Major benefits of GD&T usage
Major benefits of GD&T usage
(OP)
I would like to start this thread based on some thoughts that came to my mind during answering to Evan's post about surface interpretation of position tolerance:
http://www .eng-tips. com/viewth read.cfm?q id=286461& amp;page=1
As I said there, I have a feeling that quite a lot of GD&T users (at least the ones I work with) would say that hard gaging is the only way for checking positional tolerance at MMC. But since they know it is not really possible to use gage simulators in verifying their design (let's say because there is a company strategy that prohibits hard gaging usage), they simply do not apply M modifier, even if they feel it would be reasonable or even required from functional point of view. By doing this they intentionally or unintentionally loose one of the biggest advantages of GD&T language over traditional coordinate dimensioning - tolerance zone is becoming unnecessarily fixed in size regardless of feature of size.
That's why I wanted to ask you Guys about your experience with any other important benefits of GD&T usage that you feel are not utilized properly or not used at all in your everyday's pratice.
Any remarks are really welcomed.
http://www
As I said there, I have a feeling that quite a lot of GD&T users (at least the ones I work with) would say that hard gaging is the only way for checking positional tolerance at MMC. But since they know it is not really possible to use gage simulators in verifying their design (let's say because there is a company strategy that prohibits hard gaging usage), they simply do not apply M modifier, even if they feel it would be reasonable or even required from functional point of view. By doing this they intentionally or unintentionally loose one of the biggest advantages of GD&T language over traditional coordinate dimensioning - tolerance zone is becoming unnecessarily fixed in size regardless of feature of size.
That's why I wanted to ask you Guys about your experience with any other important benefits of GD&T usage that you feel are not utilized properly or not used at all in your everyday's pratice.
Any remarks are really welcomed.





RE: Major benefits of GD&T usage
A lot of machinist and 'purchasing engineers' these days don't understand it anyway.
There are a lot of benefits, especially making quality and accurate parts that actually fit together like they are designed to. Unfortunately making $$ overrides quality anymore. I have heard "Send it to China, who cares about GD&T. They don't understand it there anyway".
Chris
SolidWorks 10 SP4.0
ctopher's home
SolidWorks Legion
RE: Major benefits of GD&T usage
There are companies that have adapted and then shelved the application of GD&T but my philosophy on its application is a bit different from most here. This may be due to the fact that I have in the past measured products with linear tolerances using CMM equipment.
If a feature has a function and mating relationship, use it. If not, linear or angularity tolerances may be appropriate.
I do agree that many people still do not understand GD&T and the 2009 edition with its various boundary conditions for datums would blow them away. I still see datum gauges using a V-cone (RFS) where the drawing reflected MMC. Training for the last couple of years has been difficult to obtain (non-existent) with some companies just starting to have a training budget.
Dave D.
www.qmsi.ca
RE: Major benefits of GD&T usage
We need to choose our words carefully here. A company can adopt GD&T, train everybody, check drawings to ensure everyone is using it correctly, and then require their fabricators and inspectors to follow the drawing instructions. The results will be better communication between the designer, drafters, fabricators and inspectors, as well as compentently prepared drawings. All this is very desirable.
Alternately, a company can purchase the standard, write a note on its drawings stating that they conform to ASME Y14.5-2009, and then continue doing everything the way they have been doing it for the last thirty years. You have manuals, and perhaps some training, but very few benefits.
On top of that, GD&T is nothing more than a language. A language allows us communicate clearly and unambiguously, however, this is not the only thing we have to do. You have to organize the drawing clearly. You have to understand the requirements of the part. When you apply the GD&T, you have to understand the manufacturing process so that you specify realistic tolerances. If the realistic tolerances do not work, you have to re-think your design. Ultimately, the ideas you are clearly communicating in the language, can be stupid.
I am not aware of GD&T being systematically taught at the college or university level. I have been told that all my co-workers understand the standard, even though I am pretty certain that they do not.
I tell people that documentation is a link in a chain. There are a bunch of other things that have to work too.
RE: Major benefits of GD&T usage
1994 std 5.3.2.1 this note was added....
"NOTE: In certain cases of extreme form deviation (within limits
of size) or orientation deviation of the hole, the tolerance in terms
of the axis may not be exactly equivalent to the tolerance in
terms of the surface. In such cases, the surface interpretation shall
take precedence."
