DoN. Nichols wrote:
On 2008-07-10, Terry Coombs wrote:
Got an order yesterday from one of our favorite suppliers , hadda
spend more than I really wanted to get the free shipping ... but
it's all stuff I'm going to need , so I went ahead .

Congratulations.

[ ... ]

The next major need is going to be rotary indexing , and I'm a bit
baffled . Spin indexers , rotary tables , index wheels , what's good
for which task ? I expect to cut a few gears (some for the lathe ,
couple are missing teeth) and maybe a drive pulley for a cog belt
(Harley final drive belt) . Be nice to be able to index and drill or
mill a bolt circle ... or mill an eccentric slot for a cam .
So , what do I need , and how big ? I'm kinda thinking a 4" or
maybe a 6" rotary table ? I'm a bit leery of going too big , this
thing doesn't have quite the spindle/table distance (18" max) a full
sized knee mill has .

O.K. for bolt circles and for milling circles the rotary table
is the preferred tool.

The spin indexer is limited to 1 degree increments, which is
fine for some numbers of teeth, but useless for others. For gears,
you want a dividing head with a matching tailstock, and appropriate
arbors. The index head typically has a 40:1 ratio between the crank
and the spindle, though some have 20:1 and some have 60:1. Those,
combined with index disks (bunch of concentric circles of holes, each
a different integer number) allow you to get many (most) common gear
tooth numbers, but if you want to make a metric to inch transposing
gear set for a lathe, you are stuck with the 127 tooth one. The
dividing head has a pair of arms which can be set to a selected angle
to help hitting the right next hole after N crank turns, and then you
shift it so the other arm is contacting the indexing pin, and you are
set for the next. You *could* do it with a rotary table with a
calibrated collar on the crank, but the chance of making a mistake
120 teeth into the cutting of the gear is much greater.

To get the 127 tooth gear, you will need an indexing head with a
differential gear setup, which I have only seen in catalog photos, not
in real life. Or go for a CNC rotary table controlled by your
computer.

Another question , I have been reading up on hobbing gears . I
have a keyed mandrel , apparently used by a former owner of my lathe
to drive a 1/8" slitting saw . The shank is 3/4" diameter where the
collet (or holder) grips and 1" at the end the hobs would mount ,
about 4" overall length . I'm concerned about rigidity and flex in
the cutter . Should I be ?

Hmm ... as I understand it, hobs need to have the gear blank and
the hob coupled by a carefully selected set of gears so the two rotate
in the proper relationship. Most hobbing machines are rather large
and heavy. Again -- this could be done with a CNC controlled arbor
for the gear coupled to the spindle rotating the hob -- and they have
to be at just the right angle, unless you want spiral gears.

Good Luck,
DoN.

Thank you ! You've answered several questions . The gear hobs I was
thinking about were straight cut gears , like one would find in a lathe
leadscrew gear train . Or in a back gear on a Logan lathe ... I read
somewhere on the net that you can cut the teeth by advancing the blank one
tooth at a time . His hob had I think three or five cutters , so it also cut
on the flanks of the teeeh adjacent to the one under the center cutter .
You describe using a dividing head for cutting gears . From your
description I see the cutter's rotational axis perpendicular to the gear's ,
and the cutter shaped like the space between the teeth . This might be the
process I'm calling "hobbing" . Can this be done with a verticle rotary
table equipped with index discs (and a tailstock?) ? I need to visit the
guy at the machine shop next door to the cabinet shop where I work . He's
also a 'cycle rider , and loves to share his knowledge . I want to learn .
Saw an article in my Dr.'s office the other day , people who continue to
educate themselves (content/field unimportant , it's the act of learning!)
have fewer health problems , and a smaller chance of mental (Alzheimers ,
etc.) problems in their later years . There is SO much I don't know !
--
Snag
'90 Ultra "Strider"
'39 WLDD "Popcycle"
Buncha cars and a truck