Gear Hobbing computations assistance requested

[David CauntParticipant @davidcaunt67674]

Pete appears to have 'hit the nail on the head'

I have never cut a thread on a lathe let alone made a gear.

Having followed this with interest it seems pretty clear that with both cutter and blank being turned independently to a particular ratio then unless the PCD is correct the hollow versus hill cannot be the same. With a 158 teeth this would be very difficult to spot.

Wouldn't it better to grind off around the tap at the height you are cutting all but one tooth?

I do hope it comes right eventually.

[John PParticipant @johnp77052]

Posted by John Haine 01/02/2021 21:18:18

I'm just going to dive in here and probably hit my head on the bottom but…

If you were hobbing a gear you would set up the hob so its axis was parallel to the gear blank but not in the same plane, with the spacing between the hob axis and the blank axis such that when you feed the hob sideways, with both the hob and the blank rotating, the hob teeth will but to the full depth when the the hob has traversed right past the blank.

You can't do that with a worm wheel, but maybe you should be setting the hob axis co-planar with the blank and the axis spacing to get the required depth of cut, and feed the hob along its axis to engage with the blank? That way the hob should be cutting to the full depth in one pass and you should be able to avoid cutting away the teeth you just formed.
John

Hi John

I think i can see what you are thinking about here .This
illustration here fig 38 i think shows this method that you
described ,i think this is done in this way more for
speeding up production than anything else and for
gears of large pitch.
There should be no reason for any of the teeth to be
cut away providing the calculated sizes are adhered to.

You would need a specialized machine to be able to cut
a wheel in this way ,a bit beyond the average shed men.

John

[Pete RimmerParticipant @peterimmer30576]

For hobbing a worm wheel you have no choice but to feed directly in (unless using a dedicated hobber with a 4-gear or electronic setup so you can make a full depth bevel cut in one pass). My machine can cut spur gears conventionally or worm wheels straight-in. To do a worm wheel with the teeth set at the helix angle of the worm I set the head over by the helix angle and feed directly in.

[John PParticipant @johnp77052]

I just wonder if this style of gear seen here would be more
suitable for Joe's application

This video shows these spiroid and helicron gears which can be made
with zero backlash.
https://www.youtube.com/watch?v=gRriAf5snqs

A standard hob is not quite the correct profile as these spiroid and helicron
gears have an asymmetric thread form,however as can be seen
here a test gear hobbed using a 1 module cutter ,the worm would
fit in about the 11 o'clock position and have about 5 teeth in mesh at the same
time and are much stronger than a conventional worm wheel at the same size.
This gear was cut as a trial only in prototyping wax.

John

[Joseph Noci 1Participant @josephnoci1]

John ( Pace), Thank you for the time you have taken to explain and help me along. I have a much better understanding and the concept of not eroding away the teeth with my setup is also better understood. Thanks to everyone who joined in as well – this stuff is not rocket science, but appears so while the fundamentals elude one!

The penny dropped here:

The 2nd photo here is a worm wheel that would be suitable to
transmit power cut with a 0.9 module hob and is overcut
the calculation for the size was about 53 1/2
tooth but was cut at 56 tooth ,the target tooth count was 60
but the tooth shape was becoming too degraded on a test piece
and 56 was about as far as i was prepared to go to be safe.
The difference in the shape of the teeth can clearly be seen
in comparison with a tap cut wheel.

Since in my first runs I did erode away the teeth, but realise now that was because my ratios were way out for that tooth pitch and blank diameter – the teeth did not walk over each other, as I thought was happening, but per you explanation above, they simple began to thin out to the point of vanishing…

I am going to make up a handful of scrap plastic blanks , start with the computations and go and experiment to see what actually happens.

John, my application does not require any appreciable power in the transmission – zero backlash is very critical though. That is why I considered a screw thread type tooth profile as appropriate – esp as I can adjust the worm shaft easily to squeeze worm and gear together, forcing the flanks of the teeth of worm and wheel to ride hard on each other, hopefully eliminating backlash. I am also banking on that form providing some protection in the event of things going awry while in motion and the arm being forced into a hard stop – hopefully the worm wheel teeth will just give way..

Before I started with all this, I read through Ivan Law's ' Gears and Gearcutting' – but did not absorbs what I should as I did not know what I did not know… Having read Chapter 7 thoroughly again I have rekindled confidence..

Thank again to all, very much appreciated.

Joe

[Phil PParticipant @philp]

Posted by Pete Rimmer on 01/02/2021 22:16:51:

For hobbing a worm wheel you have no choice but to feed directly in (unless using a dedicated hobber with a 4-gear or electronic setup so you can make a full depth bevel cut in one pass). My machine can cut spur gears conventionally or worm wheels straight-in. To do a worm wheel with the teeth set at the helix angle of the worm I set the head over by the helix angle and feed directly in.

Pete

Are you sure about setting the head over to the helix angle for a worm wheel ?

I would have thought it should be vertical to produce the wheel, other wise the worm that eventually drives the wheel would also have to be at the helix angle ??

Phil

[Pete RimmerParticipant @peterimmer30576]

Phil,

If you set the head vertical, you'll generate teeth on the wheel set at the helix angle of the hob. If the angle of the hob is different to the helix angle of the worm, you have to adjust for the difference.

Pete.

[Phil PParticipant @philp]

Hi Pete

Yes that is true, but in the ideal world the wheel should really be hobbed with the same diameter cutter as the intended worm otherwise there will not be full contact with the teeth if they are produced with a different diameter.

I was lucky with the one in the photo's, it is the worm wheel for my model barring engine, and the only suitable hob I had was actually the same diameter as the worm. That was a real stroke of luck.

Phil

[Pete RimmerParticipant @peterimmer30576]

Oh I see what you mean now. Yes, ideally your cutter would be the same diameter as the worm for full contact, which would of course make the helix the same.

The only worm wheels I usually cut are thread chasing dial wheels which carry no load. I made one this week for a Smart & Brown lathe.

[Phil PParticipant @philp]

I probably take it for granted, but I suppose we are pretty fortunate to have hobbing machines in our workshops.

Phil

[duncan webster 1Participant @duncanwebster1]

To disappear off on a slight tangent, I reckon that if you control the Z and the blank rotate in sympathy you can cut a straight spur gear with a simple straight sided form tool like you'd use for a rack. Much easier to grind the tool but a lot of cuts to generate the form. If it's CNC just let it get on with it.

When I worked on remote handling machines we got over backlash by having a little motor fighting against the big one to take up the clearance.

[Phil PParticipant @philp]

Duncan

My late father rigged up my Alexander Master Toolmaker milling machine to generate gears with a rack form cutter by rolling the blank past the cutter a few teeth at a time. Very ingenious.

Some folks have seen these before, but its worth showing them again for the gear cutting geeks on here.

Phil

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