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Blog posts : "gd&t basics"

Basics of GD&T : SMLease Design

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 and used to define the allowable variation in form and possible size of individual features allowable variation between features. 

 


Form Control

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

 


Straightness

  • Straightness is a condition where an element of a surface or an axis is a straight line.
  • Straightness is used to control the foam of a line on the surface/feature or …

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Straightness : GD&T Tips SMLease Design

 

Straightness is a condition where an element of a surface or an axis is a straight line.

Where Straightness is Used

  • Used to control the foam of a line on the surface /  feature
  • Straightness of an axis

No Datum plane is required to define straightness and while defining axis straightness LMC and MMC modifiers can be used


Surface Straightness

It is a 2-Dimensional tolerance which is used to ensure that a part is uniform across a surface or feature. Straightness can be applied to a flat surface of a cylinder or block

 

Representation Of Surface Straightness

Tolerance Zone For Surface Straightness

Tolerance Zone will be 2 parallel lines 0.1 mm apart in 2D Plane


Measurement Of Surface Straightness

 

  • Dial Gauge is used to …

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Profile Of a Line Control : GD&T Basics SMLease Design

  • Profile of a line control describes a tolerance zone around any line in any feature.
  • Profile of a line is a 2-Dimensional tolerance range that can be applied to any linear tolerance.
  • Profile of line control : controls size, orientation, location & foam of any feature.
  • Line elements of the surface along the profile must lie within the profile tolerance zone and within a size limiting zone.
  • MMC and LMC are not applicable with profile of line control.
  • Datum Plane can be used to control the profile of a line control.

 


Tolerance Zone:


Measurement Of Profile of line Control:

Dial Gauge is followed along the true profile to record the variation keeping datum plane fixed. Profile is generally measured using CMM

 


 

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Flatness : GD&T Tips SMLease Design

  • Flatness is a condition that defines the flatness of a surface regardless of any datum feature.
  • Flatness is used to make use required surface is flat without tightening any other dimension.
  • Value of Flatness tolerance is always less than the dimensional tolerance associated with a part feature.
  • No Datum Plane is required.
  • LMC and MMC can be used.

Flatness Representation


Tolerance Zone

Reference surface should lie in between two parallel planes


 

Where Flatness Is Used

If the requirement is to make highlighted surface flat within 0.1 mm limit.  Without Flatness Tolerance one way to do this is : reduce the tolerance limit for thickness.

But this will increase overall manufacturing cost. To avoid this we increase the tolerance in thickne…

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Circularity: GD&T Tips SMLease Design

  • Circularity/Roundness is used to control the circularity of a round feature in 2D tolerance zone.
  • Circularity tolerance is independent of any datum feature.
  • Value of circularity tolerance is always less than the diameter dimensional tolerance of the part.
  • Circularity tolerance can be applied to any part (external or internal surfaces) which is circular in cross-section. 
  • MMC and LMC conditions are not applicable with circularity tolerance.

Circularity Representation on Drawing

Circularity Tolerance Zone:

 

Tolerance zone for circularity will be two concentric circles

 

Measurement of Circularity:

Circularity of any round part is measured by rotating the part by central axis and value of deviation in circular surface is measured with dial gau…

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Profile Of a Surface Control : GD&T Basics

 

  • Profile of surface control is used to control part surface. It makes a 3-Dimensional tolerance zone around a surface.
  • Profile of a surface tolerance: Controls the size, location, orientation and foam of any surface.
  • When the profile of surface control is specified, tolerance zone is uniform boundary along full length and width of the surface.
  • Usually when surface profile is required, there are no tolerances on the dimensions that describe the surface and use the GD&T callout to give the acceptable range.
  • Datum reference is required to control Profile of a Surface.

Profile of Surface Control Representation:


Profile of Surface Control Tolerance Zone


Measurement Of Profile of surface Control

 

  • Dial Gauge is followed along the true profi…

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Perpendicularity : GD&T Basics

  • Perpendicularity can be called out on any surface or axis of any feature of a part.
  • Perpendicularity tolerance on surface controls perpendicularity between two  surfaces perpendicular to each other.  Tolerance zone in this case will be two parallel planes.
  • Perpendicularity tolerance on axis controls the perpendicularity of any surface w.r.t. datum plane. Tolerance zone will be a cylindrical surface.
  • Datum plane is required to control the perpendicularity of any surface or axis.
  • MMC and LMC can be applicable with perpendicularity tolerance. 

Perpendicularity tolerance on a Surface

  • Perpendicularity tolerance on any surface controls perpendicularity between two  surfaces perpendicular to each other.
  • Perpendicularity tolerance controls the 90° angle bet…

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Position Tolerance : GD&T Basics

  • 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 the location of a feature about its exact true position.
  • Material Condition (MMC & LMC), Projected Tolerance, Tangent Planes can be used along with position tolerance.
  • Tolerance zone for position tolerance can be two parallel planes, Cylinder or a sphere.
  • Positional tolerances is used to locate features of size from datum planes such as a hole or key-way and used to locate features coaxial to a datum axis.
  • In cylindrical features, the position tolerance zone is typically a cylinder within which the axis of the feature must lie.

