Machining GD&T parts workflow 1

[Ivan Silva]

Guys, I've started learning GD&T by my own and one of my main questions is what should the machinist do to awnser the GD&T requirements of a part. About this question anyone knows a youtube channel, book or any other source of information about the machinist point of view regarding milling, turning etc of a piece where GD&T is being applied?

 

[Burunduk]

The difference between parts toleranced according to the GD&T system and parts where there are only or mainly  plus minus or limit tolerances on the print, is that the GD&T specification tells you more unambiguously what is required in terms of geometry by describing the allowed variations from the basic or nominal in a standardized way. For example, if you mill a part, you could produce the datum feature surfaces first, in a way that allows achieving the datum feature accuracies required by the drawing, then clamp/fixture the part by the datum features for machining the other features, not before you set axes zeros or the machining "home" based on your fixture to correspond with the datum reference frame. Then, if you proceed  by drilling holes, for example, their coordinate locations would be per the basic dimensions locating the holes from the datums on the print. And, you could make reliable evaluations regarding the ability to hold the tolerance per your process accuracy (the condition of the mill axes, etc.).

 

[3DDave]

The first thing a machinist should do is discuss with the engineer and the Quality Assurance people what the expectations are. Quality Assurance will have data from the processes you will use to show which ones are likely to meet the requirements and the engineer can help by analyzing the set-up steps to see that the effort isn't wasted based on that information.

There aren't many resources - in 40 years I don't recall coming across any. The main efforts are from CMM and Quality Control metrology workers. I have seen a few machines that have integrated systems - tube benders - that will change the bending program to offset errors in bending, but I have not heard of any CNC machines

Almost never is that information collected by Quality Control in a way that is useful. The most I ever saw was "First part meets print. Last part meets print. Assume all parts meet print."

The fact you are asking is proof that the company you work for doesn't have a process to collect or use that information. The usual result is either spending too much time on features that aren't required to be so precise or scrapping or reworking parts that were made too carelessly to begin with, losing both machining time and the time wasted on inspected bad parts.

What is more important is that engineering and Quality Assurance should have collaborated before it went to be machined and produced a package for you already. If it's in your hands without that collaboration, the company has failed.

 

[greenimi]

I posted this before:
Product drawing versus process drawing

Now from Mark Foster:” AGI--Applied Geometrics--

http://www.GDandT.COM

“Mark Foster • Ok, so you gentlemen have touched on the biggest, absolutely most major issue, in all of proper GD&T implementation and usage -- datums/datum features -- manufacturing versus functional. This subject is also the most misunderstood and misapplied, and is therefore is also used as fodder for arguing against implementing GD&T. Let me preface the following with a general statement regarding datum/datum feature selection and defnition. Ultimately, function wins out in any dispute between which datums and datum features should be used to define our parts/assemblies. Having said that, in the real world, we must sometimes make some compromises for the "big picture" economics of the situation. Having said THAT, here are my thoughts on datum and datum feature design, defninition, selection, etc.:

One, everyone must realize that there are 2 major definitions of the part that must be defined, understood, and most importantly, definitively related (mathematically) to one another -- design funcitonality and manufacturing process(es) -- a.k.a. "Product" versus "Process" requirements. Two, we must all also understand that there are 2 corresponding inspections that may need to occur (notice I said, "May," not "Shall.") -- functional and process. And three, there may or may not be overlap in each of those cases. That is, in some instances, the manufacturing and the functional datum features may be one in the same, and in many other cases, they will be completely different.

Once we understand those three basic concepts, we can then begin to subdivide those into specific commodities and groups of processes -- for example, the datum structure that I would use for a machined part would be heavily focused on the "product" side, where the datum structure for a weldment may necessitate incorporating manufacturing fixturing (i.e. "process") in my datum scheme in order to define my functional requirements. The Y14.5 standard pretty explicitly states that we should not state the manufacturing process requirements on our product definitions unless those manufacturing processes are critical to the definition of the engineering requirements.

I could go on forever about this subject because I am so passionate about it, but suffice it to say that you MUST define your functional requirements somewhere, and then decide how to make the part from there.”
In your particular case, it "sounds" to me as though someone is using manufacturing fixturing/tooling/process information to determine their datum structure. I would like to see the functional datum features (and corresponding datums) defined fully before resorting to manufacturing fixturing as defining the datum structure, but welded assemblies is one place where we often must consider the manufacturing process in the development of the engineering requirements and definition. So I cannot say definitively whether the choice of datum structure in your example makes sense or not without directly seeing your particular design scenario.

