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 .
Among the items I got was a 5X5 milling vise . It's really a bit too big ,
but I'm sure that I'll need the capacity eventually . Also got some
parallels , a clamp kit , and an edge finder . Already got a decent
assortment of calipers , mikes , and dial indicators .
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 .
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 ?
--
Snag
wannabe machinist who wants to spend his last few bucks wisely

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.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

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

On 2008-07-11, Terry Coombs wrote:
DoN. Nichols wrote:
On 2008-07-10, Terry Coombs wrote:

[ ... ]

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.

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 don't see why not. And it might cut larger gears than I can
do with my index head.

And the gear tooth cutters (not hobs) used for this approach are
sold in sets -- marked "#1" - "#8" for a given gear pitch and pressure
angle, and each is used for cutting teeth within a fairly narrow range
of tooth counts, because the tooth shape has to change as the count
changes. IIRC, the #8 covers from rack gears (such as you will find on
the underside of the bed lip of your lathe) down to some number of teeth
like perhaps 58 or so. (I could go downstairs and check my set of the
pitch for the gears in my Clausing lathe, but I'm comfortable up here,
and the number is not critical. The other end of the range covers only
about two tooth counts (6-7 or something like that) because the shape of
the tooth changes much more quickly with the smaller sizes.

There are add-on sets for the intermediate cutters (1-1/2,
2-1/2, etc) to get the gear tooth somewhat more accurate over the range.

A true hob starts out with something like an Acme thread being
turned in a piece of hardenable steel, then flutes are cut into it, it
is hardened, and then finish ground. This, of course, produces a slight
angle to the teeth if the shafts are at right angles to each other.
Anyway -- yes, the adjacent turns of the thread cut the other side of
the teeth at the same time -- though not to full depth, and the multiple
cuts as it rotates produce a gear tooth shape which will clear any other
tooth of the same spec engaged with it, so you don't need different
cutters for each tooth count.

There are ways to do this with a single cutter (either a
horizontal milling style shaped to the rack tooth shape, or a tool in a
metal shaper ground to the same shape), by moving the index head
sideways and rotating the gear blank just a little to progressively cut
the proper tooth shape -- but a hob on a gear hobber is a lot quicker to
use for production. :-)

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 .

Good sort to have around. Be good to him.

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 !

Well ... that maximizes your protection as you age. :-) Always
glad to learn to do something else.

Enjoy,
DoN.

--
Email: | Voice (all times): (703) 938-4564
(too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html
--- Black Holes are where God is dividing by zero ---

"Terry Coombs" wrote in message
.. .
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

I see that Don has given you good information, but to clear up a point about
hobbing: As he said, a hob starts out as a shape very much like an Acme
screw. The flanks of the "threads" are straight, just like a screw thread.
The way they achieve an involute gear-tooth shape in the workpiece is by
simultaneously rotating the cutter and the workpiece, with their axes at an
angle to each other.

Hobbing is one of several metalcutting processes called "generating." You
generate a shape through some combination of simultaneous motion. Although
there have been accessories made to do this on a lathe (by hobbyists, as far
as I know), it's mostly a stunt. It requires a dedicated hobbing machine to
do it productively, and with enough versatility to be useful. Cutting gears
on conventional machine tools is more commonly done with the kinds of
cutters that Don describes, and some kind of indexing head or table. The
cutters have involute-shaped teeth, and they cut like a conventional milling
cutter. In other words, there's no simultaneous motion. They do this in job
shops when they need just one or two gears. It's too slow and tedious for
production, which is done mostly with hobbers.

One other point that you should know about, if you get into gears: You can
cut gears on a shaper, as Don mentioned, but that is not what is called a
"gear shaper." A gear shaper is another specialized gear-cutting machine
that, like a hobber, cuts involute forms with straight-sided cutters by
simultaneously rotating the cutter and the work through an angle, as the
shaper goes through its stroke. In other words, it's another type of
generating machine. They're used in industry for various gear-cutting jobs
but the big one is cutting internal-tooth ring gears for planetary gear
sets.

This is much easier to explain in pictures, and easier still in animations
or videos.

--
Ed Huntress

This is based entirely on reading old books, I've never cut a gear
this way. I buy them.

The teeth of a hob are cut to the shape of a rack, meaning that the
tooth flanks are straight at the pressure angle. Machinery's Handbook
shows the dimensions.

First you rough out most of the tooth spaces in the gear blank with a
normal milling cutter on a rotary table. Then you put the hob on the
spindle and set the blank up beside it on an arbor that can turn
freely without play. The arbor has to be angled so it's parallel to
the hob's spiral teeth. Slowly feed the gear blank into the hob, which
will rotate the gear and automatically generate the proper involute
tooth form as the hob teeth roll in and out of mesh. Whether or not
the tooth spacing stays accurate depends on friction and how much
metal the hob has to remove. If possible the spindle should drive the
arbor through gears.

