Hole Saws Speed, Simplify Large-Hole Cutting
Published: May 2002
Mounted on an arbor equipped with a pilot drill bit, a hole saw is a versatile electric drill accessory that saves time and effort in cutting (relatively) large holes. Typically sporting saw-like teeth, a hole saw makes shorter work than conventional drill bits of cutting holes in various materials, including the mild steel used in electrical panels and enclosures, stainless steel, aluminum, copper, bronze, brass, plastic, wood and ceramic tile.
For best results and for safety, use a hole saw designed for the work material and follow the manufacturer’s recommendations on cutting speed and tooth size for the application. A hole saw that works best on nail-embedded wood, for example, won’t cut well in sheet metal; a unit that cuts well in sheet metal might strip teeth if working in stainless steel that generates a lot of heat while cutting.
A hole saw cuts away a narrow band of material that is the outside perimeter of the intended hole. Once the tool bores all the way through the material, the hole is formed, with a minimal amount of material touched to achieve removal of the central slug. The relatively light material load, compared to the load a twist drill would have to attack, facilitates fast cutting.
Sold individually and in kits, hole saws are consumable products. An average hole saw (typically cutting to a depth of 1½ inches, give or take a fraction) can accomplish between 50 and 150 holes. (Depending upon the material, a hole saw cutting wood will last a lot longer than one cutting steel.)
Because they are generally inexpensive (a 7/8-inch hole saw typically runs about $7 or $8), performance is usually more important than length of life.
Despite its straightforward appearance, much engineering design goes into the typical hole saw, and numerous characteristics affect its performance in various work materials. These include pitch or teeth per inch (TPI) in the blade, teeth height, set pattern, thickness of the backing plate, tool composition (a single metal or bimetal) and diameter.
The pitch of a hole saw blade is fixed or variable. Fixed-pitch blades have a specific number of TPI (e.g., 3, 4 or 6 TPI); variable-pitch blades, such as 4/6 or 3/4 TPI, feature teeth that get bigger or smaller (and have less or more space between them) at various lengths around the blade. Though the designation may read 4/6, the length of each segment might not be an entire inch in length. Variable-tooth patterns cut down on harmonic vibration or noise, which can contribute to stripping teeth off the saw.
In a 3-TPI hole saw, the teeth are relatively big. Each tooth, on a single pass, removes a comparatively large amount of material. This can be effective when cutting into soft materials, such as flake board and wood. With hard materials, a 3-TPI saw may be less effective than a tighter configuration because the heavy load could cause too much torque on the saw and reduce cutting efficiency. A 6-TPI hole saw works well in hard materials, with the workload distributed over a greater number of TPI. But because each tooth takes out less material, comparably, cutting is slower. In metal cutting, thickness of the material is key. A 3-TPI saw would be too aggressive in thin-gauge material and would likely strip off teeth when breaking through the material at the end of a cut. A finer tooth would not catch on the metal and therefore would not strip the teeth.
A variable-tooth design causes the number of teeth along the circumference of the blade to vary, and therefore the amount of space between each tooth, as well, in a pattern. For example, a 4/6 pitch, which is the most common pitch for variable-pitch hole saws, alternates between 4 and 6 TPI around the circumference of the blade. This tooth arrangement reduces vibration and heat buildup that can cause tooth failure.
In recent years, manufacturers have introduced other variable pitches. Advocates of variable-pitch blades suggest that variable-pitch design reduces harmonics when drilling into steel and allows efficient cutting into both thick- and thin-gauge materials without sacrificing cutting speed or cut quality.
In some hole saws, the height of the teeth, one relative to another, may vary within a pattern, facilitating each tooth working independently of the ones adjacent to it and supporting removal of smaller pieces of material, per tooth. This, say proponents of varied height patterns, speeds up the amount of material removal, overall, and allows for better wear of each tooth, without any one tooth taking the brunt of a cut.
The set pattern refers to how far to the left or right each tooth within the pattern around the circumference of the blade leans from straight. For example, two to the right, one to the left, one straight and then repeated. This enables the body of the saw to travel through the material without rubbing and causing friction. Set also affects the amount of space between adjacent teeth and the amount of material each tooth clears (called chip clearance).
Various patterns have diverse rationales, such faster cutting, minimizing of heat buildup or ability to cut a larger swatch at reduced friction, compared to straight-up teeth. In general, a hole saw with more set works well in wood; one with less set works well in metal.
A forward slant, also called a positive rake, facilitates aggressive cutting. Most hole saws have a positive rake to some extent.
The backing plate, at the base of the can, may be thick or thin, perhaps stamped out of thin sheet metal. A thick backing plate adds rigidity, holds up well when cutting larger holes and keeps chatter down.
Hole saws are available in a variety of materials. A plain carbon hole saw, with the entire unit made of one material, is often value priced.
Bimetal hole saws, with high- speed steel edges and spring-steel backing, are the most widely used type and popular among electricians because they can carry a good mix of metals for good performance, overall. One common composition: teeth made of high-speed steel for wear resistance; body and back plate made of low-carbon spring steel for strength. The teeth are welded onto the can of the hole saw.
