A Comprehensive Guide to Threading and Turning Operations on a Lathe
Threading
Threading is the process of creating helical grooves and profiles on the inside or outside of a workpiece. This is achieved by combining the rotational movement of the workpiece with the longitudinal movement of a cutting tool.
Threaded Shaft Mechanism
A threaded shaft is held within a nut array that can be opened or closed via an external lever. When the nut is closed, the carriage is engaged with the lead screw, allowing for precise longitudinal movement. Conversely, opening the nut disengages the carriage.
Derivation of Screw Revolutions
The revolutions of the threaded shaft must be synchronized with the workpiece to achieve the desired thread pitch. Standard lathes offer lead screw pitches of 3, 6, 12, or 24 mm. The appropriate rotation ratio between the threaded shaft and the workpiece can be achieved through:
Progress Box
The progress box features a table that lists the achievable thread pitches and the corresponding lever positions for the desired transmission ratio.
Cancellation of Feed
To create a thread pitch not listed in the progress box, the feed can be canceled. This results in a 1:1 ratio between the input and output speeds of the progress box. Consequently, the spindle movement relies solely on the change gears mounted on the lathe’s gear train.
Calculating Change Gears
When the feed is canceled, a gear train of two or more gears is required to transmit motion from the input shaft to the threaded shaft. To determine the appropriate gear set for a specific thread pitch, the following formula is used:
k = Desired Pitch / Lead Screw Pitch
Gears A and B must have a number of teeth that result in a ratio equal to the calculated value of k. The driving gear (A) is mounted on the input shaft, while the driven gear (B) is mounted on the progress box input shaft (which acts as the threaded shaft when the feed is canceled). An intermediate gear may be used to connect the driving and driven gears without altering the transmission ratio. Typically, a four-gear train is used, but six gears may be necessary in some cases.
Process for Calculating Change Gears
- Calculate the ratio of the desired pitch to the lead screw pitch and express it as a fraction.
- Simplify the fraction to its lowest terms.
- Multiply the numerator and denominator by a common factor to obtain gear tooth counts that correspond to available change gears.
- If two suitable gears cannot be found, decompose the numerator and denominator into products of two factors and repeat step 3.
- Verify that the selected gear train can be accommodated on the lathe’s gear train without interference.
Special Cases in Thread Pitch Calculation
- Both the lead screw pitch and desired pitch are in millimeters.
- Both the lead screw pitch and desired pitch are in inches.
- The lead screw pitch is in millimeters, and the desired pitch is in inches.
- The lead screw pitch is in inches, and the desired pitch is in millimeters.
Threading Tools
Threading tools must have a profile that matches the desired thread form. The tip angle is typically 55° for Whitworth threads and 60° for metric threads. Special gauges are used to verify the tool’s tip angle. Threading tool bits have no rake angle to prevent changes in the tip angle during sharpening.
Accurate tool placement is crucial for producing quality threads. The tool tip should be positioned at the workpiece’s center height, with its axis of symmetry perpendicular to the spindle axis. Templates are often used to ensure proper tool alignment.
Threading Procedures
When cutting screw threads, the depth of cut should be greater at the beginning and gradually decrease as the chip becomes wider. This helps to reduce vibrations and prevent the tool from digging into the workpiece. To further improve chip formation and evacuation, the tool can be inclined to cut on only one flank during each pass.
Straight Turning for Small Threads
For threads with a pitch smaller than 2 mm, straight turning with a perpendicular feed is suitable. The penetration depth is calculated as follows:
Depth = 0.7 x Desired Pitch
Side Cutting with Oblique Penetration
This method involves taking three passes with a one-flank cutting tool, with each pass advancing one-tenth of the thread pitch. The carriage is rotated to an angle equal to half the thread angle (30° for metric threads and 27.5° for Whitworth threads). Subsequent passes are made by rotating the carriage to the next thread groove.
Taper Turning
Taper Turning with a Swiveling Compound Slide
To machine a taper using a swiveling compound slide, the compound slide is rotated around its vertical axis by half the cone angle. A circular graduation on the swivel base aids in setting the correct angle. For precise taper angles, a dial indicator can be used as follows:
- Move the carriage a distance L from point A to point B.
- Note the dial indicator readings (M and N) at points A and B, respectively.
- The difference between M and N represents the length of side AC in the triangle ABC.
- Calculate the sine of the taper angle using the formula: sin(angle) = (M – N) / L
Taper Turning with a Tailstock Offset
This method is suitable for machining long, shallow tapers. The tailstock is offset perpendicular to the spindle axis, allowing for precise alignment of the workpiece centers. This ensures a cylindrical turning operation when using the lathe’s automatic feed. The tailstock offset is calculated as follows:
Offset = (D – d) / 2L
Where:
- D = Large diameter of the taper
- d = Small diameter of the taper
- L = Length of the taper
Taper Turning with a Copier
For high-volume production of tapers, a copier attachment can be used. The copier follows a template with an angle equal to half the desired cone angle, guiding the cutting tool to produce the taper.
Lathe Operations and Tools
Turning
Turning is the process of shaping a workpiece into a cylinder. It can be performed manually or with automatic feed. Right-hand turning tools with straight or layered cutting edges are commonly used. For facing operations, a blade with an angled cutting edge is employed.
Facing
Facing involves creating flat surfaces perpendicular to the workpiece axis. It can be performed on the outside or inside of the workpiece. Facing tools typically have a square cutting edge.
Parting
Parting is used to cut off workpieces of small diameter. For larger diameters, gooseneck parting tools are preferred due to their elasticity, which helps to withstand vibrations and shocks during machining. To improve cutting conditions, the leading edge of the parting tool can be sharpened with a small angle (10° to 20°) and lubricated abundantly.
Grooving
Grooving involves creating circular grooves on cylindrical workpieces. Narrow grooves are often referred to as necks. Grooving tools are similar in design to parting tools. Due to their tendency to snag and break, grooving tools are typically mounted upside down and lubricated generously.
Drilling
Small-diameter drills (less than 12 mm) are typically mounted in the lathe’s chuck. Larger-diameter drills with tapered shanks can be mounted directly in the tailstock or using reducers.
Conical Turning
Conical turning produces an oblique cylinder with respect to the workpiece axis.
Internal Turning
Internal turning encompasses various operations performed on the inside of a workpiece, such as cylindrical turning, grooving, and facing.
Eccentric Turning
Eccentric turning is used to create round surfaces with multiple centers, such as spheres and handles. The irregular curve is achieved by manually moving the cutting tool along the desired profile, combining transverse and longitudinal movements. Files and emery cloth are used for finishing the surface.
Knurling
Knurling creates roughened surfaces that enhance grip and prevent slippage. This is achieved by pressing a patterned tool, called a knurling tool, against the rotating workpiece. Knurling tools typically consist of one or two hardened steel wheels with the desired pattern engraved on their circumference.
Mounting Tools
Cutting tools are securely mounted on the lathe’s tool post using screws. The main cutting edge should be positioned at the workpiece’s center height or slightly above. If the workpiece tends to compress during machining, the tool can be positioned slightly below the center height. To prevent tool deflection, the cutting tool should not protrude excessively from the tool post.