Flat head socket cap screw dimensions are shown in the following chart according to ASME B18.3-2003 standard.
Flat head socket cap screw is a type of cap screw with a flat top
surface and a conical bearing surface with a head angle of approximately 82 deg.
Both hexagon and spline socket types are available for drive.
Some important notes from the ASME B18.3:
- The diameter range of flat head socket cap screws is from 0.06 inch to 1.5 inches according to
- Thread shall conform to Unified external threads with radius root: Class 3A UNRC and UNRF
for screw sizes No. 0 through 1 inch inclusive, Class 2A UNRC and UNRF for sizes over 1 inch through 1
1⁄2 inches inclusive.
- Alloy steel cap screws shall conform to ASTM F835 and stainless steel cap screws shall conform to ASTM F879.
Note1: Screws with nominal lengths less than the smallest length
given in the dropdown list are full form (complete) threads. In this case, the
threads shall extend to within two pitches of the intersection of the conical
portion of the head with the basic screw diameter.
Note2: For determination of min body length (LB), maximum
grip gaging length (LG) and min thread length (LT) of screws with
nominal lengths greater than the largest length given in the dropdown list, see
ASME B18.3 standard.
Note3: Screws having nominal lengths falling between those for which given in the Nominal Length Dropdown List shall have
LG and LB dimensions conforming with those of the next shorter nominal length for
the respective screw size. For example: for a 1⁄4 in. size screw, 2.13 in. long, LG
= 0.75 in. and LB= 0.50 in.
Note1: Body refers to the unthreaded cylindrical portion of the
shank for those screws not threaded to head.
Note2: Maximum sharp value is a theoretical value and not practical
to make the edges of the head sharp. This value represents the exact dia. of a
hole countersunk to 82 deg, in which a screw having max. head size will fit
Note3: The protrusion limits shown shall apply only when the gaging diameter is exactly indicated.
See appendix of ASME B18.3 for details.
Note:4 A fillet between the conical bearing surface of the head and
the shank of the screw is allowable above the max. F value.
Designation: Product Name, Designation of the Standard, Nominal Size - Thread Pitch x Nominal Length, Material, Protective Finish (if required) Ex: Hexagon Socket
Flat Countersunk Head Cap Screws, ASME B18.3, 0.250 - 28 x 1.750, Stainless Steel
Long Screw (Full Body Screw): Screw which is not threaded for full length.
Unified Screw Thread: A thread
form used by the United Kingdom, Canada, and the United States to obtain screw
thread interchangeability among these three nations. It has 60° thread angle and
dimensions are stated in inch units.
UN form screw threads: The UN
thread is intended for general purpose fastening operations.
UNC (Unified National Coarse):
Most commonly used type for general engineering applications. This thread form
is used in materials with low tensile strength which makes threads more
resistance against stripping (internal threads) .UNC give possibility for quick assembly.
UNF (Unified National Fine): External
threads of this Fine Series have greater tensile stress area than comparable sizes of the
coarse series. The Fine series is suitable when the resistance to stripping of both external
and mating internal threads equals or exceeds the tensile load carrying capacity of the
externally threaded member. It is also used where the length of engagement
is short, where a smaller lead angle is desired, where the wall thickness demands a fine
pitch, or where finer adjustment is needed.
Short Screw (Thread to Head Screw): A screw which is threaded for full length.
Thread class: A classification system to classify the threads for
interchangeability and manufacturability. Class 1 threads are loosely fitting
thread and used for ease of assembly. Most commonly used class is 2 and this
class is designed for general use. Class 3 is used for closer tolerances.
Thread Gages and Gaging :
Specifications and dimensions for the gages used on Unified inch screw thread is
defined in the ANSI/ASME B1.2-1983 standard. Types of gages including thread
plug gages, snap gages, thread ring gages and plain diameter gages for internal
and external threads are covered in same standard.