GD&T drawing feedback
GD&T drawing feedback
(OP)
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.google userconten t.com/_Awr aIBDQGDg/T WbUoStJM0I /AAAAAAAAA dk/fdPsNQc SnnU/s912/ IMG_0900%2 0small.jpg
Here is my drawing - https://lh 5.googleus ercontent. com/_AwraI BDQGDg/TWb VjTBJqJI/A AAAAAAAAdo /Ork01ZT_3 fM/s912/Gr aphics%20T xt%20Fig%2 011.51.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.
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://
Here is my drawing - https://lh
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.





RE: GD&T drawing feedback
Matt Lorono, CSWP
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RE: GD&T drawing feedback
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: GD&T drawing feedback
Matt Lorono, CSWP
Lorono's SolidWorks Resources & SolidWorks Legion
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RE: GD&T drawing feedback
RE: GD&T drawing feedback
Do you have a copy of the standard(s)?
I would suggest using position on the threaded holes. Using position on hole patterns to ensure parts matched was pretty much how GD&T started out as I understand it, so I'd stick with it. If you just attach and fcf to a thread call out you are dimensioning to the pitch diameter.
Do not dimension to hidden detail, per the current ASME stds.
In fact, I'd minimize use of hidden detail as it creates a very busy drawing - often 'less is more'.
You can't use 'basic' dimension for the diameters of holes you then put position tols on.
No you haven't indicated B & C correctly, you need to attach the same kind of anchor you'd use on a surface to the FCF.
Your flatness fcf's would probably be better coming off the surface, rather than leader to it.
It may be more appropriate to attach the datum A identifier to the flatness fcf for that surface.
You're missing some dimension, I noticed on a couple of the threaded holes but there may be others.
I'm sure that's not an exhaustive list but should keep you busy!
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: GD&T drawing feedback
I am trying for the 2009 standard, though my book and some internet information is for the previous standard which leads me to some confusion.
I understand most of your critiques, thanks! One that I couldn't find was where I dimensioned to a hidden line.
RE: GD&T drawing feedback
- for inch dimensions leading zeros should be omitted (except for angular dimensions - but this is not the point in your case);
- for multiplied features you should use something like '4X' not '4 X';
- for other missing dimensions: I can not see basic dimension for diameter of bigger collar (similar to dimension |ø2.0| on smaller collar);
- if you want to keep general note for dimensional tolerances to be within ±0.005 (which I think is not so good idea) then there is no need for putting the tolerance for four ø0.656 holes.
RE: GD&T drawing feedback
It's a debatable one but I found your drawing somewhat 'busy' with all that hidden detail and would suggest less hidden detail and a couple extra views as needed.
I realize you may be in 2D where this takes quite a bit extra work, so you have to trade off. However, if using 3D modelling extra views are cheap so within reason take advantage of it.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: GD&T drawing feedback
RE: GD&T drawing feedback
1. If all dimensions are basic on your drawing, you may simplify it by deleting the square symbolic of all basic dimensions and add a note as :
Unless otherwise specified: Untoleranced dimensions are basic
This will make your drawing looks clean and neat, not so complicated.
2. There are four places, diameter symbol Ø missed on the position tolerance callout, for all of those features the shape of the tolerance zone is a small cylinder, so the diameter symbol can't be omitted.
3. More comments in addition to pmarc :
* Use of X to indicate "Times"---a space is used between the X and the dimension, please note that no space is allowed preceding the X.
For example : 8X Ø6
* Use of X to indicate "By"---the X shall be preceded and followed by one character space.
For example : when a slot that has a given width by a specified length ( 5 X 30) or a chamfer that has equal sides (3 X 3).
4. Dimension line and extension line can't use the center line, this is the one others will confused as a hidden line ( dim 2.88 is one of the errors).
5. The four holes shoule be called out 4X Ø.656±.005, not depth symbol.
Thanks
SeasonLee
RE: GD&T drawing feedback
RE: GD&T drawing feedback
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: GD&T drawing feedback
In the first place, the part is a casting of unfinished edges. In the second place BASIC implies theoretically perfect, such as stock materials.
Making every dimension of a drawing basic is incorrect and not workable.
RE: GD&T drawing feedback
However, I agree in context it's probably not appropriate.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: GD&T drawing feedback
This statement is just not true. If this part is cast then it certainly isn't the way to dimension it but to issue a blanket statement like this is misleading. It is entirely possible to create a part using all basic dimensions and it be completely manufacturable and GD&T legal. Holes are better dealt with using size tolerances but profile of a surface isn't illegal to use.
Powerhound, GDTP T-0419
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RE: GD&T drawing feedback
While I admire your service to our country as well as your title, I stand by what I have stated and you are wrong.
I hold two degrees and was taught GD&T by one of its masters who also taught it to Caterpillar Tractor.
If you are making basic dimensions off of surfaces that are truly reference dimensions (the outsides of castings) you are doing it wrong!
I teach the subject in higher education and have taught it since 1993 after taking two 400 and 500 level courses in graduate school.
Find one example to back up your claim to this drawing being done right. It is wrong.
