The Function of a Lathe
The function of a lathe has not changed much in over 150 years since the dawn of the machine age or even earlier when lathes were powered by hand or foot. It is used to rotate a piece of stock rapidly while a cutting tool is brought to bear against it to remove material. On a wood lathe, the cutting tool is usually a hand held chisel. On a metal cutting lathe, the forces are usually too high for a hand held tool, so a sharpened steel or carbide tool is held in a tool post that is attached to a moveable table. Turning the handwheel on the crosslide moves the tool in and out, while the handwheel on the long leadscrew moves the crosslide and tool post side to side down the part. The handwheels allow very accurate cuts to be made, as they have graduations of .001″ or .01 mm on them. Remember that because you are removing material from the RADIUS of the material, advancing the crosslide .001″ into the workpiece means you will actually remove .002″ from the diameter. Some older lathes are calibrated to account for this, but because the Sherline tool system also offers a vertical milling column that allows the lathe to be used as a mill, the handwheel graduations reflect true distance movement, not amount removed from the diameter. A lathe is easier to learn to use than a mill, so it is often the first metal cutting tool a new machinist buys.
The Parts of a Lathe and What They Do
- Variable Speed Control Knob—Controls motor speed from 0 to 2800 RPM
- Headstock—Contains the spindle in two preloaded ball bearings.
- Spindle—The spindle is inside the headstock and is driven with a belt running from the motor pulley to a pulley on the rear end of the spindle shaft. The nose of the spindle is treaded on the outside to receive chucks and tapered on the inside to receive other accessories.
- Chucks—A 3-jaw or 4-jaw chuck threads onto the spindle nose to hold your work, a drill chuck is used on the tailstock to center drill your part.
- Tool Post—Attaches to the lathe table and holds a 1/4” square cutting tool
- Crosslide table—Also sometimes spelled “cross slide,” it is the table with two T-slots that holds the tool post.
- Crosslide Gib—A tapered plastic wedge that is held in place by a gib lock. It fits between the angled surfaces of the dovetail and is used to adjust for wear. As wear occurs and the table develops “slop,” the lock is loosened and the gib is pushed further into the gap, taking up any play. This allows the machine to always be kept in peak adjustment.
- Tailstock spindle—Has a #0 Morse internal taper for holding chucks and other tools. A handwheel moves it in and out for drilling.
- Tailstock locking screw—Locks the tailstock in place on the bed to keep it from moving. When loosened, the tailstock can be slid up and down the bed.
- Bed—The dovetailed steel bar that the saddle and tailstock are moved back and forth on.
- Saddle—The part that supports the crosslide table and is moved up and down the bed using the leadscrew handwheel.
- Saddle Gib—Functions like the crosslide gib to keep the saddle in tight adjustment against the dovetailed bed.
- Leadscrew—The threaded screw under the bed that controls movement of the saddle. A “saddle nut” underneath attaches the bed to the leadscrew. Turning the leadscrew handwheel moves the saddle down the bed.
- Tailstock Gib—A brass part attached to the base of the tailstock that runs on one of the bed dovetails. The brass part is expected to wear rather than the more expensive bed and can be adjusted for tightness as it wears.
- Lathe Base—The cast metal base upon which the lathe bed and headstock sit.
- Drawbolt—Goes through the hole in the spindle to draw chucks and other accessories into the headstock taper inside the spindle. A special washer locates it on center in the spindle hole.
- #1 Morse Arbor—The tailstock drill chuck normally has a #0 Morse arbor threaded into the back of it for use in the tailstock spindle. That arbor can be removed and replaced with the #1 Morse arbor so the drill chuck can be used in the headstock.
- Dead Centers—#1 and #0 Morse arbors have a 60° point and are used to locate and hold work “between centers” on the lathe. The #1 Morse arbor rotates with the headstock, but because the tailstock spindle does not rotate, the rear #0 Morse arbor is called a “dead” center. This needs to be kept lubricated because it creates friction with the moving part it is locating. Most machinists eventually replace this with a “live” center that turns on a ball bearing.
- Tommy Bars—Round steel bars used to tighten and loosen chucks and other spindle accessories. Sometimes called “Spindle Bars.”
