Geometric Dimensioning and tolerancing-Basics
GD&T |Geometric Dimensioning and tolerancing
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| Basic training on GD&T |
As in the heading, there are two words first GD&T and second Drawing Projection. So fist i am going to discuss about drawing Projection.
Here i will explain different projection method of part in 2D drawing. Whenever we read any engineering drawing we see following things:-
1. Tile block
2. BOM details
3. Drawing Notes
4. Dimensions and Tolerances
5. GD&T symbols
6. Different projection method
So when we read any drawing we see single part drawing in different -different projection. How we will identify projection of part in 2D drawing. There are 2 types of Projections which are mostly used in drawing, 1st angle projection & 3rd angle projection. These projections are basically nothing but the means of representing 3D object in 2D drawing. 1st and 3rd angle projection are nothing but the way of describing what an object look like from different direction.
First angle projection Symbol
First angle projection is orthographic projection in which we represent 3D object in different prospective projection images called views. In this projection method object is placed in first quadrant. This projection method is used internally (Mostly is Europe & Asia) but not used by USA & Australia. Graphical symbol for First angle Projection is as below:-
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| First angle Projection |
You will note from first angle projection:-
1. View from the front is in the middle
2. The view from left is on Right
3. The view from the Right is on Left
4. The view from the top is on bottom
5. The view from the bottom is on Top
6. The view from the rear is on the far Right
For more clarity, see below picture:-
Third angle projection Symbol
In this projection we place 3D object in 3rd quadrant. This projection is mostly used in Japan & USA. Graphical symbol of 3rd angle projection is as below:–
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| Third angle projection Symbol |
You will note from 3rd angle projection:-
1. View from the front is in the middle
2. The view from left is on Left
3. The view from the Right is on Right
4. The view from the top is on Top
5. The view from the bottom is on Bottom
6. The view from the rear is on the far Right
See below picture for more clarity-
You can see, point 1 & 6 are same in both projection method.
Now let’s understand GD &T in details:-
GD&T stand for Geometric dimension and tolerance, GD&T is a symbolic language used on engineering drawings. This language of GD&T consists of dimensions, tolerances, symbols that can be used to precisely communicate the functional requirements for the location, orientation, size, and form of each feature of the design model.
We use these tools while product designs. The designer’s purpose is to make the design in such a way that it is easy to read and understand, and the part manufacturing is easy by using that drawing.
For a design GD&T tools are communication tools. With GD&T help, designer gives all the instructions in the drawings, which people from different departments of the company understand and make a part and sustain quality of that part.
GD&T are direction tools which help to understand purpose of designer, what he trying to say by this symbol. Because if we understand the meaning of the symbol, we can assign a control check point for that symbol to sustain Quality.
1. Form Tolerance- All symbol under this specification are independent upon datum
2. Orientation Tolerance
3. Location Tolerance
4. Run Out Tolerance
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| All GD&T Symbols and characteristics |
Above picture is symbolic indication of tolerance feature. You can see above picture in engineering drawing, but indication of this feature could be different.
General tolerance is applicable for few GD&T symbols. Before going further in details, first understand what datum feature is?
Datum could be theoretical plane, axis or point location. It is like an anchor for entire part, where the other feature reference from.
Datum Indication in Drawing
1. On the Surface
2. On an Axis [We check Run-out, Perpendicularity, Concentricity]
3. On the point or on Hole
Datum and GD&T Symbols
Types of Datum in drawing
1. Primary Datum
2. Secondary Datum
3. Tertiary Datum
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| Datum Types in GD&T |
Datum could be indicate, basically there are 2 types of datum feature indication.
1. Integral feature indication
2. Derived feature indication
Now lets understand each GD&T symbol in detail.
For flatness, roundness and Straightness is explained, click to read.
Cylindricity
This graphical symbol describes how close an object is true cylinder at each point along entire cylindrical length. Cylindrical feature ensure that
object is round enough and straight enough along its axis. In cylindrical tolerance two concentric cylinders that run the entire length of the feature – one inner and one outer, in which all the points on the entire surface of the cylindrical feature must fall into. Cylindricity is independent of any datum feature.
We can check this tolerance feature by using height gauge. Part can be fixed in jig/Magnet block- Put Height gauge probe on surface, set zero value in height gauge and Slide height gauge along the entire surface length. Variation is checked (Let’s say we observed maximum value 0.05 so total variation will be 0.05-0=0.05 (50 micron), which is more than 0.03 (NG).
Attribute gauge can also be used to check cylindricity of an object. We will use Ring gauge.
The relation of this symbol is combined with circularity and surface straightness, but all three have different meanings. Circularity is roundness
around a single point (Just like coin), but cylindricity is roundness around a line (Like PIPE). Surface straightness is different- No bend or curve along the
surface of an object.
Profile of Line
Generally applied to parts which have varying cross sections or specific cross sections that are critical to functionality. Profile of a line describes
a tolerance zone around any line in any feature, usually of a curved shape. Profile of a line indicates how much a particular cross section of that feature can vary from a true curved radius.
A common use of line profile would be if you were comparing a curving surface such as the hood of a car, or an airplane wing. Profile of a line controls individual lines of a feature, usually having a curved shape.
Mostly we check by using CMM. Or we can verify by gauge (Mating gauge/Profile Gauge)
Relation with Surface Profile:-
Line profile is related to profile of a line on curved surface (a line start from point and end at another); we check curve line on a surface. While profile of surface is related to profile of entire curved surface; we check profile of curve around entire surface.
