27 Aug
2011
27 Aug
'11
10:24 a.m.
and have
perhaps 10 or 12 leaves (teeth to the rest of you ;-))
Something like that, yes. I don't recall the precise number and can't find the previous discussion threads that have
happened on this list
over the years.
I realize that to a trained machinist with the right tools, cutting
gears is just another day on the job. I am very much a novice when it
There are basically 2 ways to cut such a pinion.
The first used s a special milling cutter which is the shape of the cabe
ebtween the teeth. You turn a piece of rod the overall diamater of the
gear in a ltathe, and then put that rond in a dificing head. Using the
milling cutter you cut one of the inter-tooth gaps, then using the
dividning head you turn the rod throug the appropraite angle (1/12th of a
revolution if there are 12 teeth, etc), then cut the next gap and so on.
The problems are that the milling cuttter is strictly only correct for
one particular pitch _and number of teeth_ For geears of the same pitch
-- that is ons that you would mesh together, the inter-tooth gap does
change as the number of teeth changes. Normally, an apporximation is
made, the cutters some in sets of 8, ech oovering a range of numbers of
teeth. THese cutters are not cheap either. I've been quoted over \pounds
50.00 for one. No, not a set of 8, one cutter.
On something this small, you would have to take very light cuts to
avoid bending the workpiece (the rod you're making itn oa pinion), which
would slow things down a bit.
The second way is claled 'hobbinh'. Here the cutter is called a 'hob',
and is a bit like a thread-cutting tap. It is rotated, the workpiece is
also rothated at the right speed to cut the apporirate number of teeth
[1]and the 2 a brought together. Think of it as being a bit like a
worm-and-wheel drive wit hthe 'worm' actually cutting teeth in the wheel.
This needs more specialised equipment (a hobbing machine is only used for
gear cutting wheras a lathe. mill and dividing head have many other
uses), but the greatadvantag is that one hob cuts all gears of a given
pitch no matter the number of teeth,
[1] Ol;der hobbing machines did this mechanically usinga gearbox with
selectable rations (ofthen by using changewheels that you haf to fit the
right comibnation of). There have been designs using a steppre motor to
roate the workpiece contoller by a shaft encoder on the hob spindle and
some suitable electroncis. I really must have a go at making one...
through Shapeways. Because of the strictly functional
nature of the
part, there should be no worries of trademark or other IP
infringement, to head that discussion off.
I can't see how there could possibly be legal problems for soemthing like
this.
-tony