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Inside tips on gundrill geometry.

Like the carbide insert manufacturers who have improved cutting performance by originating cutting tools with special grinds, different coatings, and various chip-breaking techniques, gundrill makers have followed the same pattern in many ways.

For example, over the last five years, we have introduced an array of new gundrill designs, some unique, and some with improved cutting performance in high-temperature alloy steels and nonferrous materials.

One design finding increasing use is the double-jet gundrill. It was originally introduced in 1961 and recently redesigned for more effective chip control, Figure 1.

Many times a conventional single-jet, single-flute tool will not break a chip, particularly in steels such as 4140, 8620, and some carbon steels. Here a single-flute, double-jet style can provide effective chip breakup and removal at maximum speeds and feeds. The tool, Figure 2, has one coolant hole in the top of the tip above the cutting edge to direct cutting fluid at the cutting edge, a second hole in the conventional location for chip ejection, and a special flat-grind over the top of the tip above the cutting edge.

The tool demonstrates typical performance in roughing 15/16"-dia holes in diecast aluminum master brake cylinders. It drills 6" deep at 40 ipm, machining about 20,000 parts before the need for resharpening. The tool provides good chip control, even in extruded aluminum where long, stringy chips are encountered. It also can serve as a combination through- or blind-hole reamer where flashing is left in a cored hole such as in die-cast oil-pump bodies.

Figure 3 shows another application benefiting from the double-jet design. The diesel-engine connecting rods are gundrilled in a large multiple-spindle transfer line that has been running for 10 years. The machine takes three drilling passes for the bolt holes on forged 1049 steel connecting rods and caps hardened to 26 to 32 Rc. There is an irregular breakout because the sequence includes gundrilling one half of the depth from both sides of the connecting rod assembly, then removing the remaining material in the middle, Figure 4.

Single-flute gundrills did not give the desired tool life because not enough coolant got to the cutting edge. Poor tool life resulted, because, as the tool broke through the conical surface remaining at the end of the partially drilled hole, the pressurized coolant no longer lubricated and cooled the cutting edge. The double-jet tool solved the problem.

Intricacies of geometry

Gundrill-tip geometry, with all its special little nooks and crannies, is what sets gundrills apart from one another, especially single-flute designs. As with carbide-insert manufacturers who do a lot of on-the-floor application testing to find the best tool for the job, the gundrill specialist can many times modify a standard head design on the spot, using a special grind that makes a specific application work. Sometimes these modifications turn into standard designs. And, unfortunately, many are lost because the inventor didn't tell anyone how he did it.

Two of the more common gundrill grinds are the "bull nose" and the "fishtail," Figure 5. Typically, the former offers good chip control, and the latter drills uneven surfaces from the solid, where a conventional web-style drill might wander.

Triple threat and more

Two recent gundrill designs evolved from experiments on a customer's floor. The first is the triple grind, Figure 6. It helps single-flute tools drill lower grades of carbon steel where chips are longer and more difficult to remove by flushing.

The triple design improves tool life by distributing cutting pressure more evenly at the cutting edge, and it achieves higher penetration rates with accurate size control.

The triple grind works on tool sizes from 3/8" dia on up for machining stainless steels and high-temperature alloys. In one setup, it drills a 1/4"-dia hole 4" deep into a stainless-steel valve at 3/4 ipm. Double-grinds, also shown in Figure 6, are used for gundrills below 3/8" dia to achieve cutting-force distributions.

The tip of a double-jet tool has been modified in one case to produce radii in the bottom of blind holes to retain workpiece rigidity and strength, Figure 7.

The GL grind, Figure 8, is named for Star gundrill specialist George Larry, who discovered it. While working on an application at Dana Corp for drilling steel clutch lifter forks, he applied a two-flute, two-hole gundrill with a modified bull-nose grind.

Normally, single-flute designs are used in steel because two-flute styles can't break up chips adequately for quick removal. However, the GL grind worked well in the lifter-fork material, permitting the user to double the feed rate up to 8 ipm. Suitable steels for this drill include the 1010, 1020, 1030, and 4140.

Pin-type gundrills, Figures 9 and 10, work like a trepanning tool that cuts from the solid and generates an accurately controlled pin or core. Hydraulic-pump manufacturers use these tools to remove the weight from piston barrels while leaving a core for gundrilling another hole to serve as a lubrication channel. The drills come in 1/2"-dia sizes and larger.

Body design

Most users say head design has the most important impact on gundrill success; however, in some instances, success lies in the body design. Usually, a gundrill body is made from heat-treated 4130 tubing with a steel driver brazed to one end. Flutes are either milled or crimped into the tubular material, depending on the application.

For more severe applications (or faster penetration rates), a double-crimp design produced by swaging is preferred, because the coolant holes are larger than those in a milled flute and thus carry more coolant, Figure 11. Moreover, the flutes that are formed are much deeper than milled tools, because it is not necessary to make allowance for wall thickness between flute and coolant. These deeper flutes improve chip-removal efficiency of the tool.

Of course, addition of chipbrakers is sometimes needed to solve tough chip-control problems. Designs for both single- and double-flute types are shown in Figure 12.

Some gundrills have wear pads built into the carbide heads, Figure 13. They are useful if the user wants a slightly oversized hole, if the gundrilling machine is sloppy, or if a cast blind hole is drilled.

Diamonds in style

Another recent development is use of diamond compact to increase gundrill tool life (see T&P, December 1984, "Drilling with PDC"). In two years of application, users have gained better hole finishes, better size control, and major tool-life improvements. Most designs feature a polycrystalline insert with a carbide backing brazed into a carbide head, Figure 14. Currently, we use a triple grind for drilling high-silicon 390 aluminum.

Sharpening these tools is time consuming, but patience and diamond wheels can do the job accurately. Similar techniques will be useful when polycrystalline CBN becomes more popular for gundrills, too.

Finally, there continues to be interest in applying coatings like TiN to gundrill tips. Some are in use today without significant success. It appears that PVD coatings rub off too easily to be cost effective. And some CVD coatings have a contamination problem associated with the brazing of the carbide head to the body. Our own in-house coating division, Gold Star Coatings, has solved that problem and is testing CVD coated tools.
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Author:Stokes, Thomas
Publication:Tooling & Production
Date:Mar 1, 1985
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