RE: Major benefits of GD&T usage
Jim Sykes, P.Eng, GDTP-S
Profile Services www.profileservices.ca
TecEase, Inc. www.tec-ease.com
RE: Major benefits of GD&T usage
RE: Major benefits of GD&T usage
"they simply do not apply M modifier, even if they feel it would be reasonable or even required from functional point of view."
I think "they" make sense by purposely excluding it.
Correct me if i'm wrong, but adding that "M" modifier simply increases production costs. Simply use a realistic (large enough)tolerance zone that you know the manufacturer can easily achieve without problems.
Then, quality control is straightforeward and fast(in the zone, ok, outside not ok). In reality the holes are in the correct position because the manufacturer can achieve them easily (which he acknowledged by quoting on your drawing). Furthermore in the unlikely scenario that they are really off, the manufacturer will nearly always accept that fact and compensate/correct his mistake (again i don't think that will happen often).
In my understanding the "M" modifier can often be avoided by aplying a realistic (large enough)tolerance zone.
I believe such "overdone GD&T" profoundly works against it.
RE: Major benefits of GD&T usage
"Send it to China, who cares about GD&T. They don't understand it there anyway"
In fact, there is a great deal of sense in that statement( for the better or worse).
RE: Major benefits of GD&T usage
"Correct me if i'm wrong, but adding that 'M' modifier simply increases production costs. Simply use a realistic (large enough)tolerance zone that you know the manufacturer can easily achieve without problems."
I'd like to gently correct you on that. The "M" modifier always lowers production cost. (The only exception is the cost expended by training production personnel on how to read GD&T, but that's a different discussion.)
You can't make a "large enough" tolerance zone, because the zone that you would be talking about would have to be one designed for worst-case size (smallest hole/largest pin). The point of the "M" is to provide tolerance beyond the given amount, but only if the size of the feature is adjusted to accommodate it.
If production personnel simply ignore the "M" then they'll end up with the tolerance zone that you would have been giving them without the "M" anyhow.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Major benefits of GD&T usage
The only exception is the cost expended by training production personnel on how to read GD&T, but that's a different discussion."
And that is exactly what i mean with "extra costs". Someone has to to do these inspections, wether it be in/outside the company. Eithercase it will be reflect in the total costs.
Furthermore, the personel has to understand this principle also, which is often problematic.
Quote:
"You can't make a "large enough" tolerance zone, because the zone that you would be talking about would have to be one designed for worst-case size"
Yes, indeed, but what's "impossible" with that? The tolerance zone has to be based on "worst case" to start with.
RE: Major benefits of GD&T usage
On the tolerance zone size: I meant that without the "M" you'd be stuck with that tolerance amount only. But with the "M" there may be more tolerance allowed that what is stated on the drawing. That sure sounds like a cost-saving opportunity!
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Major benefits of GD&T usage
Again, with current (accurate)CNC machining and a realistic chosen tolerance zone, i don't see the need.
RE: Major benefits of GD&T usage
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Major benefits of GD&T usage
The modifier is well understood between GD&T inspectors on the floor, but difficulties arise when "higher up" needs to be involved in case of problems.
The modifier "forces" these managers with a concept that is IMHO not that intuitive, thus creating (some) frustration and in the end not helping the acceptance of GD&T, sort of.
RE: Major benefits of GD&T usage
I keep claiming here that any idiot can produce functional machining drawings. Machining is accurate enough to make most parts work without any special effort by the machinist.
This is true, even of the drawing is hopeless crap. At some point, the machinist can assume the drafter is an idiot. They can make the part to the nominal dimensions to machining tolerances, and no one will be the wiser.
When you get into sheet metal, castings and weldments, the standard drawing tolerances are more accurate that what can be achieved by the fabricators. Now, you can screw up badly. If you cannot apply realistic tolerances to your drawings, you cannot do a tolerance stackup, and identify and fix design problems. You cannot identify the one or two really critical tolerances the fabricator may be able to hit if were taking your drawings seriously.
RE: Major benefits of GD&T usage
RE: Major benefits of GD&T usage
Based on my experience I can tell you that I dealt with numerous applications where standard (or whatever we call them) tolerances were not enough to assure part's functioning. Manufacturer received from design dept. drawings with easy-to-achieve tolerances and was very happy of that but the produced detail did not function properly in assembly and re-work was necessary.
What I mean is that very often designers must define narrow tolerances (tighter than preffered by manufacturer) to fulfill part's function. And this is a place for M modifier and all other GD&T things which seemingly look to be "cost raising". They help manufacturers in situations when tight tolerances are difficult to meet.
