Indexing of Milling Machine

Indexing of Milling Machines Indexing is the process of evenly dividing the circumference of a circular work piece into equally spaced divisions. It is used in cutting gear teeth, cutting splines, milling grooves in reamers and taps, and spacing holes on a circle.

Indexing or Dividing head The indexing head of the indexing fixture contains an indexing mechanism which is used to control the rotation of the index head spindle to space or divide a work piece accurately. A simple indexing mechanism consists of a 40 teeth worm wheel fastened to the index head spindle, a single – cut worm, a crank for turning the worm shaft, an index plate and a sector.

Since there are 40 teeth in the worm wheel, one turn of the index crank causes the worm wheel, and the index head spindle to make 1/40 of a turn. So, 40 turns of the index crank revolve the spindle one full turn.

Index Plate The indexing plate is a round plate with a series of six or more circles of equally spaced holes. The index pin on the crank can be inserted in any hole in any circle. With the interchangeable plates regularly furnished with most index heads, the spacing necessary for most gears, bolt heads, milling cutters, splines can be obtained.

Brown and Sharp type set of index plates consists of 3 plates of 6 circles. Plate – I is having 15, 16, 17, 18, 19, 20 holes. Plate – II is having 21, 23, 27, 29, 31, 33 holes. Plate – III is having 37, 39, 41, 43, 47, 49 holes.

Cincinnati type consists of one plate drilled on both sides with circles divided as follows. First side – 24, 25, 28, 30, 34, 37, 38, 39, 41, 42, 43 holes. Second side – 46, 47, 49, 51, 53, 54, 57, 58, 59, 62, 66 holes.

To Mill a Hexagon, divide 40 by 6, which equals 6 2/3 turns, or six full turns plus 2/3 of a turn on any circle whose number of holes is divisible by 3. Therefore, six full turns of the crank plus 12 spaces on an 18-hole circle, or six full turns plus 26 spaces on a 39-hole circle will produce the desired rotation of the work piece.

Direct indexing Direct Indexing Divisions Direct indexing plate usually contains three sets of hole circles or slots: 24, 30, and 36. Number of divisions possible to be indexed are limited to numbers that are factors of 24, 30, 36. Slots Direct indexing divisions 24 2 3 4 6 8 12 24 30 2 3 5 6 10 15 30 36 2 3 4 6 9 12 18 36

To mill eight flutes on a reamer blank Since the 24-hole circle is the only one divisible by 8 (the required number of divisions), it is the only circle that can be used in this case. Slots Direct indexing divisions 24 2 3 4 6 8 12 24 30 2 3 5 6 10 15 30 36 2 3 4 6 9 12 18 36 Never count the hole or slot in which the index pin is engaged.

Simple (Plain) indexing The indexing required to cut eight flutes The indexing required to cut seven flutes The five-sevenths turn involves use of an index plate and sector arms.

Choose any hole circle that is divisible by denominator 7 Indexing for Seven Flutes Index-plate hole circles Brown & Sharpe Plate 1 15-16-17-18-19-20 Plate 2 21-23-27-29-31-33 Plate 3 37-39-41-43-47-49 Cincinnati Standard Plate One side 24-25-28-30-34-37-38-39-41-42-43 Other side 46-47-49-51-53-54-57-58-59-62-66 Choose any hole circle that is divisible by denominator 7 5/7 = /21 15 So, 5 full turns plus 15 holes on 21 hole circle.

Angular indexing Indexing for 45º Indexing for 60º 5 complete turns 6 full turns plus 12 holes on 18 hole circle

Indexing for 24’ Indexing = 24’/540’ = 4/90 = 1/22.5 1 hole on a 22.5 hole circle The nearest is a 23 hole circle. Indexing would be 1 hole on a 23 hole circle with a slight error (approximately 1/2 minute). A need for higher accuracy requires differential indexing.

Convert 9° to minutes 9°x90’ = 540’ Calculate indexing for 24º30' (24 x 60') = 1440' now add 30' = 1470‘ Convert 9° to minutes 9°x90’ = 540’ Divide 1470'/540' = 2 13/18 2 full turns and 13 holes on 18 hole circle

Differential indexing Index plate must be revolved either forward or backward part of a turn while index crank turned to attain proper spacing (indexing). Change of rotation is effected by idler gear or gears in gear train. A = approximate number of divisions N = required number of divisions

If A is greater than N, resulting fraction is positive and the index plate must move in same direction as crank (clockwise). This positive rotation uses an idler gear. If N is greater than A, resulting fraction is negative and index plate must move counterclockwise. This negative rotation required use of two idler gears.

No 57 hole circle so select number close to 57 Indexing for 57 divisions The change gears supplied with the dividing head are as follows: 24, 24, 28, 32, 40, 44, 48, 56, 64, 72, 86 The available index plate hole circles are as follows: Plate 1: 15, 16, 17, 18, 19, 20 Plate 2: 21, 23, 27, 29, 31, 33 Plate 3: 37, 39, 41, 43, 47, 49 Choose plate 2: 21 holes 5/7 would be 15 holes on 21-hole circle No 57 hole circle so select number close to 57

The fraction is negative and simple gearing is to be used, the index plate rotation is counterclockwise and two idlers must be used.

For indexing 57 divisions, a 40-tooth gear is mounted on the dividing head spindle and a 56-tooth gear is mounted on the worm shaft. Index idlers must be used. plate rotation is negative and two idlers are used. After proper gears installed, the simple indexing for 56 divisions should be followed.

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MILLING PROCESSES / OPERATIONS PLAIN OR SLAB MILLING END MILLING SLOT MILLING ANGULAR MILLING FORM MILLING STRADDLE MILLING GANG MILLING FACE MILLING

PLAIN OR SLAB MILLING

It is used for machining of flat surfaces which are parallel to the axis of the cutter. The cutters employing this type of process are hollow straight HSS cylinder of 40 to 80 mm outer diameter having 4 to 16 straight or helical equi-spaced flutes or cutting edges. They are used in horizontal arbor to machine flat surface.

END MILLING

They are commonly made of HSS. 4 to 12 straight or helical teeth on the periphery and face diameter ranges from about 1 mm to 40 mm. It is most versatile and widely used in vertical spindle type milling machines.

SLOT MILLING

FORM MILLING

It is the process of machining special contours composed of curved and straight line, or entirely of curves, at a single cut. This is done with formed milling cutters, shaped to the contour to be cut. The more common form milling operations involve milling half – round recesses and quarter round radii on the work piece. This is done by using convex, concave and corner rounding milling cutters ground to the desired circle diameter.

STRADDLE MILLING

For fast and accurate machining of two parallel vertical surfaces at a definite distance, two separate side milling cutters are mounted at appropriate distance on the horizontal milling arbor.

FACE MILLING

It is use for machining flat surfaces which are at righr angles to the axis of the cutter. The cutters employing this process are usually large in diameter (80 to 800 mm) and heavy. They are used only for machining flat surfaces in different orientations. Cutter is mounted directly in the vertical and / or horizontal spindles. They are generally used for high production machining of large jobs.

FACE MILLING Coated or uncoated carbide inserts are clamped at the outer edge of the carbon steel body.

GANG MILLING

In this type of operation, for quick production of complex contours comprising of a number of parallel flat or curved surfaces , a proper combination of several cutters are mounted tightly on the same horizontal milling arbour.

Angular Milling It is use for machining flat surfaces which are at an inclination to the axis of the cutter. A single angle milling cutter is used for angular surfaces such as chamfers an grooves. When milling dovetails, the usual angle of the cutter is 45, 50, 55 0r 60 based on the design of dovetails.