NOTE:  This is Marvins readme for the Change program.   I (David
Forsyth) have modified the program to better suite the Myford ML7 and
have also attempted to find a way to preselect gear sets as the
program finds many combinations for some ratios.   My method is to
look at the ratios between the driven and driver gears.  The result of
this calculation is printed along with the gears.  Try to use the set
with the lowest ratio in the left column and the lowest error first,
they are more likely to fit than sets with higher ratios.

Note too that I've changed it to look at all gear sets instead of
terminating early, so for some ratios you will get sets with  only 3
pairs, and others will produce 1 2 AND 3 pair sets.   Just choose a
set with a low ratio in the left column AND a low error (often zero in
this case).

Sets of gears need only be placed with 'drivers' driving and driven
gears being driven. The actual order is not important, just select
pairs so that they fit on the banjo. So long as the correct ones are
driving, the ratio will be correct.  Always cross check by threading
(just a surface scratch is needed) some scrap and measuring the pitch
if at all possible.

A further modification is the addition of 2 command line options.
/m   forces the input to be in millimeters pitch instead of TPI

I did this since I mostly use it to figure metric ratios.  

By the way, if you are not using long nuts, 25 tpi works fine for M6
and is much faster to cut than 25.4tpi (or 1mm pitch).  For high
strength applications though, use the correct pitches.

An example:

the command was
change /m

the result:
CHANGE GEAR CALCULATIONS
   change /c /m tpi | /?
      V2.2.daf
Number of change gears read = 14
Effective lead screw pitch = 8.000000 tpi
METRIC Thread pitch to cut [1.000] (mm): 1
Using pitch of 25.400 tpi
Allowable thread pitch accuracy [0.020] (%):
Required gear ratio (leadscrew/spindle) = 0.314961
leadscrew turns SLOWER than spindle
   Use gear set with lowest ratio count at left
--
.......
Data - unique gear combos in order found
 5.417 :: Drivers 12 30 35   Driven 20 40 50  ratio 0.315000 error 0.012500%
 7.500 :: Drivers 12 35 45   Driven 20 40 75  ratio 0.315000 error 0.012500%
 6.750 :: Drivers 12 35 45   Driven 20 50 60  ratio 0.315000 error 0.012500%
 6.150 :: Drivers 12 35 45   Driven 25 40 60  ratio 0.315000 error 0.012500%
 5.417 :: Drivers 12 35 45   Driven 30 40 50  ratio 0.315000 error 0.012500%
 7.333 :: Drivers 12 45 70   Driven 40 50 60  ratio 0.315000 error 0.012500%
 3.375 :: Drivers 30 35 45   Driven 40 50 75  ratio 0.315000 error 0.012500%

As you can see, the last line has the lowest count at the left and is
more likely to fit the banjo than others, though some of the others
WILL fit if needed. You'll need to adjust the 'change.dat' file
according to Marvins instructions below to suite your gear set.

Some of Marvins comment sbelow are now false.  For instance, I've
changed the program so that it only prints out unique gear sets,
instead of printing all low error ones.  It also prints ALL sets, not
just ones with low gear counts. This affect imperial thread more than
metric, like 13 tpi will give a 2 gear set that will work with 0
error, along with a lot of other sets that also work fine.   By
sorting the output you can easily find the one to use:

change 13 | sort /r

will pruduce the output sorted such that the lowest ratio is at the
endof the list where you can see it.


D.A.Forsyth  Nov 2001
-----------  start of original readme -----------

	Lots of people seem to have difficulty with change gears.  I wrote this
simple program to help deal with the problem.

	Be forewarned, I use a lathe with a quick change gear box so, if I've
overlooked something important, let me know.

	Enter the effective lead screw pitch and your available change gears
into the data file, "CHANGE.DAT".  Note that "effective lead screw pitch" is
the pitch of the thread that would be cut if the spindle rotation were the same
as the lead screw rotation (1:1 gearing).  For most lathes this is simply the
lead screw pitch but there may be some oddballs out there.

	The program asks for the thread pitch you wish to cut.  Fractional
values (e.g. 11.5 tpi) are allowable inputs.  If you're using the gears to
obtain a desired feed rate (e.g., 1/16" feed per spindle revolution), simply
convert the feed rate to the equivalent thread pitch (16 tpi in this case) and
input that.

	The program asks for an allowable error (expressed in percent) in the
final gear ratio.

	The program will first attempt to find a single gear pair that matches
the required ratio within the allowed error.  If it fails to find one it will
then attempt to find two pairs.  Failing that three pairs are examined.  Most
banjos don't have space for more than three gear pairs so the program doesn't
search beyond three pairs.

	Whenever a ratio with suitably small error is found it is printed out
along with the actual ratio and the percentage error in the pitch if that
pairing is used.  All satisfactory ratios are printed so the user can pick
the one that best fits the banjo without fouling.  Note that once an acceptable
solution is found, larger numbers of gear pairs will not be tried.

