# ACME THREAD CALCULATOR (GENERAL PURPOSE)

Acme thread calculator can be used to find dimensions of general purpose single and multiple start Acme threaded rods and nuts. Calculation can be done for tolerance classes of 2G, 3G and 4G. Tolerance class 2G is the preferred class but if less play and backlash is needed, 3G or 4G can be selected. Angle of thread of general purpose Acme thread is 29°. The thread height is half of the thread pitch as mentioned above.

Acme thread form is a trapezoidal screw thread profile used for lead screws (power screws). There are different types of Acme threads such as General purpose, Stub Acme screw and Centralizing. General purpose and Centralizing Acme threads are defined in the ASME/ANSI B1.5-1997 standard and Stub Acme screw thread is defined in the ASME/ANSI B1.8-1988 standard. The differences between these types are summarized in the “Definitions” section.

Schematic representation of the thread form of general purpose Acme thread is given in the “Definitions” section.

Note: For more information on ANSI standard General Purpose Acme screw threads including thread designation, basic dimensions, formulas for determining diameters, multi start Acme threads and limiting dimensions for different classes , please refer to pages 1973 - 1980 of Machinery's Handbook, 30th.

Allowances, Tolerances, and Clearances for General Purpose Single-start Acme Threads (All Classes)

 INPUT PARAMETERS Nominal diameter (inches) - threads per inch 1/4-16 5/16-14 3/8-12 7/16-12 1/2-10 5/8-8 3/4-6 7/8-6 1-5 1 1/8-5 1 1/4-5 1 3/8-4 1 1/2-4 1 3/4-4 2-4 2 1/4-3 2 1/2-3 2 3/4-3 3-2 3 1/2-2 4-2 4 1/2-2 5-2 Thread Tolerance Class 1 2G 3G 4G Number of Start 2 1 2 3 4

Note: 1 Preferred class is 2G.

Note: 2 Single or multiple start Acme threads.

### Definitions:

Centralizing Acme Threads: The tolerances of major diameter of external and internal threads are tighter with respect to general purpose Acme threads. In case of a lateral force, external and internal threads contact each other at the major diameter of the thread. This helps to keep alignment of the thread axis and prevents wedging.

Chamfers: General Purpose external threads (threaded rods) may have the crest corner chamfer as shown in the first figure at the top of the page.

Design Profile: The shapes of the external and internal threads in which the limits of dimensions are applied. The thread form of General Purpose Acme Thread is given in the following figure.

Gages for General Purpose Acme Threads: Gages for general purpose acme threads are defined in ASME B1.5 standard. Gage tolerances, gages for Acme threaded rods and nuts including GO/NOT GO thread ring and plug gages and necessary information related with gauging are given in this standard.

Length of Engagement: The tolerances specified in the ASME B1.5-1997 standard are applicable to lengths of engagement not exceeding twice the basic major diameter.

Major Diameter: The maximum diameter of a thread which is diameter of the crest of a male thread or root of a female thread.

Minor Diameter: The minimum diameter of a thread which is diameter of the root of a male thread or crest of a female thread.

Multi Start Acme Thread: Provides fast relative traversing motion with lower mechanical advantage. For multiple start Acme threaded rods: major, minor, and pitch diameter sizes for each class are identical to those for single-start threads. In case of multiple start Acme threaded nuts: maximum major, minor, and pitch diameter sizes are increased by a percentage of external pitch diameter allowance (es) ; 50 % of es for two-start threads, 75% of es for three-start threads and 100% for four-start threads. [Ref-2]

Stub Acme Thread: The thread height is less than general purpose and centralizing acme threads. The thread height is 0.3 pitches for Stub Acme thread. In general purpose form this is 0.5 pitches. Stub Acme thread form is provided for unusual applications where a coarse-pitch thread of shallow depth is required due to mechanical or metallurgical reasons.

Thread class: A classification system to classify the threads for interchangeability and manufacturability.

### List of Equations:

 Parameter Equation Pitch P $$P=\frac { 1 }{ n }$$ Basic Pitch Diameter D2 $${ D }_{ 2 }=D-0.5P$$ Basic Minor Diameter D1 $${ D }_{ 1 }=D-P$$ Thickness at Pitch Line t $$t=\frac { P }{ 2 }$$ Basic Height of Thread h $$h=\frac { P }{ 2 }$$ Basic Flat at Crest (internal thread) Fcn $${ F }_{ cm }=0.3707P$$ Basic Flat at Crest (external thread) Fcs $${ F }_{ cs }=0.3707P-0.259\times (PDAET)$$ Basic Flat at Root (internal thread) Frn $${ F }_{ rn }=0.3707P-0.259\times (MDAIT)$$ Basic Flat at Root (external thread) Frs $${ F }_{ rs }=0.3707P-0.259\times (MDAET-PDAET)$$ Minimum Stress Area At $${ A }_{ t }=3.1416{ (\frac { { d }_{ 1min }+{ d }_{ 2min } }{ 4 } ) }^{ 2 }$$ Minimum Shear Area per Unit Length As $${ A }_{ s }=3.1416\cdot { D }_{ 1max }\cdot (0.5+n\cdot \tan { { 14.5 }^{ ° }\cdot ({ d }_{ 2min }-{ D }_{ 1max }) } )$$

 List of Parameters Symbol Definition PDAET Pitch diameter allowance on external thread MDAIT Major diameter allowance on internal thread MDAET Minor diameter allowance on external thread

### Supplements:

 Link Usage Internal ACME Thread Dimensions Chart Tabulated form of the ACME thread dimensions calculator. External ACME Thread Dimensions Chart Tabulated form of the ACME thread dimensions calculator.

### Reference:

• Oberg, E. , Jones ,F.D. , Horton H.L. , Ryffel H.H., (2016) . Machinery's Handbook . 30th edition.  Industrial Press Inc.
• Oberg, E. , Jones ,F.D. , Horton H.L. , Ryffel H.H., (2012) . Machinery's Handbook . 29th edition.  Industrial Press Inc.
• ASME B1.5-1997, Acme Screw Threads