GD&T, when applied correctly, allows larger, more functional tolerance zones. So, if everyone involved understands the language, parts should be less expensive when properly defined.
If geometric tolerancing is not used, the designer will still hav to address those same qualities, but in another manner. There is still a need to control size, form, orientation, and location, but without GD&T it is often left to "assumptions."
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
I don't know about a general statement about being less expensive when applying GD&T but an "appropriate" application of GD&T allows the Designer to better define the part's requirements and in many cases can increase the tolerance (diametrical tolerance zone and MMC) rather than tighten it.
The overuse of GD&T, such as default tolerancing as an example, may not tend to decrease the cost of processing (could increase it) and in some applications, the datum structure may not be appropriate and could be confusing. We could actually incorporate gauges on features that are not important to the parts function and mating relationship increasing costs.
Most importantly, both the Designer and user must be competent in the application and interpretation of GD&T to capture its benefits.
You may add cost in the form of upfront drafting time and training. However, if you are currently controlling geometric relationships with lengthy notes, your drafting time may stay the same or even decrease.
We once outsourced the machining for a set of LARGE wheel hubs. The tolerances were not defined with GD&T. The bolt hole circle diameter was machined within tolerance. However, the hole pattern was off-center from the shaft pilot by about .1". The parts did not mate into the assembly properly. We could not legally reject the parts since the relationships between these features were not defined. In this case, GD&T would have saved us thousands of dollars.
This incident was the catalyst for us to start switching to GD&T.
lifttrucks - Your experience with the bolt circle being off centre from the pilot shaft is a great example of the value of utilizing GD&T (positional in your case) but when one sees a drawing that has all the dimensions in GD&T (excluding some size dimensions) with many not having any function or relationship, then the application of GD&T could become costly.
I agree. I think any company implementing a new dim and tol standard should take great care in making sure they are selecting the proper controls to assure they are defining the required relational and functional geometry of their parts. It is easy for us (engineers) to make manufacturing and inspection do more work than is actually required.
The difficulty here is comparing apples to apples.
So many non GD&T drawings (plus at least some that make poor use of it) do not anything like fully define the part.
Now often you'll get OK parts from these drawings (basically you're getting better parts than you've actually specified taking advantage of the general accuracy of the machines, work standards of the machine operators etc.) but as lift trucks example shows, you could end up with parts that technically meet the drawing requirements (or lack of requirements) but don't work.
You then give them a GD&T drawing of the part which more fully define the requirements and get charged more, giving the impression that GD&T costs more. However, this cost increase could come from a number of sources such as:
1. Person quoting not understanding GD&T properly and arbitrarily adding $ for each FCF (maybe this doesn't really happen but it seems like it).
2. More explicit requirements on the drawing (regardless of if they're specified in GD&T or note form) causing more inspection.
3. Inspection of GD&T taking more effort (not so sure on this one but I've heard it claimed).
4. GD&T needing to be interpreted for the shop floor guys.
5. The additional requirements causing them to change process to ensure they meet the requirements.
And this is assuming the GD&T was applied properly and didn't accidentally tighten tolerances because the person creating the drawing didn't understand the GD&T either, which sadly happens.
I've been involved in redrawing non GD&T drawings to include GD&T and as part of the process really looking into fit and functional requirements. Often these parts were effectively captive to one vendor - the machine shop that built the prototypes from crummy drawings with verbal input from the engineer and/or things they'd learned in the process. In some cases they actually had their own red lined prints or detailed routings that captured stuff the released drawings didn't. In the worst cases as I recall there was just one specific operator at one specific vendor that could reliably make the parts.
In many of these cases there was a net reduction in tolerance, going from general +-.002 almost everywhere to tolerances dictated by fit & function making use of GD&T - occasionally tightening underspecified tolerances/adding missing requirements but in many places loosening them if nothing else by use of circular position zones and MMC.
Depressingly often these parts came back costing more, although at least we could now get alternative sources of supply.
So based on my experience, you might get the impression that GD&T parts tend to cost more but if you look into the underlying factors, and allow for the risk/cost of having to accept bad parts, or maybe even scrapping good parts (function OK but don't meet poorly defined drawing requirements) then I'm inclined to stick with GD&T. However, that doesn't mean there isn't room in my mind for doubt.
Think of GD&T as legal protection. Most of the time things can be made with certain assumptions in place, but if you get parts that deviate from that assumption, the manufacturer can point to the drawing and say, "well you didn't specify which edge of the part is the primary datum" or other such statements. And guess what -- if GD&T is absent then he would be right and you're stuck with a batch of bad parts.
Analogy: you can write a will on scratch paper and have it witnessed. Or you can have a lawyer help you prepare a simple one. Which one could cost more in the long run?
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
As designs use more complex geometry, gd&t will be the only way to document designs. Plus/minus will be in the minority, and eventually become extinct, imho.
Well, Peter, I hope not. There are so many features on a part that have no meaning to the part's function and mating relationship that there should always be a place for a plus/minus tolerance. Many features could be out of specification and it would not affect the part whatsoever.
I realize that many trainers in this subject use the philosophy that all features excluding some features of size should have their tolerances reflected with GD&T but the current ASME Y14.5 standard examples does not support that thought.
Perhaps the physical attributes of the future products will determine how fast gd&t evolves. It seems that the idea of 'perpendicular', 'parallel', 'concentric', is slowly being replaced by 'rounded', 'soft', 'bold'... Also, objects are getting insanely small. Our definitions of physical things and how to measure them will need to evolve.
