Surface Gauges-Types of Chisel-Fitter online Note

1)-Surface gauges 

i)- The surface gauge is one of the most common marking tools.

It is used for:- 

a)- A scribing lines parallel to a datum surface, 

b)- Setting jobs on machines parallel to a datum surface, 

c)- Checking the height and parallelism of jobs, setting jobs concentric to the machine spindle.

Types of surface gauges:-

Surface gauges/scribing blocks are of two types a)- fixed and b)- universal. 

a)- Surface gauge-fixed type:-

i)- The fixed Type of surface gauge consists of a heavy flat base and a spindle, fixed upright, to which a scriber is attached with a snug and a clamp nut.

b)- Universal surface gauge:-

i)- This types of surface gauge The spindle can be set to any position. 

ii)- Fine adjustment can be made quickly. 

iii)- Can also be used on cylindrical surfaces.

Parts of the Universal Surface Gauge 

Base 

i)- The base is made of steel or cast iron.

ii)- Its 'V groove at the bottom person. 

iii)- The 'V' groove helps to seat on circular work.

iv)- The guide-pins fitted in the base, are helpful for scribing lines from any datum edge.

Rocker arm 

i)- The rocker arm is attached to the base along with a spring and a fine adjustment screw. 

ii)- This is used for fine adjustments. 

Spindle 

i)- The spindle is attached to the rocker arm. 

Scriber 

i)- The scriber can be clamped in any position on the spindle with the help of a snug and a clamping nut. 

Care and maintenance 

i)- Clean before and after the use :- 

ii)- Apply thin layer of oil to the bottom of the surface base before using for marking. 

iii)- Sharpen the Scriber if necessary. 

iv)- Do not exert more pressure while marking

2)- Chisel

i)- A chisel is a hand cutting tool.

ii)- Its shape is hexagonal or octagonal bar of tool steel.

iii)- Its one end is shaped for the purpose of cutting and other end to receive the impact of hammer strick purpose used.

iv)- The length of chisel is 150 mm to 200 mm.

v)- Its used for chipping and cutting off operations.

vi)- Chipping is an operation of removing excess metal with the help of a chisel and hammer.

vii)- Chisels are made from high carbon steel or chrome vanadium steel.

viii)-  The chisel cutting edge is hardened and tempered.

Types of chisels:-

a)-Flat chisel, b)− Cross-cut chisel, c)− Half-round nose chisel, d)− Diamond point chisel, e)− Web chisel. 

a)-Flat chisel:-

 i)- They are used to remove metal from large flat surfaces.

ii)- Its chip-off excess metal of welded joints and castings.

iii)- It is 120 mm to 150 mm long.

b)- Cross-cut chisel:-

 i)- These are used for cutting key ways, grooves and slots.

ii)- It is also called Cap chisel.

iii)- The width of cutting edge is 5 mm to 9 mm.

c)− Half-round nose chisel:-

i)- They are used for cutting curved grooves (oil grooves).

d)− Diamond point chisel:-

i)- Is used for chipping sharp corners for making 'V'-shaped grooves.

e)− Web chisel:- 

i)- These chisels are used for separating metals after chain drilling.

ii)- Chisels are specified according to their − length, width of the cutting edge, type, and cross-section of the body.

Parts of chisel

i)- The parts of chisel is Head, body, point or cutting edge.

ii)- The continuous use of a chisel deforms the shape of its head and spreads out its end is called mushrooming head.

iii)- Do not used in mushrooming head chisel.

Angles of chisels:-

Point angles and materials: 

i)- The correct point/cutting angle of the chisel depends on the material to be chipped.

ii)- Sharp angles are given for soft materials, and wide angles for hard materials. 

iii)- The correct point and angle of inclination generate the correct rake and clearance angles.

Rake angle: 

Rake angle is the angle between the top face of the cutting point, and normal (90°) to the work surface at the cutting edge.

Clearance angle:- 

i)- Clearance angle is the angle between the bottom face of the point and the tangent to the work surface originating at the cutting edge.

ii)- If the clearance angle is too low or zero, the rake angle increases. The cutting edge cannot penetrate into the work. The chisel will slip.

iii)- If the clearance angle is too great, the rake angle reduces. The cutting edge digs in and the cut will become deeper and deeper.

Crowning:- 

i)- A slight curvature is ground called “Crowning” to the cutting edge of the chisel, to prevent digging of corners, which leads to breakage of chisel point. 

ii)- Crowning” allows the chisel to move freely along a straight line while chipping.

Chisel cutting angle of material:-

Material to be cut (High Carbon Steel)- Point angle Angle (65°)  

Material to be cut (Cast iron) - Point angle Angle (60°) 

Material to be cut (Mild steel)-  Point angle Angle (55°)  

Material to be cut (Brass)-  Point angle Angle (50°)  

Material to be cut (Copper)-  Point angle Angle (45°) 

Material to be cut (Aluminium)-  Point angle Angle (30°)