On Sat, May 09, 2009 at 11:22:20PM +0100, Dave Caroline wrote:
> I have a single tooth rack form carbide cutter...I feel this should be
> relatively simple to code as a generic x module x teeth 4 axis.
Involute teeth should be able to be cut with that, if you run many
passes, moving it between passes, in Z relative to a vertical gear, as
it is stepwise rotated. (We're cutting the horizontal tooth, here.) In a
higher productivity version of that, in Model Engineering Workshop
magazine issue 72, page 54, the author used a 4 tooth _non_helical_ acme
hob to take successive passes at -,0,+ elevation with the gear rotated
proportionately, to produce a piecewise-linear approximation to the
correct form. IIRC, he did about 4 passes, then ran two such gears
together, to finish the job. (Just as you recommend) It transmitted
power to his satisfaction. I'd love to hear such gears running.
Perhaps that mimicss how the involute form was discovered? Brick-sized
wooden cogs, in gearwheels several meters across, were used in water and
windmills. With the corners cut off, they ran, but wore down, due to
sliding rather than rolling friction. Once they'd worn to shape, the
rolling instead of sliding friction preserved their shape, and the
rattling stopped.
The pain with hobbing (even in prospect) must surely be the building of
a hobbing machine. That isn't necessary any more, though. A stepper
motor suffices to rotate the gashed gear blank in synchronism with the
hob. (i.e 1 tooth for every hob rotation) In MEW issue 75, a simple
programmable divider was used to vary the "gear" ratio between hob and
blank, to determine the number of teeth. (I still haven't tried it
myself, for lack of a need for gears.)
Tilting of the hob and gear blank (to the hob's helix angle), as
described earlier in this thread, will produce a spur gear. That's maybe
easiest, because feed is along the gear's axis. But spur gears tend to
run noisily, compared to helical gears, so it would be attractive to be
able to make them. If the hob axis remains perpendicular to the gear
axis, then we are in a position to begin cutting a helical gear, I
figure. The feed needs to be at the helix angle, or there'll be no teeth
left when we finish. Also, my mental image says we need to adjust the
phase of the gear relative to the hob, as we feed across the tooth
width. That is because the middle of a helical tooth leads one side by
half a tooth pitch, and so must be cut by the hob earlier in its
rotation. A simple programmable divider can't do that, but a processor
in its place can.
> Hand grinding the rack form on flycutters will be easy enough and you
> can run the gear pairs in after making. The cycle time to make is
> going to be a bit slow though hobbing will always win there and
> hobbing cutters are pretty cheap for certain sizes we just need to
> convince EMC about geared spindles to match a real machine.
Yes, two rotary axes, driven with a fixed ratio (= desired number of
gear teeth), and a fixed phase relationship to the hob's index pulse for
spur gears, or proportional to helix angle and current feed travel for
helical, should spit out a good involute gear quite quickly.
And if one has a lathe, then there's not much need to buy a hob. Even
the hardening and annealing would be interesting. Some recommend
annealing by sliding a hot plug of copper or aluminium into the hob, so
the tips are the hardest, not softest, part.
But my mobo has just run its first latency test, and isn't in a box yet.
It'll be some time before I could dream of making gears. (Yes, my little
mill has a swivelling table, so can feed at the helix angle, relative to
the horizontal spindle. I just don't know whether it's worth converting
to CNC.)
Erik
--
Never worry about theory as long as the machinery does what it's
supposed to do.
-- Robert A. Heinlein
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