9 Questions About Annular Cutting

A Twist on Tube and Pipe Drilling

By Darwyn Jones

September 13, 2005

One way to avoid dimpling and deburring while making holes in tube and pipe is use annular cutters. Because annular cutters are hollow, there is no dead-zone resistance to overcome. Knowing how to use an annular cutter and what to watch for can help avoid problems and extend tool wear.

A portable magnetic drill allows you to take the drill to the workpiece. When you are using a magnetic drill and pipe adapter accessory, mount the adapter securely to the pipe with a tightly wrapped chain.

Using a twist drill to make holes in pipe or tubing can prompt concerns about hole roundness, location accuracy, hole finish, chip evacuation, burrs, and duration. The focal point of these concerns is the twist drill's most noticeable feature—its point. This is not really a point at all, but the tangent line where two cutting angles intersect at the web of the drill, or the "dead zone." This dead zone causes the surface speed of the cutting edges to drop to zero (see Figure 1). The lower surface speed reduces cutting efficiency and requires increased thrust, or feed pressure, for the cutting edges to push through the material.

When the material is pipe or tubing, the increased pressure causes stock deformation or dimpling around the hole. In addition, increased feed pressure creates flashing or burrs—jagged remnants of stock still attached—around the breakthrough side of the hole. As the drill point thins the material, it pushes through, usually leaving two burrs. It is possible to remove burrs from plate stock by chamfering. With tubing, however, burrs are on the inside, so deburring is more complicated and time-consuming and usually requires special tools or hand scraping.

Figure 1 A twist drill's most noticeable feature—its point—is not really a point at all, but the tangent line where two cutting angles intersect at the web of the drill, or the "dead zone." This dead zone causes the surface speed of the cutting edges to drop to zero.

One way to prevent dimpling and deburring while making holes in tube and pipe is to use annular cutters. Because annular cutters are hollow, there is no dead-zone resistance to overcome. All cutting edges are located near the periphery of the tool, and surface speeds are consistent throughout the entire cut (see Figure 2). The tool's multiple cutting edges distribute the load evenly, resulting in fast feed rates and minimizing the thrust and horsepower needed.

As an annular cutter trepans the hole, it leaves the center as a solid slug and exerts minimal force on the material. Thus, the area of cutting action—the perimeter of the hole—where metal is actually converted into chips is relatively small.

In most tube and pipe cutting applications, holes can be drilled without prespotting the location. Annular cutters require a smaller relief angle than is required in twist drills. They cut the material, rather than rip through it, leaving a clean, round, and nearly burr-free hole.
Knowing how to use an annular cutter and what to look for can help you avoid problems and extend tool wear.

1. What causes annular cutters to break or dull prematurely?
   The primary cause of broken or prematurely dull cutters is a feed rate that is too slow. Slow feed rates will reduce the life of your cutter. Using a firm, steady feed pressure throughout the cut will maximize performance and extend tool life.
   
   
2. Why do I have to clear chips from the cutter before every cut?
   As the cutter begins removing material, new chips flow out through the gullets and up the flutes. If these pathways are not cleared before cutting a new hole, packing or binding may result, causing the cutter to fill with chips and possibly break.
   
   

Figure 2 Because annular cutters are hollow, there is no dead-zone resistance to overcome. All cutting edges are located near the periphery of the tool, and surface speeds are consistent throughout the entire cut.

3. When do I use a magnetic drill?
   It can be time-consuming and cumbersome to try to jockey a large pipe into position at a stationary drill press or work center. A portable magnetic drill allows you to take the drill to the workpiece, instead of trying to bring the workpiece to the drill. A portable magnetic drill press also may be used in any position—vertical, horizontal, or overhead. The workpiece must be at least 3/8 inch thick.
   
   
4. Is the magnet on my portable drill press strong enough to drill pipe?
   When you're using a portable drill press, a support system is needed. A safety chain or strap is also recommended, in case the magnet slips from the support. When you are using a magnetic drill and pipe adapter accessory, mount the adapter securely to the pipe with a tightly wrapped chain (see lead image). Make sure the support system for the drill arbor is rigid and in good condition.
   
   
5. How do I keep my magnetic drill from slipping?
   Make sure the drill's magnetic base is clear of chips and debris and is securely attached to a clean workpiece. Uneven surfaces or large debris buildup prevents the magnet from obtaining optimal holding power, which can cause the drill to shift or lift during operation. A safety chain or strap also helps to prevent injury if the drill shifts. If it does shift or lifts during the cut, and the motor is not stopped, the cutter will break. A good magnetic drill will have a built-in safety circuit that stops the motor immediately when the drill lifts from the material.
   
   
6. What is the purpose of coated cutters, or cutters made from premium steel?
   Cutters made from M2 high-speed steel (HSS) are recommended for materials with a Brinell hardness number (BHN) of up to 275. A titanium nitride or similar coating may be applied to M2 to fortify wear resistance in materials up to 325 BHN. For materials up to 350 BHN, cobalt steel (M42) is recommended with a titanium nitride or similar coating.
   
   
7. What makes the slug stick in the cutter?
   Using lubricant or cutting fluid helps the slug to eject at the end of the cut. But when slug ejection becomes unreliable or the tube resists cutting, it probably means the cutter's cutting edges are becoming dull. Resharpen or replace your cutter.
   
   
8. Is coolant necessary?
   Always use some form of coolant when drilling to maintain cutting performance and tool life. Any form of coolant or lubrication helps—even water works better than nothing at all.
   
   
9. What happens when my portable drill press becomes loose or parts wear out?
   For the best performance, always maintain your drill as instructed in the operator's manual. Many magnetic drills feature a slide/gib/way system that requires periodic adjustments to maintain rigidity and optimal performance. Worn or damaged parts should be replaced to reduce the possibility of injury.

Darwyn Jones manages marketing communications, Hougen Manufacturing Inc., 3001 Hougen Drive, Swartz Creek, MI 48473, 810-635-7111, fax 810-635-8277, info@hougen.com, www.hougen.com.