METHODS OF INDEXING
1) Direct Indexing: Also called rapid indexing, is used making small No. of d ivisions. This can be performed in both plain and universal dividing head. The spindle and index
crank are connected by bevel gears. The required No. of divisions on the work is obtained by means of the rapid index plate generally fitted to the front end of the spindle nose.
The plate has 24 equally spaced holes, into any one of which a spring loaded in is pushed to lock the spindle with the frame. While indexing, the pin is first taken out and then the spindle is rotated by hand, and after the required position is reached, it is again locked by pin. when the plate is turned throughout the required part o a revolution, the dividing head spindle and the work are also turned through the same part of the revolution.
With a rapid index plate having 24 holes, it is possible to divided the work into equal divisions of all factors of 24 i.e.
2,3,4,6,8,12,24
Rule:
No. of holes = No. of holes in the direct index plate to be moved No. of divisions required
Q) Find out the index movement required to mill a hexagonal bolt by direct indexing.
Ans. No. of holes to be moved = 24/6 = 4
After machining one side of the bolt the index plate will have to be moved by 4 holes for 5 times to machining the remaining 5 faces of the bolt.
Video 10
2) Simple Indexing: Also called plain indexing, is more accurate and suitable for numbers beyond the range of rapid indexing. The bevel gears are replaced by a worm and worm wheel. The shaft carrying the crank has a single threaded worm and it meshes with the worm wheel on spindle having 40 teeth. 40 turns of crank are necessary to rotate the spindle thro' one revolution,
i.e one complete turn of the index crank
Rule:
Index crank movement = 40/N,
where N = No. of divisions required.
If the crank movement obtained from the formula is a whole No. the index crank should be rotated equal to the whole No. derived. If the crank movement obtained from the above formula is a whole No. and a fraction then, the numerator and denominator of the fraction are multiplied by a suitable common No. which will make the denominator of the fraction equal to No. of holes in the index plate. The new numerator now stands for the No. of holes to be moved by index crank in the hole circle derived from denominator, in addition to the complete turns of crank.
Eg: Index plates- 12, 14, 16, 18, 21 hole circles etc.
Q) Set the dividing head to mill 30 teeth on a spur wheel blank. Use 21 hole index plate.
Ans.
Index crank movement
= 40/30 = =
=
Thus for indexing, one complete turn and 7 holes
in 21 hole circle of the index plate will have to be moved by the index crank, if 21 hole plate is selected. This can also be performed with 18 hole plate [ ] or 24 hole plate [ ]
also.
Video 11
3) Compound indexing:- The indexing method is called compound due to the two separate movement of the index crank in two diff. hole circles of one (same) index plate to obtain a crank movement not obtainable by plain indexing.
4) Differential Indexing: The differential indexing may be considered as an automatic method (mechanization) of performing compound indexing. Here the Index crank is connected to milling machine feed rod through a set of gears to get continuous rotation for spindle for making helical grooves as shown.
Video 12
Setting of universal dividing head for spiral or helical grooves
TIME ESTIMATION TIME ESTIMATION
1. Time required per cut = L / (f x N) = L / f m L = L1+ ATT
L1=Length of W.P ; ATT = Added Table Travel
2. Total Milling time= Time per cut x No. of cuts(or) Indexing 3. Cutting speed, V =
πDN
/ 1000; D = Cutter Diameter4. Feed per tooth. f t = f / Z = f m/ NZ, Z = No. of teeth 5. MRR = Wdf m ; d = depth of cut; W= Width of WP
Calculation of ATT: Operations performed on the milling machines are done by peripheral cutters / slab cutters/ side and face cutters (Horizontal M/c) and face cutters or end mills (Vertical M/c).
a) For Peripheral / Slab Cutters / Side and Face Cutters
∆
= Clearance at entry /exitFRONT VIEW
FRONT VIEW General Case
ATT calculation neglecting clearance
(v) Maximum uncut chip thickness =
(vi) Average uncut chip thickness =
(vii) Peak to valley height for surface roughness = (viii) Effective no. of teeth cutting at same time = (ix) Mean Tangential Force =Fmt = K d f m W /
π
DNK = Material Constant (x) Mean Cutting Power = Fmt V
i) Tool fully engaged, Roughing Pass – doesn’t require “Full Wipe”
b) For Face Cutters/End Mills
ii) Tool fully engaged, Finishing Pass – requires “Full Wiping ” (Single pass feed) iii) Tool not fully engaged with W<D/2;
iv) Tool not fully engaged
but W ≥ D/2;
Offset CasesSpecial Cases
General Case
W/2
AL L1 OT
BROACHING BROACHING
Broach is bar type cutter with series of cutting edges gradually increasing in size to remove all materials in one stroke. In broaching there is only one motion, i.e. the primary cutting motion is provided by the machine, where
as the feed is obtained by placing the teeth progressively deeper. Since there is no f eed motion, the shape of the broach determines the shape of the machined part.
Broach is used to produce internal forms like spline holes, non-circular holes, slots, grooves, gears etc. Internal broaching is done by either pulling (or) pushing the broach
through a hole drilled in the work piece. Pulling is highly preferred to facilitate alignment and avoid buckling.
Video 1
Similar external forms can also be produced by using pot broach. Here the broach is made in segments and fixed
inside a fixture called pot fixture. The broach is stationary but the W.P. is pulled / pushed through it.
Internal broaches Pot broach
GRINDING GRINDING
Introduction: Grinding is the process of removing excess material by the abrasive action of a rotating wheel on the surface of the work piece. It is basically a finishing process employed to produce high accuracy and surface finish. The grinding wheel consists of sharp crystals called abrasives held together by suitable bonding. Natural abrasives available in nature include sand stone (natural silica), diamond, corundum and emery (natural alumina). Artificial abrasives are free from impurities and possess better performance properties. They include Al2O3, SiC, CBN etc.
Natural Silica
White Al Oxide
Corundum Emery
SiC CBN
The various bonding materials used are:
a) Vitrified bond (V) – It is made of clay and feldspar (rock forming mineral in earth’s crust). This is the strongest bond of all and is not effected by water/oils/acids. Vitrified bond is suitable for high stock removal even at dry condition. It cannot be used where mechanical impact or thermal variations are likely to occur. This bond is also not recommended for very high speed grinding because of possible breakage of the bond under centrifugal force.
b) Resinoid bond (B) – It is a synthetic thermosetting resin (phenolic resins) that becomes hard after heating. This occupies next place to Vitrified bond. Conventional abrasive resin bonded wheels are widely used for heavy duty grinding because of their ability to withstand shock load.
c) Silicate bond (S) – It is made of sodium silicate and is used for operations that generate less heat.
d) Rubber bond (R) – It is made of synthetic or natural rubber. Its principal use is in thin wheels for wet cut-off operation. They are denser than resinoid bonds but are less heat resistant. Rubber bond was once popular for finish grinding on bearings and cutting tools. They are also used for making regulating wheels in centreless grinding.
e) Shellac bond (E) – This is also an organic bond and has considerable strength. This not suitable for heavy duty work.
At one time this bond was used for flexible cut off wheels. At present use of shellac bond is limited to grinding wheels
engaged in fine finish of rolls.
Vitrified Bond Resinoid Bond
Rubber Bond
Parting Wheels Shellac Bond
Silicate Bond
Rubber Bond
Regulating Wheels