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Projected Tolerance Zone: Equivalent to Tightening the Zone?

Posted by on Feb 26, 2020 in Uncategorized

If you’re familiar with the different GD&T modifiers, then you probably know that the circled P creates a “projected tolerance zone.”  This is often used on threaded holes to keep any fastener that protrudes beyond the threaded hole from causing interference with a mating part: Without the “P” modifier, the tolerance zone exists only within the depth of the threaded hole itself.  The result is that the threaded hole could tilt, and be passed for position tolerance, yet cause interference: So “P” is a good thing.  However, when this concept is presented in our GD&T classes, someone will occasionally ask if we could — as an alternative to “P” — simply tighten the position tolerance number instead.  The dialog might go like this: “Couldn’t we just change the 0.3 to 0.2 (or 0.1) and achieve the same effect of preventing too much tilt?” “Yes, that would be legal,” I answer. “But using the P allows us to keep a larger tolerance, while preventing interference.” “But it has the same effect of tightening the position tolerance anyway,” the student might reply. This is where we have to be careful.  It’s true that projecting the tolerance zone has the effect of tightening the perpendicularity aspect of a position tolerance (because it’s extended higher), but it still...

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Common Dimensioning Symbol Errors

Posted by on Apr 23, 2014 in Uncategorized

No matter how good a dimensioning system is — GD&T, anyone? — there will still be errors encountered on drawings, simply because there will always be human beings who are behind the creation of a new drawing.  And of course we all make mistakes.  But I want to point out a few of the more common mistakes that I encounter in my travels. •   Failure to include a diameter symbol in a feature control frame when needed.  I’m thinking particularly of position and perpendicularity. When tagging these tolerances to a hole or pin, you usually need to include the diameter symbol before the number, so that the axis of the feature is contained in all directions. There are times when a hole’s position tolerance should not use a diameter symbol: if you really only want the tolerance to apply in two directions.  But that must be clearly indicated by proper using of dimension arrows.       •   The next common error I’d like to review is similar to the first — using a diameter symbol when it shouldn’t be there!  I see this in feature control frames for circularity, cylindricity, circular runout, and total runout.  It might be tempting, because each of these is applied to...

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The Importance of Blueprint Reading

Posted by on Jan 5, 2011 in Uncategorized

This website and blog naturally focus on GD&T, but it’s a good time to discuss the importance of simple print-reading skills as a prerequisite to learning GD&T.  As I travel around teaching classes on GD&T, you’d be surprised how many people don’t fully understand some of the simple rules of drafting, view layouts, and notation on drawings. First, note that there can be different terms for this skill; the title of this blog entry mentions “blueprint” reading, but nobody uses actual “blue” prints anymore.  (This name was given because at one time they really were blue, due to the chemical process used in producing these drawings; see here for more on the history of this.)    I suppose a more proper term today would be an “engineering drawing” but if you want to call them blueprints still, hey, go ahead. If GD&T is to make sense, then the object being toleranced must certainly be understood first.  Most drawings use “orthographic” projection, which is simply a fancy name referring to the straight-on, flat view of a part from a particular angle.  Think of a cube: each of the six sides can be flattened out to display six orthographic views.  Depending on the part, there may be fewer or more than six orthographic...

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What if Datums Are Not Perpendicular?

Posted by on Aug 20, 2009 in Uncategorized

Out there in the GD&T world, there is often confusion about parts that have irregular shape. We are told that the theory of GD&T requires datums to be 90º to one another. Sure, that’s great in a textbook where the examples are nice, rectangular, flat plates!  But what about those other shapes?   It’s actually very easy.  The confusion is that people mistake the term “datum” for “datum feature.  The standard defines a datum as a theoretically exact point, axis, or plane.  But a datum feature is defined as a physical portion of the actual part from which the datum is derived.   Think about those two terms, and you’ll see that irregularly shaped parts pose no problem.  Even something shaped like a blob or a potato chip has a physical surface.  It may require using datum targets, but a theoretical plane can still be constructed from those targets.   So again, it’s true that the theoretical datums mentioned in a feature control frame are perpendicular to each other. But those theoretical datums can be derived from any crazy-shaped surface.   If you have the new 2009 ASME standard, see pages 81-90 for some neat examples.  (If you have the 1994 edition, see pages 54 and 78-79.)  Stay tuned...

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Does GD&T mandate that inspection use a fixture?

Posted by on Jul 1, 2009 in Uncategorized

In a GD&T class, I often talk about (and sketch) how a sample part can be held in a fixture — this helps people understand the concept of datums, particularly if datum targets are involved. This does not imply that an inspector must use a customized fixture to check a part. I refer to fixtures and physical gaging in a class simply because people can visualize those concepts, whereas a CMM is more abstract (sometimes CMMs and similar devices are called “soft gaging” as opposed to traditional “hard gaging”). If you are using a CMM, then you ensure that the probe samples the part at the prescribed datums; this establishes a coordinate system in the computer for other measurements to be made against. But wait: the part isn’t floating around in mid-air! It is still contacting something. Perhaps it is sitting on a granite table. Here’s a key point: instead of sampling three points on the surface of the part to create the datum, you should take three points on the table, since that table simulates the true datum (as derived from the high points of the part surface). The only tricky part is when datum targets are involved. This is where the designer identifies specific points, lines, or...

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Prerequisite Knowledge before Learning GD&T

Posted by on Aug 30, 2008 in Uncategorized

When I teach a GD&T class, I have to presume that eveyone is “green” about the topic. Even if some folks have been using GD&T regularly, I find it best to start from the beginning. This ensures that everyone is on the same page, and it sets the stage for presenting the various GD&T topics that will be examined in the class. However, in order to effectively learn the GD&T system, there is some prerequisite knowledge. Before signing up for a GD&T class, make sure you are comfortable with basic blueprint reading, such as how to interpret the various views on a print (top, front, side, sections, etc.). You should also be familiar with plus/minus tolerancing (including unilateral plus or minus) and common drafting practices. Here’s a simple example: Common drafting practice tells us to assume that the corners in the right-hand view are 90 degrees, and we also assume that the inside and outside diameters are to be made on the same center line. But that raises two questions:  What is the tolerance on the 90 degree corners?  And what is the tolerance on the possible offset between the diameters? According to the general tolerance given for the print, the corners can deviate anywhere from 89-91 degrees. So...

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