Lathe Tools Used in Machining Process

In the metal working industry work pieces of different shapes, dimensions and of different materials are made by the various working processes. These processes fall into two groups, i.e. non-cutting shaping and cutting shaping. The example of non-cutting shaping are forging, pressing, drawing, etc. and cutting shaping are turning, facing, drilling, milling, etc. In this process, finished surface of desired shape and dimensions is obtained by removing a layer from the parent metal in the form of chips.

The process of metal cutting in which chip is formed is affected by relative motion between the work piece and hard edge of a cutting tool held against the work piece. These cutting tools are basically of two types, i.e. single point cutting tools and multi point cutting tools. Lathe tools are generally single point cutting tools.

Classification of Lathe Tools

Single point cutting tool are classified under the following group.

(a) According to the Method of Manufacturing.

(i) Forged tool

(ii) Tipped tool

(b) According to the Method of Holding.

(i) Solid tool

(ii) Tool  bit  inserted  in  the  tool holder

(c) According to the Operations.

(i) Turning tool

(ii) Chamfering tool

(iii) Thread cutting tool

(iv) Grooving tool

(vi) Forming tool

(vi) Boring tool

(vii) Internal thread cutting tool     

(viii) Parting off tool

(ix) Facing tool                     

(d) According to the method of applying feed.

(i) Right hand

(ii) Left hand

Elements of a Lathe Tool

The following are the main elements of a lathe tool.

(a) Shank - It is that portion of the tool bit which is not ground to form the cutting edge and is rectangular in cross section.

(b) Base - It is the underside of the shank.

(c) Heel - The heal of a single point tool is the lowest portion of the cutting and end cutting edge.

(d) Face - The face of the cutting tool is that surface against which the chip slides upwards.

(e) Nose - The nose of a tool is the conjunction of the side and end cutting edge.

(f) Flank - The flank of a cutting tool is that surface which faces the work piece.

(g) Cutting Point - It is that part of the tool which is shaped to produce cutting edges at the face.

Types of Lathe Tools

In a lathe work different application require different tools which are described in subsequent paragraphs.

(a) Facing Tool - The facing tool removes metal by its side cutting edges. So no top rake is necessary in facing tool.

(b) Roughing Tool - The main function of a rough turning tool is to remove maximum amount of metal in minimum time. This depends upon the rigidity of tool, work, and the machine. The cutting edge is so ground that it can withstand maximum cutting pressure.

(c) Finishing Tool - A finish turning tool is used to remove small amount of material. The tool angle is so ground that it can produce a very smooth and accurate surface.

(d) Round Nose Tool - A round nose tool may be fed from left to right or from right to left hand end of the lathe bed ways. For this reason they have no back rake angle and side rake. In some cases a small back rake may be provided on a tool. A round nose turning tool is generally used for finish turning operation.

(e) Parting off Tool - A parting off tool is usually forged and may be used as bits for cemented carbide tipped tools. Parting off tool is made as narrow as possible to remove minimum amount of metal. The width of the cutting edge ranges from 3 to 12 mm. The length of the cutting tool which penetrates into the work should be slightly longer than the radius of the bar stock being machined. Clearance is provided all around the tool cutting edge to prevent rubbing against the work surface.

(f) Radius Tool - Turning curved profile may be affected by using ordinary lathe tools, flat forming tools and circular forming tools. Simple forming tools having their cutting edges ground to the shape of the groove, under cut. There is no front rake provided but sufficient front clearance angle is given and it range from 100 to 150. Regrinding is always done on the top face of the tool which does not alter the shape of the tool.

(g) External Threading Cutting Tool - Metric, English, American  ‘V’ threads, Acme and Square external threads are formed by a single pointed thread cutting tool with its cutting edge ground to the shape of the thread to be cut. The shape of the tool is determined by the inclined angle at the nose of the tool which should correspond to the angle of the thread.

(h) Internal Thread Cutting Tool - The cutting edge of the tool is exactly similar to an external thread cutting tool but the front clearance angle is sufficiently increased as in boring. The tool may be ground on a solid high speed steel bar or bit type brazed on a mild steel bar.

(i) Boring Tool - A boring tool is similar to a right hand external turning tool. The tool may be a bit type inserted or brazed in boring bar or holder, or ground to the shape HSS bar.

Angle (Geometry) of a Single Point Tool

Turning, facing and profiling tools come in both right hand and left hand versions. Although, the orientation of cutting elements necessarily varies with these applications, the basic requirement on their geometry remains the same. The various tool angles are explained in the following paragraphs:

(a) Side Rake Angle - It is the angle between the base of the tool shank or holder and the face of the tool measured in a plane perpendicular to the plane through the side cutting edge and at right angles to the base. This angle gives slope of the face of the tool from cutting edge. The side rake angle is negative if the slope is towards the cutting edge and is positive if the slope is away from the cutting edge.

