- Sep 19, 2013I will post this information assuming that any interested readers will

know that a dividing plate replaces the degree/minute dial and seconds

vernier scale under the handle of a typical rotary table, and that they

will also understand that there are adjustable sector arms over the

dividing plate which can be easily set to select any set number of holes

in any selected circle, so doing divisions becomes very quick and easy

to do repeatedly.

Book Title:

Gears And Gear Cutting' by Ivan Law, #17 of the Workshop Practice Series

- - - - - Excerpt from Chapter 8, Dividing Heads, page 63 - - - - -

The author never likes to have to rely solely on tables or data charts

for information as they usually get torn or covered with oil and dirt,

making them difficult to read. It is always prudent to be able to work

things out for oneself. There is nothing magical about determining the

number of holes or which circle to use in the dividing plate, as the

following example shows.

Supposing that one wishes to cut a gear having 33 teeth using a

dividing head with a worm and wheel ratio of 60:1. The first step is to

divide the number of divisions required into the worm and wheel ratio -

in this case 60 divided by 33. This gives an answer of 1-27/33, which

indicates that each division is one complete turn of the handle plus a

further addition of 27/33 of a turn. The 27/33 of a turn can be achieved

by using 27 holes in a 33-hole circle of the division plate.

If a 33-hole circle is available then the problem is solved, but it is

unlikely that a 33-hole circle will be on the division plate so the next

step is to look at the fraction 27/33 and see if it can be factorized.

And the answer is, yes it can as both top and bottom are divisible by 3,

resulting in a new fraction of the same value 9/11. If the division

plate does not have a 33-hole circle then it certainly will not have an

11-hole one but it is most likely to to have a circle that is a multiple

of 11 and this will in all probability be a 77-hole circle. If we now

return to our 9/11 fraction and multiply both top and bottom by 7 we

arrive at 63/77. The division for producing a 33-tooth gear is therefore

one complete turn of the handle plus 63 holes on a 77-hole circle of the

division plate. Once this example is understood it is a simple matter to

substitute any division as required.

The occasion may arise when the division plate does not possess a

circle with the requisite number of holes. This is a most unlikely

occurrence as the plates are carefully planned to give a very wide range

of divisions but, if a makeshift division plate has to be made do not

worry about positioning the holes to very fine limits as any error in

the division plate is not passed on directly to the workpiece, but is

divided by the wormwheel ratio so the actual error becomes very tiny indeed.

- - End Excerpt - -

A lot of people are going to buy the increasingly common 72:1 ratio

rotary tables (R/T's) that either do not come with any dividing plates,

or in my case, only comes with one dividing plate when more than that

would be world's better.

With that said, looking around at more expensive R/T's with 72:1

ratios, I found that the best ones come with 3 dividing plates, A, B,

and C. They have hole circles as follows:

A Plate - 15,16,17,18,19,20

B Plate - 21,23,27,29,31,33

C Plate - 37,39,41,43,47,49

Save this message if you have a rotary table that goes 5 degrees per

full revolution of the handle. That would be 0 through 4, and then back

to 0 again making it 5 degrees. A 72:1 worm wheel ratio. Also, a lot of

people are going to have to search way back in memory to when they last

had to work in oddball fractions outside of the usual inch system. For

folks used to metric that may be an extra long time. :-)

To repeat the above 33-tooth gear example on a 72:1 R/T with the above

A, B and C dividing plates, that would be:

72 divided by 33 = 2.181818182 on the calculator.

So you know that is 2 full turns, mark that down and subtract 2 from

the reading on your calculator to leave only the decimal portion

showing. To turn that decimal remainder into a whole remainder, multiply

it by the same divisor just used above, in this case 33.

.181818182 X 33 = 6, as in 6/33 of a turn left over. (Always round to

nearest whole number when necessary, like when a result shows .999999999

round up, or .00000006 then round down, etc,).

So each division would be 2 full turns plus 6 holes in the 33 hole

circle on plate B. Set up the sector arms so they leave 6 holes open

between them and adjust the arm plunger to engage with the holes in the

33-hole circle. 2 full turns plus 6 holes, 33 times and done. Using a

dividing plate is easy - repetitious but easy - as long as you simply

keep track of where you are. Mainly that means don't get side tracked or

interrupted to where you forget what you were doing. Turn off the phone,

etc. :-)

How about making a dial with 40 divisions like on the cross slide of

the mini-lathe:

72 divided by 40 = 1.8, so there is 1 full turn plus

.8 times 40 = 32, as in 32/40 of a turn left over.

There is no 40 hole circle, but 32/40 can be factored down by dividing

both top and bottom by 2, so 32/40 becomes 16/20. There IS a 20 circle

hole on plate A above.

So 1 full turn plus 16 holes in the 20 hole circle. Set up the sector

arms and have at it, 20 times and done, although in making the dial you

would probably make some indicator marks longer than others, like an

extra long mark every 10th and medium long every 5th as per most dials.

Make up some more divisions to do as practice and you should hopefully

find this gets easy pretty fast. If you have an R/T with dividing plates

and a table of divisions you can double check your math that way too.

Now somebody bright is going to want to do 25, 50 or 100 divisions and

quickly find they need a 50 or 25 hole circle. Time to make a new hole

circle in another dividing plate! That would be another post, too.

Reread the last paragraph in the excerpt above. :-)

The two circles in my LMS 1810 rotary table's sole dividing plate are

15 and 28. They cover a wide range but skip way more than I'd like. The

3 plate set as per above would be much better, along with an additional

plate with a 25 hole circle. (There are reasons why 40:1 and 60:1 ratios

used to be so much more common. but if people can get little mini-mill

sized 72:1 R/T's cheap then all of the above becomes important.)

Cheers,

John Z. - << Previous post in topic Next post in topic >>