Drilling do's and don'ts

July 1, 2003
Make holemaking more productive.

Make holemaking more productive.

By Goran Melin

Edited by Patricia L. Smith
managing editor

This self-centering, solid-carbide drill from Seco-Carboloy is TiN-coated for long tool life and employs a special edge design that aids in chip evacuation.

Perfomax indexable insert drills feature a special helix angle that increases stability and reduces vibration. According to Seco-Carboloy, the tools cut tool costs up to 50% over conventional indexable-insert drills.

Seco-Carboloy's CrownLoc drill handles high-production holemaking and allows users to quickly and economically swap out its interchangeable solid-carbide crowns.

Many shops mistakenly believe that drilling operations must be done at comparatively low feeds and speeds. While this was once true, the same can't be said of today's carbide drills. In fact, users can greatly improve productivity and reduce overall costper-hole with the right choice of drills.

End users have the choice of four basic types of drills with carbide cutting edges: solidcarbide, indexable-insert, brazed carbide-tip, and exchangeable solid-carbide tip. Each has its advantages in specific applications.

The first, solid-carbide drills, are made for use on modern machining centers. Manufactured with fine-grain carbide and TiAlN coatings for long tool life, these self-centering drills employ a specially designed edge for excellent chip control and evacuation in most workpiece materials. The self-centering geometry and fine tolerances of solid-carbide drills ensure quality holes without any further machining.

Indexable-insert drills cover a broad range of diameters with depths from 2 3 D to 5 3 D. They may be used for rotating applications as well as in lathes.

With brazed carbide-tip drills, there's a strong connection of the brazed carbide to the drill body. These tools use a self-centering geometry for low cutting forces and good chip control in most workpiece materials. Brazed drills produce holes with relatively high surface finish, close diameter tolerances, and good positioning accuracy without further finishing operations. Available with throughthe-tool coolant, brazed tipped drills may be used in machining centers, CNC lathes, or other machines with sufficient stability and rpm.

The final drill type incorporates a steel body with exchangeable solidcarbide tips, known as crowns. This tool offers the precision of a brazed drill while providing greater productivity at reduced operating costs. The carbide crowns used with this new generation of drilling tools are available in precise size increments and have a self-centering geometry that produces close diameter tolerances.

Do consider tolerances and machine stability
Shops should choose a drill based on the tolerances specified for an operation. Normally, the tolerance is tighter on smaller-diameter holes. Drill manufacturers, then, classify tools by targeting the nominaldiameter hole size and above.

Of all the drill types, solid-carbide tools have the tightest tolerances. This makes them the best choice for extremely close-tolerance holes. A shop can expect tolerances of 0 to 0.0012 in. using a 0.393-in.-diameter solid-carbide drill.

Brazed drills or drills with replaceable carbide crowns, on the other hand, have tolerances from 0 to 0.0028 in. These tools are often a good choice for production drilling applications.

Indexable-insert drills are the industry workhorses. Though their initial cost is generally less than the other drills, these tools also have the largest tolerances, from 0 to 0.012 in., depending on diameter/length ratio. This means that end users need a larger tolerance to use them — or they must be prepared to finish holes with boring tools.

Along with hole tolerances, shops need to consider machine stability in the selection process. Because stability is essential for securing tool life and drilling accuracy, shops should verify the condition of the machine spindle, fixtures, and attachments.

They should also consider the inherent stability of the drill. For example, solid-carbide drills offer the best stiffness, which contributes to their high accuracy.

Indexable-insert drills, on the other hand, are prone to deflection. While these drills are fitted with two cutting inserts — one inboard insert for the center and one from the inboard insert out to the full diameter — initially only one insert enters the cut. This creates an unstable situation that causes the drill body to deflect. And the longer the drill, the greater the deflection. Therefore, shops using indexable insert drills 4 3 D and over should consider reducing feed during the first millimeter of depth and then increase to the normal feedrate.

