GD&T - Toronto Mechanical Design, Part and Assembly Drawings, Toronto CAD Drawings

9/9/2014

GD&T - Toronto Mechanical Design, Part and Assembly drawings, Toronto CAD drawings

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Geometric Dimensioning and Tolerancing 1. Datum, Datun planes, Datum features and Datum targets. A datum is a theoretical exact point, axis or plane from which the location or geometric characteristic of a part feature are established. It's a starting point or origin. A datum established by an actual physical part feature called a datum feature. A datum feature typically has an important functional relationship to the part feature being specified. On a drawing, it's identified by a special datum feature symbol. Example: A flat surface may be used to establish a datum plane. A cylindrical feature, such as a shaft, may be used to establish a datum axis. A slot may be used to establish a datum center plane. By definition, a datum is theoretically exact or perfect. However, the actual part feature used to establish the datum is not perfect. Therefore, the datum is simulated through contact with precision manufacturing or inspection equipment. This provides a more accurate stating point from which to measure. (Refer to here for more)

2. Definations: Basic Dimension - A basic dimension is a theoretically exact value used to describe the exact size, profile, orientation or location of a feature. A basic dimension should always associated with a feature control frame or datum target. Block tolerance does not apply and the applicable tolerance will be given within the feature control frame. Basic dimensions are enclosed within a box. Use basic dimensioning to locate features (e.g. holes), use tolerances on the size of features (e.g. holes).

Form Tolerances 3. Straightness

The feature control frame shows that each line element of the surface of the pin must be straight within 0.05 mm.

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This means that each lengthwise line element of the surface of the pin must lie between two parallel lines that are 0.05 mm apart. This applies to any lengthwise line element of the surface. However, the tolerance zone (0.05 mm) need not be parallel to the axis of the pin. There are two kinds of straightness tolerance - (1) Straightness of an axis or center plane and (2) surface straightness. The type of straightness is determined by the placement of the feature control frame. When the feature control frame is next to the size dimension, it is controlling the axis or center plane.

When the feature control frame is on a leader line pointing to a surface, the straightness is applied to line elements in the surface.

Another example:

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The above means:

The derived median line of the feature actual local sizes must lie within a cylindrical tolerance zone of 0.04 diameter at MMC. As each actual local size departs from MMC, an increase in the local diameter of the tolerance cylinder is allowed which is equal to the amount of such departure. Each circular element of the surface must be within the specified limit of size. (See more details here) The straightness symb ol is sometimes used to ensure mating features (e.g. a dowel or other press-fit assemb ly) will create a tight fit without the use of fasteners.

4. Flatness

The flatness control means that the surface must lie between two parallel planes that are 0.006" apart. The part thickness (1.000-1.020") must also be within the Envelope of Perfect Form unless otherwise specified on the drawing. In other words, the feature must fall within the size tolerance, e.g. at MMC (Maximum Material Condition) of 1.020", the surface must be perfectly flat. When verifying flatness, the feature of size is first measured to verify that it falls within the limits of size (1.000-1.020). Please note: Flatness applies to the entire surface while straightness applies to a single linear element. http://www.torontodrawings.com/9.html

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5. Circularity

Each circular element of the surface in any plane perpendicular to a common axis must be within the specified tolerance of size and must lie between two concentric circle (one having a radius .0.10 larger than the other). The part cannot extend beyond its envelope of perfect form. At the MMC (21 mm diameter), the part must be perfectly round and straight.

6. Cylindricity

Orientation Tolerances 7. Parallelism

Example 1.

Example 2.

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Means:

Example 3.

Means:

Example 4.

8. Perpendicular Perpendicularity of a flat surface to a datum Plane:

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GD&T - Toronto Mechanical Design, Part and Assembly drawings, Toronto CAD drawings

Perpendicularity of a cylindrical surface to a datum Plane:

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At MMC, the feature axis must lie within a cylindrical zone of 0.3 diameter which is perpendicular to and projects from datum plane A for the 14mm specified height. The feature axis must be within the specified tolerance of location over the projected height. The following print shows a part that specify the Ø1.000-inch pin perpendicular to the top surface of the part within a tolerance of .040 at MMC. A gage is also shown to inspect the part.

