Drill Points Center Up With S-Curves
With the right shape, a drill point can add quality control to
high-volume hole making.

A blended, rounded cutting edge, as on a Winslow-Bickford point, reduces breakthrough burrs.

As a manufacturer of precision-machined automotive components, Carpenter Enterprises "eats, sleeps and lives" quality control. Because, on average, more than 50% of its machining time is spent making holes, a large part of Carpenter's quality control emphasis is on drilling. The company has found that drill point geometries play an important role in controlling hole size, location, surface finish, and burrs.

Conventional drill point geometries have a strong tendency to cut off-center if the hole hasn't been center-drilled first. Cutting off-center not only causes inconsistent hole location and size, but also creates material build-up. Build-up leaves holes with poor surface finishes and, at the same time, shortens drill life before each re-sharpening. This could be why certain high-performance drill point designers are shying away from the norm and developing special geometries such as S-shaped.

S-shaped chisel edge
There are two basic methods for grinding drill points, facet grinding and form grinding.

In conventional facet grinding, the grinding wheel and drill point make contact along only one axis. As a result, the drill point has two large, relatively flat facets separated by a straight chisel edge that has a large negative cutting face. This large negative-rake cutting surface "pushes" metal ahead without actually cutting, similar to a bulldozer scraping the earth's surface. Dirt bunches up in rounded cutting front of the dozer blade edge, as on a as it is pushed away by Winslow-brute force. In contrast Bickford point, to conventional facet reduces grinding, S-shaped geometries, as on a burrs. Winslow-Helical point or a Winslow-Bickford point, are form ground.

A triaxial grinding motion creates the Helical and Bickford points on a Winslow drill grinding machine from Winslow Engineering of Fond du Lac, Wis. In triaxial drill point grinding, the grinding wheel and drill make contact through three different axes of motion simultaneously to generate a variety of point geometries. Both the Helical and Bickford points have a continuous formed relief separated by an S-shaped chisel edge.

Prominent S-shaped chisels provide a continuous cutting edge that starts at the center of the drill and blends with the cutting lip, so the drill point cuts along the entire length of its cutting edge. This distributes cutting forces more evenly and improves chip formation and flow.

S-shaped chisels include a relieved leading edge with only a small negative-rake cutting surface.

Smaller negative rakes provide the active cutting chisel necessary for cutting chips instantly and cleanly upon entering metal; making drills self-centering; reducing cutting forces to permit faster feed rates with less pressure on the workpiece; forming straighter, rounder, truer holes without separate center-drilling or secondary operations. "The S-shape seems to pull a drill right into the workpiece," says Morgan Moss, engineering manager at Carpenter. "Once we started using drills with the Winslow-Helical points, the uniformity of our hole location and size came right back."

Other drills left burrs that required manual removal before conventionally grinding the connecting rods. When these burrs were not completely removed, the parts did not sit flat on the first grind station, forcing the part to be scrapped. Moss says the Bickford point did away with a separate burr-removal operation and lowered scrap rates by making the same holes without objectionable burrs.

Darwin Dansby, Carpenter's tool grind area leader, sharpens both the Bickford and Helical points for hole sizes ranging from 3/16 to 1 in. on a Winslow Engineering model HR drill point grinder. He states that "for our high volume hole making, we sharpen same-size drills in batches of two or three dozen to keep machine operators supplied."

An accurately located point can determine drill performance and life. For example, at Carpenter's Traverse City, Mich., plant they up- graded the performance of their so-called "DB" drills (Drill Burnishing) by precisely centering the points.

This solid-carbide drill has two V-shaped, straight flutes and two flat sides, giving it a rectangular shape when viewed from the end. Similar to a reamer, radii on the two flutes burnish a hole while it's drilled, improving the surface finish.

Enhancing surface finishes further, Carpenter also grinds the Winslow-Helical point on the DB drills used for a hole in an ABS (Automatic Brake System) component. And, according to Moss, they're holding a better than 125 rms finish and consistently maintaining tolerances of 0.0015 to 0.002 in. for hole size and location.

Since the Helical point on the DB drills eliminates the center-drilling operation, the cost of a separate rotary-dial index machining station for center drilling is nonexistent. The point also generates less heat, giving the DB drills up to four times more life. "The cost savings don't come just from needing fewer tools and less grinding time," says Moss. "We also lower the number of machine interruptions needed to change dull drills."

Carpenter Enterprises switched from a standard OB drill (drill burnishing), on the left, to a Winslow-Helical point geometry, on the right, for improved surface finishes