Thread chasing is the process of cutting a thread on a lathe with a chasing tool that comprises several single-point tools banked together in a single tool called a chaser. Thread chasers are shown in Figure 5.7. Chasing is used for the production of threads that are too large in diameter for a die head. It can be used for internal threads greater than 25 mm in diameter. The chaser moves from the headstock. The chaser is moved radially into the WP for each cut by means of the cross-slide screw. Thread chasing reduces the threading time by 50% compared with single-point threading. However, thread chasing is a relatively slow method of cutting a thread, as a small depth of cut is used per pass. Depending on the size of the thread, 20-50 passes may be required to complete a thread. Multiple threads, square threads, threads on tapers, threads on
FIGURE 5.7 Thread chasers: (a) flat (shank type), (b) block, and (c) circular. (From Rodin, P., Design and Production of Metal Cutting Tools, Mir Publishers, Moscow, 1968. With permission.)
diameters not practical to thread with a die, threads that are not standard or those that are so seldom cut that buying a tap or die would be impracticable, or threads with a quick lead are all cut by chasing.
Chasing lends itself better to nonferrous materials rather than ferrous ones. Multistart threads can be chased without any indexing of the WP. Taper threads can be generated by chasing, if the chasing attachment is used in conjunction with a taper attachment. For high-speed steel (HSS) cutters, a cutting speed of the order of 40 m/ min and upward should be used. Feed varies from 5 to 7.5 cm/min for coarse threads in tough materials to 20-25 cm/min under more favorable conditions. Figure 5.8 shows the different methods of thread chasing.
Thread tapping is a machining process that is used for cutting internal threads using a tap having threads of the desired form on its periphery (Figure 5.9). There are hand taps and machine taps, straight shank and bent shank taps, regular pipe taps and interrupted thread pipe taps, solid taps, and collapsible taps. A tap has cutting teeth and flutes parallel to its axis that act as channels to carry away the chips formed by the cutting action. Hand taps are furnished in three sets—taper, plug, and bottoming (Figure 5.10). These three are identical in size, length, and vital measurements, differing only in chamfer at the bottom end. Standard taps are furnished with four flutes and are used for iron and steel. These do not provide sufficient chip room for certain soft metals, such as copper, in which case two- or three-fluted taps should be used. The tap cuts threads through its combined rotary and axial motions. The cost of tapping increases as the work material hardness becomes greater. Fine threads of 360 tpi in 0.33 mm diameter holes and coarse threads such as 3 tpi in 619 mm diameter pipe fitting are possible (Metals Handbook, 1989).
Tapping machines are basically drill presses equipped with lead screws, tap holders, and reversing mechanisms. Lead screws convert the rotary motion into a linear one so that the axial motion of the tap into the hole to be threaded conforms with the pitch of the thread. Lead screw control is often used with larger tap sizes to ensure
FIGURE 5.8 Thread chasing methods: (a) right-hand external and (b) right-hand internal.
FIGURE 5.9 Tap nomenclature. (From Rodin, P„ Design and Production of Metal Cutting Tools, Mir Publishers, Moscow, 1968. With permission.)
high-quality threads. However, such an arrangement has the following two major disadvantages:
- • It is necessary to return to the starting point to begin each cycle and to stop the rotation between cycles.
- • Changing the taps for different thread sizes requires time-consuming changes in the feed-controlling members.
Tension or compression tapping spindles and attachments provide axial float and compensate for any differences between machine feed and correct tap feed. This provides the possibility to tap different thread pitches at the same time with a single
FIGURE 5.10 Straight flute hand taps. (From Standard Tool Co., Athol, MA.)
machine feed rate. Self-reversing tapping attachments eliminate the need for reversing motors for tap retraction. Nonreversing tapping attachments are generally used with machines equipped with reversing motors. Figure 5.11 shows the components of a tapping attachment. Tapping machines include the following:
- 1. Drill presses. Simple to set up, easy to operate, and can be provided with lead-control devices that regulate the tap feed rates. When a solid tap is used, the drill press must be supplied with a self-reversing tapping attachment or a reversing motor having a tension compression tap holder. With a collapsible tap, the tapping attachment is not required, because the tap automatically collapses at the required depth and returns without stopping or reversing the spindle.