9. Angularity

Profile Tolerances 10. Profile of a Line A two-dimensional tolerance zone that controls individual line elements of a feature or surface. Profile of a line is usually applied to parts with varying cross-sections, or to specific cross sections critical to a part's function. Examples of parts where profile of a line could be applied include aircraft wings and housings used to seal out dust or water.

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11. Profile of a Surface A geometric tolerance that controls how much a surface can deviate from the true profile. Profile is a three-dimensional tolerance that applies in all directions regardless of the drawing view where the tolerance is specified. It is usually used on parts with complex outer shape and a constant cross-section like extrusions.

Means: The top surface of the part must lie within a profile tolerance zone of 1.5 mm on each side of the basic profile. This tolerance is applied to the basic print dimension of 30 mm measured from datum plane "A

(Refer to here for more details)

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The entire surface of the die cavity must lie within a profile tolerance zone of 0.015 outside the true profile.

The entire surface of the punch must lie within a profile tolerance zone of 0.015 inside the true profile. Note that if the leader from a profile feature control frame points directly to the true profile, the tolerance specified is equally disposed about the true profile . If the leader from a profile tolerance points directly to a segment of a phantom line extending, outside or inside, parallel to the profile, then all the tolerance is outside or inside the true profile. Note that if the design requires a smaller radius than the radius allowed by the profile tolerance, a local note such as, “ALL CORNERS R.015 MAX,” or “R.015 MAX” is directed to the radius with a leader.

12. Circular Runout Runout is a measure of how perfectly a circular part rotates about its axis.

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Circular runout applies independently to each circular element on the surface of a part either constructed around a datum axis (left image above) or perpendicular to a datum axis (right image above) as the part is rotated 360° about its datum axis. Runout is measured as the FULL INDICATOR MOVEMENT (FIM). For example, if the needle on the dial moves from -1 to +1, the FIM is 2 mm. The boxed symbols can be read "each circular element of this surface must have full indicator movement (FIM) of less than 0.05 relative to datum A".

The above image shows a sample measurement taken at one cross section, but multiple measurements are required to verify runout. Note that the indicator is applied perpendicular to the measured surface, and that this tolerance controls only individual circular elements and not the whole surface simultaneously. Where applied to surfaces of revolution, circular runout controls a combination of variations in circularity and coaxiality.

Circular runout example:

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Control the left shaft with a circular runout tolerance of .003 to both the left and right shafts. Control the right shaft with a circular runout tolerance of .003 to both the left and right shafts. Control the middle wheel with a circular runout tolerance of .015 to both the left and right shafts. This means that both datum features are used to establish a common datum axis. The controlled feature must not have more than 0.003(0.015) indicator movement in relation to the common axis established by datum features "A" and "B".

When inspecting circular runout, the feature, first, must fall within the specified size limits. It must not exceed the boundary of perfect form at maximum material condition. The datum feature is then mounted in a chuck or a collet. With a dial indicator contacting the surface to be inspected, the part is rotated 360◦ about its simulated datum axis. Several measuring positions are inspected. If the full indicator movement (FIM) does not exceed the specified runout tolerance, the feature is acceptable. Runout tolerance may be larger than the size tolerance. If the runout tolerance is larger than the size tolerance and no other geometric tolerance is applied, the size tolerance controls the form. If the size tolerance is larger than the runout tolerance, circular runout refines circularity as well as controls coaxiality. The same preliminary checks required for circular runout are also required for total runout. Just as when inspecting circular runout, a dial indicator contacts the surface to be inspected, but the dial indicator is moved along the full length of the feature’s profile as the part is rotated 360◦ about its simulated datum axis. If the FIM does not exceed the specified runout tolerance, the feature is acceptable.

13. Total Runout http://www.torontodrawings.com/9.html

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Total runout is similar to circular runout with the exception that the dial indicator is moved back and forth over the entire controlled surface while the part is rotated. The full indicator movement on the dial indicator over the entire surface of the controlled feature cannot be more than 0.02 mm. This also controls cumulative variations of straightness, roundness and taper of the surface. Where applied to surfaces constructed around a datum axis, total runout controls a combination of surface variations such as circularity, straightness, coaxiality, angularity, taper, and profile. Where applied to surfaces at a 90° angle to the datum axis, total runout controls a combination of variations ofwobble, flatness and perpendicularity to the datum axis.

14. Position

15. Concentricity

16. Symmetry Circular runout inspection: Torontodrawings.com

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