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Hi: A dividing head is used for accurately rotating workpieces on a milling machine when making repetitive, evenly spaced (usually) cuts, like gear teeth or flats on a nut. It is usually mounted in the t slots of the bed of a milling machine. The workpiece is often mounted on a mandrel which is supported at one end by a chuck or center (in the indexing head), and the other by a footrest, like a center in a tailstock of a lathe. Head and footstock must be accurately in line or the accuracy of your work will be non existent. The numbered,holed,plates are used to accurately keep track of the turning of the handle on the indexing head. The handle is usually at a 40:1 gear ratio to the chuck that holds the mandrel or workpiece under the milling machine cutter. Therefore, if you make one turn of the handle (clockwise), the workpiece will rotate 1/40 or a turn. You can accurately cut 40 sides on your workpiece by doing this. But, you say, I want to cut 10 sides, not 40 sides. In that case, turn the handle 4 times. 40/10 =4. 8 sides, 5 turns, 40/8=5.See how it works? You can usually come up with any number of sides you need with the formula: Required turns of handle = 40/n, where n = number of evenly spaced cuts you wish to make. This only works if you have a 40:1 ratio. Wide range dividing heads usually have a finer 80:1 ratio or more, which gives them more possible indexing combinations. Mark your indexing head and turn the handle until the mark rotates around fully to the position in which it started, The number of turns required to make one complete revolution will tell you your gear ratio. Put the resulting number in the above formula instead of 40. Say you want to make a six sided nut. For a 40: 1 dividing head: Turns=40/n = 40 /6 = 6.66666 66....or 6 and 2/3 turns. Whoa, you say, how do I make 2/3 of a turn then? Easy. If you do the reverse of reducing a fraction to lowest terms, you will eventually come up with a fraction that has a denominator that is the same as the number of holes in one of the circles on one of your plates. (????? ????) Say, for arguments sake, that the largest number of concentric holes on your plate is 18. ( I know it probably is much higher than that but bear with me.) 2/3 will equal what out of 18? 12. 12/18 is the same as 2/3. As I said, it's basically like doing the opposite of reducing a fraction to lowest terms. So, if you set the pin in the handle to run in the circle with 18 holes in it,and you turn your handle 6 full turns, plus 12 out of 18 holes, you will have turned the workpiece 1/6 of a complete turn under the milling machine cutter. If you put the pin in the circle with 36 holes, it will be 6 full turns + 24 out of 36 holes. Or 6 full turns + 36 out of 54 holes. They will all turn the workpiece 1/6 of a turn. I was taught to always use the circle with the largest # of applicable holes (54 in this case). I don't know why. Instilling a work ethic perhaps. Usually, the indexing head will also have two little sector arms under the handle, just over the plate. Space them apart the correct distance so you don't have to count holes every time. Just turn your handle the correct number of full turns, then forward till it reaches the arm spaced the correct number of holes forward (i.e. 12/18 holes for the above example).Do NOT include the hole that the pin is currently in when setting the spacing of the sector arms. Then after you put the pin in the hole in the plate, rotate the arms forward (clockwise)to indicate the hole the pin should end up in after the correct number of turns have been completed. Always push the arm farthest from the pin clockwise toward the pin. If you push the arm closest to the pin, the distance between the sector arms may decrease and lead to indexing errors as you progress through the job. This may be hard to visualize I know, and your head may be constructed differently than the ones I have used, but that is what a dividing (indexing) head does. It allows you to accurately rotate a workpiece on a milling machine to position it for a cut. I am sure there are many others out there who can explain in more depth the intricacies of indexing. The real fun begins when you come up with a fraction that doesn't correlate to any of the numbered circles, or you want to cut a spiral flute or a helical or hypoid gear!
Experiment a bit. You will soon get the hang of it.
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