Mechanical Design Interview Questions and Answers
Mechanical Design Interview Questions and Answers
Mechanical Design Interview Questions and Answers
Answers
What are engineering tolerance?
Engineering Tolerances are allowable variation from given dimensions. In other words, it is
the total amount by which a given dimension may vary without affecting product function.
The production of closely mating parts, without tolerances is economically unfeasible. This
will increase the rejection rate and manufacturing cost. Therefore, tolerances are used with
part dimensions to facilitate manufacturer and reduce cost.
Tolerance Stackup analysis is a design tool used to analyse and optimize product assembly. It
calculates the cumulative effects of part tolerances in an assembly.
1. Unilateral Tolerance
2. Bilateral Tolerance
The term fit refers to the permissible clearances between mating parts. The clearance between
mating parts determines the type of Fit.
1. Clearance Fit
2. Interference Fit
3. Transition Fit
Because of the limitations of second and fourth angle projection system. Only First and Third
angle projections are used for making engineering drawings. In this article we will discuss the
limitations of 2nd and 4th angle projection.
To understand why 2nd and 4th angle projection are not used? Firstly we need to understand
the concept behind orthographic projection system.
As per rule of projection, to bring drawing views From three dimensional to two
dimensional plane, horizontal plane (HP) is rotated in the clockwise direction.
As shown, Point A in 1st quadrant lies in between observer and projection plane. Therefore
Front view of point A will be in vertical plane (V.P.) and top view will be on horizontal
plane.
Similarly Point B in 3rd quadrant can be projected on the horizontal and vertical plane,
Why Second and Forth angle Projection System are not used ?
To understand why 2nd and 4th angle projection system are not used. We will repeat above
experiment in second and fourth quadrant.
Let’s consider rectangular parts X and Y are placed in 2nd and 4th quadrant respectively.
In Second quadrant, vertical plane (VP) lies in between object X and observer. Therefore
front view of object x will lie on vertical plane whereas top view will lie on horizontal plane.
As per rule of projection when horizontal plane is rotated 90 degree in clockwise direction,
top and front view will overlap.
Overlapping projection views create confusion in the drawing. Therefore 2nd angle
projection system is not used.
Similarly when object is placed in 4th quadrant both top and front view will overlap.
Therefore fourth angle projection is also not used.
Conclusion
To sum up, Because of overlapping of front and top views 2nd and 4th angle projections are
not used. First and third angle projection systems are recommended for orthographic
projections.
It is the smallest value measured with the measuring instrument. For example, least count for
a normal scale is 1 mm whereas for vernier scale it is 0.01 mm.
Least Count= Value of one main scale Division – Total Number of Vernier scale division
What is least count for Vernier Scale, Micrometer and normal scale?
FOS describes the load carrying capacity of a system beyond the expected or actual loads. In
another words, FOS represents : how much stronger the system is compared to intended load.
FOS is calculated considering design load maximum value. Value of FOS is always greater
than one.
1. Machine Screw
2. Thread Forming
3. Thread Cutting Screw
A datum is theoretical exact plane, axis or point location that GD&T or dimensional
tolerances are referenced to.
Geometric dimension and tolerance ( GD&T ) is a system to define nominal and allowable
variations in part and assembly geometry. They are used along with linear
tolerance. ASME Y14.5-2009 standard has defined GD&T symbols in detail.
GD&T allows comparatively larger tolerance zone. Therefore part rejection rate and cost also
decreases.
Five types of GD&T tolerance are used to define a part design intent accurately. This
includes 14 symbols that controls the features and geometry of a part.
1. Form Control
2. Profile Control
3. Orientation Control
4. Location
5. Runout
Form Control
Foam control geometric tolerance controls the form of the feature of a part. Location and
Orientation tolerances can also used to control the foam of a feature.
Datum reference is not used with form control geometric dimension and tolerance. Form
tolerance are further classified in four types.
1. Straightness
2. Flatness
3. Circularity
4. Cylindricity
Straightness
Straightness is a type of form control geometric dimension control tolerance in. It controls a
condition where all element of a surface or an axis of a feature lies in a straight line.
Flatness
Flatness is a type of form control GD&T tolerance. It controls the variations in flat surface,
regardless of any datum feature.
Circularity / Roundness
Circularity controls the roundness of a circular feature in two dimensional tolerance zone.
Cylindricity
Cylindricity Tolerance
Representation
Profile Control
Profile control tolerance in “Geometric Dimension and control Tolerance” defines a uniform
boundary around a surface. All points of controlled surface must lie within defined
boundary.
“Profile of a Line Control” GD&T tolerance creates a 2-dimensional tolerance zone around
any line of a feature.
Profile of a Surface Control Tolerance creates a uniform 3D boundary tolerance zone along
full length and width of the surface.
Orientation Control
Orientation Control GD&T tolerance controls the orientation of the part feature with respect
to another feature or datum. They can be classified in three categories.
1. Parallelism
2. Perpendicularity
3. Angularity
Parallelism
Parallelism Tolerance
Representation
Parallelism tolerance does not controls the angle of the referenced feature. But it
creates a tolerance zone where all points of the feature must lie.
Datum plane is required to control Parallelism Tolerance
Perpendicularity
Penpendicularity Tolerance
Representation
When perpendicularity is applied over surface, Tolerance zone will be two parallel
surfaces/planes/lines perpendicular to datum plane.
Whereas when applied to an axis. Tolerance zone will be a cylinder boundary around
a true axis. Axis of referenced feature must lie in this cylinder boundary.
Angularity
Angularity tolerance in GD&T controls the orientation of one feature with respect to datum at
specified angle.
Angularity tolerance
Representation
Location Control
Location control in “Geometric Dimension and Control Tolerance” defines the deviation of a
feature from the actual location. It is further classified in three types.
1. Position Tolerance
2. Concentricity
3. Symmetry
Concentricity and symmetry controls the center distance of feature whereas position tolerance
controls coaxiality of a features.
Position Tolerance
Position tolerance in gd&t controls the variation in the location of a feature from exact true
position. It is the total permissible variation in feature location from its exact true position.
MMC, LMC, projected tolerance, tangent planes can be used along with position tolerance.
Concentricity
Concentricity Tolerance
Representation
Concentricity tolerance controls the central axis of a cylinder or sphere with respect to datum
plane/axis. In other words, it controls the median points on high precision cylindrical parts
such as transmission gears.
Runout
Runout in “geometric dimension and control” controls the variation in a feature when the part
is rotated 360° around the datum axis. It also measures the wobbling of a part. Runout can be
controlled with two symbols :
1. Circular Runout
2. Total Runout
Circular Runout
Circular Runout Representation
Circular runout creates a 2-Dimensional circular tolerance zone defined by a datum axis.
It controls the total variation in controlled surface, when the part is rotated around the
datum true axis.
Runout is used to control features of a rotating part such as drill, gears, shafts, axles
and machine tool parts.
Total Runout
Total Runout Representation
Total runout creates a 3-dimensional cylindrical tolerance zone defined by datum axis.
It controls the total variation in the reference surface, when the part is rotated around the
datum’s true axis.
Conclusion
To sum up, Geometric Dimension and tolerance (GD&T) has the advantage of
communicating part design and assembly intent in engineering drawing. It also helps in
increasing tolerance zone as well. To calculate required tolerance, tolerance stack up
calculator can be used.