CoilThread inserts are normally installed 3/4 to 1-1/2 pitch below the surface of the parent material in countersunk holes.  Modification of this set-down depth may be made to suit certain limitations.  Without a countersink the inserts are typically installed 1/4- to 1/2 pitch below the surface.  However, for smoother and easier installation a countersink is strongly recommended.

Drilling the Hole: (step 1)

Drilled Hole Dimensions are tabulated and are available on the Technical Data Index Page or click on the link above to view the tables immediately.  The minor diameters tabulated conform to MIL Spec NASM33537 and must be met after tapping.

The sizes shown are recommendations only, based on expected material behavior and for increased tap life in harder materials.  Actual individual conditions may require varied drill sizes in order to product the correct minor diameter after tapping.

The Minimum drilled hole depth is designed to allow for:
The use of standard countersink as tabulated.
The use of standard Screw-Thread Insert (STI) plug or bottoming tap for the maximum tool life.
The user of the standard installation se-down of 3/4 to 1-1/2 pitch below the work surface.

The "F" dimension minimum is based on the installed insert length, plus set-down, plus tap chamfer length, plus chip clearance.

For plug taps with a nominal diameter of 5/16" or smaller the "F" dimension minimum allows for a make center on the tap equal in length to nominal tap diameter divided by 2.

The nominal insert length shown in the Insert Dimensions table is the minimum through hole length (material thickness) without countersinking and with the insert located 1/4 to 1/2 pitch below the top surface.  With normal countersink and set-down, the minimum through hole material thickness equals "H" dimension, the minimum full thread depth...or minimum tapping depth shown in the Tapped Hole Dimensions table.

Edge Distance Minimum:

For most materials, a minimum edge distance equal to the nominal bolt diameter may be used when measured from the centerline of the hole.

Countersinking: (step 2)

Countersinking values are also tabulated in the Tapped Hole Dimensions table.  These values conform to NASM33537.

Before tapping, it is good practice to countersink 120°±5° to "M' diameter in order to prevent a feather edge at the start of the hole.  The 60° per side countersink angle is used because it is the same as the angles of the tapped thread, insert cross-section and inserting tool prewinder tip.  This combination provides for smooth and rapid production installation.

The "F" and "H" values are calculated with a countersink depth of 1/2 pitch.

Tapping: (step 3)

The pitch diameter limits shown in the Tapped Hole Dimensions table conform to NASM33537 and must be met in order to assure proper thread fit after installation of the CoilThread Insert.