GD&T interview questions
GD&T interview questions

The GD&T interview questions cover a wide range of topics which are very interesting and easy to understand for the candidates. Most of the times, these questions are designed with multiple choice format where the candidate is asked multiple question simultaneously.

GD&T interview questions are formulated by a team of technocrats, all expert in different aspects of CAD designing. Their knowledge in every facet of CAD is such that it can easily be identified by the candidates who are inexperienced in this field. So, they may miss the key points of GD&T and get confused which can easily lead to wrong decisions.

These questions will help the candidates to understand the what they may expect in the interview. The Geometric dimensioning and tolerancing are very important features for the CAD applications. Each GD&T have its own characteristics which the candidate must not ignore.

if you need little help for the GD&T here is our extensive guide on the topic.

What are the expectation of interviewer from you

GD&T or Geometric Dimensioning and Tolerancing, enables engineers to define and communicate engineering tolerances for a workpiece by including GD&T specifications in engineering drawings and models.

You should be able to explain the reason for and the significance of using GD&T control types, the characteristics they describe, how they are used in engineering drawings to define features and tolerances, and the difference between the different control types and symbols.

For example, the first GD&T control type describes form, shape or size features in an engineering drawing and includes four GD&T characteristics: flatness, straightness, circularity/roundness, and cylindricity. You should also be familiar with feature control frames, what they contain, and how they are used to define GD&T specifications.

You should also be able to tell the difference between a datum axis, and a datum plane, and which datum is used for each GD&T symbol.

Another question you might face during an interview could be whether you can use GD&T basic dimensions without a corresponding tolerance feature control frame. This is legitimate if a basic dimension can be defined without a corresponding feature control frame, for example when a basic angle is specified on an angled surface, and a size dimension controls tolerance zone size.

Another case would be when basic dimensions are used to specify the location of datum targets.

GD&T Questions with answers

Q1. What is meant by GD&T ?

Answer: 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.

Q2. Describe how you go about calculating feeds and speeds and how  you determine cut depths and cut widths for a particular operation?

Answer: 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.

Q3. Why is GD&T and Why it is Important?

Answer:

  • 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

Q4. What is Form Control in GD&T?

Answer: Form tolerances are applicable to single (individual) features or elements of single features. Form tolerances are not related to datum.

  • Straightness
  • Flatness
  • Circularity
  • Cylindricity

Q5. When to use GD&T?

Answer:

  • 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

Q6. What is Profile Control of GD&T?

Answer:

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

Q7. What is meant by TERMINOLOGY REVIEW of GD&T?

Answer:

  • 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.

Q8. What is Tolerance in GD&T?

Answer: 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

Q9. What is Limits Of Size of GD&T?

Answer: 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).

Q10. What is meant by Datum in GD&T?

Answer: 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.

Q11. What are Characteristics & Symbols cont’d in GD&T?

Answer:

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

Q12. What is Geometric Tolerance?

Answer: The general term applied to the category of tolerances used to control form, profile, orientation, location, and runout.

Q13. What is Feature Control Frame?

Answer:

  • 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.

Q14. What is Bilateral Tolerance?

Answer: A tolerance in which variation is permitted in both directions from the specified dimension.

Q15. What is Unilateral Tolerance?

Answer: A tolerance in which variation is permitted in one direction from the specified dimension.

GD&T interview question list

  • State 14 GD&T symbols with form, orientation, location, profile and run-out.

  • GD&T Purpose, Benefits and Utility

  • Apply Feature control frame for various characteristics

  • why not Datum required for straightness, flatness, circularity and cylindricity.

  • Explain MMC, LMC, RFS ( Modifiers), Virtual Conditions.

  • Why MMC is preferred? Typical use of LMC.

  • Types of dimensions, Why Basic dimensions?

  • Angle of Projection system, its difference, Critical fitments vs non-critical fitments.

  • Limits, fits and tolerance, Hole basic vs shaft basic system, need of tolerance, IT grades.

  • FCF, Zone descriptor, Datum, Datum Feature, Datum Target, Datum Simulators, References selection for FCF.

  • Inspection Methods for all Characteristics, Available tolerances, and GD&T applications on components. e.g., piston rings, cluth plate, shaft keyways, hole positioning, spindles, cylinder, Dovetail slot, Pulleys, gears, journals

  • How to control taper by GD&T?

  • Symbol for projected pin method, Tangent plane method, unidirectional control.

  • Difference between circular runout and total run out, Profile of line n surface with examples.

  • Why Position Tolerance is given over co-ordinate dimensioning.

  • What tolerances are covered in position characteristics, apply with MMC and explained, calculate available tolerance

  • GD&T is a very important topic and and now days has become a sure topic to be discussed during interview and group discussion. I will try to enumerate all the points one by one. First of all make yourself familiar with all the definition of the terms used in gd&t and what is difference between them because there are certain which are easily to be confused like concentricity, circularity and cylindricity. Similarly flatness and straightness.

  • Runout and total runout.

  • Secondly know how would you differentiate between a datum axis and a datum plane and what and which datum is used for what gd&t.

  • Thirdly learn about MMC max material condition and LMC least material condition and how do they affect the gd&t and how tolerances to be adjusted if a design intent is changed from MMC to LMC.

  • Why cant the primary datum be on the opposite face of the bracket.

  • How will you make a gauge for this part.

  • Which hole slot will you inspect first.

  • Why dont we use circularity , concentricity and flatness for all functional features.

  • What benefit does MMC give compared to RFS.

  • What characteristics of a feature are required for it to qualify as a primary datum

  • What are datum shifts

  • What are datum targets

  • When do you use composite tolerancing and when do u use compound.

Tricky (GD&T) Questions

Question 16: GD&T axis straightness along with an orientation or position tolerance

When using GD&T axis straightness along with an orientation or position tolerance, can the specified straightness tolerance value be greater then the specified orientation or position tolerance values? If not, then why not?

Answer .: The correct answer to the question is…No!

GD&T interview questions 1

When used alone with a size dimension, axis straightness does allow the feature to go outside the MMC boundary envelope. However, according to the Y14.5M standard, “Where the straightness tolerance is used in conjunction with an orientation or position tolerance, the specified straightness tolerance value shall not be greater that the specified orientation or position tolerance values.” (Para. 6.4.1.1.2, pg 158). This makes sense; there is no way an axis may fit within an orientation or position cylindrical tolerance zone if the axis is “bent” more than the allowed orientation or position tolerance value. Congratulations if you answered the Test Your GD&T Knowledge question correctly!

Question 17: GD&T basic dimensions without feature control frames

Answer.: Is it proper to specify (show) a basic dimension on a drawing without a corresponding tolerance in a feature control frame? If so, in what situations would it proper?

The correct answer to the question is…Yes! There are two situations where a basic dimension can be specified without a corresponding feature control frame. A basic angle may be specified on an angled surface as shown in Figure 4, where a size dimension controls the size of the tolerance zone. (See ASME Y14.5M-1994, 2.12). Basic dimensions may also be used when specifying the location of datum targets as shown in Figure 5. The tolerance for the location of the datum targets is determined by “Gagemakers Tolerance” or “Toolmakers Tolerance” (established tooling tolerances). (See ASME Y14.5M-1994, 4.6.2).

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