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.
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