cutting spur gears on a mill

Could we stop calling it a tap when we mean hob? Except of course when talking about trying to use a tap as a hob.

Brian, from this response:

"Pay attention at the back

Or maybe you don't mean herringbone but just have two straight gears with some angular offset to take up the backlash."

...can we assume that indeed you didn't mean herringbone?

Since the original post was regarding using a tap as a hob I will continue to refer to the process of using a tap as a hob. This is clearly not a tap but a bolt but the point is the same regardless, if you change the angle of the tap relative to the axis of the blank you change the pitch of the resulting gear. As I said earlier there is no quick and simple way of cutting accurate gears.

Martin C

Edited By Martin Connelly on 22/08/2021 08:34:21

This is how I see it. In the same way that the pitch of the straight spur gear is dictated by the hob. The hob helix angle setting the set over angle. I agrre that the axial pitch of the helical gear will not be the same as the straight spur gear just by Pythagoras. However once you choose the helix angle of the gear you fix the set over angle of the hob. The pitch is then dictated by the pitch of the hob. You cannot just vary the set over angle to obtain different pitches. If you do you cease to generate an involute and merely alter the shape of the resulting teeth.

I'm not clear on how helical gears are specified but the pitch perpendicular to the helix (I would guess at calling it the helical pitch) will be the same as the pitch of the hob.

regards Martin

I've experimented with making a particular sort of helical gear - "helical lever" - which have simple straight sided teeth - they were used by a Victorian clockmaker and can realise large reductions in one stage as you can make one-tooth pinions. I believe that you can hob helical involute gears with a standard hob. From what I recall -

• The "helical pitch" which is measured orthogonal to the teeth is the same as that of the hob.
  
• The circumferential pitch is increased by a factor 1/cos(helix angle) so the diameter of the gear has to increase as well (as does that of the mating gear of course), by the same factor, to fit in all the teeth.
  
• Looking at the tooth form along the helix angle it is involute because that's what the hob cuts. This is also what is "seen" by the mating tooth of the other gear which is also helical. It may not be the same as the profile of a cross-section of the tooth.
  
• You have to angle the hob by whatever its own helix angle is to cut a straight tooth - for a helical tooth you add or subtract the helix angle depending on the handedness you want.

Purely pedantry, but a helical gear is not a herringbone gear.

It can be paired with a similar one of the opposite helix to produce a herringbone gear, with the Vee tooth pattern.which gives rise to the name.

WHY?

A helical gear, by virtue of its helix angle produces an axial thrust on the shaft. By mounting a gear which is identical, but of opposite helix, the two end thrusts cancel out.

Skew gears are helical gears where the axes of the shafts carrying them are not parallel.

Engineering implies precision. An imprecise description may result in an answer to a question which wasn't asked, and cause confusion.

Howard.

So at least two of us are in agreement.

regards Martin

Trying not to go of topic here. The OP 's aim is to be able to cut gears
using a tap by free hobbing , whilst this can be done as some have shown
on here quite successfully there is always the level of uncertainty as to the finished
result The photo here is a 90 tooth wormwheel cut using a 5/16" tap the finished
job works well using a piece of threaded rod, because this is done using a hobbing
unit you get 90 teeth every time even if the blank is over or under size.

90 tooth worm wheel

It is possible to cut a spur gear using a tap providing the tap is set over at the helix
angle and using the hobbing unit to time the motion .I think is less likely to be able to
do the same with a tap or even a proper hob in a free hobbing situation ,the situation
is worse with a tap and only 4 cutting edges as one cutting edge is disengaged
before the next is engaged so the blank has to revolve 1/4 tooth pitch on its own
with no driving mechanism,the most likely event is the cutting edge strikes the
tooth wiping it out or gouging some of it away.

Gear cutting hobs as seen here are mostly all threaded right hand at least all of
mine are ,the helix angle is indicated on the side of the hob,the largest here at
2.5 mod is 2 deg 20 min and the smallest 0.3 mod just over 1/2 deg .Usually
12 tooth the one on the left 0.9 mod made from silver steel flycut form relieved
in 3axis 8 tooth 3 deg helix ,again right hand could just as easily have been cut
left hand reverse the sense of one axis and incline the head 3 deg in the opposite
direction. To the right 0.5 mod 15 tooth Micron hob brand new bought of a junk
stall at ME show for 50 p ,what a bargain.

Gear hobs

In setting up the hob is inclined over at the angle indicated so the cutting action
is parallel to the table traverse ,from this spur gears and helical gears may be
generated in left and right hand using the same hob with no errors in pitch.
The spur gears will mesh and run with the helical gears with no problems at all.
The illustration here show the possible combinations and shaft angles.

Shaft angles.

Why is this the pitch of the hob dictates the pitch of the gear teeth and since
the set over angle remains the same nothing changes in making any of these gears.
The size of a helical gear will change for a given tooth count according to the
angle of the helix and is calculated by blank divide by cos , angle.

