Basics of Tolerance 121
Basics of Tolerance provides a comprehensive overview on part tolerancing, including different types of tolerances and the relationship between tolerances and part dimensions. Every manufactured part must meet certain specifications. Tolerances describe the range of acceptable measurements in which a part can still perform its intended function. Tolerance ranges typically describe a linear measurement. Surface texture can require a certain tolerance as well. Tolerances attempt to balance the use of a product with the cost required to produce that product.
Improper tolerancing can result in parts that do not function in the way they were intended or parts produced with dimensions that are more precise than necessary, adding unwanted cost to production. After the class, users will be able to describe common methods used for part tolerancing, as well as the impact tolerances have on part production and quality.
Number of Lessons 14
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- The Importance of Measurements
- What is Tolerance?
- Tolerance and Use
- Tolerance and Cost
- Tolerance Review
- Tolerances for Various Operations
- Types of Tolerance
- Machining Tolerance Limits
- Advantages of Limit Dimensions
- Tolerances for a Hole
- Limit Dimensions and Allowance
- Surface Tolerance
- Surface Texture
- Dimensioning and Tolerance
- Identify why measurements are important in a manufacturing environment.
- Define tolerance.
- Identify how tolerance is determined.
- Describe the impact of tolerance on cost.
- Compare tolerances possible in machining operations.
- Identify methods of describing tolerance.
- Identify advantages of different tolerance methods.
- Identify elements of tolerance for holes.
- Describe elements of surface tolerance.
- Describe elements of surface tolerance.
- Identify the relationship between dimensions and tolerance.
A material or tool consisting of hard particles used to wear down, rub away, or machine material. Abrasive materials are often used in finishing operations.
The difference between a measurement reading and the true value of that measurement. The less error present in the measurement, the more accurate the results.
The difference between the smallest permissible hole and the largest permissible shaft. An allowance creates a certain kind of fit between mating parts.
A tolerance method using an equal plus and minus deviation from the specified dimension. Bilateral tolerances are preferred if deviation in either direction risks exceeding the absolute limits and can be equal or unequal.
A document that contains the instructions necessary to manufacture and/or assemble a part. The designer includes the degree of tolerance necessary for the part to function as intended in the blueprint.
A manufacturing process that involves pouring a heated liquid material into a hollow mold until the material cools into a solidified shape. Casting creates a part surface with no clear lay.
A fit that allows intentional space to exist between a hole and the shaft inserted into it. For a clearance fit, the shaft diameter is designed to be slightly smaller than the hole.
The desired measurement of a feature on a part. A dimension is listed as a number given in the appropriate units.
The process of determining the desired measurement of a feature on a part. The ideal dimensions for a part can never be met, requiring the addition of tolerances.
The use of a drill to cut a round hole into a workpiece. The drill may rotate to cut a stationary workpiece, or a stationary drill may be fed into a rotating workpiece.
An unintentional irregularity that may be random or repeating. In surface inspection, flaws are random surface defects that are generally not included in the measurement of the surface.
The use of an abrasive tool or wheel to wear away at the surface of a workpiece and change its shape. Grinding produces a finish that is smoother than both sawing and milling.
A tight fit designed where the smallest permissible shaft is larger than the largest permissible hole. Force is required to assemble parts with an interference fit.
An abrasive process that removes the last bit of unwanted material. A lapping process sometimes uses an abrasive paste that is rubbed across the part to gradually smooth it.
The overall direction of the pattern created by the production process. Lay cannot be measured because it indicates only a direction.
A tolerancing method using an absolute maximum and minimum allowable dimension. Limit dimensioning specifies a range of acceptable measurements instead of a target dimension.
The process of removing metal to form or finish a part. Machining can occur using traditional methods, like turning, drilling, milling, and grinding, or with less traditional methods that use electricity, heat, or chemical reaction.
The use of a rotating multi-point cutting tool to machine flat surfaces, slots, or internal recesses into a workpiece. Milling produces a finish that is smoother than sawing but rougher than grinding.
The degree to which an instrument will repeat the same measurement over a period of time. Precision is also called repeatability, as it will show the same results under unchanged conditions.
The inherent, fine, closely-spaced irregularities remaining on a part surface after manufacturing. Roughness is created by the production process.
The allowable deviation in surface texture. Surface tolerances should only be used for surfaces that affect the use of a part.
The total amount a specific dimension is permitted to vary. The object will still meet specifications if it remains within its tolerance.
Parts may have different tolerances for different features. A cast part can be milled to produce a flatter surface within a hair’s thickness of 0.001 in. (0.03 mm). Likewise, a machined surface can be ground afterward to meet tighter tolerance restrictions within 0.0003 in. (0.008 mm).
A fit toleranced for an allowance that permits both a clearance and an interference fit. For a transition fit, a clearance fit occurs with the smallest shaft diameter and the largest hole diameter, but an inference fit occurs with the largest shaft diameter and the smallest hole diameter.
A machining operation used to make cylindrical parts. During turning, a single-point cutting tool passes along the outer surface of a rotating cylindrical workpiece and gradually removes a layer of material.
A tolerancing method using a deviation in only one direction, either plus or minus, from the specified dimension. Unilateral tolerances are used if variation in only one direction risks exceeding the absolute limits.
The repeating, widely spaced irregularities of surface texture. Waviness is the result of machine deflections and vibration.