ASME/ANSI BUTTRESS INCH SCREW THREADS DIMENSIONS
ANSI/ASME buttress Inch screw thread is a form of thread with load angle of 7 degrees and a clearance angle of 45 deg from the normal to the axis.
The basic height of thread engagement for buttress thread is 0.6p. The buttress thread form has certain advantages for the applications where
high stress occurs only in one direction along the thread axis. The buttress form of thread is particularly applicable when tubular members screwed together.
Breech assemblies of large guns, airplane propeller hubs and columns for hydraulic presses are some of the examples of buttress threaded rods and nuts.
Following calculator has been developed in accordance to ANSI B1.91973 “Buttress Inch Screw Threads”
standard. The standard provides information about preferred
diameterpitch combinations, formulas for calculation pitch diameter tolerances, tolerances for major and minor diameters,
a system of allowances between external and internal threads, recommended methods of measuring and gauging, dimensional
acceptability of buttress threaded rods and nuts.
Note: For more information on buttress threads including diameterpitch combinations, thread designation, basic dimensions, formulas, thread tolerances and allowances for easy assembly , please refer to pages 1945  1952 of
Machinery's Handbook, 29th.

Standard 7° / 45° Buttress Thread with 0.6p Basic
Height of Thread Engagement and Round Root 
Calculator:
Note: ^{1} Class 2: Standard grade, Class 3 : Precision Grade
RESULTS 
Parameter 
Symbol 
Value 
Unit 
Nominal Diameter 
D 


in 
Threads per inch 
n 


 
Thread Tolerance 
 


LIMITS OF SIZES FOR BUTTRESS THREADED ROD 
Maximum Major Diameter 
D_{s_max} 


in 
Minimum Major Diameter 
D_{s_min} 


Maximum Pitch Diameter 
E_{s_max} 


Minimum Pitch Diameter 
E_{s_min} 


Maximum Minor Diameter 
K_{s_max} 


Minimum Minor Diameter 
K_{s_min} 


LIMITS OF SIZES FOR BUTTRESS THREADED NUT 
Maximum Major Diameter 
D_{n_max} 


in 
Minimum Major Diameter 
D_{n_min} 


Maximum Pitch Diameter 
E_{n_max} 


Minimum Pitch Diameter 
E_{n_min} 


Maximum Minor Diameter 
K_{n_max} 


Minimum Minor Diameter 
K_{n_min} 


TOLERANCES & ALLOWANCES 
Tolerance ^{1} 
tol. 


in 
Allowance 
G 


BASIC DIMENSIONS 
Pitch 
p 


in 
Height of sharpV thread 
H 


Basic height of thread engagement 
h 


Max. root radius (theoretical) 
r_{max} 


Min. root radius (theoretical) 
r_{min} 


Max. root truncation 
s_{max} 


Min. root truncation 
s_{min} 


Max. flat width for flat root form 
S_{max} 


Min. flat width for flat root form 
S_{min} 


Crest truncation 
f 


Crest width 
F 


Height of thread of internal thread 
h_{n} 


Height of thread of external thread 
h_{s} 


Note: ^{1} Tolerance on major diameter of external thread, pitch diameter of external and internal thread, and minor diameter of internal thread.
Definitions:
Gages for Buttress Screw Threads:
Recommended gages and gauging practice for
external and internal buttress thread are defined in ASME B1.9 standard.
Gage maker's tolerances, gages for buttress threaded rods and nuts including GO/NOT GO plain ring gages,
threaded rings, setting plug gages and necessary information related with gauging is
given in this standard.
Allowance: An allowance (clearance) should be provided on all
external threads to secure easy assembly of parts.
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.
Roots of Threads: There are 2 alternatives for thread roots. These are rounded and flat roots.
Rounded Roots: Equal radii, at the roots of the external and
internal basic thread forms tangential to the load flank and the clearance flank. The rounded root form of the external and internal thread shall be a continuous, smoothly
blended curve within the zone defined by 0.07141p maximum to 0.0357p minimum radius. The resulting curve shall have no reversals
and sudden angular variations, and shall be tangent to flanks of the thread. There is, in practice, almost no chance that the thread
forms will be achieved strictly as basically specified,
that is, as true radii.
Flat Roots: Equal flat root of the external and internal
thread. In instances where absence of root radius is not detrimental
to the requirements for strength, and where it is more economical
to provide tools which do not produce a radius at
root, flat root buttress threads may be specified.

Standard 7° / 45° Buttress Thread with 0.6p Basic
Height of Thread Engagement and Flat Root 
Threads per inch: Number of full
thread per an inch length.
Threads designation: When only the designation, BUTT is used, the
thread is "pull" type buttress (external thread pulls)
with the clearance flank leading and the pressure flank
7° following. When the designation, PUSHBUTT is
used, the thread is a push type buttress (external
thread pushes) with the load flank 7° leading the 45°
clearance flank following.
List of Equations:
Parameter/Condition 
Symbol 
Max. Material
(Basic) 
Min. Material 
Height of sharpV thread 
H 
H = 0.89064p 
 
Basic height of thread engagement 
h 
h = 0.6p 
 
Root radius (theoretical) 
r 
r= 0.07141p 
r_{min} = 0.0357p 
Root truncation 
s 
s = 0.0826p 
s_{min} = 0.0413p 
Flat width for flat root form 
S 
S = 0.0928p 
S_{min} = 0.0464p 
Height of thread engagement 
h_{e} 
h_{e_max} = h  0.5G 
h_{e_min} = h_{e_max } (0.5 tol.
on major dia. external
thread + 0.5 tol. on
minor dia. internal thread) 
Crest truncation 
f 
f = 0.14532p 
 
Crest width 
F 
F = 0.16316p 
 
Major diameter of internal thread 
D_{n} 
D_{n} = D + 0.12542p 
D_{n_max} = E_{n_max}+ 0.80803p 
Major diameter of external thread 
D_{s} 
D_{s} = D G 
D_{s_min} = D  G  D_{tol} 
Pitch diameter of internal thread 
E_{n} 
E_{n} = D  h 
E_{n_max} = D  h + PD_{tol} 
Pitch diameter of external thread 
E_{s} 
E_{s} = D  h  G 
E_{s_min} = D  h
 G  PD_{tol} 
Minor diameter of external thread 
K_{s} 
K_{s} = D  1.32542p  G 
K_{s_min} = E_{s_min}  0.80803p 
Minor diameter of internal thread 
K_{n} 
K_{n} = D  2h 
K_{n_min} = D  2h + K_{tol} 
Height of thread of internal thread 
h_{n} 
h_{n} = 0.66271p 
 
Height of thread of external thread 
h_{s} 
h_{s} = 0.66271p 
 
List of Parameters 
Symbol 
Definition 
p 
Pitch 
G 
Allowance 
D 
Major Diameter 
D_{tol} 
Major diameter tolerance 
E 
Pitch diameter 
PD_{tol} 
Pitch diameter tolerance 
K 
Minor diameter 
K_{tol} 
Minor diameter tolerance 
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.

ANSI B1.91973 Buttress Inch Screw Threads