five things you need to know about gd&t - amazon s3 · five things you need to know about...
TRANSCRIPT
Five Things You Need to Know
About GD&TJohn-Paul Belanger
Certified Sr. GD&T Professional
July 17, 2014
Agenda
Today, we will be discussing the following topics:
• What is GD&T?
• The true benefits of GD&T usage
• More than learning some symbols
• Dispelling some myths
• Five things to remember!
• Questions/Answers
2
What Is GD&T?(One answer: A language)
Basics of GD&T
• Stands for “geometric dimensioning and tolerancing”
• Shown on a print with symbols
• Think of it as a language
• Sometimes creates confusion (not the fault of GD&T!)
4
Sample GD&T Drawing
Why Would That Print Be Confusing?
• I am not fluent the GD&T language
• Some of the GD&T may be incorrect / illegal
• The GD&T requirements are not practical for the part’s function
• The GD&T doesn’t tell me how to make the part
6
Consider the Following…
7
Suppose I’m an art collector, and I want a replica of “Starry Night”
What requirements would I demand of the person making the replica?
Requirements
It depends on what I will use it for, and also on what I am willing to pay!
• If it will only be seen from a distance, then it doesn’t need to have
textured brush strokes; a scan of the real picture will suffice
• If it will be seen up close, I might require textured brush strokes
• The textured strokes will make the replica more expensive
• Regardless, I don’t dictate the procedure, only the final requirements
8
The Same with Designing a Product!
9
-- Function drives the dimensioning and tolerancing
FUNCTION is what should drive the tolerancing. If anything else is
the main driver, then we may be artificially driving up cost
Think of the simple block that was shown before – suppose the
function is to assemble against another part, and then simply
receive a mating pin…
The First Key Point:It’s all about the function
Without GD&T – Is the Tolerancing Based on Function?
Coordinate Tolerancing
A square tolerance zone
doesn’t make much sense
when we think about the
FUNCTION of the hole.
Sample GD&T Drawing
Sample GD&T Drawing The shape of the tolerance
is now cylindrical
The Second Key Point:It doesn’t make inspection more difficult
How to Inspect This Part
Inspection Methods
17
Just based on the GD&T, there are 4 things to be checked:
o Location of the 3 bolts
o Profile of the triangular perimeter
o Flatness of the mating surface
o Location of the large tube
In addition to the regular dimensions (not shown)
If we didn’t use GD&T, would it be easier to inspect?
No!
• Those qualities would still have to be inspected, but they would be
documented by different tolerance expressions (plus/minus, etc.)
• What makes parts difficult to inspect is their functional requirements,
not the language used to express those requirements!
18
GD&T Often Makes Inspection Easier
• The GD&T system spells out the datums – those features that the
measurements will be taken from. Traditional tolerances sometimes
resulted in ambiguous datums!
• If it seems like the inspection is made more difficult, it might just be that
the designer has overconstrained some things. Remember, it should
always go back to the functional requirements.
19
Ambiguous Datum Sequence
Ambiguous Datum Sequence
Ambiguous Datum Sequence
Ambiguous Datum Sequence
The Third Key Point:It’s not just about memorizing symbols, but overall fluency
Anyone Can Memorize the GD&T Symbols
But fluency in GD&T is the
key! You need to
understand the rules and
concepts that form the
“glue” of GD&T – just like
any other language.
More Than Just Learning Symbols – Another Example
Defined by a Technical Standard: ASME Y14.5
1994 edition 2009 edition
The Fourth Key Point:GD&T can actually reduce part cost
How Can GD&T Make the Part Less Expensive?
• We’ve already seen how GD&T clarifies the datums that dimensions are
taken from
• GD&T allows for many different shapes to a tolerance – more versatile
than the plus/minus system
• By giving the designer more tools in the toolbox, certain tolerances can
be isolated and given larger or smaller values as needed
29
And… Another Unique Way that GD&T Reduces Cost
• Using the circled M modifier (“maximum material condition”), a GD&T
callout can actually get more tolerance than what’s shown on the
drawing
30
M refers to “maximium
material condition,” which
in this case is the smallest
possible hole (more
material on the part)
Using the “M” ModifierOnly a hole made at “MMC” must be within the 0.4…
Ø17.9Ø18.2
Ø18.5
The Fifth Key Point:GD&T is here to stay
A Long History, and a Long Future
• The GD&T system in its earliest form dates from the late 1940s
• Even small shops need to know this language to stay competitive
• On a personal level, fluency in GD&T is a great skill to have
33
GD&T Skills Are Essential for Staying Competitive
34
– Even after all this if you don’t think GD&T seems valuable, your
suppliers and customers do!
– It’s been around for more than 50 years, and it is here to stay.
Summary
• Geometric Dimensioning & Tolerancing provides more tools in our design toolbox.
• The result should be better communication between design, manufacturing, and inspection, and reduced costs.
• Because GD&T is a language, it requires knowledge and practice.
Learn More About GD&TWe can provide a GD&T solution that is right for your needs. We have a range of resources available, including:
• Instructors
• Online classes
• Books
• Videos
Check out our upcoming Practical GD&T Workshop
• October 22-23 in Chicago, IL
• Attend virtually or in person!
• Go to toolingu.com/gdtworkshop for more info
37
If you would like to learn
more, please contact:
Tooling U-SME
866.706.8665
www.toolingu.com