Reading a Production Drawing
Engineering Drawing vs. Production Drawing
Engineering Drawing is what most of us have learnt during the preliminary years of engineering — Isometric drawings to orthographic projection and vice versa. A small example of such practice can be:
But these drawings are incomplete not very useful during the actual manufacturing of the desired part. During production, a lot of additional information needs to be given to the manufacturer.
For example:
i. the allowable amount of deviation from actual dimensions or geometry,
ii. The standard dimensions and number of holes (if any),
iii. Information on threading- internal or external, pitch, etc.,
iv. Information on surface finish.
For this purpose, GD&T is used on engineering drawings or mechanical drawings to convert it into a production drawing.
What is GD&T?
GD&T stands for Geometric Dimensioning and Tolerancing — a system to define tolerances. It is a symbolic language used on engineering drawings and computer generated 3D models to define the allowable deviation of the feature’s geometry from its nominal dimensions.
It is us engineers’ very own code language! Although knowing GD&T is a must for Design Engineers, it is must that each and every engineer should know GD&T to properly read and decode a production drawing.
Let’s understand production drawings..
Production drawings are complete a sets of drawings that have detailed description of the manufacturing and assembly of designed products. The main purpose of production drawings is to define the size, shape, location and production of the component. Machine operators, production line workers and supervisors all use production drawings.
Design engineers use orthographic as the basis for both the component and assembly drawings. Production drawings contain graphic information prepared by the design team for use by the production team. Orthographic projections are supplied in production drawing, giving views of machine parts and their assembly in an accessible form. Sometimes an exploded from is given to explain the individual components.
Additional characteristics of production drawings:
- The production drawings may describe the preferred order in which to assemble components.
- If the engineering drawings call for a screw fastener to be tightened, the production drawings would typically describe the tool to be used and how it should be calibrated.
- Material and component specifics are also commonly provided in the title block of a production drawing.
- Assemblies of components are usually shown and the production drawings may specify where each assembled component will be built.
- Production drawings also record the number of parts that are required for making the assembled unit and may be required to authorise the production of the item described.
Take a look at the example of production drawing of a Companion Flange and notice how it is different from engineering drawing:
This component actually looks something like this :
Notice how the drawing specifies all geometrical and dimensional constraints that a manufacturer should consider while manufacturing the component. To understand what the constraints are, and what each symbol means, let us first study what is meant by tolerance.
What is meant by Tolerance?
The permissible variation of a size is called tolerance. It is the difference between the maximum and minimum permissible limits of the given size. Tolerance can be given in two directions of basic size. On that basis, it can be:
1. Unilateral Tolerance: If the variation is provided on one side of the basic size, it is termed as unilateral tolerance.
2. Bilateral Tolerance: If the variation is provided on both sides of the basic size, it is known as bilateral tolerance.
Great care and judgement must be exercised in deciding the tolerances which may be applied on various dimensions of a component. If tolerances are to be minimum, that is, if the accuracy requirements are severe, the cost of production increases. In fact, the actual specified tolerances dictate the method of manufacture. Hence, maximum possible tolerances must be recommended wherever possible.
Types of Tolerance:
1. Dimensional Tolerances:
It is the total amount of a specific dimension permitted to vary from its basic size, which is the difference between maximum and minimum permitted limits of sizes of that feature.
Dimensional tolerances are shown in drawings by three methods:
Eg: The maximum and minimum values for radius of the companion flange is shown by method 1, as highlighted in the drawing below.
The dimensional tolerance over the radius of the component can be at maximum, 100 mm and at minimum, 99 mm.
2. Geometrical Tolerances
Geometrical tolerances are used to convey in a brief and precise manner the complete geometrical requirements on engineering drawings. They should always be considered for surfaces which come into contact with other parts, especially when close tolerances are applied to the features concerned.
Geometrical tolerances are indicated on drawings by symbols, tolerances and datum references, all contained in compartments of a rectangular frame, which looks like follows:
This set of representation is known as a drawing callout. A lot of useful information about the tolerances over a feature can be given via a callout.
As we saw earlier, GD&T is an engineer’s code language. Each type of geometric tolerance has its own specific symbol.
Refer to the table below to understand what geometric characteristics can be given a tolerance and what are the symbols to represent them.
Now, let us consider the above example of production drawing of a Companion Flange (used in drive shafts for effective power transmission) and go through each tolerance mentioned in it and understand what it means:
Let’s take a look at all the dimensional tolerances mentioned in the side view of the drawing:
All the highlighted parts above indicate the dimensional tolerances on the features indicated.
Now, let us move on to the geometric tolerances:
1. Perpendicularity:
The highlighted callout describes the perpendicularity of feature w.r.t. datum A, which is 0.08 mm in terms of diametric dimensions.
In other words, two parallel planes or lines are oriented perpendicular to the datum feature or surface 0.08 mm apart. These planes are held perpendicular to the datum, but only ensure that the entire feature falls within the tolerance zone of 8 microns.
2. Parallelism:
This callout represents that the indicated feature is parallel to datum A within 0.06 mm. Surface Parallelism is a tolerance that controls parallelism between two surfaces or features. The surface form is controlled similar to flatness with two parallel planes acting as its tolerance zone. In this case, the tolerance zone being 0.06 mm on either side of nominal dimension of the feature.
3. Threading and Positioning:
In this callout, M42 refers to Metric thread of 42 mm diameter and having pitch 1.5 mm. 6g is the tolerance class on the major diameter of the thread. The geometric tolerance is of the position of the centre of the M42 hole.
Position tolerance is defined as the total permissible variation that a feature can have from its “true” position. This callout signifies that the centre of the M42 thread is restricted to be within an imaginary cylinder of 0.1 mm diameter with the datum B (highlighted axis) as the centre.
The symbol of M enclosed in a circle stands for Maximum Material Condition.
4. Flatness:
Flatness is a straightforward GD&T symbol indicating how flat a feature should be regardless of any datum. The flatness tolerance references two parallel planes (parallel to the surface that it is called out on) that define a zone where the entire feature surface must lie. Flatness tolerance is always less than the dimensional tolerance associated with it.
In this case, the indicated surface must lie enclosed within two planes defined at 0.01 mm on both sides of the feature, i.e., flatness of entire surface is constrained within 0.02 mm.
5. Radial Run-out:
This type of tolerance is typically used for features which rotate about an axis, i.e., circular parts.
Runout is how much one given feature can vary with respect to another datum when the part is rotated 360° around a datum’s axis. It is essentially how much “wobble” the feature’s surface has when it is rotated about the axis of the datum.
So, in this case, for the 20 mm diameter hole, runout is specified as 0.14 mm from datum A and C, which means when rotated about these datums, the surface of the hole should lie within 140 microns. Similarly, for the circular feature of diameter 31.6 mm, radial runout is 100 microns w.r.t. datum B.
And finally, moving on to the callout on the front view,
6. Positioning and Holes:
The callout says 8X, i.e., the tolerance mentioned is the same for 8 holes, which are given to be 45◦ apart placed symmetrically around the circular part.
The diameter of the holes have a dimensional tolerance between 7.9 mm — 8.1 mm. The position of the centre axis of the holes are restricted to be within 0.14 mm of the axes of datums A and C.
The dimension of Φ86 is a basic dimension and does not have tolerance. However, it must be kept as close as possible to given dimension for proper alignment of holes in the part.
Hope this walkthrough of basic tolerances has been helpful for you future engineers!
