As a follow-up to my April column, here are additional tips for enhancing the operation of a manual lathe.
Multipurpose tool bits, such as this one that turns ODs, chamfers IDs and ODs and faces part surfaces, can save time when manually changing tools. All images courtesy T. Lipton.
Make a spring-type toolholder if you need to do large radii, use form tools with a broad cutting edge or use any tool that tends to chatter. The spring-type toolholder with its pivot above the centerline backs the cutting edge off when the tool bites and starts to chatter.
Wire EDM special tool profiles and difficult-to-grind geometries.
- Be aware of the reduction in clamping force when spinning a chuck at high speed. You can lose 50 percent of the clamping force when running at a high rpm.
- Orient rough-cut blanks from oxy-fuel, waterjet and plasma cutting with the large end of the taper in the cut to the back of the chuck. All of these processes produce varying amounts of taper, depending on the material thickness.
- When roughing, try to get completely under the bark, or outer skin, of a bar on the first pass. This extends tool life.
- Most lathes have 0.001"-dia. divisions on the cross-slide dial. A decent operator can control diameters to 0.0003" by interpolating between the divisions—if the lathe is in good shape. With that said, a lathe should repeat to half the smallest division on the cross-slide dial.
- Set the compound rest at a small angle to dial increments smaller than the cross-feed dial dimensions. A 5.73° angle off the Z-axis will take off 0.0002" on the diameter on the X-axis for each 0.001" dialed on the compound.
- Try to grind special lathe tools so the tool post can stay square to the machine axes. This reduces setup time, but is not always possible.
- Grind a couple of multipurpose tool bits. This can save time when manually changing tools.
- Make an old-school, spring-type toolholder if you need to do large radii, use form tools with a broad cutting edge or apply any tool that tends to chatter. The spring-type toolholder, with its pivot above the centerline, backs the cutting edge off when the tool bites and starts to chatter. Typically, you can double the cutting speed using this setup. Contrary to popular thinking, sometimes more rigidity is not the answer. In the old days when planers were used to produce surfaces on plates and other large, flat workpieces, the final finishing tool was a wide, flat tool held in a gooseneck, or spring-type, toolholder. Form tools are an excellent and fast way to duplicate complicated geometry in a manual lathe.
- For jobs your shop might find overly challenging, have your local wire EDM shop cut special part profiles and difficult-to-cut geometries. Some of these are a nightmare to accurately hand grind. Good luck trying to find commercially made tooling to machine some of the crazy stuff part designers create.
- Pull back on the tool post with a few pounds of force when backing tools in the Z-axis. This prevents leaving tool tracks in a turned surface. This also works with boring bars on the ID, but you have to push instead.
- Make a couple of aluminum face plates that fit in a 3-jaw chuck. These can be resurfaced dead flat dozens of times and set up quickly. They can be welded or bolted together. If you bolt them, be sure to sink the heads well below the surface so you don’t face the screw heads.
- A nifty hand tapping guide that fits in the tailstock chuck works well for manual or slow-speed tapping of small threads.
- Three-jaw backing plates are great for backing up a part for heavy drilling. If you make several thicknesses, they can be used to quickly position thin, disc-shaped parts. Add three jacking screws for adjusting the plate in relation to the chuck face to hold thin, disc-shaped parts where you want them in the jaws against a parallel surface.
- Use light cuts for a disc-shaped part by pressure-plating against a face plate. You can even use the top of the chuck jaws if you have a little help. Three pieces of double-stick tape make all the difference. Open the jaws to a radius just below what you will be turning to for maximum holding capability. Use a smaller disc with a center hole to push against the tailstock center. This method works great when you can’t apply a center drill or have a center mark in the workpiece. Double-stick tape always works better when the mating surfaces are cleaned with alcohol and the blank is squeezed into the tape. CTE
About the Author: Tom Lipton is a career metalworker who has worked at various job shops that produce parts for the consumer product development, laboratory equipment, medical services and custom machinery design industries. He has received six U.S. patents and lives in Alamo, Calif. Lipton’s column is adapted from information in his book “Metalworking Sink or Swim: Tips and Tricks for Machinists, Welders, and Fabricators,” published by Industrial Press Inc., New York. The publisher can be reached by calling (888) 528-7852 or visiting www.industrialpress.com. By indicating the code CTE-2012 when ordering, CTE readers will receive a 20 percent discount off the book’s list price of $44.95.
Related Glossary Terms
- backing
backing
1. Flexible portion of a bandsaw blade. 2. Support material behind the cutting edge of a tool. 3. Base material for coated abrasives.
