Gundrill makers load up their arsenal.
When the gundrill makes precision holes, as opposed to straight holes, tool wear becomes more important. We thus look at tools with less back taper, coated tools, or even diamond-impregnated tools."
In 1982, Precision Drill & Reamer, Div Winco Industries Inc, Tipp City, OH, built a universally adaptable gundrilling machine with sensors to measure loads and cutting forces. It reaches spindle speeds of 12,000 rpm, feed rates to 90 ipm, developing coolant pressure and flows to 1000 psi and 25 gpm. They use it to evaluate their tool designs and have produced gundrills for machining aluminum that feed at 60 ipm while holding size and roundness within 0.0003" and surface finishes of single-digit micros. They have researched polycrystalline diamond reamers and gunreamers with cubic boron nitride (BZN) inlays.
Sealed Power, Muskegon, MI, is a major producer of valves and valve guides. They have done their own research to increase gundrilling speeds in steel from 2 ipm to 10 ipm. Engineer Jeff Beach doesn't use traditional oil coolants. "It costs too much to clean the parts after machining. That's why we switched to water-soluble coolant for all our gundrilling operations." We are told by another source that a 5 percent soluble coolant works just as well as an oil coolant for gundrilling.
In gundrilling, the cooling effect is the most needed characteristic of the cutting fluid. Water works as well as oil, but it needs twice the pressure. Where ordinary gundrilling machines provide 300 to 400 psi coolant supplies, Sealed Power requires 800 to 1000 psi. In processing high-alloy cast iron and some steel, the only major change in the gundrill tooling is extra back clearance for the oil. The holes they drill are usually 0.3" dia or smaller, using special equipment that provides the high pressure needed.
Several firms now provide gundrills with polycrystalline diamond compact (PDC) tips for high-speed, consistent drilling of nonferrous materials. Crafts Co, Canton, MA, for example, documents case histories of PDC tools lasting up to 50 times longer than carbride in fiberglass golf-club heads, and 65 times the life of carbide in drilling glass-reinforced phenolic. Diamond also serves well in drilling hard-rubber bushings and graphite liners.
What about superabrasives for gundrilling steel? Gordon Collier, product manager--CBN, Specialty Materials Dept, General Electric Co, Worthington, OH, said, "PDC got all the publicity, because it came out first, but now we're offering polycrystalline CBN (cubic boron nitride) for use in gundrills and other tooling, and we expect it to find wide use. We call it BZN.sup.TM Compact blanks and inserts.
"BZN doesn't handle the crushing load at the zero-velocity point of a common twist drill, but this is not so much of a problem with gundrills. Gundrills avoid many of the zero-velocity problems because they sweep beyond the center. Also, the high-pressure coolant protects the tool. Thus BZN is a good material for a gundrill.
"Although we have an excellent product for drilling steels up to 65 Rc, there's very little demand. Most engineers choose to drill while the workpiece is soft, then harden it. Carbide works fine for that.
"The future for BZN is in cast iron. Although castings are relatively soft, they can have unexpected hard spots. After they are poured, a scattered few may suffer a chill while cooling--from a draft, for instance. A 20-ft-long cast roll is highly susceptible to chill. Any resulting hard spots, of course, can ruin ordinary cutting tools.
"Many cast-iron parts have rough skins made worse by traces of embedded sand. Rough edges, cavities, or blow holes in castings act like interrupted cuts, which can dull edges of carbide tools rapidly.
"Then there's carbon. When carbon is used in excessive quantities (more than will stay in solution), the extra precipitates out, becoming intercellular carbide, i.e., extremely hard particles embedded in a relatively soft matrix. They can tear up many so-called tough tools.
"CBN or BZN cutting tools, however, can handle such problems. They survive interrupted cuts, and hold tolerances for a long time. Thus they are suggested for mass-produced cast-iron products machined in automated lines. Without long-wearing tools, an automated line will produce too much scrap before it shuts down after unexpected tool wear or breakage. BZN tools will give longer life with less failures from sudden shocks, hard spots, etc. They are more predictable over a longer service life. We expect to do a lot of business when manufacturing engineers find this out!"
How small can we drill?
According to Eldorado Tool & Mfg Corp, Div Quamco Inc, Milford, CT, the Mega 50 system allows gundrilling diameters from 0.0550" to 0.0781", at depths to 3.5". The system provides the same precision achieved by conventional larger-diameter tools. Even in high-volume production, the tools give hole straightness to 0.001"/ft of depth, and diameter tolerance to [plus-or-minus]0.0005". Surface finishes are said to be as low as 4 microinches. Materials drilled include aluminum, stainless steel, tool steel, titanium, copper, and low and medium carbon steels. Drilling conditions vary, but 10,000 rpm at 1 ipm is typical.
DeHoff also has built machines and provided tooling for drilling down to 0.055" dia, and they point out that the design requires a sophisticated high-pressure coolant system as well as very-high-speed spindles.
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|Author:||Miller, Paul C.|
|Publication:||Tooling & Production|
|Date:||Mar 1, 1985|
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