- 2. Single-spindle tapping machines. Used for small to medium production lots. The simpler modes have no lead-control devices but depend on the screw action of the tap in the hole to control the feed (see Figure 5.12).
- 3. Multiple-spindle tapping machines. Used for high-volume production lots. They may have up to 25 spindles that are rotated by a common power source. Holes of different sizes can be tapped simultaneously. Spindles having axial float compensate for differences between the lead of the tap and the feed of the spindle. Thus, different thread pitches can be cut simultaneously on the same machine (see Figure 5.13).
FIGURE 5.11 Tapping attachment.
- 4. Gang machines. Permit in-line drilling, reaming, and tapping operations and are generally used for low-volume production lots.
- 5. Manual turret lathes. Used for small production lots. Because the WP rotates, they are more accurate than machines that rotate the tap. The machine capability permits drilling, boring, and tapping on the same machine. A lead-control device is used when tapping on the turret lathe.
- 6. Automatic turret lathes. Tapping may be included among the many other operations of an automatic turret lathe or in a single multiple-spindle bar or chucking-type machine. These machines require long setting times and are therefore used for large production lots. These machines use lead-control devices for regulating the feed.
The selection of a tapping machine depends on the following factors:
- • Size and shape of the WP
- • Production quantity
- • Tolerance
- • Surface finish
- • Number of related operations
- • Cost
FIGURE 5.12 Herbert flash tapping machine with automatic cycle. (From Alfred Herbert Ltd., Coventry, UK.)
Generally, small diameters and fine-pitch threads are cut on machines of relatively low power, and larger threads in harder materials require heavier machines with large power.
Thread Tapping Performance
Figure 5.14 summarizes the different factors that affect the performance measures of tapping in terms of quality, productivity, and cost. These include the following:
WP characteristics. The use of free-cutting metals is more recommended where better accuracy and surface finish at higher production rates and lower cost are achieved. General purpose HSS taps are used when the WP hardness is about 30 or 32 HRC; otherwise, highly alloyed HSS is recommended. The work material composition may affect the preparation of the hole before tapping. In this regard, reaming the hole improves the accuracy and finish in aluminum, although stainless and carbon steels do not require such a reaming process (IMetals Handbook, 1989). Tapping problems occur with WPs that are too weak to withstand tapping forces. Under such circumstances, a loss of dimensional accuracy, bad surface quality,
FIGURE 5.13 Jones and Shipman multiple-spindle automatic drilling and tapping machine.
FIGURE 5.14 Factors affecting threading performance.
and WP damage may occur. For tapping blind holes, a clearance between the last full thread and the bottom of the hole should be compatible with the tap chamfer length. Such a clearance provides room for the produced chip to avoid tap breakage or hole damage by the compressed chip under the advancing tool.
Thread features. Thread size, pitch, and percentage of full depth to which the threads are cut determine the volume removed during the tapping operation. Larger volumes have a direct effect on the process efficiency and tool life. Conditions that cause dimensional variations in the tapped threads cause rough surface finish of threads. These include concentricity error between the tap holder and the spindle and the WP center. Worn tapes, chip entrapment in the tapped hole, and chip build-up on the cutting edges and flanks of the tool also cause dimensional variations and deterioration of the surface finish.
Tapping conditions. The WP material has the greatest effect on the tapping speed. The following recommendations should be followed (Metals Handbook, 1989):
- • As the depth of the tapped hole increases, the speed should be reduced because of chip accumulation.
- • In short holes, taps with short chamfers run faster than taps with long chamfers.
- • As the pitch becomes finer, for a given hole, tapping speed can be increased.
- • The amount of cutting fluid and the effectiveness of its application greatly influence the cutting speed.
During tapping, the teeth of the tap are more susceptible to damage by heat generated during threading and the chips that are more likely make the tape congested. Cutting fluids are, therefore, used in tapping all metals except Cl. However, for tapping holes longer than twice the diameter or blind holes in Cl, a cutting fluid or an air blast is recommended (Metals Handbook, 1989).