My advice to all is to absolutely, positively, and completely define your FUNCTIONAL requirements up front, and then make economic decisions from there. If you don't completely define your functional requirements somewhere, there is a good chance that you will lose sight of them and end up chasing a lot of problems that never needed to exist and that you will have a tough time resolving.
I agree that it is "best" when you can have manufacturing datum features and functional datum features be the same. But the reality of life is that they rarely are. Usually manufacturing must produce the datum features, and very often, they would prefer to leave the part in some fixture that is locating the part off of features that will not, in the end product, be functional features. That is why I brought up the other issues in my previous post.

But absolutely the end product must meet its truly functional requirements from the functional datum features when the part is finished, or it will not function. What good is it if something is very easy to make and measure (according to the manufacturing datum features) if it doesn't work (or we cannot tell if it will work until we try it).
We all need to understand that there are two major considerations in the production of a part -- the product definition and the process definition -- and we can choose to use GD&T to describe either or both of these considerations. However, we must be careful to be clear when we are talking about one or the other.

Of course, the best of all worlds is when those two scenarios can make use of the same datum features in the same order of precedence, but that situation is rarely practical or even possible. So we must distinguish when we are talking about one or the other situation. I am a huge advocate of FUNCTIONAL (i.e. how the part physically interacts with its mating parts) datum features (and precedence) for PRODUCT definition -- i.e. no matter how the part is produced. Because, no matter how the part is produced, if it does not meet its functional dimensional requirements from its functional datum features, then it won't work as intended. What good does that do anyone?

But you certainly can use GD&T to define PROCESS requirements as well. However, in *MY* company (if we were a manufacturing firm), these drawings would be controlled by Manufacturing (i.e. NOT Design), and they would the words, "PROCESS DRAWING" in big red letters stamped across the drawing, so as to have no misunderstanding that what is being depicted are the PROCESS requirements, and NOT the PRODUCT requirements.

In short, PRODUCT requirements are the goals to achieve, not the instructions for how to make the part. And the PROCESS requirements are the instructions *for one particular supplier's chosen method* to achieve those goals that were stated on the PRODUCT drawing. The PRODUCT drawing is the legal requirement and obligation of the supplier. The PROCESS drawing is the chosen method for a given supplier, but another supplier could chose a different process to achieve the same PRODUCT requirements.

Having said all that, we teach that design people should begin their designs by first imagining the datums that they require for the product to function as desired, then DESIGN their datum features to be physically capable of arresting the degrees of freedom that they need arrested for the part to function as desired, and then dimensionally control features relative to those datum reference frames that they designed."

End of Mark's post

 

[TheTick]

As a former machinist, I say that one of the biggest advantages of GD&T is that you know what the datums are. I've made more than a few batches of non-GD&T parts where the difference between passing or failing inspection was creative fixturing.

 

[metrologic]

@greenimi

Interesting post, greenimi.

 

[Ivan Silva]

@Burunduk

It's clarify my mind. Thank you!

 

[Ivan Silva]

@3DDave

The communication between the Quality Control and engineering teams is in fact a necessary thing although does the discussing of machining and inspection workflow for each and every "GD&T part" is not a laborius matter? If it is happening maybe the drawing is not so clear as it should, is not?

Another question, since I'm not american and english is not my main language which do you mean by "First part meets print. Last part meets print. Assume all parts meet print.". What does mean "print"?

 

[Belanger]

Print = drawing = blueprint = engineering drawing

 

[powerhound]

What does mean "print"?

Same as "Dibujo" if your language is Spanish. Same as "Desenho" if it's Portugese.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[CWB1]

I always recommend Alex Krulikowski's "Fundamentals of GD&T" as the best and most common book I've seen on the subject, good for both learning GD&T and as a reference.

GD&T or not, the dimensioning/tolerancing scheme is rather irrelevant because the part either meets every requirement on the print or it doesn't. GD&T is simply another method of communicating those requirements. The ONLY expectations of the shop should be that they make parts which meet print and communicate problems, so to answer directly.... make parts, inspect parts, and call or email with issues.