The Centre Lathe: Removing Metal

To remove metal from work effectively you must pay some attention to the rate of metal removal; this is affected by the speed of the lathe, the speed of cutting, the depth of the cut and the feed.

Lathe speed

Lathe speed, measured in revolutions per minute. Is the speed at which the lathe spindle rotates and consequently at which the work rotates. The range of available lathe speeds will depend on the lathe you own; a typical range is about 50-800rpm. Some of the more sophisticated lathes allow speed-changing devices to be fitted; these enable speeds to be changed almost instantly. Others need to be stopped before you can change the lathe speed. Six speed lathe A typical lathe with six speeds comprises an electric motor carrying a pulley with a belt to drive a larger pulley on a countershaft. Which also carries a stepped pulley. A belt from the countershaft in turn drives a stepped pulley, which rotates on the lathe spindle. You can lock this stepped pulley onto the spindle by means of a pin; it will then drive the spindle. To select one of the three higher speeds you move the belt from one to another of the three steps on the pulley; with the belt on the smallest diameter step of the spindle pulley the spindle will turn at its highest speed. To achieve low speeds on the lathe you will have to engage the back gear; this is a shaft which carries gears, mounted behind or below the lathe spindle. When engaged, one of its gears connects with a gear on the stepped pulley and the other end of its shaft has another gear which connects with a gear on the lathe spindle. You disengage the locking pin which holds the stepped pulley to the lathe spindle and the drive is transmitted to the spindle via the back gear, resulting in a lower speed.

centre-lathe-removing-metal

The arrangements for engaging the back gear will vary slightly on different models of lathe, but they all involve disengaging the stepped pulley from the lathe spindle at the same time as you engage the back gear. While it can be tedious to change speeds when it involves adjusting belts and back gears, you should remember the slower the belt is running, the lower the horse power it will transmit; this can be important if you intend making heavy cuts.

Cutting speed

This is the speed at which the surface of the metal travels past the edge or point of the particular cutting tool you are using. Normally measured in metres per minute or feet per minute (one metre per minute equals 3.28 feet per minute), the cutting speed depends on the lathe speed and the diameter of the work. As a general rule, the higher the cutting speed, the shorter the life of the tool before it requires regrinding; this is because friction between the tool and the work generates a large amount of heat which softens the cutting edge. If, however, the cutting speed is much below the correct speed for the materials of the tools being used and the work being machined, it will be difficult — if not impossible — to obtain a good finish on the work.

For soft or ductile materials such as aluminium, copper and brass, use a high cutting speed — say above 30m/min; for the harder materials such as mild steel, cast iron and carbon steels, use lower cutting speeds. When you have to remove a large amount of metal (a roughing cut). use a low speed and use a high speed for the light finishing cuts. You will find for most work the cutting speed is not critical; if you are uncertain in any way you should reduce speed. The graph gives an indication of approximate speeds to use with various metals. If you are making heavy cuts, it would be best to reduce the speeds shown in the graph.

Revolutions per minute are estimated in metric at 318 times the cutting speed divided by the diameter of the work in millimetres, and in Imperial at four times the cutting speed divided by the diameter of the work in inches.

Depth of cut

The depth of cut is the distance from the machined surface to the surface of metal where machining began, whether you are turning or facing the work. When turning, the depth of cut is the distance the tool is moved into the work as it moves across the lathe; the diameter is reduced by twice the depth of cut. When facing work, its length is reduced by the depth of cut. As a rough

guide to the depth of cut, you can consider cuts of 3-6mm in depth as heavy cuts and 0.2mm (0.008in) as a light finishing cut. The depth of cut possible depends on the material you are turning and the power available to drive the lathe; practise with scrap materials to discover the combination of speeds and cuts your lathe will accept when using different materials. Until you have found the capabilities of your lathe it is best to begin by taking light cuts, since the speed at which metal is removed from the work is rarely important in the home workshop. As with all jobs on the lathe, practice is essential for the best results.

Feed

To reduce the diameter of your work you move the cutting tool along its length towards the headstock of the lathe. The rate at which you move the tool in this direction is known as the feed. If the work is being faced, the tool is moved across the lathe; in this case the feed is the rate of movement across the lathe. For a great deal of work the tool will be fed into the work by hand; in this case the rate of feed is determined by observing the work and by the ‘feel’ on the handwheel used to feed the tool into the work.

Automatic feed

On screw-cutting lathes it is possible to drive the leadscrew from the headstock spindle; by means of the carriage engagement lever (also called screw-cutting lever, feed lever or half-nut lever), you can connect the carriage to the lead-screw. The tool will then advance automatically along the length of the lathe.

Warning

When using automatic feed you must disengage it in time to avoid the carriage or the top slide colliding with the chuck or face plate.

Rate of feed The rate of feed is given by the lathe manufacturer as the distance the carriage moves towards the headstock for each revolution of the headstock spindle. A typical range of feeds would be 0.25-0.09mm per revolution. You can select the required feed by use of the gearbox on the more expensive lathes or by altering the change wheels if a gearbox is not provided. Only very expensive lathes have a power feed to the cross slide. In choosing rate of feed a deep cut might require a fine feed of about 0.1mm (0.004in) per revolution, while a light cut could be taken with a coarser feed of about 0.2mm (0.008in) per revolution. In general, light tools and tools with sharp points should have a slow rate of feed.

Coolant

There are several reasons for using coolant; it helps to keep the cutting tool and workpiece cool, which lengthens the life of the cutting edge; it reduces friction between tool and work and therefore reduces wear and improves the finish; and by applying coolant you can also flush the swarf away from the work.

Types of coolant

The most common coolant is soluble cutting oil, an emulsion of oil and water. Mix the oil with water, following the supplier’s directions very carefully since the proportions can vary from about six to fifty parts water to one of oil. After using soluble oil you should clean the lathe thoroughly because the presence of water can lead to rusting. An alternative to soluble cutting oil is to use ‘suds’ — merely a soapy water solution with a little washing soda added to improve the wetting properties of the solution. However, it is best to avoid using suds if possible because they increase the risk of rust appearing on the work and the lathe. Use soluble oil for all work with steel and paraffin for turning aluminium; brass and cast iro arc best turned dry. Soluble oil can be used on copper and the best coolant for tapping and screw-cutting is lard oil, if available; mineral oil (engine oil) can be used as a substitute, although it is not an ideal lathe coolant.

Applying coolant

Larger lathes are fitted with coolant pumps; on smaller lathes you will have to apply the coolant with a brush. Take care not to trap the bristles of the brush in the rotating parts. You can apply the coolant from an engineer’s oil can instead of an ordinary can. Avoid getting the coolant under the saddle because the water will evaporate and the saddle will be difficult to move.

Finishing

To achieve a good finish on turned work you should ensure the final cut removes only a very small amount of metal (a small depth of cut) and the cuts made in the work are very close together (a fine feed). Make sure the tools used are sharp and correctly set to the centre height of the work; you can use a knife tool or a round nosed tool for finishing.

A tool ground to the correct angles for work on steel can be used for all the materials listed below, except on brass when you must use a tool ground for use on brass. It may be necessary to experiment to find the best speed for some materials; if you have to do this, make sure you do not leave the experimentation until you come to the final cut on the work. A guide to the requirements for different metals is shown below.

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