The sketch here shows the hobbing unit work head and the positions to cut
gears of the same hand and opposite hand ,this is really more to do with
accessibility than anything else as there are instances where it is difficult
to either fit the gear blank or get the cutter to a position to make the gear.
As the hobber is connected to the hob electronically there is no problem
in connection with the drive,the work head swings in an arc of about 250 deg.

Work head

There is however a variation in pitch if the hob is inclined at greater angle,
an explanation of this can be seen in Ivan Law's book Gears and
gear cutting on page 97 but has no real significance in cutting helical
or spur gears as this can only be used when cutting worm wheels.
At angles below about 3 deg this variation is ignored ,if the angle is greater
the worm is cut to match the worm wheel.

John

Well this topic has certainly addled a few FES pates, like stirring up the suds sump

So to demystify the limits of plausible conjecture and give a platform to the deniably involved

What else on lockdown Sunday morning from the desk within my Anderson Shelter

I hope you can see the double helix (truncated Whitworth form BTW). I skewed 6 deg for fun and made from 2 identical blanks chived at the same time

For the follically scarified, note that the yellow gear has been flipped over 180 to make the red

also if you skew one gear wrt the other fractionally then you can mitigate backlash

Now with the appliance of science I shall endeavour to make an animated version showing meshing and also the reviled tap attacking gear blanks

My biggest challenge will be to post mpg or gif on this board, I havent seen any other Disneys on here

Anyone know how its done - maybe Dropbox?, Ive never done a Youtube upload yet (do you have to sell your soul?)

Sorry JP missed your eminent description above, most interesting

As you point out, this OP is not about using hobbing gear and all the expensive industrial attachments involved, way off topic

The point being that a hobbing method ties the spindle mechanically to the mandrel in much the same way as screw cutting on a lathe (excluding the tedious divider plate method)

This is examining whether FREE HOBBING has a place for the hobbyist NPI.  Its clearly demonstrated in making simple worm gears so this is just an extension of the topic. As seen above there are a lot of armchair misconceptions.

One point to note is that theoretical errors can creep in

eg

a) say you want to make a ruler with a 1" block.  Use it to scribe 12 marks then do you have 1ft - well clearly it depends on the accuracy of the block.  Hope you can see the implication of cumulative errors

b) Instead use the basic tech drawing trick use the block to make 12 ticks then project that onto a 1ft line and you have your accurate ruler with 12 inches

So the tap method is using method a)  and the dividing plate (or hob) is using method b)

Now thats the theory but what happens in practice

Edited By brian jones 11 on 22/08/2021 12:44:19

Edited By brian jones 11 on 22/08/2021 12:46:01

But it does not work like that with a metal gear. simply flipping it over won't have your teeth running in the opposite direction, that is why I thought you may have a left hand tap or as John has shown will need to cut a left and rright helix pair and butt them together

Easy way to check it is to draw a diagonal line on the side of a disc or cylinder and then flip it over with line still facing you and you will see it is still at the same angle.

OMG the Monsignor is RIGHT

10^3 apologies, I have flipped my gear blank wrongly????????. I will have to investigate

You do indeed have to angle the tap anti clockwise 11.4 deg for a 10 tpi (5.711deg ha)

GOT IT

My gear flipping created a mirror image which reverses LH to RH

Wow thats an easy mistake to make

Edited By brian jones 11 on 22/08/2021 13:45:03

All my years in CAD and I never appreciated or noticed the difference between flipping and rotating 180

Gobsmacked - how many hidden errors did that create?

Still my by bridges are still standing but Hammersmith is in trouble but I have a cunning plan

I thought I could pluck a ready made model of a whitworth threaded rod/bolt easily, turns out to be all clickbait stuff so had to start from scratch which also turned out i will gouge some flutes tomorrow

I still have a cunning plan

I wonder if a spiral tap might solve the problem

Edited By brian jones 11 on 23/08/2021 20:37:46

Brian looking at your sketch t looks like you have the tap(hob) vertical which is OK but you also seem to have shown the gear blank with it's axis horizontal rather than tilted by the helix angle of the tap. Unless the much larger diameter tap you are now using has a very shallow helix angle that means the tilt of the gear can't be seen. If you don't tilt the gear axis the tap will act like a roughing mill and just chew its way into the gear.

This is with a 1/2" tap

Yes JB indeed my model needs more realistic sizing. By the time I had sorted my cad problems I was knackered

What do you think of a spiral tap?

I love than for their intended purpose

As a couple of other posters have already stated they are likely to make better hobs than a standard tap as there should always be part of the tap in the cut so the free hobbing will be less likely to get out of phase.

..we'll be expecting to see a shot like this at the end of this thread..

Edited By DiogenesII on 23/08/2021 20:57:53

Specifically spiral flute, as opposed to spiral point.

Andrew

Yes working on it but its straining my cad app (sketchup - it was originally for the architectural artists impression market and was never intended for engineering but the user fan club took to it and developed it in remarkable ways - Ive been a fan since 2004 )

which cad app do you use.

I use Alibre and a bit of Fusion360

New to me

OMG did you pay £1200 for a one year licence?

Is it one of those full cad/cam packages to CNC?

Looks like youd spend a year learning it before cutting metal

Edited By brian jones 11 on 24/08/2021 18:19:37