Although you have to harden and grind the hob and may have to try a
few times to get a good gear, hobbing lets you make gears of any tooth
count with correct involute teeth starting from a lathe bit ground to
a simple straight-sided shape.

Jim Wilkins wrote:
This is based entirely on reading old books, I've never cut a gear
this way. I buy them.

The teeth of a hob are cut to the shape of a rack, meaning that the
tooth flanks are straight at the pressure angle. Machinery's Handbook
shows the dimensions.

First you rough out most of the tooth spaces in the gear blank with a
normal milling cutter on a rotary table. Then you put the hob on the
spindle and set the blank up beside it on an arbor that can turn
freely without play. The arbor has to be angled so it's parallel to
the hob's spiral teeth. Slowly feed the gear blank into the hob, which
will rotate the gear and automatically generate the proper involute
tooth form as the hob teeth roll in and out of mesh. Whether or not
the tooth spacing stays accurate depends on friction and how much
metal the hob has to remove. If possible the spindle should drive the
arbor through gears.

Although you have to harden and grind the hob and may have to try a
few times to get a good gear, hobbing lets you make gears of any tooth
count with correct involute teeth starting from a lathe bit ground to
a simple straight-sided shape.

OK , this is closer to what I had in mind . The website I got my info from
is : http://helicron.net/workshop/gearcutting/involute/ . They demonstrate a
method for cutting spur gears that doesn't require gearing the blank to the
cutter , you advance the blank in one tooth increments to generate the
proper tooth shape .
--
Snag
'90 Ultra "Strider"
'39 WLDD "Popcycle"
Buncha cars and a truck

On Wed, 9 Jul 2008 21:26:47 -0500, "Terry Coombs"
wrote:
snip
I expect to cut a few gears (some for the lathe , couple are missing
teeth)
snp
=============
To avoid rediscovering fire and reinventing the wheel, buy the
book "Gears and Gear Cutting" by Ivan Law. Specifically written
for the home/hobby machinist and covers making your own cutters,
single tooth and b&s style.
several sources in the us but one example is
http://www.amazon.com/Gears-Gear-Cut...78963&sr= 1-1


Also on generating a 127 tooth gear. For best accuracy from the
typical rotary table / dividing head, the best best seems to be
to use the table to make as big a plate as will fit your head
with 127 holes using the degree markings and vernier and use this
to generate the gear. The 127 hole plate is not generally
available and the other methods are (a) a pain, and (b) not
generally possible with the less expensive dividing heads
available for home/hobby shop use.

On Sat, 12 Jul 2008 11:12:50 -0500, F. George McDuffee
wrote:

On Wed, 9 Jul 2008 21:26:47 -0500, "Terry Coombs"
wrote:
snip
I expect to cut a few gears (some for the lathe , couple are missing
teeth)
snp
=============
To avoid rediscovering fire and reinventing the wheel, buy the
book "Gears and Gear Cutting" by Ivan Law. Specifically written
for the home/hobby machinist and covers making your own cutters,
single tooth and b&s style.
several sources in the us but one example is
http://www.amazon.com/Gears-Gear-Cut...78963&sr= 1-1


Also on generating a 127 tooth gear. For best accuracy from the
typical rotary table / dividing head, the best best seems to be
to use the table to make as big a plate as will fit your head
with 127 holes using the degree markings and vernier and use this
to generate the gear. The 127 hole plate is not generally
available and the other methods are (a) a pain, and (b) not
generally possible with the less expensive dividing heads
available for home/hobby shop use.


Greetings Terry,
What George said about the book by Ivan Law is correct. This book is a
treasure trove of information about gears that is geared (like the
pun?) toward the hobbiest. Ivan Law has written this book in a fashion
that makes it very understandable for the average novice who has never
cut a gear. He also writes about indexing and how to achieve indexing
with lathe change gears with the result being more accurate that
needed. There is one table of numbers in the book that has some
numbers mis-labeled. I don't have the book in front of me right now so
I can't tell you which one it is. This table is not needed for all
types of gear cutting so you may not even use it. But if you do it
will become obvious which numbers are mis-labeled and you can then
write the corrections in your book. If you want to cut gears with a
straight sided tool then you can generate the tooth form in discrete
steps. You can use a fly cutter ground to the proper pressure angle.
First make a cut to the proper depth. Then raise the cutter a little
and rotate the gear blank a little. Then make another cut. The amount
the cutter is raised and the blank is rotated depends on how many
passes you want to make with more passes making a profile closer to
ideal. If you search the group uk.rec.models.engineering you will find
posts there that explain the process in more detail.
ERS