tech 104 – technical graphics communication week 8: tolerancing practices

TECH 104 – Technical Graphics Communication

Week 8:

Tolerancing Practices


Week 8: Tolerancing

Here’s what we talked about last time…..


Dimension elements….

Week 8: Tolerancing

Dimensions are used to show an object’s:

1. Overall: Width

Depth

Height

2. The actual size of features (rounds,

fillets, holes, arcs, etc.)


Week 8: Tolerancing


3. And where features are located such as centers, angles, etc.

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Dimensions should be stacked in a “broken chain” format.

“Breaking the Chain” refers to leaving out one dimension as shown above so that manufacturing tolerances are maintained.

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As a general rule…Stay off the object as much as possible.

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Extension lines can be shared and even broken to clarify crowded dimensions.

3.

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Some features are dimensioned from their center lines.

The center line may also be used as an extension line.

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Leaders with dimensions are used to show negative cylinders (holes).

The leader should always be placed to penetrate the center of all round features.

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Features such as counterbores, countersinks and spot faces are all dimensioned using a leader.

Note: Each of these features has a special dimensioning symbol that can be used to show:

a. Diameterb. Shapec. Depth


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Arcs are always dimensioned as a radius. Full circles are dimensioned showing their diameter value.

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When dimensioning a part, always start with the inner-most dimensions and work to the outer-most values.

Remember:

Dimensions are used to show both the size and location of features.

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Always dimension features and not lines…..and remember….

NEVER, NEVER, NEVER dimension to hidden lines!

Week 8: Tolerancing

Today’s Lecture - Week 9:Tolerancing


∙ Tolerances are used to control the variation that exists on all manufactured parts.

∙ Toleranced dimensions control the amount of variation on each part of an assembly.

∙ The amount each part is allowed to vary depends on the function of the part and of the assembly. For example: the tolerances placed on a swing set is not as stringent as those placed on jet engine parts.

∙ The more accuracy needed in the machined part – the higher the manufacturing cost.


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∙ Tolerances are represented as Direct Limits (A) or as Tolerance Values (B).

Representing Tolerance Values

∙ Tolerance is the total amount a dimension may vary and is the difference between the maximum and minimum limits.

(A) Tolerance = .04(B) Tolerance = .006

Which part costs more to manufacture?


Week 8: Tolerancing

Tolerances can also be expressed as:

2. Notes Referring to Specific Conditions.

3. A General Tolerance Note in the Title Block.

Example: ALL DECIMAL DIMENSIONS TO BE HELD TO ± .002”

1. Geometric Tolerances.“GDT”


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Plus and Minus Dimensions

With this approach, the basic size is given,followed by a plus/minus sign and the tolerance value.

Notice that a Unilateral Tolerance varies in only one direction,while Bilateral Tolerances varies in both directions from the basic size.


Week 8: Tolerancing

Important Terms of Toleranced Parts

A System is two or more mating parts.

Nominal Size is used to describe the general size (usually in fractions).The parts above have a nominal size of 1/2”

Basic Size – theoretical size used as a starting point for the application ofTolerances. The parts above have a basic size of .500”


Week 8: Tolerancing


Limits – the maximum and minimum sizes shown by the tolerance dimension.The slot has limits of .502 & .498, and the mating part has limits of .495 & .497.The large value on each part is the Upper Limit, the small value = Lower Limit.

Actual Size is the measured size of the finished part after machining.The Actual Size of the machined part above is .501”


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Allowance – the tightest fitbetween two mating parts.

For these two parts, the allowance is .001,meaning that the tightest fit occurs when theslot is machined to it’s smallest allowable sizeof .498 and the mating part is machined to itslargest allowable size of .497. The differencebetween .498 and .497, or .001, is the allowance.

(The minimum clearance or maximum interference).


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Tolerance – the total allowable variance in a dimension;the difference between the upper and lower limits.

The tolerance of the mating part is .002” (.497 - .495 = .002)

The tolerance of the slot is .004”(.502 - .498 = .004)


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Maximum Material Condition (MMC)The condition of a part when it contains the greatest amount of material. The MMCof an external feature, such as a shaft,is the upper limit. The MMC of an internalfeature, such as a hole, is the lower limit.Least Material Condition (LMC)The condition of a part when it contains he least amount of material possible.The LMC of an external feature is the lower limit. The LMC of an internal feature is the upper limit.


