Cutting Tool Applications, Chapter 11: Reaming and Tapping

Twist drills do not make accurately sized or good finish holes; a reamer of some type is often used to cut the final size and finish. A reamer will not make the original hole; it will only enlarge a previously drilled or bored hole. It will cut to within +0.0005 inches of tool size and give finishes to 32 micro inches.

Reamers are usually made of High Speed Steel, although solid carbide and carbide tipped reamers are made in many sizes and styles. Regular chucking reamers are made in number and letter sizes, in fractional inch sizes and in millimeter sizes. They can be purchased ground to any desired diameter.

Screw threads are used for a variety of purposes and applications in the machine tool industry. They are used to hold or fasten parts together (screws, bolts and nuts), and to transmit motion (the lead screw moves the carriage on an engine lathe Screw threads are also used to control or provide accurate movement (the spindle on a micrometer), and to provide a mechanical advantage (a screw jack raises heavy loads).

When defining a screw thread, one must consider separate definitions for an external thread (screw or bolt) and an internal thread (nut).

An external thread is a cylindrical piece of material that has a uniform helical groove cut or formed around it. An internal thread is defined as a piece of material that has a helical groove around the interior of a cylindrical hole. This chapter will discuss internal threads and tapping, the operation that produces such threads.

Reaming — Reaming has been defined as a machining process that uses a multi-edged fluted cutting tool to smooth, enlarge or accurately size an existing hole. Reaming is performed using the same types of machines as drilling.

A reamer is a rotary cutting tool with one or more cutting elements used for enlarging to size and contour a previously formed hole. Its principal support during the cutting action is obtained from the workpiece.

Reamer nomenclature — Here's the basic construction and nomenclature of reamers. The illustration shows the most frequently used style for holes up to 1 inch, called a chucking reamer.

Solid reamers do almost all their cutting with the 45-degree chamfered front end. The flutes guide the reamer and slightly improve the finish. Therefore, reamers should not be used for heavy stock removal.

Axis: The axis is the imaginary straight line that by rotating the reamer between centers.

Back taper: The back taper is a slight decrease in diameter, from front to back in the flute length of reamers.

Body: The body is: 1) The fluted full diameter portion of a reamer, inclusive of the chamfer, starting taper and bevel; 2) the principal supporting member for a set of reamer blades, usually including the shank.

Chamfer: The chamfer is the angular cutting portion at the entering end of a reamer.

Chamfer length: The chamfer length is the length of the chamfer measured parallel to the axis at the cutting edge.

Chamfer relief angle: The chamfer relief angle is the axial relief angle at the outer corner of the chamfer. It is measured by projection into a plane tangent to the periphery at the outer corner of the chamfer.

Clearance: Clearance is the space created by the relief behind the cutting edge or margin of a reamer.

Cutting edge: The cutting edge is the leading edge of the land in the direction of rotation for cutting.

Flutes: The flutes are longitudinal channels formed in the body of the reamer to provide cutting edges, permit passage of chips, and allow cutting fluid to reach the cutting edges.

Flute length: Flute length is the length of the flutes not including the cutter sweep.

Land: The land is the section of the reamer between adjacent flutes.

Margin: The margin is the unrelieved part of the periphery of the land adjacent to the cutting edge.

Neck: The neck is a section of reduced diameter connecting shank to body, or connecting other portions of the reamer.

Overall length: The overall length is the extreme length of the complete reamer from end to end, but not including external centers or expansion screws.

Shank: The shank is the portion of the reamer by which it is held and driven.

Straight shank: A straight shank is a cylindrical shank.

Taper shank: A taper shank is a shank made to fit a specified (conical) taper socket.

Types of reamers — Reamers are made with three shapes of flutes and all are standard.

Straight flute: Straight flute reamers are satisfactory for most work and the least expensive, but should not be used if a keyway or other interruption is in the hole.

Right-hand spiral: Right-hand spiral fluted reamers give freer cutting action and tend to lift the chips out of the hole. They should not be used on copper or soft aluminum because these reamers tend to pull down into the hole.

Left-hand spiral: Left-hand spiral fluted reamers require slightly more pressure to feed but give a smooth cut and can be used on soft, gummy materials, since they tend to be pushed out of the hole as they advance. It is not wise to use these in blind holes, because they push the chips down into the hole.

All reamers are used to produce smooth and accurate holes. Some are turned by hand, and others use machine power.

Machine reamers — Machine reamers are used on both drilling machines and lathes for roughing and finishing operations. Machine reamers are available with tapered or straight shanks, and with straight or helical flutes. Tapered shank reamers fit directly into the spindle, and the straight shank reamer, generally called the chucking reamer, fits into a drill chuck.

