SHARPENING DRILLS By Larry Corbin (EAA 34458, Designee 1525)

Corner ""

Edited by Chuck Larsen, EAA Designee

Director

Figure 1 shows a standard drill point. Notice that the included angle of the cutting lips is 118 degrees and the end clearance angle is 3 degrees. If properly ground, the angle on the peak of the web is 45 degrees and is centered. This makes a nice visual check. Figure 2 shows two of the most common mistakes made in sharpening drills. "A" illustrates what happens if the drill is fed straight into the grinding wheel. A concave surface results which is weak. "B" illustrates a negative end clearance angle which means the drill will rub instead of cut. Figure 3 illustrates a simple homemade gage. It is made from sheet metal. The chisel marks are not critical since they only serve as references to assure that both cutting lips are uniform in length. Figure 4 shows the correct way to grind a drill. Three motions are required to get the right shape. The drill must, of course, be fed into the wheel, but with only a light pressure. It must also be rotated and lifted at the same time. After all, the drill is round so it makes sense to roll the shape onto the end. This takes practice. It is much like learning to fly. Most people overcontrol the elevators, undercontrol the ailerons and completely forget the rudder. Similarly, most people feed the drill too hard, lift it too little and completely forget to rotate it. Perhaps the most important part of drill sharpening is to avoid overheating. Dip the drill into water frequently

between light grinding passes. If the drill turns blue or if it boils the water rapidly, you are too late. Not only can the drill lose its hardness, small microscopic cracks will form and the cutting edge can self destruct as soon as you begin to use it.

CALENDAR CE EVENTS JANUARY 26-29 — LAS VEGAS. NEVADA — 1st Annual World Cup

Championship Ultralight Show and Convention. Presented by S. Sports, Inc. and The Sahara Hotel MARCH 9-10 — CAPE CANAVERAL. FLORIDA — Valiant Air Command

Air Strike '84. Tico Airport Contact Bob Reid, 8970 Mahogany Way.

Our realization of flight requires much of a technical nature and the application of numerous shop skills to uphold our standards of quality. The following article explains how Larry sharpens drill bits to meet the hole drilling requirements we all encounter in our shops.

Figure 5 illustrates a special point for drilling wood. Wood will splinter very badly when drilled with a standard point. Changing the included angle of the cutting lips to 60 degrees will help considerably. However, it is still best to back up the workpiece with a piece of scrap wood. It should be noted that this drill point will dull rapidly and "lead off' if used on steel. Figure 6 shows a point that will make a very smooth, round and almost burr free hole in sheet metal. The center point is higher than the cutting lips and so will center the drill in a center punch mark. The outer part of the cutting lips then act as little fly cutters. The center point must be accurately located in order for the hole to be on size. If a slightly oversize hole is desired, the center point can be ground off center and the cutting lip on the short side can be ground back so it does not cut. The amount of offset is somewhat limited due to the wobbling that will result. Figure 7 shows a drill point on which the rake angle has been reduced. The major use for this type of point is in drilling soft materials such as nylon or brass. In these materials, the standard drill tends to dig in and grab the workpiece. Grinding a small flat on the leading edge of the cutting lips will eliminate the problem. This drill point is also used on extremely hard materials or where hard spots exist in a piece. Figure 8 shows a drill with a "thinned point". Most of the force required to push a drill into the workpiece is needed to push the blunt part of the point. Since the web gets thicker as the drill gets shorter, the blunt part gets wider and the required force becomes higher. This in turn can lead to drill breakage or increase the tendency to "lead off'. interest to all pilots. Contact Neil Hymk, Minn. DOT, 612/296-1141; Tim

Griffith, Iowa DOT, 515/281-8166 or Duane Esse. Wise. DOT, 608/2667347.

MAY 18-20 — HAYWARD. CALIFORNIA — Hayward to Las Vegas Proficiency Air Race. Contact Hayward Air Race Committee, 20301 Skywest Dr., Hayward, CA 94541 or Lou Chianese 415/581-2345. Ext. 5285.

MAY 26-27 — GULFPORT, MISSISSIPPI — Annual Shade Tree Fly-In. Sponsored by EAA Chapter 479. Airstrip located 8 mi N/NW Gpt. VOR/ TAC, Lyman, MS. Contact "Hap" Bryant, 601/832-7713. MAY 25-27 — ATCHISON, KANSAS — 18th Annual Fly-In sponsored by

Greater Kansas City Area Chapter of the AAA. Amelia Earhart Airport. Contact Lynn Wendl 913/888-7544 or John Krekovich 913/648-1279.

Plantation, FL 33324 - 305/472-2356 MARCH 11-17 — LAKELAND, FLORIDA — Sun 'N Fun '84. Help Sun

JUNE 15-17 — OSHKOSH, WISCONSIN — 3rd Annual EAA Ultralight Convention. Contact EAA Headquarters for Information.

'N Fun celebrate its 10th birthday! Contact Fly-In Office 813/644-

JUNE 28 - JULY 1 — HAMILTON. OHIO — 25th National Waco Reunion Fly-In. National Waco Club, 700 Hill Ave., Hamilton. OH 45015, 513/ 868-0084.

2431

or 313/665-6374, 813/644-9319, 813 665-7955.

MARCH 15 — LAKELAND. FLORIDA — Winter Frolic Silver Wings Fraternity. Central Holiday Inn. Contact Jack Corliss 813/848-2616 MARCH 21-22 — ROMEOVILLE, ILLINOIS — 10th Annual Midwest Main-

tenance/Exhibit Seminar

Lewis University Campus. Contact Charlie

Wells, 217/753-4400, ext 522 APRIL 7 — MILWAUKEE, WISCONSIN — 4th Annual Aerobatic Safety

and Information Seminar sponsored by IAC Chapter 8. 9 a.m. to 5 p.m. Phone 414/461-3222 or 414 282-3250. APRIL 9-11 — BLOOMINGTON, MINNESOTA — 8th Annual Midwest Regional Professional Pilots Seminar Marriott Hotel. Varied subjects of

JULY 19-22 — DAYTON. OHIO — Dayton International Airshow and Trade Exposition - 10th anniversary. Contact Shawnee Lee Culbertson 513/ 898-5901. JULY 28 - AUGUST 4 — OSHKOSH, WISCONSIN — 32nd Annual Fly-

In Convention - Start making your plans now to attend the World's Greatest Aviation Event. SEPTEMBER 1-3 — STANLEY, NOVA SCOTIA. CANADA — Stanley Air-

port Annual Fly-In sponsored by EAA Chapter 305 Contact Wayne Hatch 902/865-3642.

SPORT AVIATION 29