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The Anatomy of a Gundrill
 The Gundrill– depicted here is your basic, traditional single flute tool. As the tool is drilling, high-pressure oil or coolant is fed through the hollow shank, lubricating and cooling the cutting area and carrying the chips and heat away by means of the V-shaped flute.
The single flute also comes in other variations, such as a cartridge tip for quick changeover of larger size tools without removing the shank from the machine, and carbide-inserted tips where solid carbide would be cost-prohibitive.
There are two other predominant types of drill: the two-flute and the half-round drill (sometimes called the D-Drill.) Each of the latter has limited application as compared to the single flute, but in some instances they may prove the better choice.
The above are all illustrated and explained in great detail in the Gundrilling Handbook.
Tip– The drill tip is made of solid carbide and is the 'worker bee' of the process. The tip and contour shown here are the basic workhorse of Gundrilling– N-8, R-1. The "N-8" indicates the most universal nosegrind: 30 degree outside angle, 20 degree inside angle, and 35 degree duboff. "R-1" is the basic contour.
Although there are probably in excess of 100 nosegrinds (or 'geometries') that have been developed over the years, the N-8/R-1 is the geometry of choice for about 80 to 90% of all gundrilling applications, and is the geometry you'll get if you don't specify otherwise.
Six additional popular nosegrinds and their application are depicted and discussed at length in the Handbook, including the facet grind, as well as grinds for interrupted cuts (stacked work and intersecting holes), flat-bottom holes, cast iron, copper, aluminum and wood.
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Driver– The basic driver is 3/4" dia. by 2-3/4" long, but there are many variations as well as a multitude of custom drivers to accomodate your specific process requirements.
Shank– The shank is the 'shaft' of the tool. It's made of a mildly heat-treated alloy steel tubing. The shank is silver brazed on one end to the Driver, and on the other end to the Tip. The shank tubing is formed into a vee-shaped cross-section prior to brazing to form the flute. The shank is always slightly smaller in diameter than the tip.
Flute– The Flute acts as a passageway to flush the coolant and generated chips and heat back and away from the cutting area.
Flute Length– The distance from the runout of the flute groove to the tip of the drill. The flute runout must always be greater than the depth of the hole you're drilling, or else the chips will get trapped and bad things start to happen, like twisted shanks and broken drills.
Overall Length– This is the way you normally specify the length of the drill. "OAL" is usually equal to depth of hole + 3/4" (safe distance from flute runout to bottom of driver) + driver length (usually 2-3/4") + 5/8" for regrinds + length allowances for chip box, whip supports, etc.
Diameter– The diameter of a gundrill tip is a closely-held dimension based of the diameter of hole you wish to drill. If you specify a 3/8"(.3750") dia. drill, the size will be held to .3750 +.0000 –.0002". It's not necessary to specify to four places unless you need a non-standard drill size: four place accuracy is understood in the industry.
Some contours, the R-1 included, have 'non-micable' diameters, meaning that the wear pad/margin relationship does not allow a 180 degree plane for direct measurement with a micrometer or vernier caliper. (If you mike a tool with an R-1 contour, you'll get a reading of about .0002 to .0003" under the nominal diameter.)
Note: If you do try to gauge a gundrill diameter, always measure across the outside corner. Any measurement below the corner will result in further error generated by the back taper.
Not to worry! Correct diameter of your purchased tool will be within tolerance 99.99% of the time.
Back Taper– The normal back taper on a gundrill is .0006 to .0008" per inch. Back taper can be reduced to a lesser value (ie: .0002 to .0004") in certain instances where additional burnishing is required for optimum surface finish, but this is not usually necessary.
Unlike geometry, which you can change in house by regrinding, back taper is established in the manufacturing process and cannot be altered.
Contour– The configuration of the wear pad/margin combination. The contour shown here is the basic R-1.
The wear pad performs two functions: it acts as a counter-support to the pressures being exerted on the outside angle and the outer corner; and it provides a burnishing action on the I.D. surface of the hole as it's being drilled.
There are several alternate contours available depending on material being drilled and surface finish requirements. These are all described at length in the Handbook.
The contour, like back taper, is 'baked in' at the factory and cannot be changed in the field.
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Drill Bushing (not shown)– The primary function of the Gundrill bushing is momentary, but extremely critical. The bushing must be in intimate contact with the workpiece– support of the gundrill by the bushing lasts for only a fraction of an inch. Shortly after the drill starts to advance, support from the bushing is lost as the nominal diameter of the tool passes out of the bushing and is followed by the diminishing diameter of the back taper. If the bushing is not touching the workpiece, the drill tip starts to 'whip', causing bellmouthed holes and potential tool damage.
The bushing also performs a secondary function by supporting the shank. Although the shank is somewhat less in diameter, the bushing still helps to control shank whipping.
When specifying tooling, the correct nomenclature for the bushing is "GDI" (Gundrill Insert). The tolerance on a GDI is nominal diameter +.0002 –.0000". They may be had in tool steel or in carbide. I prefer tool steel– the word 'carbide' can sometime lull one into a state of false security whereby one forgets to check for bushing wear from time to time. (Carbide ain't like diamond– it doesn't last forever! Some folks just think it does.)
Standard configurations are off-the-shelf and will satisfy most process requirements. However 'specials', featuring non-standard OD's and/or lengths are an option; and lead time and additional cost are reasonable.
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