Drilling

Machinists have the choice of four basic types of drills that are made with carbide cutting edges: solid carbide drills, drills with indexable-inserts, drills with brazed carbide tips and drills with exchangeable solid-carbide tips. Each of these drills has advantages for specific applications.

Solid-carbide drills are made to be used on modern machining centers. These drills are manufactured with fine-grain carbide and titanium aluminum nitride (TiAlN) coatings to provide long tool life. As self-centering drills, they have a specially designed edges that help to control chips and chip evacuation in most workpiece materials. The self-centering geometry and fine tolerances of solid-carbide drills ensure quality holes without additional machining.

Indexable-insert drills cover a broad range of diameters with depths two times diameter to five times diameter. They may be used for rotating applications as well as in lathes.

Drills with brazed carbide tips rely on the strong connection of the brazed tips to the drill bodies. 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 additional finishing operations. These drills are available with through-the-tool coolant, and can be used in machining centers, CNC lathes or other machines that have sufficient stability and rpm.

Drills that have exchangeable solid-carbide tips incorporate a steel body with the exchangeable tips that are known as crowns. These tools offer the precision of brazed drills, and offer increased 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 to produce close diameter tolerances.

Drilling tolerances and machine stability

Machinists should choose a drill based on the tolerances specified for an operation. Normally, the tolerance is tighter on smaller-diameter holes. Drill manufacturers classify their tools by targeting the nominal diameter of a hole.

Of all of the types of drills that are available, solid carbide drills can hold the tightest tolerances. That makes these tools 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 and drills with replaceable carbide crowns, on the other hand, can hold tolerances from 0 to 0.0028 in. These tools often are a good choice for production drilling applications.

Indexable-insert drills are the industry workhorses. Although their initial cost usually is lower than the costs for other drills, these tools also have the loosest tolerances, from 0 to 0.012 in., depending on the ratio of length to diameter. This means that machinists who choose them must consider the need for a larger tolerance window and they often must be prepared to finish holes with boring tools.

Along with hole tolerances, machinists need to consider machine stability when they are selecting drills. Machine tool stability is essential for extending tool life and for achieving accurately drilled holes, so machinists must verify the condition of the machine spindle, fixtures and attachments.

They also should 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. The longer the drill, the greater the deflection that can be caused. Therefore, machinists who use indexable insert drills to four times diameter and higher should consider reducing feed during the first millimeter of depth and then increase to the normal feedrate.

Brazed drills, as well as replaceable crown tools, are designed with two symmetrical cutting edges that form 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 percent to 50 percent 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, solid carbide 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 does not matter which drill is being used.

Shops often use an external coolant supply to remove chips, but this works only when the hole depth is less than one times diameter of the drill 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 0.47 in. to 0.78 in. (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 regrind a solid-carbide drill only seven to ten times, and a brazed drill only three or four times. The steel body of a crown-type drill, on the other hand, lasts for at least 20 to 30 crown changes when used to cut steel.

There also is 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 solid carbide or brazed drills. In many cases, reground drills do not match the performance of a new drill. That is because it is hard to match the cutting edge shape and edge preparation in regrinding. Drills with improper edge preparations often chip more easily, require more torque or power to operate, or create more heat, thereby shortening tool life.