So I have to fully disagree with the statement that: "<M> modifier simply increases production costs". This is one of the myths which probably stems from poor GD&T knowledge amongst people involved in development of different types of products (designers, manufacturers, inspectors, managers etc.) and is repeated very often.
It was also nice to read that every idiot can produce functional machining drawings
RE: Major benefits of GD&T usage
" I keep claiming here that any idiot can produce functional machining drawings."
I'm baffled to see somebody with practical experience make such a statement.
Furthermore, I was trying to make the case that the M modifier is less important when there is a large diameter differences between the holes and bolts. When not, it DOES make sense, i never said it did not.
I tried to state these different bewteen such situations, but discussion is pointless sad to say, i won't further participe.
RE: Major benefits of GD&T usage
The diameter difference and the positional tolerance should be under your control. If you have one, but not the other, you are in trouble.
Problem:
Specify a tapped hole and a clearance hole for an M6 screw clamping a thin plate to a base.
I call up an M6X1 H6 hole with a positional tolerance of Ø0.3mm. I do not bother with MMC/MMB here because the bonus tolerance does not help me assemble the thing. If I had some of the inspection tools described above, I might look into it.
My clearance hole will be positioned to zero, at MMC/MMB. This means that my perfectly located hole must be just big enough to clear an maximally inaccurate screw. I need to clear Ø6.3mm, and I add a little in case the tapped hole is not perfectly perpendicular. I could have added a projected tolerance to my tapped hole. My minimum hole is Ø6.4mm. My maximum clearance allows for maximum error in location and drilling. I allow 0.5mm for drilling, and 0.3mm for a positional tolerance. My maximum hole is Ø7.2mm.
I have never discussed this with our machine shop, but I have one complaint only about tolerances like this. He accepted my explanation. Obviously, I do not anticipate receiving a part with a hole that measures Ø6.4mm.
If I do not call up MMC/MMB, I must specify a feasible positional tolerance, how about Ø0.3mm? My smallest hole must allow for this, plus a small safety margin. Ø6.7mm. My large hole accounts for the drilling accuracy. Ø7.2mm.
The zero positional tolerance provides a larger, more flexible error budget.
RE: Major benefits of GD&T usage
It looks like you've taken your ball and gone home but in case you're still lurking just to check out any further responses, I'm going to add my two cents.
JP was giving you tremendous amount of leeway when he conceded that any increased costs associated with the M modifier could be attributed to training. Thats WAAAAAY more than anyone else would have likely given you. The M modifier reduces cost...period. If you don't use it just because you're afraid someone won't understand it, you're still not making a less expensive part. This argument is more accurately directed at GD&T in general. You can't just have a training class in the use of the M modifier for bonus tolerance. That class would take all of 5 minutes. Explaining its use regarding datum shift is a totally different thing, but that's not what we're talking about. The bonus tolerance allowed by MMC is very easily understood once you sit down and think about it.
Your argument about the M modifier not being intuitive is true, but only in the sense that nothing about GD&T is intuitive. Intuitive means that you can just look at something with no prior knowledge and be able to tell how it's used.
No matter how offensive it sounded, drawoh was spot on with his comment. Shop floors are rife with screwed up prints that drafters think are perfect but the machinists have just learned to deal with. When good parts are produced, the drafters think the prints are good.
If you continue your current thought process that just because we are in the modern CNC era, you can apply unnecessarily tight tolerances with no added cost, you are going to cost your company more money than any MMC modifier ever could. If you really want to control costs, you should start with that...AND you should listen to what people like JP and others are saying. They've been on this board for years and have dealt with these issues on a daily basis. You are the only one on this thread that thinks the MMC modifier adds cost. You should consider that someone else might know something more about this than you.
Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X4
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Major benefits of GD&T usage
At the same time, it provides a simple and common means to communicate design intent. Whenever a standard tolerance band applies, by all means use it. But by using GD&T, including the "bonus" tolerance, you can dramatically reduce costs. How many times are parts found to be defective only to then be treated as "use-as-is"? Why, because the default tolerance was easy to use and understand, but did not fully communicate what the design could tolerate. Or parts carefully and methodically CNC machined to ultra-tight tolerances (because they CAN), but at the cost of being less tolerant of how the part actually functions?