Surface Profile
In surface profile we check entire surface where the radius is has to fall within the tolerance zone. Surface Profile controls all the points along the surface within a tolerance range.
Measurement of surface profile:-
Inspection could be done by fixing object on magnet block and checking variation by moving height gauge & puppy dial arrangement.
When we check profile of surface on an object, we check how much curve on surface. So when we check flatness, we ensure each point on a plane should align within tolerance of surface plane.
Parallelism
This GD&T Symbol describes a parallel orientation of one referenced feature to a datum surface or line. It relates the orientation of one surface plane parallel to another datum plane.
It is tolerance that controls parallelism between two surfaces or features. The surface form is controlled similar to flatness with two parallel planes.
It is a tolerance that controls how parallel a specific parts central axis needs to be to a datum plane or axis. The axis form is controlled by a cylinder around a theoretical perfectly parallel axis.
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| Parallelism in GD&T |
We check parallelism of a surface with respect to datum or reference plane. For that we have to freeze or fix the datum( Make sure datum is properly aligned with zero deviation) and check with height gauge. Put height gauge probe on surface check the reading on different-different location. If variation is beyond the tolerance then result is NG.
8. Perpendicularity
This symbol describes the orientation of one surface plane perpendicular to another datum plane. This requires the referenced surface or line to be perpendicular or 90° from a datum surface or line.
It is a tolerance that controls Perpendicularity between two 90° surfaces, or features.
It is a tolerance that controls how perpendicular a specific axis needs to be to a datum.
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| Perpendicularity symbol in GD&T |
Surface Perpendicularity can be check with the help of Height gauge. In this measurement datum is fix 90 degree to base plate (using magnet)- move puppy
dial probe across the surface which is perpendicular to datum (just like we check flatness). Or we check surface flatness with the help of Right angle gauge.
For Axis perpendicularity, Maximum material condition is mostly considered while checking perpendicularity by using a gauge. Axis perpendicularity with respect to datum could be in two conditions 1) There is Hole in body 2) There is shaft on body (Cylinder on Plane surface). Accordingly we have to check with gauge, so for condition of 1st we will use Pin gauge for condition 2nd we will use Hole gauge.
Size of those gauges is decided by using these rules:-
1. Gauge size for an internal feature (like a hole):
Gauge Ø (pin gauge)= Min Ø of hole (MMC) – Perpendicularity Tolerance
2. Gauge size for an external feature (like a pin):
Gauge Ø (hole gauge) = Max Ø of pin (MMC) + Perpendicularity Tolerance
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| Perpendicularity measurement method |
Angularity
Angularity is the symbol that describes the specific orientation of one feature to another at a referenced angle.
Angularity related to Datum & as well as MMC & LMC.
Perpendicularity and Parallelism are just special cases of Angularity.
Perpendicularity is 90° to a datum and parallelism is 0° to a datum. Angularity controls a surface (non feature of size), a center plane or an axis of a feature of size to a specified angle. These tolerances control the orientation of features to a datum plane or axis.
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| Angularity symbol in GD&T |
Measurement of angularity is done with the help of Sine gauge or angle protector. As explained angularity is basically orientation w.r.t to any datum plane and which has two special cases first -Perpendicularity and second- Parallelism. So we not only check angle but also ensure perpendicularity & parallelism (depend upon case to case condition).
We fix the datum and then we check angle with gauge, when we do this we not only check angle that time we have to ensure parallelism within tolerance. You can understand by below image.
True Position
Some people say only Position, but why true position?. You can understand this by definition.
Definition:- Position is defined as the total permissible variation that a feature can have from its “true” position. This symbol is position of tolerance. It is exact coordinate or location defined by basic dimensions.
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| Position Symbol in GD&T |
Measurement can be done with the help of height gauge & CMM. Also attribute gauge can be used to verify true position of any tolerance feature.
For Location of feature:- We fix the datum as base with zero. And check dimensional variation with respect to datum for the position of hole or pin. Formula can be use to check if this position is ok or not?
Square root of [(Actual X-True X)²+( Actual Y-True Y)²] result from this formula should be less than position tolerance.
For MMC & LMC:- We ensure the true position of feature in MMC & LMC condition by using attribute gauge, here gauge size matter. Hole size cannot be change, only its position could change, to issue position of hole within MMC we can check by mating pin.
For position of Hole: – We have to set diameter of pin. This will be (Minimum of hole-Position tolerance).
Run-Out
It is amount of variation of reference feature with respect to another datum when the part is rotate 360˚ around the datum axis. In other words, it indicates what extent surface of rotation when being rotated around its axis. And run-out tolerance gives the permissible deviation in the surface when it is rotated around its axis (datum).
Total Run-out same thing, only difference we measure along the whole surface during rotation while in run-out we check at one section of the surface.
For checking, we have to fix puppy dial on the surface where we have to check, now rotate the object with respect to its axis. Check variation in dial gauge.
Run-out is combination of Concentricity & circularity. [Run-out=Concentricity + Circularity]
If object is perfectly round, then run-out will be equal to the concentricity.
Total Run-Out
As explained we have to check permissible deviation on the whole surface when that object is rotate at 360 degree through it axis.
Concentricity
It is also called coaxially. When we check concentricity of an object we basically ensure its axis variation with respect to central axis of object.
If object is perfectly round, concentricity could be half of run-out.
So this was short description of GD&T, hope you find this article helpful.
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