The Relationship Between H-Limit and Class of Fit
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Table 1. Permissible deviations for linear dimensions except chamfered parts (external radii and chamfer heights, see Table 2.) Unit : mm Tolerance class
Designation Division of basic dimension 0.5 (1) Over 3 Over 6 Over Over Over Over Over up to 3 up to 6 up to 30 up 120 up 400 up 1000 2000 30 to 120 to 400 to up to up to 1000 2000 4000 Description Permissible deviations f fine 0.05 0.05 0.1 0.15 0.2 0.3 0.5 - m medium 0.1 0.1 0.2 0.3 0.5 0.8 1.2 2 c coarse 0.2 0.3 0.5 0.8 1.2 2 3 4 v very coarse - 0.5 1 1.5 2.5 4 6 8 Note (1) : For basic dimensions below 0.5 mm, the deviations shall be indicated adjacent to the relevant basic dimension(s). Table 2. Permissible deviations for linear dimensions of chamfered parts (Rounding of corner and chamfered dimension) Unit : mm Tolerance class Division of basic dimension Designation Description 0.5 (1) Over 3 up to 3 up to 6 Over 6 Permissible deviations Note (1) : f fine m medium c coarse v very coarse 0.2 0.5 1 0.4 1 2 For basic dimensions below 0.5 mm, the deviations shall be indicated adjacent to the relevant basic dimension(s). Table 3. Permissible deviations for angular deviations Tolerance class Deviation of length (unit : mm) of the shorter side of the angle concerned Up to 10 Designation Description Over 10 Over 50 Over 120 up to 50 up to 120 up to 400 Over 400 Permissible deviations f fine m medium c v 1° 30c 20c 10c 5c coarse 1°30c 1° 30c 15c 10c very coarse 3° 2° 1° 30c 20c Table 4. General tolerances on straightness and flatness Unit : mm Straightness and flatness tolerances for ranges of nominal lengths Tolerance Up to 10 class Over 10 Over 30 Over 100 Over 300 Over 1000 up to 30 up to 100 up to 300 up to 1000 up to 3000 General tolerances on straightness and flatness H 0.02 0.05 0.1 0.2 0.3 0.4 K 0.05 0.1 0.2 0.4 0.6 0.8 L 0.1 0.2 0.4 0.8 1.2 1.6 Table 5. General tolerances on perpendicularity Unit : mm Perpendicularity tolerances for ranges of nominal lengths of the shorter side Tolerance class Up to 100 Over 100 Over 300 Over 1000 up to 300 up to 1000 up to 3000 General tolerances on perpendicularity H 0.2 0.3 0.4 0.5 K 0.4 0.6 0.8 1 L 0.6 1 1.5 2 Table 6. General tolerances on symmetry Unit : mm Symmetry tolerances for ranges of nominal length Tolerance Up to 100 class Over 100 Over 300 Over 1000 up to 300 up to 1000 up to 3000 General tolerances on symmetry H 0.5 K L 0.6 0.6 1 0.8 1 1.5 2 Table 7. General tolerances on circular run-out Unit : mm Tolerance class Circular run-out tolerance H 0.1 K 0.2 L 0.5
H limits are used to properly size a tap for the threaded hole to be produced. They are selected based upon the tolerance required for the part. These tolerances are defined by the symbols class 1B, 2B, or 3B. Class 1B has the broadest tolerance and is generally applied to DIY (Do It Yourself) type nuts and bolts. Class 2B is the most common and is used for general fasteners and threaded parts. Class 3B is the tightest tolerance and used for close fit and high strength fastening applications, primarily in the automotive and aerospace industry.
Once the class of thread and part tolerance has been defined, an H limit is selected to produce a thread that is within the minimum and maximum limits for that class if fit. These limits are the same as the Go and Not Go thread plug gage dimensions. The goal is to select a tap with an H-limit that is near the middle of the part tolerance. For instance, if the total tolerance was .005', the tap should be approximately .0025' larger than the minimum limit of the part and .0025' smaller than the maximum. However, to handle the widest variety of tapping conditions, the '40% rule' is commonly used. Using this rule, the tap is placed at 40% of the part tolerance. For example, if the part tolerance is .005', multiplying .005' by 0.40 equals .002'. Thus, the tap would be .002' larger than the minimum limit of the part or Go thread gage.
With the position of the tap in relationship to the part tolerance established, the selection of an H limit number, such as H3, H4, H5, etc. is possible. H limits are a sequence of size 'steps' in .0005' increments beginning at the minimum size limit of the part, starting with H1. In other words, an H1 limit is one .0005' increment larger than the minimum limit or Go gage, an H2 is two .0005' increments (or .001) larger than the minimum limit, an H3 is three .0005' increments (or .0015') and so on. In the example above, a tap that is .002' larger than minimum limit, is four .0005' increments larger, or an H4. This would be the tap H limit recommendation for this tolerance.
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TRID Fee Placement and Tolerance Chart As of 1/1/2016 By VS Loan Estimate ZERO Tolerance 10% Tolerance NO Tolerance Requirement Section A. Origination Charges Section B. Services You Cannot Shop For Section E. Taxes and Other Government Fees Section E. Taxes and Other Government Fees Section C. Preferred fits and tolerance table for hole and shaft basis systems which are. H7/g6, G7/h6, Sliding fit not intended to run freely, but to move and turn freely.
If after selecting the proper H limit, an oversize or undersize thread exists, or if shrinkage due to heat treating or plating will occur, larger or smaller H limits may be required to adjust to the condition.
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