Transcript
1. Understand the fundamental concepts of tolerance of position; the definitions and conventions, the advantages and the basic theories.
2. Interpret RFS and MMC tolerance of position applications.
3. Draw cartoon gages for tolerance of position (MMC) applications.
Definitions and Conventions
• True position is the theoretically exact location of a FOS as defined by basic dimensions.
• Tolerance of position (TOP) control is a geometric tolerance that defines the location tolerance of a FOS from its true position.
Advantages of TOP
1. Provides larger tolerance zones; cylindrical tolerance zones are 57% larger than square zones.
2. Permits additional tolerances-bonus and datum shift.
3. Prevents tolerance accumulation.
4. Permits the use of functional gages.
5. Protects the part function.
6. Lowers manufacturing costs.
Types of Part Relationships that can be controlled with TOP
1. The distance between features of size, such as holes, bosses, slots, tabs, etc.
2. The location of features of size (or patterns of features of size) such as holes, bosses, slots, tabs, etc.
3. The coaxiality between features of size.
4. The symmetrical relationship between features of size
Common TOP RFS Applications• TOP used on an RFS Basis
• Three conditions :1. The tolerance zone applies to the axis (or
centerline) of the FOS.
2. The tolerance value applies regardless of the size of the tolerance feature of size.
3. The requirement must be verified with a variable gauge.
RFS Tolerance Zones• Two tolerance zones : A fixed diameter
cylinder and two parallel planes a fixed distance apart.
• The diameter of the tolerance zone cylinder or the distance between the parallel planes is equal to the tolerance value specified in the TOP callout.
• The location of the tolerance zone is always centered around the true position of the FOS.
The Location of a Hole Controlled with Tolerance of Position (RFS)
The Location of a Pattern of Holes Controlled
with Tolerance of Position (RFS)
• The following conditions apply:
– The shape of each tolerance zone is cylindrical.
– The tolerance zones are located by the basic dimensions.
– The tolerance zones apply RFS.
– The tolerance zones also control the orientation of the holes relative to the primary datum reference from the TOP callout
– The tolerance zones are at an implied basic 900 to datum A.
The Location of Coaxial Diameters Controlled with Tolerance of Position (RFS)
Inspecting TOP Applied at RFS• Three separate checks required
1. Size of the hole
2. TOP requirements
• Necessitates the use of Variable Gauges: a gauge what is capable of providing a numerical reading of a part parameter.
E.g.: CMM, Height gauges, Expanding mandrels, and Dial indicators
• Steps in Inspection1. The location of the hole is established by contacting the
datums in the sequence of the TOP callout
2. A best fit gauge pin is placed in the hole representing the AME.
Common TOP MMC Applications• TOP is specified on MMC basis, when the part
function is assembly or when the effects of bonus tolerance and/or datum shift would not have a detrimental effect on the function of the part
• MMC basis is a more liberal control than an RFS application
• Three conditions apply1. The tolerance zone is considered a boundary zone.2. A bonus tolerance and/or datum shift is permissible.3. The requirement can be verified with a functional
gauge.
MMC Tolerance Zones• Two tolerance zone shapes:
– Virtual condition cylindrical boundary
– Virtual condition parallel plane boundary
• The location of the tolerance boundary is always centered on the true position of the FOS.
VariableFunctional (fixed)Gauging
NoYesBonus tolerance
permissible
An axis zone
(or center plane)A boundary zoneTolerance zone
RFSMMC
The Location of a Hole Controlled with Tolerance of Position (MMC)
• The following conditions apply:
– The shape of the tolerance zone is a virtual condition cylindrical boundary.
– The tolerance zone is located by the basic dimensions from the datum planes.
– The relationship between the centerline of the hole and datum plane A is an implied basic 900 angle.
– A bonus tolerance is permissible.– The tolerance zones also control the orientation
of the holes relative to the Primary' datum reference from the TOP callout
The Location of a Hole Pattern Controlled with Tolerance of Position (MMC)
Inspecting TOP Applied at MMC
• A TOP applied at MMC can be verified in number of ways: Variable gages, Open inspection, CMM, and functional gauging.