(b) End Relief Angle - It is also known as front rake angle. It is the angle between the position of the end flank immediately below the end cutting edge and a line perpendicular to the base of the tool, and measured at right angle to the end flank.

(c) Side Relief Angle -  It is angle between the position of the side flank immediately below the side cutting edge and a line perpendicular to the base of the tool, and measured at right angle to the side flank.

(d) Side Cutting Edge Angle - Side cutting edge angle, is also known as lead angle, is the angle between the side cutting edge and side of the tool shank. The complimentary angle of side cutting edge angle is called the approach angle.

(e) Back Rake Angle - It is angle between the face of the tool and the base of the shank or holder, measured in plane through the side cutting edge and at right angles to the base. This angle is positive, if the tool faces downwards from the point towards the shank and is negative if the side cutting edge faces upwards towards the shank. Positive back rake angle takes the chips away from the machined surface, where as negative back rake angle directs the chips on to the machined surface.

(f) Nose radius - Nose radius is favorable to long tool life and good surface finish. A sharp point on the end of a tool is highly stressed, short lived and leaves a groove in the path of cut. There is an improvement in surface finish and permissible cutting speed as nose radius is increased from zero value. Too large a node radius will induce chatter.

(g) Positive Rake - A tool has a positive rake when the face of the tool slopes away from the cutting edges and slants towards the back or side of the tool. In most cases, tools are provided with a positive rake.

 (h) Negative rake - A tool has a negative rake when the face of the tool slops away from the cutting edge and slants upwards towards the back or side of the tool. Negative rake is used in turning metal with heavy cuts in mass production work. Turning with a negative rake angle has the following advantages and limitations.

(i) Advantages - The following are the advantages of negative rake:

(aa) It has more strength permitting to take heavier depth of cut

(ab) Cutting force gives a compressive load on the tip. The negative rake tools due to its shape can withstand  these loads easily.

(ac)  It can work against a very high cutting speed. (ad)  It decreases tool wear and increases the tool life.

(ii)        Limitation.

(aa) High Speed: The machine needs to be operated at high speed to take full advantage of negative rake tools.

(ab) Rigidity of the Machine:  The machine and the tool holding devices must be sufficiently rigid to resist vibrations that may set up in the machine when it runs at high speed.

Commonly Recommended Angles of Turning Tools - The following table shows the different angles required on a turning tool for turning various materials:

Tool Angles

Material to be Machined	Top Rake in deg	Side Rake in deg	Front Clearance in deg	Side Clearance in deg	Nose Radius in mm
Mild Steel	0 to 7	6 to 7	5 to 10	5 to 8	0 to 1
Medium Carbon Steel	0 to 7	6 to 7	5 to 10	5 to 10	0 to 1
Brass	0 to 5	8 to 5	6 to 8	6 to 8	0.3 to 0.5
Cast iron	0 to 7	6 to 7	5 t0 8	5 to 8	0.5 to 1.1
Aluminium	0 to 10	10 to 20	6 to 10	6 to 10	0.5 to 1.5

Chip Breaker

Under normal conditions with high speed tools and comparatively low cutting speeds the continuous not much dangerous if these are controlled and directed by suitable front and side rakes. If the turning operation is carried out at high speed, then the hot sharp edged chips are very dangerous. It becomes necessary break these chips during the operation otherwise it will restrict the free tool cutting action as well as spoil the surface finish.

The chip breakers are designed on the cutting edge of the tool. These are of the following types:

(a) Groove Type - It consist of grinding a groove on the face of the tool, behind the cutting edge, leaving a small land near the tip.

(b) Step Type - It consist of grinding a step on the face of the tool, adjacent to the cutting edge.

(c) Clamp  Type - This type of chip breaker is very common with carbide tipped tools. The chip breaker is a thin and small plate is either brazed to or held mechanically on the tool face.

Tool Setting and Effect on Job

For successfully performing any type of turning or facing operation on a lathe it is essential that  the  tool should be  held at correct height during operation. For this, correct setting of the cutting tool is very necessary. The correct setting is that the tool nose should be exactly at the centre height of the work piece. If the tool is set even slightly above or below the centre line, its front rake angle will change. If the tool is set above the centre height, the front rake angle is reduced and the tool, instead of cutting, will rub against the work piece. If it is set below the centre height, the front rake angle will increase as a result the tool will dig into the surface of the work piece. If it is set at the centre a neat and clear cutting action will take place which will provide good finished surface.

Built-up Edge - The built up edge may occur when cutting soft material, soft ductile material. The built up edge is formed on the cutting edge of the tool where highly deformed material particles are welded at the tool tip. This causes due to affinity of the material to the tool while machining. The built up edge occurs due to the following reasons:

(a) Ductile material

(b) Low speed

(c) Insufficient cutting lubrication

(d) Affinity between the tool and the chip material

(e) High pressure and friction at tool face.