Centre Lathe Cutting Tools

Cutting tools used for turning on a lathe come in a number of forms. The simplest form is a piece of square section high speed steel (HSS) which is ground to the correct cutting shape. You can buy ready hardened HSS in various sizes up to about 12.5mm square. After grinding small section tool bits to shape, you can mount them in toolholders (for external turning) or in bars known as boring bars (for internal turning); the use of tool-holders or boring bars will help keep down the amount of HSS you will need. Generally, however, carbon steel shanks are used with high speed steel butt-welded onto the end.

Throwaway tips

These come in various types and consist of a shank with the cutting material, which may be cemented carbide or ceramic, clamped in position. The tips are ground and lapped ready for use; they may be rectangular with two or four cutting edges or triangular with up to six cutting edges. When one edge becomes blunt, the tip is rotated and a fresh edge is presented to the work. These tips are not usually reground.

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Grinding tools

A powered grinding machine is necessary as part of a turner’s equipment. If you wish to grind your own tools to shape from HSS stock, a machine is obviously necessary; but even if you buy a set of tools, they will need grinding from time to time. The tools should always be as sharp as possible and it is best to regrind the edges lightly whenever the tool appears to be losing its cutting edge. The bulk of the grinding should be carried out on the front and side so the top is only ‘licked’: grinding off the top weakens the tool.

Tool angles

All cutting tools must approach the work at the correct angle if they are to move metal effectively. They must also be ground so they do not rub against the work as they cut. You can take the clearance angle required on lathe tools as five degrees, both at the front and side of the tool. When the tool is used for internal turning, the clearance should be at least eight degrees and on some jobs it may be necessary to grind a second clearance if the tool is to clear the work; this is known as secondary or additional clearance.

Rake angle

Metal is actually removed from the work by a wedging action and this action will depend on the rake angle. Some materials will produce continuous chips in small spirals which leave the work as it is turned; this is common with aluminium, copper and mild steel and a top rake from about seven to twelve degrees is suitable for general purposes. Ideally a steeper rake angle should be employed for these softer materials, but this would mean keeping several sets of tools for use with different materials or frequently regrinding the tools. On materials such as brass, cast iron and medium and high carbon steels, the metal leaves the work in small separate pieces or discontinuous chips; a top or side rake of about two degrees is sufficient for these materials.

Accuracy

Without special grinding equipment you will find it is not possible to maintain exact cutting angles when grinding a tool; but a slight error in the suggested angles should make little difference to the results you obtain. You do need to avoid grinding excessively large clearance and rake angles because the combined effect of this will be to weaken the cutting edge of the tool. The one case in which accurately ground tools are essential is when you carry out screw-cutting on the lathe.

Tool profiles

Lathe tools are usually described by their shape in plan view (looking from the top of the tool). For general purpose work it is not necessary to have a large number of tool shapes; but you will need two sets of tools, one set having its top rake ground to an angle suitable for use with continuous chips and the other for use with materials which produce discontinuous chips. Most tools are used cutting from the right-hand side of the lathe towards the headstock; for this reason they are called RH tools. A basic set of tools consists of a knife tool, a roughing tool, a round-nosed tool, a boring tool and a recessing tool.

Knife tool

This type will deal with a very large amount of turning work. It has no top rake, but has a side rake of up to 30 degrees; use about two degrees only for brass. It is set so the cutting edge is at right-angles to the direction of the feed; for this reason the action is often called orthogonal cutting.

Roughing tool

A modified form of the knife tool, the roughing tool is used for rapid removal of metal. Viewed from above the cutting edge of this tool is ground at an angle to the direction of cutting; this gives a longer and therefore stronger cutting edge than the knife tool. The plan approach angle should be large — between 70 and 80 degrees — for heavy cuts and about 20 degrees for light cuts; this is known as oblique cutting.

Round-nosed or radiusing tool

Use this type when you require a radius in a corner. The tool should have an included angle of 60 degrees and the tip should have a radius of about 1.5mm. For steel, the top rake should be about 10 to 12 degrees; for brass, a top rake of two degrees is suitable. The round-nosed tool can be cranked or straight.

