# tolerances

Student
15 de Apr de 2015
1 de 5

### tolerances

• 1. ASSIGNMENT-5 1. Understand why tolerances are needed. Explain with a case study? It is almost impossible (and sometimes uneconomical) to maintain the strict degree of accuracy as listed on a plan. To accommodate this, it is normal to display measurements with a plus or minus (+/-) tolerance which allows for some margin of error. Care needs to be taken however when determining such +/- tolerance, particularly where there are mating parts. For example, a shaft which is machined to its maximum tolerance may not fit a gear center that has been machined to it minimum tolerance or an unsatisfactory loose fit would result from the shaft being machined to its minimum tolerance with the gear center machined to its maximum tolerance. Usually, the dimensional tolerance is decided at the design stage and a Machinist must take care to apply the required dimensional tolerance and to ensure that discrepancies are not introduced as a result of poor workmanship of measuring techniques. Case study: Imagine you are working on a design for a high efficiency windmill. Due to its complexities you need various custom parts, so you send out manufacturing drawings to various vendors for them to be made. Several weeks later you receive all the parts, but some do not fit. Suppose,One of your special shafts that should be 7/8 in. in diameter does not fit in its mating bearing. So, you grab your Vernier callipers and measure the section of the shaft only to discover that it has a diameter greater than what you requested, but by only 0.004 in. Yes, four thousandths of an inch can make a difference. Any interference, defined here as the diameter of a hole that is smaller than the diameter of a shaft, will prevent parts from sliding together. They might have to be pressed on. If too large of an interference exists, it will degrade system performance, especially in bearings. You specified the diameter of the shaft as 0.875 in., but the machine shop made the part to a 0.879 in. diameter. Why the difference? Some machine shops will apply a standard tolerance of 3 decimal places (±0.005) to un-toleranced dimensions, especially if they do not know the design intent. Now, you’ve lost weeks of time while you wait for reworked parts. Such a scenario can be avoided. While many machine shops use due diligence to verify non- toleranced dimensions, it is critical to understand the importance of tolerances, and how to use them correctly. Since parts need to be made either from larger pieces of material or built up from a powder or liquid, there’s no guarantee they will be exactly the size you want.
• 2. ASSIGNMENT-5 2. Understand Importance of Fits and Tolerances in an assembly.Explain with case study? Fit: When two parts are to be assembled, the relation resulting from the difference between their sizes before assembly is called a fit. A fit may be defined as the degree of tightness and looseness between two mating parts Tolerance: Tolerance can also be defined as the amount by which the job is allowed to go away from accuracy and perfectness without causing any functional trouble, when assembled with its mating part and put into actual service. Fit can be divided into three classes: (i) Clearance Fit. (ii) Interference Fit. (iii) Transition Fit Clearance fit: In clearance fit, an air space or clearance exists between the shaft and hole as shown in Figure. Such fits give loose joint. A clearance fit has positive allowance, i.e. there is Minimum positive clearance between high limit of the shaft and low limit of the hole. Interference fit: A negative difference between diameter of the hole and the shaft is called interference. In such cases, the diameter of the shaft is always larger than the hole diameter. In Figure Interference fit has a negative allowance, i.e. interference exists between the high limit of hole and low limit of the shaft. In such a fit, the tolerance zone of the hole is always below that of the shaft. The shaft is assembled by pressure or heat expansion Transition fit: It may result in either clearance fit or interference fit depending on the actual value of the individual tolerances of the mating components. Transition fits are a compromise between clearance and interference fits. They are used for applications where accurate location is important but either a small amount of clearance or interference is permissible. As shown in Figure 3.3, there is overlapping of tolerance zones of the hole and shaft.
• 3. ASSIGNMENT-5 CASE STUDY:
• 4. ASSIGNMENT-5 3. What is the importance of Geometric Tolerance? Give Few Examples? Geometrical tolerances refer to the shape of the surfaces (tolerance of form) as well as the relative location of one feature to another (tolerance of position). Geometric means geometric forms such as a plane, cylinder, square, etc. Geometrical features are : flatness, straightness, squareness etc. These tolerances are specified by special symbols (refer Tables below Geometrical tolerances are specified for geometrical features, in addition to linear tolerances. Data about the tolerances on the shape and location of surfaces are indicated on drawings in a rectangular box divided into two or three parts. For example “Lack parallelism between two surfaces is within 0.1 mm” can be written as
• 5. Examples of geometrical tolerances: Parallelism (Figure a) It indicates the requirement, “Surface A is parallel to opposite face within 0.1 mm”. Straightness (Figure b) It indicates the requirement, “Straight within 0.02 mm”. Squareness (Figure c) It indicates the requirement, “Square within 0.03 mm total”. Flatness (Figure d) It indicates the requirement, “Flat within 0.002 mm total”. Roundness (Figure e) It indicates the requirement, “Taper round within 0.01 mm”. (a) (b) (c) (d) (e) A 50 20.02 20.00 // 0.1 A  0.0 2  0.03 0 0.0  0.02 ASSIGNMENT-5