Description of LATHE

Description

The purpose of a lathe is to rotate a part against a tool whose position it controls. It is useful for fabricating parts and/or features that have a circular cross section. The spindle is the part of the lathe that rotates. Various workholding attachments such as three jaw chucks, collets, and centers can be held in the spindle. The spindle is driven by an electric motor through a system of belt drives and/or gear trains. Spindle speed is contolled by varying the geometry of the drive train.The tailstock can be used to support the end of the workpiece with a center, or to hold tools for drilling, reaming, threading, or cutting tapers. It can be adjusted in position along the ways to accomodate different length workpices. The ram can be fed along the axis of rotation with the tailstock handwheel.
The carriage controls and supports the cutting tool. It consists of:

• A saddle that mates with and slides along the ways.
• An apron that controls the feed mechanisms.
• A cross slide that controls transverse motion of the tool (toward or away from the operator).
• A tool compound that adjusts to permit angular tool movement.
• A toolpost T-slot that holds the toolpost.

  Choosing a Cutting Tool

  The figure above shows a typical cutting tool and the terminology used to describe it. The actual geometry varies with the type of work to be done. The standard cutting tool shapes are shown below.

  • Facing tools are ground to provide clearance with a center.
  • Roughing tools have a small side relief angle to leave more material to support the cutting edge during deep cuts.
  • Finishing tools have a more rounded nose to provide a finer finish. Round nose tools are for lighter turning. They have no back or side rake to permit cutting in either didection.
  • Left hand cutting tools are designed to cut best when traveling from left to right.
  • Aluminum is cut best by specially shaped cutting tools (not shown) that are used with the cutting edge slightly above center to reduce chatter.

    Installing a Cutting Tool

    Lathe cutting tools are held by tool holders. To install a tool, first clean the holder, then tighten the bolts.
  • The tool post is secured to the compound with a T-bolt. The tool holder is secured to the tool post using a quick release lever.

  • Positioning the Tool

    In order to move the cutting tool, the lathe saddle and cross slide can be moved by hand.
    
    There are also power feeds for these axes. Procedures vary from machine to machine.
    A third axis of motion is provided by the compound. The angle of the compound can be adjusted to allow tapers to be cut at any desired angle. First, loosen the bolts securing the compound to the saddle. Then rotate the compound to the desired angle referencing the dial indicator at the base of the compound. Retighten the bolts. Now the tool can be hand fed along the desired angle. No power feed is available for the compound. If a fine finish is required, use both hands to achieve a smoother feed rate.
    The cross slide and compound have a micrometer dial to allow accurate positioning, but the saddle doesn't. To position the saddle accurately, you may use a dial indicator mounted to the saddle. The dial indicator presses against a stop.
    
    

    Feed, Speed, and Depth of Cut

    Cutting speed is defined as the speed at which the work moves with respect to the tool (usually measured in feet per minute). Feed rate is defined as the distance the tool travels during one revolution of the part. Cutting speed and feed determines the surface finish, power requirements, and material removal rate. The primary factor in choosing feed and speed is the material to be cut. However, one should also consider material of the tool, rigidity of the workpiece, size and condition of the lathe, and depth of cut. For most Aluminum alloys, on a roughing cut (.010 to .020 inches depth of cut) run at 600 fpm. On a finishing cut (.002 to .010 depth of cut) run at 1000 fpm. To calculate the proper spindle speed, divide the desired cutting speed by the circumference of the work. Experiment with feed rates to achieve the desired finish. In considering depth of cut, it's important to remember that for each thousandth depth of cut, the work diameter is reduced by two thousandths.