Boring tool

This is usually made from round material which is cranked and rough-ground before it is hardened. Because of the large overhang on this tool there is a possibility of tool ‘chatter’ (vibration) in use, so the tool shank should be fairly rigid. Grind the top rake to between 10 and 12 degrees for steel or two degrees for brass. If you are using a boring bar, grind the boring tool to shape from a piece of square section HSS and clamp it into the bar.

Recessing tool

This tool, also known as a parting-off tool, completes the basic set of tools. Use this tool for cutting off the work once it has been turned to shape or for machining grooves in the work. Compared to the other tools, the recessing tool is relatively fragile and you should be careful both during grinding and in use. If you do not maintain the side and front clearances, the tool is likely to jam in the work. You can buy recessing tools with special holders which require very little grinding to obtain the correct tool profile.

The differences in the two recessing tools you will need are again related to the top rake. For use with continuous chips, a top rake of about five degrees is needed; it sometimes helps with chip clearance if a radius is ground on the top of the tool just behind the cutting edge. You do not need a top rake for discontinuous chips.

Tool posts

The tool post of a centre lathe has several jobs to do. It has to hold the cutting tool on the top slide (or cross slide), allow adjustment of the tool so the cutting edge is at the correct height in relation to the work and permit adjustment of the tool so the cutting edge approaches the work at the correct angle.

To achieve the correct height of cutting edge, with most tool posts it is necessary to use packing strips or pieces. These are steel strips in various thicknesses which are known to have parallel faces; by selecting the correct combination of packing pieces, you can bring the tool point to the required height.

American type

This type of tool post is designed to eliminate the need for packing strips and to provide rapid height and angular adjustments. The tool fits into a slot and is clamped down onto a piece of metal which is curved along the bottom; this fits into a dished washer. When the clamp is slackened. The metal strip (called the ‘boat’ because of its shape) can be tilted to adjust the height and angle of the tool. The main disadvantage of this type of tool post is the tool overhang is often excessive and the top slide may collide with work or with the chuck jaws during use.

Spring-loaded clamp

This is a common type of tool post; a stud, which may be screwed or slotted into the top slide, carries a clamping plate supported by a spring. The tool and the necessary packing are fitted under the clamping plate; you adjust the plate so it is horizontal by means of a screw and nut which are adjacent to the main stud. When you have set the tool correctly, tighten a domed nut down onto the clamp plate. After much use the surface of the top slide may become indented because of the pressure from the packing and from the adjusting screw.

Box type

The box type of tool post carries the clamping equipment for the tool and the packing in the tool post itself. It provides good support for the tool and ensures no damage occurs to the face of the top slide.

Four tool turret

The most useful tool post, but one of the most expensive, is the four tool turret. Tools can be mounted simultaneously; as in the box tool post, once tools are mounted the turret is held down by a quick-action clamping device. After one tool has been used you release the clamp and swing the turret round so the next tool is brought into action and the turret can be reclamped down. The turrets are usually provided with positive locking devices so they can be securely clamped in as many as eight positions. Four tool turrets are particularly useful when doing repetition work; once the tools have been set up, the cycle of turning operations can be repeated many times over without resetting the tools.

Tool setting

You will find it is quite easy to set a lathe tool if you remember a few points. The tool point must be at the same height as the centre of the lathe spindle; the tool must be set so it is horizontal and so its length is at right-angles to the centre line of the work; and remember if the tool point is too far from its support in the tool post this is likely to cause tool chatter.

When making adjustments to avoid tool chatter take care when you reach the end of the cut that the work or the chuck does not collide with the top or cross slides.

To set a tool to the correct height, place a centre in the lathe spindle and set the tool height to the height of the centre’s tip.

Checking height

To check the tool point is at the correct height, place a piece of bar with a flat face into the chuck. Use the cross slide to move the tool so the point scribes a line on the face of the bar; rotate the chuck through 180 degrees and draw the tool point back along the bar face. If the two lines do not coincide, the tool is not set to the centre line of the spindle. You can set tools horizontally and at -right-angles to the work by eye, but you must check the height setting again if you change the other adjustments.

It is permissible to set the tool point just below the lathe centre height when using the recessing tool. This will minimize any tendency the tool may have to ‘dig in’; however the recessing tool must be at right-angles to the work being cut. If not set correctly, the tool is likely to bind on the sides of the groove it cuts.

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