GD&T Interview Questions and Answers

1. What is meant by GD&T ?
Geometric Dimension & Tolerance (GD&T) is a system for defining engineering tolerances. GD&T is very important part of mechanical product design.
GD&T defines degree of accuracy and precision required on controlled feature of part. GD&T is used to define the nominal (theoretically perfect) geometry of parts and assemblies, to define the allowable variation in form and possible size of individual features, and to define the allowable variation between features.

2.Describe how you go about calculating feeds and speeds and how  you determine cut depths and cut widths for a particular operation.
Everyone has their own ideas on this one (what, everyone doesn’t use G-Wizard Calculator?), so your goal is to see how compatible the applicant’s approach is with what your shop needs.  We’ve got an article on how to optimize Cut Depth and Cut Width for 2 1/2D machining.  When you’re 3D profiling the rules change (does your applicant know the strategies for both?).  CAM developer Robert Grzesek wrote a fabulous guest post for us on how to choose your stepover for 3D work.
Don’t be afraid to drill down and ask about subjects like High Speed Machining or Chip Thinning if it looks like a productivity way to explore your applicant’s deeper skills set.

3.Why is GD&T and Why it is Important

  • Saves money
    • For example, if large number of parts are being made – GD&T can reduce or eliminate inspection of some features.
    • Provides “bonus” tolerance
  • Ensures design, dimension, and tolerance requirements as they relate to the actual function
  • Ensures interchangeability of mating parts at the assembly
  • Provides uniformity
  • It is a universal understanding of the symbols instead of words

4.What is Form Control in GD&T
Form tolerances are applicable to single (individual) features or elements of single features. Form tolerances are not related to datum.

  • Straightness
  • Flatness
  • Circularity
  • Cylindricity

5.When to use GD&T

  • When part features are critical to a function or interchangeability
  • When functional gaging is desirable
  • When datum references are desirable to ensure consistency between design
  • When standard interpretation or tolerance is not already implied
  • When it allows a better choice of machining processes to be made for production of a part

6.What is Profile Control of GD&T?

  • Profile of a line Control
  • Profile of a Surface Control

7.What is meant by TERMINOLOGY REVIEW of GD&T

  • Maximum Material Condition (MMC): The condition where a size feature contains the maximum amount of material within the stated limits of size. I.e., largest shaft and smallest hole.
  • Least Material Condition (LMC): The condition where a size feature contains the least amount of material within the stated limits of size. I.e., smallest shaft and largest hole.
  • Tolerance: Difference between MMC and LMC limits of a single dimension.
  • Allowance: Difference between the MMC of two mating parts. (Minimum clearance and maximum interference)
  • Basic Dimension: Nominal dimension from which tolerances are derived.

8. What is Tolerance in GD&T?
Tolerance.The total amount by which a specific dimension is permitted to vary. The tolerance is the difference between the maximum and minimum limits. They are three types:

  1. Unilateral Tolerance
  2. Bilateral Tolerance
  3. Geometric Tolerance

9.What is Limits Of Size of GD&T?
A variation in form is allowed between the least material condition (LMC) and the maximum material condition (MMC).
The actual size of the feature at any cross section must be within the size boundary.
No portion of the feature may be outside a perfect form barrier at maximum material condition (MMC).

10.What is meant by Datum in GD&T
A theoretically exact point, axis, or plane derived from the true geometric counterpart of a specified datum feature, A datum is the origin from which the location or geometric characteristics of features of a part arc established.

11.What are Characteristics & Symbols cont’d in GD&T?
Maximum Material Condition MMC

  • Regardless of Feature Size RFS
  • Least Material Condition LMC
  • Projected Tolerance Zone
  • Diametrical (Cylindrical) Tolerance Zone or Feature
  • Basic, or Exact, Dimension
  • Datum Feature Symbol
  • Feature Control Frame

12.What is Geometric Tolerance?
The general term applied to the category of tolerances used to control form, profile, orientation, location, and runout.

13.What is Feature Control Frame?

  • Uses feature control frames to indicate tolerance
  • Reads as:  The position of the feature must be within a .003 diametrical tolerance zone at maximum material condition relative to datums A, B, and C.
  • Uses feature control frames to indicate tolerance
  • Reads as:  The position of the feature must be within a .003 diametrical tolerance zone at maximum material condition relative to datums A at maximum material condition and B.

14.What is Bilateral Tolerance?
A tolerance in which variation is permitted in both directions from the specified dimension.

15.What is Unilateral Tolerance?
A tolerance in which variation is permitted in one direction from the specified dimension

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