Brazed drills, as well as replaceablecrown tools, are designed with two symmetrical cutting edges forming a self-centering geometry. This highly stable cutting design allows the drill to enter the workpiece at the full feedrate. The only exception is on setups with an angled entrance where it is recommended to reduce feed by 30% to 50% when entering and exiting. The slight deflection that occurs with a steel-body drill allows the tool to also be successfully used on lathes where stiff, solidcarbide drills may easily break, especially if they are not perfectly centered with the workpiece.

Don't forget chips
Where many shops run into trouble is in chip evacuation. In fact, poor chip evacuation is the most common problem in drilling, especially in low carbon steel. And it doesn't matter what drill is being used.

Shops often use an external coolant supply to remedy the situation, but this works only when the hole depth is less than 1 3 D and when using reduced cutting data. Otherwise, they must direct the appropriate coolant at the recommended flow and pressure to match the drill diameter. For machines not equipped with coolant through-the-spindle, shops should use a coolant inducer.

Remember, the deeper the hole, the harder it is to remove chips, and the more coolant pressure is needed. Always check the manufacturer's recommended minimum coolant-flow level. At lower flow, it might be necessary to reduce feed.

Do examine lifecycle cost
Productivity or cost-per-hole is the biggest trend affecting drilling today. This means that drilling-tool manufacturers must find ways to combine some operations and also develop tools that can cope with higher feeds and speeds.

The latest drills with exchangeable solid-carbide heads offer exceptional economy. Instead of replacing the whole drill body, end users only purchase carbide heads, which cost about the same as regrinding a brazed or solid-carbide drill. These crowns swap out easily and accurately, and shops can use a single drill body with multiple crowns to drill several different hole sizes.

This modular drilling system reduces drill-inventory costs for tools ranging from 12 to 20 mm in diameter. In addition, it eliminates the cost of backups needed when regrinding brazed or solid-carbide drills.

In reviewing cost-per-hole, shops should also take overall tool life into account. Typically, shops can only regrind a solid-carbide drill 7 to 10 times and a brazed drill only 3 or 4 times. The steel body of a crowntype-drill, on the other hand, lasts for at least 20 to 30 crown changes when machining steel material.

There is also an issue of productivity. Brazed and solid-carbide drills must be reground; therefore, shops have a tendency to run these tools slowly to avoid chipping them. But regrinding is unnecessary with exchangeable-head drills, so shops can operate them at maximum feed and speed without fear of chipping the carbide material.

New crown-type drills also deliver more consistent tool life than solidcarbide or brazed drills. In many cases, reground drills do not match the performance of a new drill. That's because it's hard to match the cutting edge shape and edge preparation in regrinding. Drills with improper edge preps often chip more easily, require more torque or power to operate, or create more heat, thereby shortening tool life.

A crowning achievement

When tightened by the clamping screw, the CrownLoc's pull rod exerts such a strong pull that the crown/body combination can cope with high feeds as well as a brazed drill can.

A precision-ground interface of interlocking V-grooves in the CrownLoc drill ensures rigidity and exact, repeated positioning of each crown.

Seco-Carboloy's CrownLoc drilling system features a steel body and exchangeable solid-carbide tips (crowns). The tools sport a precision-ground interface of interlocking V-grooves that ensures the tool's rigidity while also guaranteeing the precise, repeated positioning of each crown.

The locking pattern, which allows only one correct fit, is easy to find once a crown is mounted on the mating pull rod. When tightened by a clamping screw, the pull rod exerts such a strong hold that the crown/body combination can withstand the high feeds and speeds necessary for high-production holemaking.

Seco-Carboloy also uses an advanced injectionmolding process to make the crowns. This process produces a completely homogenous tungsten carbide so that the crowns have uniform strength and thermal properties.

The cutting tool company currently offers three special geometries of CrownLoc drill: a universal geometry with normal edge honing, land margins, and back taper for most steel materials; a geometry with wider land margins and chamfered corners for cast iron drilling; and a geometry that reduces heat and workhardening in drilling stainless steel and superalloys.

Mr. Melin is drilling products manager at Seco-Carboloy, Warren, Mich.

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