Machinery's Handbook, 31st Edition
2066
Centralizing Acme Screw Threads Table 8a. American National Standard Centralizing Acme Single-Start Screw Threads—Formulas for Determining Diameters ANSI/ASME B1.5-1997 (R2014)
D = Nominal Size or Diameter in Inches P = Pitch = 1 ÷ Number of Threads per Inch
No.
Classes 2C, 3C, and 4C External Threads (Screws)
1 2 3 4 5 6 7 8 9
Major Dia., Max = D (Basic).
Major Dia., Min = D minus tolerance from Table 12, columns 7, 8, or 10. Pitch Dia., Max = Int. Pitch Dia., Min (Formula 9) minus allowance from the appropriate Class 2C, 3C, or 4C column of Table 10. Pitch Dia., Min = Ext. Pitch Dia., Max (Formula 3) minus tolerance from Table 11. Minor Dia., Max = D minus P minus allowance from Table 12, column 3. Minor Dia., Min = Ext. Minor Dia., Max (Formula 5) minus 1.5 × Pitch Dia. tolerance from Table 11. Classes 2C, 3C, and 4C Internal Threads (Nuts) Major Dia., Min = D plus allowance from Table 12, column 4. Major Dia., Max = Int. Major Dia., Min (Formula 7) plus tolerance from Table 12, columns 7, 9, or 11.
Pitch Dia., Min = D minus P /2 (Basic).
10 11 12
Pitch Dia., Max = Int. Pitch Dia., Min (Formula 9) plus tolerance from Table 11.
Minor Dia., Min = D minus 0.9 P .
Minor Dia., Max = Int. Minor Dia., Min (Formula 11) plus tolerance from Table 12, column 6.
Major and Minor Diameter Tolerances: The tolerances on the major and minor diameters of the external and internal threads are listed in Table 12 and are based upon the formulas given in the column headings. An increase of 10 percent in the allowance is recommended for each inch or fraction thereof that the length of engagement exceeds two diameters. For information on gages for Centralizing Acme threads the Standard ANSI/ASME B1.5 should be consulted. Pitch Diameter Tolerances: Pitch diameter tolerances for Classes 2C, 3C and 4C for var ious practicable combinations of diameter and pitch are given in Table 11 . The ratios of the pitch diameter tolerances of Classes 2C, 3C, and 4C are 3.0, 1.4, and 1, respectively. Application of Tolerances: The tolerances specified are such as to insure interchange ability and maintain a high grade of product. The tolerances on the diameters of internal threads are plus, being applied from the minimum sizes to above the minimum sizes. The tolerances on the diameters of external threads are minus, being applied from the maxi mum sizes to below the maximum sizes. The pitch diameter tolerances for an external or internal thread of a given class are the same. Limiting Dimensions: Limiting dimensions for Centralizing Acme threads in the pre ferred series of diameters and pitches are given in Table 8b and Table 8c. These limits are based on the formulas in Table 8a. For combinations of pitch and diameter other than those in the preferred series the formulas in Table 8b and Table 8c and the data in the tables referred to therein make it possible to readily determine the limiting dimension required. Designation of Centralizing Acme Threads.— The following examples are given to show how these Acme threads are designated on drawings, in specifications, and on tools and gages: Example, 1.750-6-ACME-4C: Indicates a Centralizing Class 4C Acme thread of 1.750- inch major diameter, 0.1667-inch pitch, single thread, right-hand.
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