Hand thread chasing is a traditional technique in woodturning that allows turners to create decorative and functional screw threads on wood. Hand thread chasing requires precision and a bit of patience but is so much fun! This technique has been used for centuries, dating back to when woodturners crafted everything from everyday items to ornamental pieces with threads. In this article, we will explore the essential aspects of hand thread chasing, including the tools, techniques, and tips for achieving success.

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At its core, hand thread chasing involves using specialized tools, known as chasers, to cut threads into a piece of wood while it is mounted on a lathe.

Thread chasers for wood come in various pitches with 10, 12, 16 and 20 threads per inch (TPI) being the most common. The choice of pitch depends on the size and purpose of the project and the wood being used. For instance, finer threads (higher TPI) are often used for delicate, decorative items, while coarser threads (lower TPI) are more suitable for functional objects like boxes or lids that require a secure fit.

Most chasers are made from M2 HSS with one cutter for cutting external threads (the "male" chaser) and another for internal threads (the "female" chaser). Most come as a pair of separate tools. The Carter and Son's chasers (12 or 16TPI) come as one piece with a cutter on each end. I like this convenience of having just one tool. Carter and Sons thread chasers are made of the harder M42 steel and hold an edge longer than M2 HSS.

I recommend starting with 16TPI as a good compromise size for hardwood boxes with 12TPI a close second. 20TPI is too fine for most domestic wood, but works well on synthetics. 10TPI chasers have to traverse the wood twice as fast as 20TPI chasers and remove more material so are harder to learn and take more skill to use. The 10TPI threads can look rather coarse and industrial if used on smaller boxes.

The success of hand thread chasing largely depends on the type of wood used. Some of the best woods tend to be imported exotic woods. Dense, close-grained, slow growing hardwoods such as boxwood, lignum vitae, and African Blackwood are ideal for this technique. Someone referred to these as “uranium enriched” woods because they tend to be pricey. These woods provide the necessary strength and density to hold the threads without tearing or crumbling. Softer woods, or those with an open grain, are generally not suitable for hand thread chasing, as they tend to produce weak and unreliable threads or crumble when being cut. Some turners have success stiffening the threads with CA glue or sanding sealer.

It is also essential that the wood is well-seasoned and free from defects such as knots or cracks. Any imperfections in the wood can compromise the integrity of the threads and lead to failure during the chasing process. The grain needs to run parallel with the lather axis. Cutting threads in cross grain blanks does not work because the wood movement will cause the pieces to go oval and lock the pieces together when the humidity goes up! My best tip for turners just starting to thread chase is to buy a 2”x2”x12” blank of wood known to be excellent for thread chasing ($$$) and practice until it is gone! This will eliminate a major variable when learning. Some turners find inexpensive Schedule 40 PVC pipe useful for initial practice.

The lathe should run at a relatively slow speed, typically between 200 to 450 RPM to allow for better control over the chaser tools. I find that 300 RPM lathe speed gives me a comfortable rate of travel (CRT) for me to move and control a 16TPI chaser. If I use a 12TPI chaser at 300 RPM I am going to have to move it faster than I am used to or can comfortably control. So I slow the lathe down to about 240 RPM to get the chaser moving at the same CRT I would use with a 16TPI chaser. Conversely, I could speed the lathe to 400 RPM and get the same CRT with a 20TPI chaser. Changing the lathe speed to accommodate a different thread pitch allows me the ability to be consistent in how I move the chasers with a different pitch.

The top of the tool rest must be smooth and polished to allow the chaser to slide as easily as possible. A short tool rest with a hardened rod that does not require maintenance is ideal so the chaser will slide smoothly without catching on nicks. Cast iron or soft steel tool rests may require some maintenance with a file and sandpaper to make smooth. A light application of candle wax along the rest can also help the smooth travel of the chaser. Proper tool rest height is also important. Typically, the tool rest should be set just below center height, allowing the chasers to cut efficiently on center.

The process begins by preparing the wood surface where the threads will be cut. This surface should be smooth and cylindrical, with a diameter slightly larger than the intended thread size. Do not sand the wood as imbedded grit can dull the chasers. The chasers should rest comfortably on the tool rest, with the teeth engaging the wood at the correct angle. The corner of the wood to be struck needs to have a 45 degree chamfer or be slightly rounded on the corner edge for both inside and outside threads. The initial strike is at a 45 degree angle on male and a bit less on female threads.

External threads are easier for most turners to cut than internal so I recommend starting your practice with external threads. However, we typically start a threaded project doing the interior threads first. It is easier to recover from mistakes with external threads. If the external threads are not cut properly, perhaps by getting a double thread by moving the chaser to quickly, or become wavy, known as “drunken” thread, the wood can be shortened and the thread cutting can be started again.

1. “Strike” the Thread: Begin by positioning the tool inside the opening where the threads will be cut, holding it at a slight angle. Make a few practice dry run motions to get the feel for the speed. With the chaser moving, gently bring it into contact with the rotating wood in order to “strike” the thread. You should engage the third or fourth tooth on the chamfered corner at the mouth. The tool must be kept parallel with the floor or you risk altering the thread pitch.

2. Make a Recess or Stop Gap: You will need to create a recess where the threads will end. For most projects, you want to shoot for about 5 complete internal threads. This will provide some margin for error to make adjustments for grain alignment. The recess needs to be a bit wider and deeper than the threads. This stop gap gives you time to remove the chaser before the tool bottoms out and rips your threads. Even better than a recess, for internal threads, is chasing into open space like a collar or hollow out beyond where the threads will end.

4. Is an armrest necessary? No. Many turners are able to do fine without one when cutting threads. An armrest is certainly not needed when cutting threads in a collar for an urn or hollow form when cutting straight into an open space when there is no need for a recess.

1. Size the tenon: Before chasing the male thread, you must get the size right for the male tenon. With calipers, measure the female recess and lock in this setting. Using this measurement, turn a 1/8” long spigot where the male threads begin. This short spigot should fit snuggly in the recess against the interior threads. The surface of this spigot is now a reference point that will eventually be the bottom of your male thread. To the left of this spigot, establish a new level with a diameter increased by the depth of the male threads.

2. Strike the First Thread: With the handle raised and the chaser teeth parallel with the chamfer, with the chaser moving gradually, make the initial contact to “strike” the thread.

3. Make a Recess or Stop Gap: As with internal threads, cut a recess or stop gap with a recess tool after a full thread is struck in case of a bad initial strike. This makes recovery a lot easier if you create “drunken” or double threads with the initial “strikes.”

4. Advancing the Tool: As with internal threads, slowly advance the chaser tool along the wood's length, maintaining consistent pressure. It is important to keep the tool moving at a steady pace to ensure even thread spacing and depth.

Thread Chasing

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, . 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.

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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

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, ).

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, . 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, ). 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, ):

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• • 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.