THE LATHE 

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The lathe operates on the principle of the work being rotated against the edge of a cutting tool. It is one of the oldest and most important machine tools. The cutting tool is controllable and can be moved lengthwise on the lathe bed and into any desired angle across the revolving work.

LATHE SIZE

Lathe size is determined by the SWING and LENGTH OF THE BED. The swing indicates the largest diameter that can be turned over the ways

(flat or V-shaped bearing surface that aligns and guides movable part of machine). Bed length is entire length of the ways.

Bed length must not be mistaken for the maximum length of the work that can be turned between centers.  The longest piece that can be turned is equal to the length of the bed MINUS the distance taken up by the headstock and tailstock.

As an example, consider the capacity and clearance of a modern 13 in by 6 ft (325mm by 1800mm) lathe:
Swing over bed                            13 in (325rnm)
Swing over cross slide                 8 3/4 in. (218mm)
Bed length                                     72 in. (1800mm)
Distance between centers           50 in. (1240mm)

MAJOR PARTS OF A LATHE

The chief function of any lathe, no matter how complex it may appear to be, is to rotate the work against a controllable cutting tool. Each of the lathe parts in Fig. 10-4 falls into one of the following categories:
 

1. Driving the late.
2. Holding and rotating the work.
3. Holding, moving and guiding the cutting tool.

Driving the lathe

Power is transmitted to the various drive mechanisms by belt drive and/or gear train.
 

Holding and rotating the work

The headstock contains the spindle to which the various work holding attachments are fitted.   The spindle revolves in heavy/duty bearings and is rotated by belts, gears or a combination of both. It is hollow with the front tapered internally to receive tools and attachments with taper shanks.   the hole permits long stock to be turned without dangerous overhang.

Work is held in the lathe by a chuck, faceplate, collet or between centers.

The outer end of the work is often supported by the tailstock. It can be adjusted along the ways to accommodate different lengths of work. The tailstock mounts the "dead" center, and can be fitted with tools for drilling, reaming and threading.    It can also be offset for taper turning.
 

Holding, moving and guiding the cutting tool

The bed is the foundation of a lathe. All other parts are fitted to it. Ways are integral with the bed. The V-shape maintains precise alignment of the headstock and tailstock, and serves as rails to guide the travel of the carriage. The cutting tool is mounted on the carriage.

The carriage controls and supports the cutting tool and is composed of:

1. The saddle is fitted to and slides along the ways.
2. The apron contains the drive mechanism to move the carriage along the ways using hand or power feed.
3. The cross slide permits transverse tool movement (movement toward or away from the operator).
4. The compound rest permits angular tool movement.
5. The tool rest mounts the cutting tool.

Power is transmitted to the carriage through the feed mechanism.

Power is transmitted through a train of gears to the quick change gear box which regulates the amount of tool travel per revolution of the spindle. The gear train also contains gears for reversing tool travel.

The quick change gear box is arranged between the spindle and the lead screw. It contains gears of various ratios which makes it possible to machine various pitches of screw threads without changing loose gears. Longitudinal (back-and-forth) travel and cross (in-and-out) travel is controlled in the same manner.

An index plate provides instructions on how to set the lathe shift levers for various thread cutting and feed combinations. It is located on the face of the gear box. The large numbers on the index plate indicate the number of threads that can be cut per inch or pitch of metric threads. The smaller figures indicate the carriage longitudinal movement, in thousandths of an inch or in mm for each spindle revolution.

The lead screw transmits power to the carriage through a gearing and clutch arrangement in the carriage apron. Feed change levers on the apron control the operation of power longitudinal feed and power cross feed.

When place din neutral, the half-nuts may be engaged for thread cutting. The gear arrangement makes it possible to engage power feed and half- nuts simultaneously. The half-nuts are engaged ONLY for thread cutting and are NOT used as "automatic" feed for regular turning.

PREPARING LATHE FOR OPERATION

Before an aircraft is permitted to takeoff, the pilot or crew must go through a check-out procedure to determine whether the engines, controls, and safety features are in first-class operating condition. The same applies to the operation of a machine tool such as a lathe. the operator should inspect the machine for safe and proper operation.
 

