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.

 

[ewh]

You lost me with this one.

...BASIC implies theoretically perfect, such as stock materials

I have never come across a mateial spec that claimed stock size was theoretically perfect.

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

 

[DrCADD]

So when you buy something like 1 inch round bar stock, you do not expect it to be 1 inch?

I have trouble getting this through to my students all the time...

I use two other examples to help them understand... one is mechanical and the other architectural:

When you go to the hardware store to buy a screw to replace one you lost, do you not expect it to work in the empty hole when you get home? It is made basic to a trusted and universal standard.

When you go to the lumber yard to buy a 4x8 sheet of plywood, do you not expect it to be 4 by 8 feet?

Basic is like a given. You have to be able to trust it to always know what you are going to get.

 

[ewh]

When you are working with tightly toleranced assemblies (±.0005 and tighter), stock tolerances ARE taken into account and NOTHING is assumed to be "perfect".

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

 

[DeanD3W]

Basic dimensions are best thought of as locating and orienting tolerance zones for location or orientation tolerances... Nothing is ever "made basic".

If basic dimensions are placed all over a drawing, with no tolerance specifications, then they have no meaning and they do nothing. Unneeded basic dimensions should be deleted from a drawing, but other than adding clutter, they don't have any negative effect.

Dean

http://www.d3w-engineering.com

 

[ewh]

One exception:
Datum target points can be dimensioned using basic dimensions without any geometric control. Tolerances then default to "established tooling or gaging tolerances" (ASME Y14.5-1994 para 4.6.2; Y14.5-2009 para 4.24.7).

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

 

[fcsuper]

So, considering the entire solid model as BASIC is wrong, even though ASME Y14.41 specifically says this is how one treats a solid model?

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

 

[powerhound]

DrCADD,
You clearly have your own set of standards that are independent of any established and documented national standards. Your argument has changed from "Making every dimension of a drawing basic is incorrect and not workable." to "you do not just randomly throw BASIC dimensions at everything." These are two different arguments.


Go to any lumber yard and try to buy a 2 X 4 that actually measures 2 X 4. Try to buy a 1/4" thick sheet of acrylic that actually measures .250.

I get the feeling that neither I nor anyone else on this board is likely to convince you that you need to rethink your understanding of basic dimensions.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[ewh]

This may be a reflection of the many posts regarding GD&T training in higher education.

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

 

[DrCADD]

"You clearly have your own set of standards that are independent of any established and documented national standards."

Find me one drawing in a text or ASME manual dimensioned like the one that began this blog. You won't! It is done wrong.

"Unneeded basic dimensions should be deleted from a drawing, but other than adding clutter, they don't have any negative effect."

Yes and no. The negative effect is you are asking someone to hold to that dimension and nothing else will do. That is why guys down in the shop want to slap guys like us that can put anything down on paper with little concern as to how it will be made.

Think of it this way; You have a pattern of holes to be made in a plate to positional tolerance. You make a drill jig to locate this pattern of holes. The jig ain't changing. You may get some drill chatter, or wobble, which is why the holes need tolerances of size and location, but the jig is offering you the centers of those holes BASIC!

You guys talk like you are clueless about how things get made... You never over spec dimensions tighter than something needs to be acutally made to function. It ups the cost.

Basic means we really want something to that dimension and assume it to be there. The outside of a casting would never be called to be basic unless a flat spot was ground into it such as a datum target zone from which a basic dimension could begin.

My example of 2x4's are good because more people have built things like garden sheds and dog houses rather than made things in a machine shop. While they are not 2x4 any more, they are all made 3.5x1.5 stock size or basic. Their lengths are reference as they seldom all are cut to the exact same length.

Glad to help.

 

[ewh]

Your example of 2x4's is horse**** in the manufacturing world. Stock size IS NOT basic! It may be fine for building a shed but total garbage when designing a precise fuel metering valve or nano range measuring equipment.

...the jig is offering you the centers of those holes BASIC!

