no standard is specifically mentioned on the drawing), ASME Y14.41 is the overriding ASME standard that would apply. It does invoke ASME Y14.5, and it covers the 3D solid model realm, which may be implied by the original drawing by including the general profile FCF. The issue with the drawing is that it doesn't invoke any standard, nor does it remove ambiguity, not that there are too many BASIC dims. Y14.4 declares everything BASIC, which contradicts some of the comments by some others in this thread. That's really why I brought it out at this point.








Matt Lorono, CSWP
Lorono's SolidWorks Resources & SolidWorks Legion
Follow me on Twitter

 

[DrCADD]

That is a very good point, Frank. I forgot to mention that.

I have such a bad time getting my students to look down into the title block for general notes and tolerances.

One could say that a basic dimension is a dimension to override any general tolerances.

This is a good point I will have to remember to use in lecture next year. Thank you.

 

[Belanger]

Then a reference dimension is a special subset of a basic dimension? (Ref dims also have no tolerance.)



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

 

[DrCADD]

A reference dimension is a dimension that is non-critical to the making of the part. As an example; Very often a casting may shrink and somebody did not calculate the shrink factor correctly. Perhaps we wanted it to be 8 inches and it comes out somewhat smaller. Still, the part is machined from ground datums off of the outside so the overall casting does not really matter. The engine block in your car serves as a good example. The fact that your block may be slightly larger or smaller than the exact same model next to yours doesn't really matter. What matters is what is going on inside the block.

I always tell my students a basic dimension is critical where a reference dimension is non-critical. The reason for the parens around it is to indicate any general tolerances do not apply to it.

 

[fsincox]

I am not saying we can live without tolerances, just they are really relative. For example, If I specify 70 H7 the 70 is basic (the basic size) without a box noted there. To put it another way tolerances are there only because we've put them there they are not automatic.
Frank

 

[Belanger]

Dr. -- I know what a ref dim is; my question was meant to illustrate why it might be dangerous to say that a basic dim is any dimension that overrides general tolerance.

I'm not sure about ISO, but in ASME when you say basic dimension most people have a specific idea in mind: a dimension that is theoretically perfect, but one where a feature will get a tolerance from GD&T (with the exception of datum targets and other gage/fixture dims).

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

 

[fsincox]

So you are all thinking so what is the point. So to "can all dimensions be basic?" my argument is without a title block or notes they always were/are.
Frank

 

[DeanD3W]

DrCADD,
Please make this a discussion in which you actually consider what others say... Maybe some of those posting here are at the level of your former mentor... Wouldn't you then feel a bit silly for calling them names? This is a good forum, which will be degraded by any name-calling and super-ego attitudes. Because someone disagrees with you does not give you any right to flame them. Consider the possibility that it may be you that is not quite correct in your terminology.

I think the drawings you would produce might be fine, based upon what you have said, but you really do seem to have some misconceptions about basic dimensions. Please consider this with an open mind.

A basic dimension does not override a general tolerance unless a geometric tolerance that imposes a tolerance zone that is located or oriented by the particular basic dimension also appears on the drawing. So, it is not the basic dimension that overrides a default tolerance, it is the geometric tolerance that does so. The requirement to have a basic dimension follows the application of the geometric tolerance.

So again, basic dimensions, by themselves, do absolutely nothing. Only when a geometric tolerance appears that requires a basic dimension to locate and/or orient its tolerance zone does the basic dimension have any effect upon anything. Unused basic dimensions that might appear on a drawing only consume ink and add a bit of clutter, and if dealing with the "partially informed", may cause some confusion. Unused basic dimensions should not appear on the drawing, I think we all agree, but the issue seems to be what effect their accidental inclusion would have. Unless dealing with those partially informed folks, I assure you that the effect is non-existent.

Dean

http://www.d3w-engineering.com

 

[dingy2]

DrCADD:

Wow - I have read some of the posts here and that is all I can say.

Some thoughts of basic dimensions from ASME Y14.5-2009:

7.2 (b) Basic dimension establish the true position from specified datums and between interrelatted features."

8.2 "A true profile is a profle defined by basic radii, basic angular dimensions, basic coordinate dimensions, basic size dimensions, undimensioned drawings, forumulas, or mathematical data, including design models."

4.24.7 Datum Target Dimensions

"The location and size, where applicable, of datum targets are designed with either basic or toleranced dimensions."

Just some other application of basic dimensions.

Dave D.

http://www.qmsi.ca

 

[fsincox]

JP,
Without implied tolerances reference dimensions are irrelevant. A basic bolt circle with six holes does not need to say 5X 60 deg it says 6X 60 deg. Reference dimensions exist only to say: "ignore me for tolerancing". Basic dimensions, assuming they are correct, can be redundant.
Frank

 

[Belanger]

Frank,
The statement was made earlier that "a basic dimension is a dimension to override any general tolerances."
My point was that parentheses around a number are also an indication to override general tolerances (which is exactly what you are saying, "ignore me") Yet that doesn't mean that a reference dimension is equivalent to a basic dimension.

Somehow I think we're saying the same thing, but I was presenting it in a way to dispel some misconceptions about basic dimensions.

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