1. Clean and lubricate the machine.
2. Be sure all guards are in position and locked in place.
3. Turn the spindle over by hand to be sure it is NOT locked nor engaged in back gear.
4. Move the carriage along the ways. There should be no binding.
5. Check cross slide movement.
6. Mount the desired work holding attachment.
7. Adjust the drive mechanism for the desired speed and feed.
8. If the tailstock is used, check it for alignment.
9. Clamp the cutter bit into an appropriate tool holder and mount it in the tool post. Do NOT permit excessive compound rest overhang as this often causes tool "chatter" and results in a poorly machined surface.
10. Mount the work. check for adequate clearance between the work and the various machine parts.
11. In addition, to the above procedures, the operator must take some precautions. sleeves should be rolled up and rings, jewelry and necktie or necklace removed.

            
 
 

             
 
 

WARNING! Lathe chips are sharp; do NOT remove them with your hands.
DANGER! an air hose should NEVER be used to remove chips. The flying particles might injure you or a nearby person.
 

1. No attempt should be made to operate a lathe until you know the proper procedures and have been checked out on its safe operation by your instructor.
2. Dress appropriately!   Remove necktie, necklace, wrist watch, rings and other jewelry, and loose fitting sweaters. Wear an apron or a properly fitted shop coat. Safety glasses are a must!
3. Clamp all work solidly! Use the correct size tool and work holding device for the job. Get help when handling large sections of metal and heavy chucks and attachments.
4. Check work frequently when it is being machined between centers. The work expands as it heats up and could damage the tailstock center.
5. Be sure all guards are in place before attempting to operate the machine.
6. Turn the faceplate or chuck by hand to be sure there is NO binding or danger of the work striking any part of the lathe.
7. Keep the machine clear of tools!
8. Stop the machine before making measurements and adjustments.
9. Remember--chips are sharp! Do NOT try to remove them with your hands when they become "stringy" and build up on the tool post. Stop the machine and remove them with pliers.
10. Do NOT permit small diameter work to project too far from the chuck without support from the tailstock. Without support, the work will be tapered, or worse, spring up over the cutting tool and/or break.
11. Be careful NOT to run the cutting tool into the chuck. Check any readjustment of work or tool for ample clearance when the cutter has been moved left to the farthest point that will be machined.
12. Stop the machine before attempting to wipe down, a machine surface.
13. Before repositioning or removing work from the lathe, move the cutting tool clear of the work area. This will prevent accidental cuts from the cutter bit.
14. Avoid talking to anyone while running a lathe! Do NOT permit anyone to fool around with the machine while you are operating it. You are the only one who should turn the machine on or off, or make adjustments to the lathe.
15. If the lathe has a threaded spindle nose, never attempt to run the chuck on or off the spindle using power. It is also dangerous practice to stop such a lathe by reversing the direction of rotation. The chuck could spin off and cause serious injury to you. There is also the danger of damaging the machine.
16. You should always be aware of the direction of travel and speed of the carriage before engaging the half-nuts or automatic feed.
17. Always remove the key from the chuck. Make it a habit NEVER to let go of the key until it is out of the chuck and clear of the work area.
18. Tools must NOT be placed on the lathe ways. Use a tool board or place them on the lathe tray.
19. When filing on the lathe, be sure the file has a securely fitting handle.
20. Stop the machine immediately if some off sounding noise or vibration develops during operation.   If you cannot locate the trouble, get help from your instructor.   Under no condition should the machine be operated until the trouble has been corrected.
21. Remove sharp edges and burrs from work before removing it from the machine.
22. Plan your work thoroughly before starting.    Have all needed tools on hand.
23. Use care when cleaning the lathe. Chips sometimes stick in recesses. Remove them with a brush or short stick., NEVER clean a machine tool with compressed air.

DANGER! Stop the machine before making measurements or cleaning out chips!

SAFETY NOTE! Under NO condition should a lathe be reversed to brake it to a stop!