If your industry or design doesn't really need them then don't use them. But you will find in high performance or well optimized designs, they will have a place. The cost savings of understanding how to communicate the design through effective GD&T truly outweighs the investment in training. But if your company insists that CNC operators just turn the machines on and off and shouldn't need to understand tolerances, then GD&T isn't for you. Those are parts most likely to be made in China. Sloppy designs, either over-specified (expensive) or underspecified (expensive-prone to not function as needed) are just ridiculous. Robust design analysis should tell you when you need GD&T. Otherwise don't use it.
RE: Major benefits of GD&T usage
RE: Major benefits of GD&T usage
In many cases, it can save money and is quicker to draw on a drawing. In some cases (as when its being used improperly), it can cost money. However, there are legitimate uses that may be time consuming to figure out how to do them correctly if you aren't familar with that portion of the standard.
Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion
&
RE: Major benefits of GD&T usage
"As the clearance hole gets bigger, it's location can vary more and still allow the screw/bolt through"
Really simple, if the holes bigger, it doesn't need to line up as well. Now applying this properly is a bit trickier but not rocket science.
M allows you more tolerance, I struggle to see how this doesn't tend toward being cheaper.
You can choose to inspect ignoring M. If it passes great, if it fails, well if you take M into account it may now pass. Or you can decide to scrap the part. Your choice.
321GO, from a US (and to an extent as I experienced it UK) perspective, Function is the primary driver of dimensioning & tolerancing. The most important thing being that parts work.
With the Iso system, at least 2768, it seems that manufacturing is given priority in driving tolerance scheme. 2768 is entirely based around 'what typical shops can achieve' etc.
This perspective, and the predominance of US based posters on this site may explain some of the disagreement.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Major benefits of GD&T usage
yes, the German DIN-ISO-2768 is in essence indeed based on the "shop capabilities". The idea is to simplify the drawing(nothing new there).
To be precize, it has 2 parts(1,2). The first part covers tolerances for linear/angular and chamfer dimensions based on "shop capabilities".
The second part covers GD&T also again in different classifications based on shop capabilities.
Generaly it would be stated as below on a drawing:
Tolerances and GD&T according: DIN-ISO-2768 mK (the "m" is a indication for the toleance class and the "K" for the GD&T class)
To ensure a correct fit, one can specify overruling tolerances and GD&T on specific features when needed.
(KENET), is the US approach not nearly indentical in essence?
RE: Major benefits of GD&T usage
"The ability / mechanism to accept as many functional parts as possible using cost effective tolerancing".
My list of GDTadvantages:
Datums to more clearly define / convey part interface, feature function; inspection setup; mfg;
Cylindrical tolerance zone vs. Cartesian
Form controls
Orientation controls
Rule #1 controls perfect form at MMC (unless accepted by note)
Bonus tolerance relative to MMC / LMC
Composite flatness
Runout
I have been around long enough to have had to use various "verbiage"
on drawings prior to using the 1973 ANSI Y14 standard to "describe" the list above, which can be
open to more varied interpretation than the standard's symbols. concepts and definitions.
In short a good design and communication of that design needs to be
understood by a myriad of people. Whether or not it seems difficult to explain
the concepts, definitions, and application of the standard, just try to do it without the
symbol language of GDT. It is very possible to put out a simple drawing without GDT
but more than likely it will not consider nor communicate the design intent nearly
as well or completely compared to using GDT.
RE: Major benefits of GD&T usage
ASME Y14.5M-1994 - the Dimensioning & Tolerancing standard - puts the emphasis clearly on function.
2768 - which I am unfortunately moderately familiar with - has a bit at the end which (to paraphrase) essentially says a part doesn't have to meet the requirements of the standard, it should be accepted or rejected based on function.
Well if the drawing doesn't pretty much capture the required the functional requirements, then what does?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Major benefits of GD&T usage
RE: Major benefits of GD&T usage
Comparing 14.5 with 2768 is not even "apples to oranges". It's rather "apples to orange peels".
14.5 is EVERYTHING about dimensioning and tolerancing.
2768 is merely dealing with dimensions without explicit tolerances attached to them.
In that sence the closest thing to 2768 would be ANSI B4.3, which unfortunately now is "missing in action".
And about "non-rejection" the standart clearly sais "Unless otherwise stated", so it is your right (and responsibility) to state oterwise. You can even enter some legalese like "Clause A.4 does not apply" in your contract with manufacturer.
They probably don't ask for your advise when writing a contract (I know they don't care about mine), but that's different problem.
RE: Major benefits of GD&T usage
Just the concept that ASME accentuates function, while 2768 accentuates manufacturing without regard to function.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?