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.

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