I can find you all kinds of examples in the two books I use by Goodheart-Willcox Publishing that show the correct use of basic dimensions but that would be a violation of copyright and you can go buy them for yourself... and you should also take a class from someone who "thinks" they know it.
BASIC dimension - a dimension that is considered to be theoretically perfect.
A dimension of location used to dimension a feature of size having a tolerance.
I have worked in industry as an engineer while teaching. NEVER have I ever had an application where I did not use this for a pattern of holes to true position them.
RE: GD&T drawing feedback
Huh? Fair use covers the limited scope of the examples you could show. Or, you make briefly create a similar one yourself as an example.
"BASIC dimension - a dimension that is considered to be theoretically perfect."
Theoretically perfect, but not perfect. Right?
In practice, 3D solid models are entirely BASIC. There's really no way to use a 3D solid model effectively unless it is considered BASIC. ASME Y14.41-2003 covers this exact topic in para. 7.2
Matt Lorono, CSWP
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RE: GD&T drawing feedback
You: "Find one example to back up your claim to this drawing being done right."
Me: I did not claim that this drawing was done right. Please re-read my post.
You: "NEVER have I ever had an application where I did not use this for a pattern of holes to true position them."
Me: What does this have to do with what we're talking about? You make it sound like I said you couldn't position holes with basic dimensions.
You: "If you are making basic dimensions off of surfaces that are truly reference dimensions (the outsides of castings) you are doing it wrong!"
Me: I agree with this. Again, please re-read my post. Start paying attention somewhere around the second sentence.
Try this link for some support of my ludicrous notion:
http://www.tec-ease.com/gdt-tips-view.php?q=87
Just for example, in the drawing done by panzer there is a .76 basic dimension in the lower left corner of the lower left view. The tolerance block has a default profile tolerance of .010 to A, B, and C. That means there is a .010 wide tolerance zone equally disposed about a true profile located .76 from datum A. Apply this same principle to the rest of the part. Tell me what the problem is with that.
I will remind you that I said in my original post that if this is a casting, this is not the way to do it. My issue was with your blanket statement that "Making every dimension of a drawing basic is incorrect and not workable."
Powerhound, GDTP T-0419
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RE: GD&T drawing feedback
Basic dimensions in the problem of this post should only be used off of the flange edges that would have presumably been ground flat to locate the patterns of holes in true position.
My point is you do not just randomly throw BASIC dimensions at everything. That is not the correct application of GD&T. Any dimension taken off of the rough edge of a casting cannot be basic. They are treated as reference dimensions as they are random and will varie from part to part.
The example you offered was a round part that was turned. That is not the same as a rectangular casting. Apples and oranges.
RE: GD&T drawing feedback
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." - Robert Hunter
RE: GD&T drawing feedback
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.
RE: GD&T drawing feedback
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: GD&T drawing feedback
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
www.d3w-engineering.com
RE: GD&T drawing feedback
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." - Robert Hunter
RE: GD&T drawing feedback
Matt Lorono, CSWP
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RE: GD&T drawing feedback
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
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RE: GD&T drawing feedback
"Good to know you got shoes to wear when you find the floor." - Robert Hunter
RE: GD&T drawing feedback
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.
RE: GD&T drawing feedback
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." - Robert Hunter
RE: GD&T drawing feedback
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.
RE: GD&T drawing feedback
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
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RE: GD&T drawing feedback
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:
h
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.
RE: GD&T drawing feedback
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." - Robert Hunter
RE: GD&T drawing feedback
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
www.d3w-engineering.com
RE: GD&T drawing feedback
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.
RE: GD&T drawing feedback
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
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RE: GD&T drawing feedback
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.
RE: GD&T drawing feedback
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.
RE: GD&T drawing feedback
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'.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: GD&T drawing feedback
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
RE: GD&T drawing feedback
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.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: GD&T drawing feedback
I have to confess that I still have problems with some of your terminology...
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." - Robert Hunter
RE: GD&T drawing feedback
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.
RE: GD&T drawing feedback
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." - Robert Hunter
RE: GD&T drawing feedback
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
RE: GD&T drawing feedback
Matt Lorono, CSWP
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RE: GD&T drawing feedback
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
www.d3w-engineering.com
RE: GD&T drawing feedback
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
RE: GD&T drawing feedback
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
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RE: GD&T drawing feedback
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.
RE: GD&T drawing feedback
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: GD&T drawing feedback
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.
RE: GD&T drawing feedback
Frank
RE: GD&T drawing feedback
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
RE: GD&T drawing feedback
Frank
RE: GD&T drawing feedback
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
www.d3w-engineering.com
RE: GD&T drawing feedback
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.
www.qmsi.ca
RE: GD&T drawing feedback
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
RE: GD&T drawing feedback
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
RE: GD&T drawing feedback
I hope so.
I have seen so many problems caused by the limitations of implied title block tolerances over the years that it is hard not to speak up. It seems the forward/preface to the new standard says almost the same, saying something like, direct tolerance methods are only really apropriate for FOS. I don't have it here with me now.
Frank