The limits and tolerances in this Standard relate to single-start
ACME screw threads and may be used, if considered suitable, for
multiple-start ACME screw threads. The latter screw threads are
used to provide relatively fast traversing motion when necessary.
The three classes-of-fit (2G, 3G, and 4G) of general purpose screw
threads have clearances on all diameters for free movement. This
screw thread relies on the screw thread flanks to maintain concentric
The three classes of centralizing ACME screw threads have a limited
clearance at the major diameters of the external and internal screw
threads so that a bearing at the major diameter maintains approximate
alignment of the screw thread axis and prevents wedging on the flanks
of the screw threads.
For any combination of the three classes of screw threads covered
in this Standard, some end play or backlash will result. This is
unavoidable for interchangeable products. When backlash or end play
is objectionable, some mechanical means should be provided to eliminate
the condition. In any case, sufficient end play must be left to
provide a close running fit.
In addition to limiting dimensions for the standard series of diameters
and pitches of ACME single-start screrw threads, tables of tolerances,
in terms of pitch and diameter, provide for a wide choice of diameters
for a given standard pitch. By using the formulas for diameter and
pitch increments, the pitch diameter tolerances for special diameters
and pitches can be determined for each class. Formulas and data
are also provided for allowances on external screw threads and major
and minor diameter allowances and tolerances.
The Appendices provide text and dimensions on the following:
(a) Alternate Centralizing ACME Screw Threads. Appendix A uses
the minor diameter to ensure concentric operation.
(b) Multiple-Start ACME Screw Threads. Appendix B includes formulas
for limits of size. Multiple-start screw threads may require additional
allowances and/or tolerances for satisfactory operation.
(c) General Purpose ACME Screw Threads, Class 5G. See Appendix
(d) Centralizing ACME Screw Threads, Classes 5C and 6C. See Appendix
(e) Three-Wire Method of Measurement of Pitch Diameter of 29°
External ACME Screw Threads. See Appendix E.
(f) Ball Methods for Internal Pitch Diameter Measurement of 29°
ACME Screw Threads. See Appendix F.
(g) Go Gage Compensation, Calculation of Flank Angle, Limit Gaging
of Setting Rings, and Gaging Problem Areas. See Appendix G.
(h) Tolerances for ACME Screw Thread Gages Over 5 in. See Appendix
(i) Determining Limits of Size for Special Diameter/Pitch Combinations.
See Appendix I .
This data is accurate to the best of my knowledge as of 10/15/09.
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