Hand reamers — Hand reamers are finishing reamers distinguished by the square on their shanks. They are turned by hand with a tap wrench that fits over the square. This type of reamer cuts only on the outer cutting edges. The end of the hand reamer is tapered slightly to permit easy alignment in the drilled hole. The length of taper is usually equal to the reamer's diameter. Hand reamers must never be turned by machine power, and must be started true and straight. They should never remove more than 0.001" to 0.005" of material. Hand reamers are available from 1/8" to over 2G in diameter and are generally made of carbon steel or high-speed steel.

Operating conditions — In reaming, speed and feed are important; stock removal and alignment must be considered in order to produce chatter-free holes. Reaming speeds Speeds for machine reaming may vary considerably depending in part on the material to be reamed, type of machine, and required finish and accuracy. In general most machine reaming is done at about 2/3 the speed used for drilling the same material.

Reaming feeds: Feeds for reaming are usually much higher than those used for drilling, often running 200 to 300 percent of drill feeds. Too low a feed may result in excessive reamer wear. At all times it is necessary that the feed be high enough to permit the reamer to cut rather than to rub or burnish. Too high a feed may tend to reduce the accuracy of the hole and may also lower the quality of the finish. The basic idea is to use as high a feed as possible and still produce the required finish and accuracy.

Stock to be removed: For the same reason, insufficient stock for reaming may result in a burnishing rather than a cutting action. It is difficult to generalize on this phase as it is tied in closely with type of material, feed, finish required, depth of hole and chip capacity of the reamer.

Alignment: In the ideal reaming job, the spindle, reamer, bushing, and hole to be machined are all in perfect alignment. Any variation from this tends to increase reamer wear and detracts from the accuracy of the hole. Tapered, oversize, or bell-mouthed holes should call for a check of alignment. Sometimes the bad effects of misalignment can be reduced through the use of floating or adjustable holders. Quite often if the user will grind a slight back taper on the reamer it will also be of help in overcoming the effects of misalignment.

Chatter: The presence of chatter while reaming has a very bad effect on reamer life and on the finish in the hole. Chatter may be the result of one of several causes, some of which are listed: • Excessive speed; • Too much clearance on reamer; • Lack of rigidity in jig; • Insecure holding of work; • Excessive overhang of reamer or spindle; • Too light a feed.

Reaming operations can be performed on lathes, drills and machining centers.

Tapping

Tapping has been defined as: A process for producing internal threads using a tool (tap) that has teeth on its periphery to cut threads in a predrilled hole. A combined rotary and axial relative motion between tap and workpiece forms threads.

Tap nomenclature — Screw threads have many dimensions. It is important in modern manufacturing to have a working knowledge of screw thread terminology. A "right-hand thread" is a screw thread that requires right-hand or clockwise rotation to tighten it. "Thread fit" is the range of tightness or looseness between external and internal mating threads. "Thread series" are groups of diameter and pitch combinations that are distinguished. from each other by the number of threads per inch applied to a specific diameter. The two common thread series used in industry are the coarse and fine series, specified as UNC and UNF.

Chamfer: Chamfer is the tapering of the threads at the front end of each land of a chaser, tap, or die by cutting away and relieving the crest of the first few teeth to distribute the cutting action over several teeth.

Crest: Crest is the surface of the thread which joins the flanks of the thread and is farthest from the cylinder or cone from which the thread projects.

Flank: Flank is the part of a helical thread surface which connects the crest and the root, and which is theoretically a straight line in an axial plane section.

Flute: Flute is the longitudinal channel formed in a tap to create cutting edges on the thread profile and to provide chip spaces and cutting fluid passage.

Hook angle: The hook angle is the angle of inclination of a concave face, usually specified either as "chordal hook" or "tangential hook."

Land: The land is one of the threaded sections between the flutes of a tap.

Lead of thread: The lead of thread is the distance a screw thread advances axially in one complete turn. On a single start tap, the lead and pitch are identical. On a multiple start tap the lead is the multiple of the pitch.

Major diameter: This is the diameter of the major cylinder or cone, at a given position on the axis that bounds the crests of an external thread or the roots of an internal thread.

Minor diameter: Minor diameter is the diameter of the minor cylinder or cone, at a given position on the axis that bounds the roots of an external thread or the crests of an internal thread.

Pitch diameter: Pitch diameter is the diameter of an imaginary cylinder or cone, at a given point on the axis of such a diameter and location of its axis, that its surface would pass through the thread in a manner such as to make the thread ridge and the thread groove equal and, as such, is located equidistant between the sharp major and minor cylinders or cones of a given thread form. On a theoretically perfect thread, these widths are equal to one half of the basic pitch (measured parallel to the axis).

Spiral point: A spiral point is the angular fluting in the cutting face of the land at the chamfered end. It is formed at an angle with respect to the tap axis of opposite hand to that of rotation. Its length is usually greater than the chamfer length and its angle with respect to the tap axis is usually made great enough to direct the chips ahead of the tap. The tap may or may not have longitudinal flutes.

Square: Square is the four driving flats parallel to the axis on a tap shank forming a square or square with round corners.

