There are three factors that make up the cutting conditions: cutting speed, depth of cut, and feed rate. In this lesson we will concentrate on feed rate factors as they affect single point lathe tooling. The table feed rate for turning is given in terms of inches per revolution (IPR). Inches per revolution is the rate at which the tool will advance for every revolution of the workpiece (Figure 1). The feed rate, is determined by the size of the chip that the tool can withstand. The feed rate in inches per tooth is also known as chip load. Because turning tools have only one cutting edge, the chip load or feed rate per revolution is the feed rate setting of the quick-change gearbox on the lathe (Figure 2).
Figure 1 Feed per revolution
The recommended values for chip load are based on the cutting tool material and the hardness or machinability rating of the workpiece material. The recommended values for I.P.R (chip load) can be found in charts in the Machinery’s Handbook, your textbook, and charts given to you by your turning tool salesperson. A typical feed in inches per tooth chart can be found in Table 3.
Figure 2 Quick change gear box setting
Table 3 Recommended Feed Rate Selection in Inches Per Revolution for Turning
|
Material |
High-Speed Steel |
Carbide |
||
|
Roughing |
Finishing |
Roughing |
Finishing |
|
|
Low Carbon Steel |
0.010 to 0.020 |
0.002 to 0.008 |
0.008 to 0.035 |
0.006 to 0.010 |
|
Med. Carbon Steel |
0.008 to 0.018 |
0.002 to 0.008 |
0.008 to 0.030 |
0.006 to 0.010 |
|
High Carbon Steel |
0.008 to 0.015 |
0.002 to 0.008 |
0.008 to 0.030 |
0.006 to 0.010 |
|
Cast Iron |
0.010 to 0.025 |
0.003 to 0.010 |
0.010 to 0.040 |
0.008 to 0.012 |
|
Bronze |
0.015 to 0.025 |
0.003 to 0.010 |
0.010 to 0.040 |
0.008 to 0.012 |
|
Aluminum |
0.015 to 0.030 |
0.003 to 0.012 |
0.015 to 0.045 |
0.008 to 0.012 |
While the recommended feed rates found in these charts represent good fundamental machining practice, they are only recommended values. Deviations from these values may be necessary due to certain circumstances, such as long, small diameter workpieces. The feed rate used on small diameter workpieces may need to be reduced. The work-holding technique has a great deal to do with the feed rate selection. Setups, which lack rigidity, may require a slower feed rate. The distance that the unsupported part sticks out of the work-holding mechanism must be kept to a minimum to assure proper rigidity. The required workpiece finish will also affect the feed rate selection. Finer finish requirements on the part will require a slower feed rate selection. When using carbide-turning tools, the available horsepower and the rigidity of the spindle bearings will always influence the feed rate.
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