Creating the perfect hole

Posted by on Jul 24, 2015 in Uncategorized | 0 comments

If you need to make a lot of holes of moderate depth (up to about 4 times the diameter of the drill) indexable insert drills are your first choice. Indexable drills combine the high speed capacity and accuracy of carbide cutting edges with the ability to index the inserts and reuse the drill body without regrinding. Indexable drills make efficient use of the machine tool’s horsepower and permit higher penetration rates and concurrent reductions in cycle times. A few application tips can help you gain maximum benefit from the use of indexable insert drills.

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Optimize for specific workpieces

An indexable insert drill employs at least two inserts (Fig. 1). The inside insert cuts the inner portion of the hole, and the outside insert creates the final hole diameter. Because the cutting speed at the centerline of any drill is zero, the cutting edge at the point must be durable enough to withstand the compressive force of machine feed without breaking. The outside insert, on the other hand, must combat higher speeds and needs to be more wear resistant. For general use, the same insert grade and geometry in both inner and outer insert pockets will do the job well. In some cases, however, the use of different grades for the inner and outer inserts will provide optimum productivity.

Users can select the appropriate carbide grades, insert radii and chip breakers to optimize the drilling process for a particular workpiece. The drills can offer increases in drilling efficiency from 300 to 500 percent, and the low cost per part they provide is most beneficial in high-production situations. Indexable drills can also drill surfaces that aren’t flat or are inclined up to 45 degrees.

The basic principals of indexable insert drill design are similar, but manufacturers offer their own variations. For example, Drill-Fix indexable insert drills from Kennametal can drill from solid, bore, face, and chamfer, and also have the capability to be offset in the +X direction. This enables the drills to produce hole sizes larger than the nominal drill diameter, permits fewer drills to cover a wide application range, and often eliminates the need for costly special-diameter drills.

First consideration is rigidity

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As with any tool, correct application is the key to gaining maximum productivity from indexable drills. The first consideration is sufficient rigidity and power in the machine tool on which the drill will be used.

creating2Cracking and chipping of insert edges (Fig. 2) usually is a result of insufficient rigidity in the machine tool or inadequate fixturing of the workpiece. Check the machine alignment and tool clamping accuracy. Too much overhang of the drill may also reduce rigidity of the setup. If tool clamping can’t be improved, machine rigidity is questionable, or both, a reduction in feed rate can help reduce insert chipping. A tougher carbide grade–with higher transverse rupture strength–can also withstand the impacts resulting from unwanted drill and workpiece movement relative to each other.

creating3Excessive insert wear (Fig. 3) may be a result of excessive cutting speed. Reducing speeds is one way to reduce wear, or it often can be minimized by increasing coolant pressure and volume. Another alternative is substitution of a more wear-resistant (harder) carbide grade, especially for the outer insert.

Many factors create a perfect hole

Uncontrolled, the factors above can result in poor hole quality. In addition, rough cutting action and unacceptable surface finish can result from feed rates that are too high. Lowering feed rates, or sometimes increasing speed, can overcome rough and rumbling cutting.

Another cause of poor hole quality is trapped chips (Fig. 4). Good coolant pressure and volume can help dissipate heat and evacuate chips. Increased coolant pressure and volume can also result in a cooler part and longer tool life.

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One of the major advantages of indexable insert drills is the availability of chip control geometries. Your manufacturer’s representative or literature can help you choose the geometry best suited to your workpiece material. But if long, stringy chips remain a problem, try increasing cutting speed while maintaining the feed rate. However, if the chips turn dark blue, a symptom of excessive heat, either reduce the cutting speed or increase the supply of coolant. If chip formation is still unsatisfactory after increasing cutting speed, try increasing the feed rate until chips break adequately.

On the other hand, if chips are short, thick, and flat, the feed rate may be too high in relation to the cutting speed. You should try lowering the feed, increasing cutting speed, or both. Increasing coolant pressure and volume, and careful inspection of tool clamping, machine rigidity, and workholding setup can also contribute to better chip control and improved hole quality.

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A safety note

Any application guidelines for indexable insert drills should be accompanied by a safety note regarding through-hole operations. As the drill breaks through the workpiece a slug is formed (Fig. 5). Because the drill is stationary and the workpiece is rotating, centrifugal force may hurl the slug from the chuck with great force. Provide adequate shielding to protect all bystanders.

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