Maybe I'd better explain the maths after saying it's reasonably easy, it'll show why the dividing plates are useful...
Say you want to cut a 51-tooth gear, or 51 splines, or evenly space 51 holes on a bolt circle: 360/51 gives 7.05882 degrees, or 7 degrees, 3 minutes, 31.8 seconds - and your rotab measures to (f'rinstance) 1/10 of a degree - tricky to get it right!
Your rotary table has 5 degrees per turn of the handle, so dividing 360 by 5, you have a 72:1 ratio;
you rummage through your dividing plates, looking for something with a common factor with 51 - its factors are 17 and 3, so you want a multiple of 17 (the 3 is a factor of 72 so that's covered already) and you find a plate with a 34 hole circle ( 2 x 17) and fit that;
Next comes working out how many holes on the plate for each cut, again fairly simple!
You picked the 34 because it had a factor of 17, which you've used, the remaining factor is 2, so that goes in the pot;
The table's ratio is 72, and you've used the factor of 3 in it, leaving a factor of 24 - into the pot and multiply by the 2, giving 48 holes "but I've only got 34 holes!" you cry!
48 holes is a full turn plus another 14 holes, so set the dividing sector arms with 14 SPACES between them - the first hole counts as 0 as you haven't moved from it, so you'll have 15 HOLES between the arms, then bring the "lagging" arm up to the detent pin, take a full turn and the fraction to the "leading" arm - you've done 48 spaces, exactly what you want! now make your cut, move the sector arms around again, take a full turn and a bit, next cut etc. etc. etc.
Because you're mechanically DIVIDING rather than guessing using the dial on the handle, your cuts will be in exactly the right places (important for gears or dials!) - if you don't have the right plates you can do much the same to make 'em, each time you go through the dividing process the accuracy will improve:
Say you need a 50-hole plate (maybe making a dial for a tailstock handwheel), 50 isn't in the set of plates, nor 25... So you make a rough plate (old CDs work well
) and approximate the 25 or 50 holes, then use that plate to divide another, more accurate one. Say one of your holes is 2 degrees off, the table is rotating through (e.g.) 1/72 the angle of the handle on the plate, so the resulting hole you're marking / drilling is now 1/36 degree off - you drill your plate, then swap that onto the handle side, do it again, the "inaccurate" hole is now 1/36 x 1/72 degrees off, 1/2592 = 0.000386 degrees off - probably good enough for Government work, unless it's for NASA, in which case you might want to do another round of dividing, to get to 1/186624th = 0.00000536 degrees...
This is pretty much how the first dividing plates (and, incidentally, threads) were made, repeated division of the errors, and why we don't need a "master plate" in a national standards lab' somewhere to compare our divisions against
Hope this helps, explains rather than confuses!
Dave H. (the other one) (who'll be setting up to cut some odd gears if he wins that mill on ebay - currently the highest Buddha)