With carbide-tipped hole saws, the tip of the teeth or cutting surface is made of carbide, which holds up very well to heat and, when used on fiberglass, cement board, particle board, stainless steel or other soft metals and composite materials. Carbide lasts up to 10 times longer than bimetal hole saws in these types of materials.
Other hole saw materials include grit or carbide grit, suitable for ceramic, plaster (as long as not cutting through wood or metal lathe), sheetrock and wood when the user wants to have a smooth finish on both sides of the material and synthetic diamond, suitable for ceramic. The operator does not have to worry about filling the rake because the material just becomes dust.
Hole saws are available in several standard diameters, in sizes ranging from 9/16 inch through 6 inches, and larger for specialized models. Popular sizes for electrical work range from 7/8- to 2½-inch diameters, with models usually available at 1/8- or ¼-inch increments from a particular manufacturer. The diameters of the most prevalent hole saw sizes correspond to conduit diameters, with the hole saw diameter measuring in the range of 3/8 inch—or sometimes more—than the conduit diameter. Whatever the depth of the cut, it is possible to double it by cutting from both sides after first drilling a complete hole with the pilot drill.
Two rules of thumb: the harder the work material, the slower the drilling speed; and, the larger diameter the slower the recommended revolutions per minute (rpm).
While many cordless drills can run up to 1,800 rpm and many corded drills can run up to 2,000 rpm, hole saws should be run at lower rpms. For example, the recommended speed for a 7/8-inch hole saw in mild steel is 390 rpm: with a 2-inch hole saw the recommended rpm is 170 rpm. To maximize saw life and keep the quality of the hole good, it is important to keep the speed down and be careful not to exceed manufacturers’ recommendations.
When working in deep material, speed may be slowed by chips lodging in the gullets, the spaces at the base of the blade between the teeth. The more elongated the space between the teeth, the larger the gullet. While a smaller gullet compacts with waste material relatively quickly, a larger, or deeper gullet maximizes chip clearance and allows packing in of more shavings without compromised operation and faster clearing out of those shavings. Periodically stopping the drill, removing the saw from the hole and (with power off) clearing the chips should bring the cut back up to speed and help extend the life of the saw.
LENOX hole saws, from American Saw & Mfg. Company, feature patented 4.5/5.5 variable-tooth design, which also has variable-tooth height. The company said that each tooth works independently and helps distribute the cutting force evenly over all the teeth, maximizing cutting efficiency in thick- and thin-gauge materials, without sacrificing cutting speed, tooth strength or cut quality.
Milwaukee Electric Tools’ Quick-Change Sheet Metal Carbide Tooth Hole Saw is a carbide-tip hole cutter for material up to 3/16-inch thick, ideal for cutting holes in electrical boxes, thin-gauge steel, stainless steel or structural-steel studs. The carbide-tipped teeth provide for a longer life (four times the life of) and faster cutting time than a conventional hole saw. The tool, available in diameters ranging from 5/8 inch through 3 inches, uses an interchangeable arbor that fits all cutters.
Gardner Bender’s Special Electrician’s Hole Saw Kit, for 2½- to 4-inch pipe entrance sizes, contains four large bimetal hole saws and one arbor. The teeth have a pattern and a variable pitch of 4/6 TPI.
Klein Tools’ hole saws, ranging from 9/16 inch to 6 inches in diameter, feature a thick-cup design that provides strength and long-lasting performance by eliminating pilot-hole elongation that results in excessive vibration and chatter. The hole saws are recommended for a variety of materials, from steel to plastic. A Journeyman’s 13-piece set contains nine high-speed hole saws, two arbors, a pilot drill and an arbor adaptor.
Greenlee Textron’s Kwik Change Carbide-Tipped Hole Cutters feature a Kwik Change arbor design that allows change of the cutter with a push and turn of the head, rather than threading it on. Suitable for making fast holes in stainless steel, copper, aluminum and mild steel, the hole cutters are available in sizes from 5/8 inch to 3 inches, individually packaged or in kits.
HILTI bimetal hole saws are available individually in diameters ranging from 9/16 inch to 6 inches, each with a cutting depth of 1½ inches. The 8-piece Electrician’s Hole Saw Kit features six bimetal hole saws with diameters ranging from 7/8 inch to 2½ inches and two arbors.
Suitable for wood, stainless steel, cast iron, copper, aluminum, brass and plastics, Irwin variable-pitch hole saws, with 11/8-inch cup depth, available in several diameters from American Tool Companies, sport high-speed steel teeth welded to the alloy steel body. The one-piece construction eliminates the need for a separate arbor.
The DeWalt Industrial Tool Co., 8-piece Electrician’s Kit includes six bimetal hole saws (7/8- to 2½-inch diameters), a ¼-inch mandrel and a 7/16-inch quick-change mandrel. A seamless-cup design allows cutting through a 2x4 in one pass. The bottom cap, made from hardened metal, resists wear from the mandrel pins. EC
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