Types of taps

Taps are manufactured in many sizes, styles, and types. Figure 11.11 shows some of the taps discussed here.

Hand Taps: Today the hand tap is used both by hand and in machines of all types. This is the basic tap design: four straight flutes, in taper plug, or bottoming types. The small, numbered machine screw sizes are standard in two and three flutes depending on the size.

If soft and stringy metals are being tapped, or if horizontal holes are being made, either two- or three-flute taps can be used in the larger sizes. The flute spaces are larger, but the taps are weaker. The two-flute especially has a very small cross section.

The chips formed by these taps cannot get out; thus, they accumulate in the flute spaces. This causes added friction and is a major cause of broken taps.

Spiral point tap: The spiral point or "gun" tap is made the same as the standard hand tap except at the point. A slash is ground in each flute at the point of the tap. This accomplishes several things:
• The gun tap has fewer flutes (usually three), and they are shallower. This means a stronger tap.
• The chips are forced out ahead of the tap instead of accumulating in the flutes, as they will with a plug tap.
• Because of these two factors, the spiral point tap can often be run faster than the hand tap, and tap breakage is greatly reduced.

This tap has, in many cases, replaced the "standard" style in industry, especially for open-ended trough holes in mild steel and aluminum. Both regular and spiral-point taps are made in all sizes including metric.

Spiral flute tap: The spiral flute-bottoming tap is made in regular and fast spirals, that is, with small or large helix angle. They are sometimes called "helical-fluted" taps. The use of these taps has been increasing since they pull the chip up out of the hole and produce good threads in soft metals (such as aluminum, zinc, and copper), yet also work well in Monel metal, stainless steel and cast steel. They are made in all sizes up to 1 1/2" and in metric sizes up to 12 mm.

While the "standard" taps will efficiently do most work, if a great deal of aluminum, brass, cast iron or stainless steel is being tapped, the manufacturer can supply "standard" specials that will do a better job.

Pipe taps: General purpose pipe taps are used for threading a wide range of materials both ferrous and non-ferrous. All pipe taps are supplied with 2-1/2 to 3-1/2 thread chamfer. The nominal size of a pipe tap is that of the pipe fitting to be tapped, not the actual size of the tap. Ground Thread Pipe Taps are standard in American Standard Pipe Form (NPT) and American Standard Dryseal Pipe Form (NPTF). NPT threads require the use of a "sealer" like Teflon tape or pipe compound. Dryseal taps are used to tap fittings that will give a pressure tight joint without the use of a "sealer."

Fluteless taps: Fluteless taps do not look like taps, except for the spiral "threads." These taps are not round. They are shaped so that they "cold form" the metal out of the wall of the hole into the thread form with no chips. The fluteless tap was originally designed for use in aluminum, brass, and zinc alloys. However, it is being successfully used in mild steel and some stainless steels. Thus, it is worth checking for use where BHN is under 180. They are available in most sizes, including metric threads.

Collapsing Taps: These taps collapse to a smaller diameter at the end of the cut. Thus, when used on lathes of any kind, they can be pulled back rapidly. They are made in sizes from about 1 inch up, in both machine and pipe threads. They use three to six separate "chasers" which must be ground as a set. The tap holder and special dies make this assembly moderately expensive, but it is economical for medium and high production work.

Operating options — Some threads, both external and internal, can be cut with a single-point tool as previously shown. However, most frequently a die or tap of some type is used because it is faster and generally more accurate.

Taps are made in many styles, but a few styles do 90 percent of the work. The cutting end of the tap is made in three different tapers.

The "taper tap" is not often used today. Occasionally, it is used first as a starter if the metal is difficult to tap. The end is tapered about 5 degrees per side, which makes eight partial threads.

The "plug tap" is the style used probably 90 percent of the time. With the proper geometry of the cutting edge and a good lubricant, a plug tap will do most of the work needed. The end is tapered 8 degrees per side, which makes four or five incomplete threads.

The "bottoming tap" is used only for blind holes where the thread must go close to the bottom of the hole. It has only 1-1/2 to 3 incomplete threads. If the hole can be drilled deeper, a bottoming tap may not be needed. The plug tap must be used first, followed by the bottoming tap.

All three types of end tapers are made from identical taps. Size, length, and all measurements except the end taper are the same.

Material used for taps is usually high-speed steel in the M1, M2, M7 and sometimes the M40 series cobalt high-speed steels. A few taps are made of solid tungsten carbide.

Most taps today have ground threads. The grinding is done after hardening and makes much more accurate cutting tools. "Cut thread" taps are available at a somewhat lower cost in some styles and sizes.

Just like reaming operations, tapping can be performed on lathes, drills and machining centers.

George Schneider, Jr., is the author of Cutting Tool Applications, a handbook to machine tool materials, principles, and designs. He is the Professor Emeritus of Engineering Technology at Lawrence Technological University, and former Chairman of the Detroit Chapter of the Society of Manufacturing Engineers.