Time to come down from your ivory tower. The jig is not and has never offered up features located to perfect"BASIC" dimensions, only features APPROACHING the theoretical basic. NOTHING, NOTHING is perfect, only good enough. Toolmaking tolerances are much tighter than those used in design by necessity, but they are still tolerances.
Take some time and read "Zen and the Art of Motorcycle Maintenence" and maybe you'll start to understand that the world we live in is far from PERFECT.

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

 

[DrCADD]

Okay... I tried to educate some people here who pretty much think they know it all but show they have yet to truly see the world.

Many times have I gotten my butt chewed by my elders for coming off as you are to me now.

You think you know it all but you do not. Offer some proof rather than rattle your gums.

I never meant to imply a 2x4 as being basic. What I said was I use the example of a 2x4 to contrast basic dimensions with reference dimensions. The size is assumed as a set standard. The lenghts are reference as you just can't count on them to be as close to perfect as the stock size.

Take a class before you lip off to me again. You truly do not know what you are talking about.

The drill jig I offered up as a machine example before is a perfect example of the association of basic dimensioning to positional tolerancing of a pattern of holes.

If you have not worked in industry before, and I do not think you have, here is another good one for you that you can understand. The studs on the hub of your car. The wheel is going to look to mate with those studs. You can grab it an shake it and hear some slop but once you lug it down, off you go. The studs ain't going to change. They are basic. Wheels on the other hand, well... One time I tried to put Firebird wheels on a Camaro I had. They went on but were not safe to drive that way. The positional tolerance was off to the studs... too much slop. Even to lug them down would not make them safe. The studs did not change. The pattern of holes was out of tolerance to the basic dimensions of the stud pattern. Location and size. Basic dimensions are about location. Size dimensions are about such things as positional tolerances. Look it up.

 

[powerhound]

"Find me one drawing in a text or ASME manual dimensioned like the one that began this blog. You won't! It is done wrong."

Let's start with Fig. 6-12 in ASME Y14.5M-1994. From there, turn the page to find Fig. 6-13. Oops, that's actually two examples, sorry. I don't have Y14.41-2003 but if I did, I would probably find one or two more examples.

Seriously, what is up with your arrogance? The rest of us here play nice and actually try to learn from one another. You, however, seem to have it in your head that there's nothing left for you to learn because you have not one, but TWO degrees.

Your example of the drill jig was kind of funny. Did you learn that in wood shop?

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[DrCADD]

That must be why I have the 2009 standards and have pitched the other two you alude to.

I will try to figure out if those figures you mention are still in there and what you are talking about.

All I am trying to do here is have a teaching moment where you do not go out of here and get railed by some ole vet in the CNC shop that has been around the block a lot more times than you have.

Here is a link to a drawing making proper use of basic dimensions:

http://www.designengineerlife.com/wp-content/uploads/2010/08/gd-and-t_basic-dimension_1.jpg

One could imagine this same drawing as a casting of this post and the outside dimensions would be referenced or left identified as neither basic or reference.

 

[ewh]

You may not have meant to imply a 2x4 as being basic, but you did state (not imply)that the stock size was basic.
Just because the studs don't change doesn't mean that they are basic. BASIC is defined as a perfect number with no variation. The variation comes in through the geometric tolerance. Referring to studs as being basic makes no sense. The dimensions that they were fabricated to may be basic, but the studs themselves are "good enough" by meeting the geometric tolerance allowed. A fundamental rule of understanding a language (and GD&T is a language) is to understand the definition of terms. Basic refers to diminsions, not features.
Have you ever actually done precise mechanical or tooling design?
Tossing standards may be great in academia, but in the real world, those other standards are still legitimate, depending on the data they have to work from.
Actually accusing me of not having worked in industry just goes to show that your position is weak, and it's time to bring on your straw men. No thanks on your offer of education. There are many on this forum who understand GD&T much better than I, and while I may not always agree with them, I am still learning much from them. Many of them are also wisely bypassing this thread.
...wood shop...

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

 

[DeanD3W]

Hello Again,

DrCADD - Do you have a copy of ASME Y14.5 on hand and are you willing to cite the sections that would support your assertions? It will be much better to keep the discussion here technical and polite. Our degrees, work experience, membership on standards committees, etc lend no weight to an argument. Only the technical merit of our assertions and the support we provide for them has merit. We are all learning, including you and I.

