Fitting and Turning Level 4
Milling machine – Indexing calculations
INDEXING EQUIPMENT:
Indexing is done on the milling machine using a dividing head. The dividing head is a very important tool used to divide the circumference of a work piece into equally spaced divisions. This is important when milling gears, splines, hexagons and so on. The dividing headset consists of the headstock with footstock, chuck, center rest, index plates and change gears.
The chuck clamps the work piece when milling while the footstock supports the work piece on the opposite side of the dividing head when milling between centers.
The center rest supports long work pieces when milling between centers.
SIMPLE INDEXING:
Simple indexing is performed in the following manner.
Have the worm and worm gear engaged.
Calculate the indexing
Place the adjustable plunger into the required hole on the chosen index plate.
Adjust the sector arms to count the required number of holes.
Turn the index crank handle the number of full turns and part of a turn determined by the sector arms and put into the set hole
Move the sector arms and repeat the procedure until the required number of divisions or sides have been cut.
Two types of dividing heads have been developed namely Brown and Sharp as well as the Cincinnati.
Brown and Sharp: The dividing head id supplied with three index plates:-
Plate 1: 15, 16, 17, 18, 19 and 20 holes
Plate 2: 21, 23, 27, 29, 31 and 33 holes
Plate 3: 37, 39, 41, 43, 47 and 49 holes
The Cincinnati dividing head is supplied with only one indexing plate that is reversible:
Side 1: 24, 25, 28, 30, 34, 37, 38, 39, 41, 42 and 43 holes
Side 2: 46, 47, 49, 51, 53, 54, 57, 58, 59, 62 and 66 holes
It is also important to note that the crank handle must turn through 40 full revolutions before the work piece and worm wheel is turned through one revolution.
Out of this information we use the following formula for indexing.
Formula =40/N where N = the number of divisions or sides to be machined.
Let us do two examples. In one example the divisions is more than 40 (in which case there will be no full turns of the handle) and in the next the divisions is less than forty (in which case there will be some full turns of the handle.)
Example 1
We want to cut a gear with 44 teeth. In our formula N will be equal to 44 and we are using the Cincinnati dividing head.
Formula =40/N
=40/44 We can simplify this
=10/11 × 66/1 (on the Cincinnati plate 66 is the first number of holes dividable by 11)
= 60
Our full answer will now be: 0 full turns of the crank handle, 60 holes on a 66-hole circle plate.
Example 2
We want to cut a gear with 12 teeth on a Brown and Sharp dividing head.
Formula =40/N
=40/12 We can simplify this
=3 1/3 × 15/1 (on the Brown and Sharp plates 15 is the first number of holes dividable by 3)
=1/3 × 15/1 (we ignore the three and calculate only the fraction)
Type equation here.
= 5
Our full answer will now be: 3 full turns of the crank handle, 5 holes on a 15-hole circle plate.
ANGULAR INDEXING:
If we want to cut grooves or slots at an angle in a work piece. If we are working with angles it is important to remember that 40 full turns of the crank handle will0 rotate the work piece through only one full turn. (360°) To work out how many degrees the crank handle will turn the work piece in one full turn of the handle, we must divide 360° by 40. This will give us 9°.
The formula for indexing when working with degrees will therefore be N/(9°)
For example:- Using the Cincinnati head, calculate the indexing to cut a gear with 38° teeth.
Our Formula =N/(9°)
=38/9
=4 2/9
=2/9 × 54/1 (we ignore the 4 and 54 is the first number of holes dividable by 9)
= 12
Our full answer will now be: 4 full turns of the crank handle, 12 holes on a 54-hole circle plate.
DIFFERENTIAL INDEXING:
Sometimes it is impossible to calculate the indexing because the plates do not provide a number of holes that are divisible by the number of teeth or divisions. To overcome this the headstock can be attached to a number of interchangeable gears. The gears will in fact slightly change the number of turns the crank handle make for one full revolution of the work piece.
How do we do these calculations?
Step 1: Round of the divisions either up or down.
Step 2: make a note that, if you have rounded up, the plate must turn the same direction as the crank handle and if you have rounded down, the plate must turn in the opposite direction as the crank handle. (make use of an idler gear to change direction.)
Step 3: Use the following formulae: Indexing =40/n and Gear Ratio (n-N) × 40/n (Where N = Real number of divisions and n = rounded number of divisions.
The change gears supplied are the following: 2 of each 24, 28, 32, 40, 44, 48, 56, 64, 72, 86, 100
EXAMPLE:
Using the Cincinnati dividing head calculate the indexing for 99 divisions.
Round the divisions to 100
Indexing =40/n
=40/100
=4/10 × 20/1 (20 is the first divisible number of holes on Cincinnati plate)
= 8
Gear Ratio =(n-N) ×40/n
=(100-99) ×40/100
=1×40/100
=40/100
=4/10 ×10/10
=40/100
Our full answer will be: Indexing = 0 full turns of the crank handle, eight holes on a 20-hole circle plate with a gear ratio of 40/100 and the index plate turning in the same direction as the crank handle.
RAPIT INDEXING:
No difficult calculations needed. You want to cut a hexagon or a square
EXAMPLE:
Determine the indexing to complete six sides on you work piece.
For rapid indexing follow these steps:
Using the handle on the dividing head, disengage the worm from the worm wheel.
On the 12 slot plate, mark every second slot with chalk.
Place the plunger in the first marked slot and cut the first division.
Remove the plunger from the slot, turn the spindle by hand to the next marked
slot.
Engage the plunger and cut the next division.
Repeat the proses until all six divisions have been cut.
thx
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