- boring
boring
Enlarging a hole that already has been drilled or cored. Generally, it is an operation of truing the previously drilled hole with a single-point, lathe-type tool. Boring is essentially internal turning, in that usually a single-point cutting tool forms the internal shape. Some tools are available with two cutting edges to balance cutting forces.
- center drill
center drill
Drill used to make mounting holes for workpiece to be held between centers. Also used to predrill holes for subsequent drilling operations. See centers.
- chatter
chatter
Condition of vibration involving the machine, workpiece and cutting tool. Once this condition arises, it is often self-sustaining until the problem is corrected. Chatter can be identified when lines or grooves appear at regular intervals in the workpiece. These lines or grooves are caused by the teeth of the cutter as they vibrate in and out of the workpiece and their spacing depends on the frequency of vibration.
- chuck
chuck
Workholding device that affixes to a mill, lathe or drill-press spindle. It holds a tool or workpiece by one end, allowing it to be rotated. May also be fitted to the machine table to hold a workpiece. Two or more adjustable jaws actually hold the tool or part. May be actuated manually, pneumatically, hydraulically or electrically. See collet.
- cutting speed
cutting speed
Tangential velocity on the surface of the tool or workpiece at the cutting interface. The formula for cutting speed (sfm) is tool diameter 5 0.26 5 spindle speed (rpm). The formula for feed per tooth (fpt) is table feed (ipm)/number of flutes/spindle speed (rpm). The formula for spindle speed (rpm) is cutting speed (sfm) 5 3.82/tool diameter. The formula for table feed (ipm) is feed per tooth (ftp) 5 number of tool flutes 5 spindle speed (rpm).
- electrical-discharge machining ( EDM)
electrical-discharge machining ( EDM)
Process that vaporizes conductive materials by controlled application of pulsed electrical current that flows between a workpiece and electrode (tool) in a dielectric fluid. Permits machining shapes to tight accuracies without the internal stresses conventional machining often generates. Useful in diemaking.
- face plate
face plate
Flat, round workholder with slots used to hold regular- or irregular-shaped stock. If stock is markedly asymmetrical, counterbalances may be needed to prevent vibration.
- finishing tool
finishing tool
Tool, belt, wheel or other cutting implement that completes the final, precision machining step/cut on a workpiece. Often takes the form of a grinding, honing, lapping or polishing tool. See roughing cutter.
- flat ( screw flat)
flat ( screw flat)
Flat surface machined into the shank of a cutting tool for enhanced holding of the tool.
- inner diameter ( ID)
inner diameter ( ID)
Dimension that defines the inside diameter of a cavity or hole. See OD, outer diameter.
- lathe
lathe
Turning machine capable of sawing, milling, grinding, gear-cutting, drilling, reaming, boring, threading, facing, chamfering, grooving, knurling, spinning, parting, necking, taper-cutting, and cam- and eccentric-cutting, as well as step- and straight-turning. Comes in a variety of forms, ranging from manual to semiautomatic to fully automatic, with major types being engine lathes, turning and contouring lathes, turret lathes and numerical-control lathes. The engine lathe consists of a headstock and spindle, tailstock, bed, carriage (complete with apron) and cross slides. Features include gear- (speed) and feed-selector levers, toolpost, compound rest, lead screw and reversing lead screw, threading dial and rapid-traverse lever. Special lathe types include through-the-spindle, camshaft and crankshaft, brake drum and rotor, spinning and gun-barrel machines. Toolroom and bench lathes are used for precision work; the former for tool-and-die work and similar tasks, the latter for small workpieces (instruments, watches), normally without a power feed. Models are typically designated according to their “swing,” or the largest-diameter workpiece that can be rotated; bed length, or the distance between centers; and horsepower generated. See turning machine.
- parallel
parallel
Strip or block of precision-ground stock used to elevate a workpiece, while keeping it parallel to the worktable, to prevent cutter/table contact.
- tapping
tapping
Machining operation in which a tap, with teeth on its periphery, cuts internal threads in a predrilled hole having a smaller diameter than the tap diameter. Threads are formed by a combined rotary and axial-relative motion between tap and workpiece. See tap.
- toolholder
toolholder
Secures a cutting tool during a machining operation. Basic types include block, cartridge, chuck, collet, fixed, modular, quick-change and rotating.
- turning
turning
Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.
- wire EDM
wire EDM
Process similar to ram electrical-discharge machining except a small-diameter copper or brass wire is used as a traveling electrode. Usually used in conjunction with a CNC and only works when a part is to be cut completely through. A common analogy is wire electrical-discharge machining is like an ultraprecise, electrical, contour-sawing operation.
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