Given that, when two parts mate, they both have tolerance associated with all mating features. To view one part as "basic" and the other at toleranced, or varying is not possible. This is actually clear using common sense, rather than any standard. The studs in a hub vary in location and orientation just as the holes in a wheel do. I believe that I'm speaking of rather undeniable truth here, aren't I? Most lug holes have a tapered seat for conical lug nuts, so it's not necessarily as simple as dealing with cylindrical features, but either way both parts have tolerance associated with them. The role of basic dimensions is to define the theoretically exact location orientation, and for profile tolerances, the shape of, tolerance zones for those tolerances on all of the features of each of the parts in the assembly.

fcsuper - Yes, you're correct about Y14.41... Those basic dimensions from the solid model are queried when needed for a specific call-out and laying in wait to be queried if not needed for a specific call-out.

Dean

http://www.d3w-engineering.com

 

[DrCADD]

Well, you are a smart ass who thinks he knows it all so I will admit I was wrong in trying to offer your something to keep you from making an ass out of yourself one day.

It is pretty plain to me you have yet to work in the field.

The drawing submitted with every dimension made basic is in error. Anybody that truly knows GD&T would spot this in a heart beat.

With people like you as our next generation, we all had better start to learn Chinese.

 

[powerhound]

Thanks DrCADD, but maybe you should spend more time trying to have a learning moment.

I've been a CNC machinist since 1989 and have been programming since around 1993. I began delving into the finer details of GD&T somewhere around 1995 and got my ASME Technologist certification in it in 2007. At this point I AM the ole CNC vet and I'm positive that you have no idea how many times I've been around the block so don't presume to know that too. The only one that's gonna get railed by the ole CNC vet is you.

I figured you'd have an excuse as to why you can't refer to the figures I provided.

Again, your arrogance is profound.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[DrCADD]

Dean,

How do you think they inspect parts in industry?

The example of the wheel hub and studs with two similar but different wheels is a great way to understand basic dimensions and positional tolerancing.

The studs, center marks of the holes located by basic dimensions, are assumed to be perfect. The wheel, on the other hand, can be a go or no go. When I was a young punk like ewh, I had this idea Firebird wheels would fit on my Camaro as they were the same car made in the same plant in Ohio. I got a quick lesson in GD&T the hard way.

I posted a correct drawing using basic and positional tolerance.

I am done arguing until somebody posts something to show me wrong. This is a waste of my time.

 

[DrCADD]

Not as bad as you Powerhound. I have been in this since 1975 and my professor was on the ANSI board. He helped Caterpillar go metric and go GD&T.

Try again.

As my good ole professor used to say, "There are a lot of bad GD&T drawings out there done by people who think they know what they are talking about but don't."

I have been teaching this since 1993.

My professor, now retired, has his own consulting firm in GD&T and travels the world teaching this topic.

One of the greatest lessons of GD&T is about tolerance. You sir, need to learn that.

 

[KENAT]

OK knock if off the lot of you.

DrCADD, you are new posting to this forum so I initially tried to use kid gloves and then avoided posting all together but some of the aspersions you've cast at contributors who I've come to appreciate & respect are not appropriate.

The drawing in the OP certainly had issues, no one is debating that. For the intent of it throwing basic dims everywhere was almost certainly not appropriate. However, appropriate/cost effective GD&T/dimensioning isn't always quite the same thing as technically correct/in compliance with the standard.

You unfortunately used terms in too general/sweeping of a way, without making clear that you were just talking specifically about the OP drawing. You also used terms and examples differently from how I and others have usually seen them used.

You also rubbed a bunch of people the wrong way by flashing your alleged qualifications around and rubbing them in peoples faces.

Most of the regular contributors here do have a pedigree in industry, though some may be from a drafting background, some from and Inspection background and others degreed engineers. Coming in as a new member and decrying the input of others with some pedigree on this site (see the MVP list on the right) is hardly a good way to 'make friends & influence people'.

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