- Faceplate—A cast plate that threads onto the spindle nose. A workpiece can be bolted to it as an alternative to using a chuck. It has three slots to drive a drive dog.
- Drive Dog—Also called a “Lathe Dog,” this part is attached to a piece of bar stock by means of a screw that goes through the side and the long point is placed into one of the slots in the faceplate. The part is located between the lathe centers (live or dead) and when the faceplate turns, the dog actually drives the piece to rotate it for cutting. It also acts as a universal joint when turning a part between centers when the headstock is rotated to a slight angle, allowing a tapered part to be cut.
- Headstock Locking Screw—Holds the headstock in place. The screw is a pointed set screw. The point engages a tapered groove in the pin that sticks up out of the lathe bed. When the screw is tightened, it pulls the headstock down onto the alignment key and holds it tight against the lathe bed.
- Alignment Key—A precision ground key that fits in slots in the top of the bed and bottom of the headstock to keep the headstock aligned straight with the tailstock. Removing this key and rotating the headstock allows tapers to be cut.
- V-belt—A Kevlar-reinforced Urethane belt that drives the spindle by means of the pulleys.
- 2-position Pulley—The motor turns a maximum of about 6000 RPM. Putting the drive belt in the normal (rear) position gears the motor down about 2:1 for a maximum speed of about 2800 RPM. The “High Torque” position (closest to the headstock) gears it about 4:1 for lower speed but more torque when needed for heavy cuts.
Other Lathe and Machining Terms
- Adjustable “Zero” Handwheels—On base model machines, plain handwheels are used. They are laser engraved with 50 marks (inch) or 100 marks (metric) and numbers for reference. Adjustable zero handwheels allow you to stop at any given point, loosen a knurled wheel in the center of the handwheel and rotate the laser engraved collar back to the zero mark before starting the next cut without moving the position of the handwheel. This means each time you are starting from zero rather than from a random number, making your depth and cut calculations easier. This eventually means less mistakes. 4400- and 4500-series lathes and 5400- and 2000-series mills include these upgraded handwheels as standard equipment.
- Compound Slide—A device found on many lathes that allows the cutting tool to be brought into the part at an angle for cutting tapers. A compound slide is optional on Sherline lathes, but is not included as standard because the rotating headstock feature allows a Sherline lathe to cut tapers without the use of a compound slide. The optional compound slide is P/N 1270 (inch) or 1280 (metric).
- DRO—Stands for “Digital Readout”. Digital readouts incorporate an electronic box with a screen that reads out numbers rather than you looking at the graduations on the handwheels to determine movement. If offers two advantages: For those with poor eyesight it is easier to read than the little marks on the handwheel and 2) it keeps track of accumulated distance so you don’t have to count handwheel revolutions when making longer movements. This helps eliminate a common source of errors. Any Sherline lathe or mill can be ordered fitted with DRO or it can be added later. Also incorporated in the readout is a sensor and RPM indicator for the spindle to eliminate guesswork regarding spindle speed.
- CNC—Stands for “Computer Numeric Control.” Instead of you turning the handwheels, a computer determines the speed and distance and drives DC motors called “stepper motors” or “servos” to move the lathe for you. Any Sherline machine can be ordered ready for the application of CNC or as a complete CNC system with steppers, controller, computer, software and everything. CNC can also be fitted at a later date to any Sherline lathe or mill.
- HSS—Stands for “High Speed Steel.” This is the most common material for cutting tools used by the home machinist. It is inexpensive and easily sharpened on a bench grinder and can be resharpened with a stone or grinder when it becomes dull. It can also be ground into different shapes for special purposes like boring, threading or cutting special shapes. (Called a “Form” tool.)
- Carbide Insert Tooling—Tool holders that clamp a small, replaceable carbide tip into place for cutting harder materials like stainless steel or titanium. Often used in professional machine shops because of the higher metal removal rates they allow, many home machinists like using carbide tools too. An alternative is “Brazed tip Carbide” tooling where the piece of carbide is brazed to the tool shank and is not removable. Generally, once chipped, carbide tooling is disposed of because it is not easily sharpened.