• Functional Gauge: Functional gages a gage that verifies functional requirements of part features as defined by the geometric MMC tolerances. Also called attribute gauge or a fixed gauge.
• A functional gauge only provides a "pass" or "fail" assessment of a part feature, i.e. does not provide a numerical reading of a part parameter.
• The functional gage represents the virtual condition of the tolerance FOS
Benefits of functional gages
1. The gage represents the worst-case mating part.
2. Parts can be verified quickly.
3. A functional gage is economical to produce.
4. No special skills are required to read the gage or interpret the results.
5. In some cases a functional gage can check several part characteristics simultaneously.
Cartoon Gauge
• Functional gage does not exist in the design stage
• A cartoon gage is a sketch of a functional gage to analyze a part in the design stage
• It does not represent the actual gage construction of a functional gage.
TOLERANCE OF POSITION (TOP) SPECIAL APPLICATIONS:
•TOP Locating Holes those are Non-Parallel.
•Bi-Directional TOP (Locating a Hole in Two Directions).
•TOP with Zero Tolerance at MMC.
•TOP with the LMC Modifier.•Using TOP to Control Symmetrical Relationships.
•Top Using a Projected Tolerance Zone.
•Using TOP to locate an elongated hole.
Bi-Directional TOP (Locating a Hole in Two Directions):
In this application, the following conditions apply;-The tolerance zones are parallel boundaries in the direction of the TOP control.-The shape of tolerance zone is_________.
Bi-Directional TOP (Locating a Hole in Two Directions):
In this application, the following conditions apply;-The tolerance zones are parallel boundaries in the direction of the TOP control.-The shape of tolerance zone is rectangular.
-Bonus tolerance are permissible.-The elongated hole must also meet its size requirements.
Using TOP to locate an elongated hole (Contd..)
Interpret the drawing and design a gauge pin?
Using TOP to locate an elongated hole (Contd..)
In this figure, the TOP control limits the spacing between the holes and the square ness of the holes relative to datum plane A, but the TOP control does not control the location of the hole pattern
Using TOP to Control Spacing and Orientation of a Pattern of Holes
The fixed fastener formula is:
Where:
T = Position tolerance diameter
H = MMC of the clearance hole
F = MMC of the fastener
H = F + 2T or
T = (H –F)/2
Fixed fastener assemblies
Floating Fastener Assemblies
Floating fastener assembly is where (two or more) components are held together with fasteners(such as bolts and nuts), and both components have clearance holes for the fasteners.This type of assembly is called a floating fastener assembly because the fasteners can “ float ”(move) in the holes of each part.
Floating fastener formula:-
where: T= position tolerance diameter(for each
part) H=MMC of the clearance hole F= MMC of the fastener
T = H-F
Floating Fastener Assemblies
For the above two types of assemblies
• The formula used in both condition ensures that the part will assemble.
• This results in a “No interference , No clearance”
condition when the components are at MMC and located at their extreme position.
• MMC(worst condition of assembly) modifier is used to arrive the TOP. This allows additional position tolerance as the holes depart from MMC.
Floating Fastener Assemblies
1. True position is the theoretically exact location of an FOS as defined by ……… dimensions.
1. True position is the theoretically exact location of an FOS as defined by basic dimensions.
2. The advantages of tolerances of Position are:
• ……….% larger tolerance zone
• Additional ………. tolerance & datum shift if specified
• Permits use of ………….. gauging
• Ensures part ……………
• Helps ………….. cost of manufacturing
2. The advantages of tolerances of Position are:
•57% larger tolerance zone
•Additional bonus tolerance & datum shift
•Permits use of functional gauging
•Ensures part functionality
•Helps reducing cost of manufacturing
3. …………….. Gauge can be used to verify a part quickly.
3. Functional Gauge can be used to verify a part quickly.
4. When Tolerance of Position is specified on RFS basis, a functional gauge can be used. True or False.
4.When Tolerance of Position is specified on RFS basis, a functional gauge can be used. False.
5. ………… basis is a more liberal control than RFS.
5. MMC basis is a more liberal control than RFS.
6………. Control may be specified to control co-axiality.
6.Position Control may be specified to control co-axiality.
The End of Chapter 6
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