Representation Of Position Tolerance


Tolerance Zone For Position Tolerance

 

 …

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Cylindricity : GD&T Basics SMLease Design

  • Cylindricity is used to constrain the part surface close to true cylinder. Cylindricity forms a cylindrical tolerance zone around the object.
  • Cylindricity is 3 Dimensional tolerance used to control the overall form of a cylindrical feature to ensure that it is round enough and straight enough along its axis.
  • Datum Feature are not required for Cylindricity tolerance.
  • Value of cylindricity tolerance is always less than the diameter dimensional tolerance of the part.

Representation of Cylindricity Tolerance: 


Tolerance Zone :

Tolerance zone for cylindricity tolerance will be two concentric cylinders across the entire length of the feature  equal to the value of cylindricity tolerance.

 


How to Measure Cylindricity Tolerance:

 

Cylindricity …

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Parallelism : GD&T Basics

  • Parallelism can be used to control parallelism between two surfaces or the parallelism of two axis.
  • Parallelism describes a parallel orientation of one referenced feature to a datum surface or line in 3D tolerance zone.
  • Parallelism does not control the angle of the referenced feature, but it creates a tolerance zone in which the feature must lie.
  • Tolerance zone will be two parallel planes  that are parallel to the datum feature or surface.
  • MMC and LMC conditions are applicable with parallelism tolerance.
  • Parallelism is required to make sure two surfaces/features work in sync with each other and constant distance between them is maintained.‚Äč

Tolerance Zone:

 


Application Of Parallelism

Parallelism is used to make sure two surfaces are in synchronised w…

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Angularity : GD&T Basics

  • Angularity is used to constrain the orientation of  one feature w.r.t. datum at specified angle.
  • Angularity tolerance zone will be two parallel planes /surfaces in 3D. All points on controlled feature should lie within defines planes.
  • Maximum material condition can be used along with angularity tolerance.
  • Datum plane is required to define angularity tolerance.
  • Angularity tolerance can also be used to control the axis of any feature w.r.t datum plane.
  • Tolerance zone for angularity for an axis will be a cylinder around theoretically exact axis w.r.t. datum plane.
  • Angularity can be used to control the critical feature at an angle where assembly is happening at an angle.

 


Angularity Representation 


Tolerance Zone

Tolerance zone for angularity wi…

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Concentricity : GD&T Basics

  • Concentricity  tolerance is used to control the central axis of a cylinder or sphere w.r.t. a datum plane/axis.
  • Datum Plane /Axis is required to control Concentric tolerance.
  • Concentric tolerance is used where high precision is required to control median points on a cylindrical part such as transmission gears where gears need to be concentric with  mounting.
  • Runout or position tolerance is used instead of concentric tolerance as it is very difficult to measure concentric tolerance.
  • Concentricity tolerance makes a 3-Dimensional cylindrical tolerance zone.
  • LMC and MMC is not applicable with concentric tolerance.

 


Representation Of concentric Tolerance 


Concentricity Tolerance Zone

Concentricity tolerance makes a cylindrical tolerance zone around d…

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Maximum Material Condition : GD&T Basics

Maximum Material Condition (MMC) is a condition of a feature (hole or pin) of a part where maximum material  condition exists.

 


 

Maximum Material Condition For a Hole

Maximum Material Condition for a hole will be smallest hole size. E.g. for a hole of 15 mm diameter with tolerance limit of ± 0.2 mm, MMC will be 14.8 mm 

 

 


 

Maximum Material Condition (MMC) For Shaft

Maximum Material Condition for a pin will be largest size of the pin. E.g. For a pin of 15 mm diameter with tolerance limit of ± 0.2 mm, MMC will be 15.2 mm

 

 

 


Maximum Material Condition Can be used with Following Tolerance:


Why Maximum Material Condition i…

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Least Material Condition : GD&T Basics

Least Material Condition (LMC) is a condition of a feature (hole or pin) of a part where minimum material  condition exists. 

 


 

Least Material Condition For Shaft/pin

LMC for a hole will be largest hole size. E.g. for a hole of 15 mm diameter with tolerance limit of ± 0.2 mm, MMC will be 15.2 mm

 


 

 

Least Material Condition For Shaft/pin

LMC for a pin will be smallest size of the pin. E.g. For a pin of 15 mm diameter with tolerance limit of ± 0.2 mm, LMC will be 14.8 mm

 


 Maximum Material Condition Can be used with Following Geometric Tolerance:


 

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