Metrology & Statistical Quality Control

Metrology & Statistical Quality Control 5th Term, Batch: 2010 lec# 3 & 4

Standards Of Measurement (Cont..) Characteristics of line Standards: • Accurate engraving on the scales can be done but it is difficult to take full advantage of this accuracy. Eg: a steel rule can be read up to ± 0.2mm of the true dimension. • It is easier & quicker to use a scale over a wide range. • The scale markings are not subject to wear although significant wear on leading end leads to undersizing lec# 3 & 4

Standards Of Measurement (Cont..) Characteristics of line Standards (Cont..): 4. There is no ‘built in’ datum in a scale which would allow easy scale alignment with the axis of measurement, this again leads to under sizing. 5. Scales are subjected to the parallax effect, a source of both positive & negative reading error lec# 3 & 4

Standards Of Measurement (Cont..) Characteristics of End Standards • Except for applications where microscopes can be used, scales are not generally convenient for the direct measurement of engineering products, where as slip gauges are in every day use in tool-rooms, workshop, & inspection departments throughout the world • Modern end standards consists fundamentally of a block or bar of steel generally hardened, whose end faces are lapped flat & parallel to within a few millionth of a cm. lec# 3 & 4

Standards Of Measurement (Cont..) Characteristics of End Standards (cont..) • Highly accurate & well suited to close tolerance measurements • Dimensional tolerances as small as 0.0005mm can be obtained • Subjected to wear on their measuring faces • To provide a given size, the groups of blocks are “wrung” together. Faulty wringing leads to damage • There is a “built-in” datum in end standards, because their measuring faces are flat & parallel and can be positively located on a datum surface • As their use depends on “feel” they are not subject to the parallax error lec# 3 & 4

Standards Of Measurement (Cont..)Relative Characteristics of Line and End Standards lec# 3 & 4

Limits, Fits & Tolerance • Tolerance: is the difference in size b/w limits, prescribed in order to allow reasonable errors in workmanship 1.1. Unilateral Tolerance: one which is disposed either below or above the basic size 1.2. Bilateral Tolerance: one which is disposed both above & below the basic size lec# 3 & 4

Limits, Fits & Tolerance (cont..) 2. Interference: is the difference in size b/w female feature & male mating feature when the later feature is larger 3. Clearance: is the difference in size b/w female feature & male mating feature when male feature is smaller 4. Allowance: is the specified difference b/w low limit of size of female feature & the high limit of size of male feature lec# 3 & 4

Limits, Fits & Tolerance (cont..) • Limits: The extreme allowable range in any manufacturing feature , Or maximum & minimum allowable errors above the basic size 6. Fits: Literal meaning is to adjust. The fit b/w the two mating features is the relationship which results with respect to clearance or interference obtained lec# 3 & 4

Limits, Fits & Tolerance (cont..) 7. Limits System: It consists of series of tolerances & allowances arranged to suit a specific range of sizes, so that limits of size may be selected & given to pairs of mating feature to ensure specific class of fit lec# 3 & 4

Limits, Fits & Tolerance (cont..) Definitions: • Shaft: it refers not only the diameter of a circular shaft but to any external dimension on a component • Hole: it refers not only the diameter of a circular hole but to any internal dimension on a component lec# 3 & 4

Limits, Fits & Tolerance (cont..) Definitions(cont..): 3. Actual size of a shaft: this is the measured dimension of the part 4. Basic Size: the basic size is a standard size for the part & is the same for both the hole & its shaft 5. Zero line: this is the line which represents the basic size so that the deviation from the basic size is zero lec# 3 & 4

Limits, Fits & Tolerance (cont..) Classes of Fits: • Clearance Fit: A Clearance fit could be obtained by making the lower limit on the hole equal to a larger than the upper limit on the shaft. Any hole & any shaft made to these tolerances would assemble with a clearance fit with certainity lec# 3 & 4

Limits, Fits & Tolerance (cont..) Classes of Fits (cont..): • Interference Fit: An Interference fit would be obtained with equal certainty by making the lower limit on the shaft equal to or larger than the upper limit on the hole lec# 3 & 4

Limits, Fits & Tolerance (cont..) Classes of Fits (cont..): • Transition Fit: Transition fit, b/w these two conditions lies a range of fits known as transition fit. These are obtained when the upper limit on the shaft is larger than the lower on the hole, & the lower limit on the shaft is smaller than the upper limit on the hole. It must be realized that transition fits exist only as a class; any actual hole & shaft must assemble with either a clearance or interference fit. lec# 3 & 4

Limits, Fits & Tolerance (cont..) Basis of Fits (or limit) System: This system may be arranged on the following basis: • Hole Basis System: Hole basis system is one in which the limits on the hole are kept constant & the variation necessary to obtain the classes of fit are arranged by varying those on the shaft. lec# 3 & 4

Limits, Fits & Tolerance (cont..) Basis of Fits (or limit) System (cont..): • Shaft basis System: Shaft basis system is one in which the limits on the shaft are kept constant & the variations necessary to obtain the classes of fit are arranged by varying the limits on the holes. lec# 3 & 4

Limits, Fits & Tolerance (cont..) Preferred Basis of Fits (or limit) System: In present day industrial practice hole basis system is used because a great may holes are produced by standard tooling, for example reamers, drills etc, whose size is not adjustable. Subsequently the shaft sizes are more readily variable about the basic size by means of turning & grinding operations. Thus the hole basis system results in considerable reduction in reamers & other precision tools as compared to a shaft basis system, because in shaft basis system due to non-adjustable nature of reamers drills etc. great variety (of sizes)of these tools are required for producing different classes of holes for one class of shaft for obtaining different fits lec# 3 & 4

Limits, Fits & Tolerance (cont..) Systems of Specifying Tolerances: For Detail refer book Designation of Holes, Shafts & Fits: A Hole or Shaft is completely described if the basic size, followed by the appropriate letter & by the number of tolerance grade, is given. Eg: - A 25mm H-hole with the tolerance grade IT8 is given as: 25mm H8 or simply 25H8 - A 25 mm f-shaft with the tolerance grade IT7 is given as: 25mmf7 or Simply 25f7 lec# 3 & 4

Limits, Fits & Tolerance (cont..) Designation of Holes, Shafts & Fits (cont..): A ‘Fit’ is indicated by combining the designations for both the hole & shaft with the hole designation written first, regardless of the system (i.e. hole system or shaft system), eg: 25H8-f7 Or 25H8/f7 lec# 3 & 4

Limits, Fits & Tolerance (cont..) Commonly Used Holes & Shafts: In several engineering applications the fits required can be met by a quite small selection from the full range available in the standards. The holes & the shafts commonly used are as follows: Holes: H6, H7, H8, H9, H11. Shafts: c11, d10, e9, f7, g6, h6, k6, n6, p6, s6. IS: 919 gives the most commonly used holes & shafts up to 500mm for the purpose of general engineering lec# 3 & 4

Limits, Fits & Tolerance (cont..) ISO System of Limits & Fits: ISO has presently been universally adopted & as a matter of fact IS:919 is almost in line with this system Other characteristics such as fundamental deviations & tolerance unit etc are same in both the systems lec# 3 & 4

Limits, Fits & Tolerance (cont..) Types of Fits: • Selective Fit • Push Fit • Driving Fit • Forced or Pressed Fit • Shrinkage Fit • Freeze Fit lec# 3 & 4

Metrology & Statistical Quality Control