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Piece toleranceThe difference between the upper and lower limits of a single part(.002 on the insert in this example, .004 on the slot.).

System tolerance The sum of all the piece tolerances.For this example (.006)


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Clearance & Interference fitsbetween two shafts and a hole

Shaft A is a Clearance fit, shaft B is an Interference fit

Fit Types:


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Fit Types: Transition Fit

A Clearance Fit occurs when two toleranced mating parts willalways leave a space or clearance when assembled.

An Interference Fit occurs when two toleranced mating parts willalways interfere when assembled.

A Transition Fit occurs when two toleranced mating parts are sometimesan interference fit and sometimes a clearance fit when assembled.


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Functional Dimensioning

Functional features are those that come in contact with other parts,especially moving parts. Holes are usually functional features.

Functional Dimensioning begins with tolerancing the most important features.

Then, the material around the holes isdimensioned (at a much looser tolerance).


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Tolerance Stack-upOccurs when dimensions are takenfrom opposite directions of separateparts to the same point of an assembly.

Dimensionedfrom the

left.

Dimensionedfrom theright.

AVOID THIS!!!


Week 8: Tolerancing

Tolerance stack-up canbe eliminated by carefulconsideration and placement of dimensions.(Dimension from same side).

Better still, relate the twoholes directly to each other,not to either side of the part.The result will be the besttolerance possible of ±0.005.

AvoidingToleranceStack-up


Week 8: Tolerancing

Basic Hole System

The basic hole system is used toapply tolerances to a hole and shaftassembly.

The smallest hole is assigned thebasic diameter from which thetolerance and allowance is applied.

Creating a Clearance FitUsing The Basic Hole System

Using the basic hole system,assign a value of .500” to thesmallest diameter of the hole,which is the lower limit.

The allowance of .004” is subtracted from the diameter of the smallest hole to determine the diameter of the largest shaft, .496”, which is the upper limit.

The lower limit for the shaft is determined by subtracting the part tolerance from .496”. If the tolerance of the part is .003”, the lower limit of the shaft is .493”

The upper limit of the hole is determined by adding the tolerance of the part to .500”. If the tolerance of the part is .003”, the upper limit of the hole is .503”

The parts are dimensioned on the drawing.

Using the assigned values results in a clearance fit between the shaft and the hole. This is determined by finding the difference between the smallest hole (.500” lower limit) and the largest shaft (.496” upper limit), which is a positive .004”. As a check, this value should equal the allowance used in step 2

The difference between the largest hole (.503” upper limit) and the smallest shaft (.493” lower limit) equals a positive .010”. Because both the tightest and loosest fits are positive, there will always be clearance between the shaft and the hole, no matter which manufactured parts are assembled.

Check the work by determining the piece tolerances for the shaft and the hole. To do so, first find the difference between the upper and lower limits for the hole. Subtract .500” from .503” to get .003” as a piece tolerance. This value matches the tolerance applied in Step 4. For the shaft, subtract .493” from .496 to get .003” as the piece tolerance. The value matches the tolerance applied in Step 3.

The system tolerance is the sum of all the piece tolerances. T o determine the system tolerances for the shaft and the hole, add the piece tolerances of .003” and .003” to get .006”


Week 8: Tolerancing

Creating an Interference Fit Using The Basic Hole System

Follow the same sequence of steps as you did for a Clearance Fit,except that you ADD the allowance in Step 2, instead of subtract.

ADD here

This Week’sAssignments

This Week’sCADAssignment

Assignment # 26 Page 173 (SUPPORT) Graph.

Comm. for Engineers

Draw on CAD as an Offset Section and Dimension

w/tolerances

NOTE: Change diameter of the two smaller holes to .56

(Add Revision Block)


Assignment # 27 Page 173 (WHEEL)

Graph. Comm. for Eng.

Draw on CAD as a Half Section and Dimension

w/tolerances

NOTE: Change OD of the Hub to 1.00” (Add

Revision Block)


Assignment # 28 Page 173 (BUSHING)

Graph. Comm. for Eng.

Draw on CAD as a Full Section and Dimension

w/tolerances


Week 9:

Midterm Exams

tech 104 – technical graphics communication week 8: tolerancing practices

Documents

tolerancing tech

tolerancing slide

tolerancing tolerances

depth tech

tolerancing practices

tolerancing heres

todays lecture week

location of features