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## 26742Re: [GrizHFMinimill] RE: Rotary Table Book or PDF

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• Sep 19, 2013
I 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.
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