In view of the great number of materials suitable for gundrilling as well as other variables, a table of specific grinds for all situations is not feasible.  Instead, the gundrill user must understand the function of drill geometry well enough to use available tables as a starting point and them make refinements to suite the particular setup, work material and hole specification.

The cutting edge angles of the conventional gundrill determine chip shape, size, finish, tool life, and therefore, determine tool efficiency.  Generally, softer materials require a smaller included angle.  Hard materials require a larger included angle.  No single pair of angles in best for all materials, but the 30°/20° slash angle combination will drill almost any workpiece material.  For some materials, it may not be 100% efficient, and changes in the included angles and lengths of cutting angles can improve performance.

Ordinarily, the inside angle needs a primary clearance of 10° to 15° while the primary clearance for the outside angle will vary from 12° for hard steels, to 15° for aluminum or magnesium.  In addition, the outside angle must have a form relieved secondary clearance of 15° to 25°.  The secondary clearance area for the outer angle is important because it prevents heeling and it provides controlled access for the cutting fluid to reach the cutting edge.

As a principle of conventional gundrill design, the edge forming the inside angle must extend 0.0005” to 0.032” past the center of the tool, depending on diameter.

Coolant Hole Configurations

Coolant pressure and coolant hole size play an important part in gundrill performance.  The higher the penetration rates, the more important coolant pressure and flow become to evacuate chips and lubricate the cutting edge and wear margins.

Star SU provides three styles of coolant holes with the Double Jet gundrill.

Single plain round coolant holes can be provided in all gundrills.  Elongated coolant holes are standard in sizes .0787” through .3750”.  Plain round coolant holes must be specified in these areas if elongated coolant holes are not desired.

To cut steels or difficult to machine materials, a good sulfur based coolant is recommended.  Coolant temperatures should be maintained below 120° F.

Star SU gundrills are furnished in C-3 micro grain carbide and can also be furnished in C-2 carbide on request.

To Find Out More...

You can download our gundrill catalog or visit our gundrill page on our Star Cutter website.