BASIC SETUP OF LATHE MACHINE

BASIC SETUP OF LATHE MACHINE

A knowledge of the basic setup is required if you are to become proficient in performing machine work with a lathe. Some of these setups are considered in the following sections.

Cutting Speeds and Feeds

Cutting speed is the rate at which the surface of the work passes the point of the cutting tool. It is expressed in feet per minute (fpm).

Feed is the amount the tool advances for each revolution of the work. It is usually expressed in thousandths of an inch per revolution of the spindle. Cutting speeds and tool feeds are determined by various considerations: the hardness and toughness of the metal being cut; the quality, shape, and sharpness of the cutting tool; the depth of the cut; the tendency of the work to spring away from the tool; and the strength and power of the lathe. Since conditions vary, it is good practice to find out what the tool and work will stand and then select the most practical and efficient speed and feed for the finish desired.

When ROUGHING parts down to size, use the greatest depth of cut and feed per revolution that the work, the machine, and the tool will stand at the highest practical speed. On many pieces where tool failure is the limiting factor in the size of the roughing cut, you may be able to reduce the speed slightly and increase the feed to remove more metal. This will prolong tool life. Consider an example where the depth of cut is 1/4 inch, the feed 0.020 inch per revolution, and the speed 80 fpm. If the tool will not permit additional feed at this speed, you can drop the speed to 60 fpm and increase the feed to about 0.040 inch per revolution without having tool trouble. The speed is therefore reduced 25 percent, but the feed is increased 100 percent. Thus the actual time required to complete the work is less with the second setup.

For the FINISH TURNING OPERATION, take a very light cut, since you removed most of the stock during the roughing cut. Use a fine feed to run at a high surface speed. Try a 50 percent increase in speed over the roughing speed. In some cases, the finishing speed may be twice the roughing speed. In any event, run the work as fast as the tool will withstand to obtain the maximum speed during this operation. Be sure to use a sharp tool when you are finish turning.

CHATTER

Chatter is vibration in either the tool or the work the finished work surface appears to have a grooved or lined finish instead of a smooth surface. The vibration is set up by a weakness in the work, work support, tool, or tool support and is probably the most elusive thing you will find in the entire field of machine work as a general rule, strengthening the various parts of the tool support train will help. It is also advisable to support the work by a center rest or follower rest.

The fault may be in the machine adjustments. Gibbs may be too loose; hearings may, after a long period of heavy service, be worn; the tool may be sharpened improperly, and so on. If the machine is in excellent condition, the fault may be in the tool or tool setup. Grind the tool with a point or as near a point as the finish specified will permit; avoid a wide, round leading edge on the tool. Reduce the overhang of the tool as much as possible. Be sure all the gab and bearing adjustments are properly made. See that the work receives proper support for the cut and, above all, do not try to turn at a surface speed that is too high. Excessive speed is probably the greatest cause of chatter. The first thing you should do when chatter occurs is reduce the speed.

Direction of Feed

Regardless of how the work is held in the lathe, the tool should feed toward the headstock. This causes most of the pressure of the cut to bear on the work-holding device and the spindle thrust bearings. When you must feed the cutting tool toward the tailstock take lighter cuts at reduced feeds. In facing, the general practice is to feed the tool from the center of the work piece outward.

PRELIMINARY PROCEDURES

Before starting a lathe machining operation, always ensure that the machine is set up properly. If the work is mounted between centers, check the alignment of the dead center and the live center and make any necessary changes. Ensure that the tool holder and cutting tool are set at the proper height and angle. Check the work-holding accessory to ensure that the work piece is held securely. Use the center rest or follower rest to support long work pieces.

PREPARING THE CENTERS

The first step in preparing the centers is to see that they are accurately mounted in the headstock and tailstock spindles. The centers and the tapered holes in which they are fitted must be perfectly clean. Chips and dirt left on the contact surfaces prevent the bearing surfaces from fitting perfectly. This will decrease the accuracy of your work. Make sure that there are no burrs in the spindle hole. If you find burrs, remove them by carefully scraping and reaming the hole with a Morse taper reamer. Burrs will produce the same inaccuracies as chips or dirt.

A center’s point must be finished accurately to an angle of 60°. Figure 9-18 shows the method of checking this angle with a center gauge. The large notch of the center gauge is intended for this purpose. If this test shows that the point is not perfect, you must true it in the lathe by taking a cut over the point with the compound rest set at 30°. You must anneal the hardened tail center before it can be machined in this manner, or you can grind it if a grinding attachment is available.

CHECKING ALIGNMENT

To turn a shaft straight and true between centers, be sure the centers are aligned in a plane parallel to the ways of the lathe. You can check the approximate alignment of the centers by moving the tailstock up until the centers almost touch and observing their relative positions.

To test center alignment for very accurate work, take a light cut over at each end with a micrometer and, if readings are found to differ, adjust the tailstock accordingly. Repeat the procedure until alignment is obtained.