(NOTE: I got this as direct mail from John. It was apparently addressed
to pwcs!phile@hoosier.cs.utah.edu. I did not get the one he sent to
british-cars@autox.team.net nor any of my mail from any mailing list. The
usual bounces from Randy and the one confirmation from someone on the list
using a delivery-confirmation program did find their way back to me. So I
know you have been getting my messages, but I have not been getting yours.
This is apparently due to a communication problem between my site and our
registered Internet gateway. Our Sysop is looking at it. When they get
it fixed, I will probably get all the flames you all have sent me since
Saturday. Meanwhile, I am talking with my ears plugged.)
John R. Lupien writes > > >
> > > Gears are easy - you can cut them on most lathes, without any CNC.
> > > Cams, cranks, & so on are much tougher.
I wrote > >
> > You can cut gear BLANKS on a regular lathe.
> > To cut the teeth, you need a hob.
John R. Lupien writes >
> I made a nice straight-cut gear on a lathe, it really was easy.
> Cutting the blank is the hardest part. To cut the teeth, you use
> this many-toothed rotary cutting device that clamps onto the
> screw thread drive. They have both straight-cut and helical
> tools for this, I think, though I have only used the straight-cut
> version.
That is a hob. According to my dictionary: "A cutting tool used for
cutting the teeth of worm wheels or gear wheels."
"Most lathes" are not going to have a hob attachment. Where were you
working that had one of these? I have never seen a hob attachment on a
regular lathe, but then, I have only worked in about 5 shops and seen a
maybe 50 while working in machine-tool sales. I really would be
interested in hearing exactly how this works. Of course, I can speculate...
Often a hob is a dedicated machine that uses what looks like a multi-row
side mill which runs in concert with the gear blank as it turns. Some
hobs for straight-cut gears use straight profile cutters on an endless
chain belt. The cutters advance in a straight line whilst the gear blank
is "rolled" across them, rotating as its axle moves laterally. It is
probably easier to build a machine to quickly cut helical spur gears,
because they are essentially VERY fast multiple screw threads, and the
axle of the gear need not move.
I have seen in a crane company (American Hoist and Derrick, who later
abandoned Saint Paul with a lot of our money to move to the Sun Belt
somewhere) a hobbing machine for the big "bull gear" used to rotate the
cab. In the old days, these large ring gears were merely cast, teeth and
all, then cleaned up by a guy with a hand grinder. Older cranes with
these are easy to spot by the horrendous grinding noise when they rotate.
This disappeared when they started casting the blanks and hobbing them.
The ring gear blank is clamped to a large indexing table. The
carbide-insert milling head rotates vertically and is advanced downward
through the blank. The head retracts and returns to its starting position
and the indexing table rotates to the next gear position and they do it
again. This would seem to be the way to go for even smaller gears in a
short-run or one-off situation. An adaption of this method could be made
to work on a lathe.
I can envision cutting straight-cut gears on a lathe by cutting each tooth
with the lathe spindle not turning. Sort of using the lathe as an
indexing head, and the long feed on the lathe to advance the cutting wheel
on the hob. To run this off the lead screw, the lathe spindle would have
to be disconnected from the power and the lead screw powered. The lead
screw would then set a relationship between the speed of the cutter and
the long feed, although I don't see why that would be necessary. The
cutter wheel would have to run horizontally, so the cross feed would be
used to set the depth of cut and bring the gear to its final working
diameter. The disconnected spindle would need an indexing device of some
sort to make it halt rigidly at each tooth position. Probably an indexing
plate on the opposite end of the spindle from the cutter. Lathe spindles
typically extend all the way through the machine, and placing the indexing
plate here would give you a lot more room to work.
> > A simple straight-cut gear you could do on a milling machine with an
> > indexing head and a mill shaped to fit the required profile.
This still seems to me the easier way to do this.
Phil Ethier, THE RIGHT LINE, 672 Orleans St, Saint Paul, MN 55107-2676 USA
h (612)224-3105 w (612)266-6244 phile@stpaul.gov
Ask me the time and I will tell you how to make a watch.
Or maybe the gears for a large clock.
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