Gear hobber (mechanical)

[AjohnwParticipant @ajohnw51620]

Sorry about that Gustav. I replied in the way I did because I am well aware of the work involved in producing the hobbing machine having wondered myself when the articles it's based on first appeared.

John

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[Michael GilliganParticipant @michaelgilligan61133]

Posted by Bazyle on 23/02/2016 13:45:36:

It would be nice to know who designed the modifications if not Tom Jacobs to give credit where it is due.

The maths is going to be fun.

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Bazyle,

Thank you for the concise "how it works'

I think the notes and software linked on this page may be of interest.

"reading between the lines" I think he is implying that the 'differential gear' is a standard part of production hobbing machines … so maybe Mr Jacobs modelled it from some full-scale prototype. [?]

MichaelG.

[AjohnwParticipant @ajohnw51620]

A couple of links that might help, or confuse further as googling differential hobbing brings up several areas

**LINK**

**LINK**

John

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[Michael GilliganParticipant @michaelgilligan61133]

Posted by Ajohnw on 24/02/2016 11:21:06:

googling differential hobbing brings up several areas

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It sure does, John … mostly to do with cutting helical gears

Gustav's original question was related to prime number spur-gears, and I thought the Duncanamps link that I posted might be pointing us in the right direction … but I remain hopelessly lost.

For my own simple needs, it's probably best to concentrate on non-hobbing methods !!

MichaelG.

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Edit: See the closing lines of 10.3.15 here

Edited By Michael Gilligan on 24/02/2016 11:41:25

[Michael GilliganParticipant @michaelgilligan61133]

Wow … this is clever:

**LINK**

But, not what Gustav is looking for.

MichaelG.

Edited By Michael Gilligan on 24/02/2016 11:51:58

[AjohnwParticipant @ajohnw51620]

It seems that the same sort of arrangement can be used to produce primes so that info may be around.

That looks like and Enco, manual on the web. This is the table of primes they provide in it

I'm pretty sure that I have seen an arrangement where the gearing is behind the plate rather than how this one and others are arranged.

John

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[Engine BuilderParticipant @enginebuilder]

I have been following this thread with interest.

Can anyone state the formula for calculating the ratio of hob speed to number of gear teeth required.

I have cut worm gears using the free hob method but cannot see anywhere the details for hobbing with a proper hobbing tool.

[Michael GilliganParticipant @michaelgilligan61133]

Can't help you, Engine Builder … but I'm bumping this, in hope.

MichaelG.

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P.S. This differential gear looks a bargain [if only we knew how to use it]

[Gustav ThiesenParticipant @gustavthiesen53240]

To Engine Builder:

I'm not able to give you a formula. The mechanical way to do this is based on two arms:

1. you have a gear blank and the cutting hob

2. you have a dividing gear connected with a worm

The two are connected by an idler gear. If the dividing gear has 40 teeth, the cutting hob will also cut 40 teeth. If you want to have a 60 teeth gear, you must replace the simple idler gear by compound approach with two gears which gives a translation of 1,5. I hope you can understand what I mean, I have learned English more than fifty years ago.

[Roderick JenkinsParticipant @roderickjenkins93242]

The simple answer, as I understand it, is that the hob rotates N times for each rotation of the blank, where N is the number of teeth to be cut. **LINK**

HTH,

Rod

[Michael GilliganParticipant @michaelgilligan61133]

Good link, Rod

… and the external PDF linked at the foot of that page is excellent.

MichaelG.

[BazyleParticipant @bazyle]

Correction to my earlier post. The 'very big gear' is probably directly keyed to the hob shaft not the sleeve. Makes more mechanical sense but doesn't change the operation of the device.

Slight clarification of Gustav's statement "If the dividing gear has 40 teeth, " that is the worm you see driving the blank. So formula is (desired number of teeth)/worm = (ratio of change gears on the banjo) You can use the link MG gave to calculate ratios.

Is anyone else thinking about the calculations for the differential which seems a bit less straight forward than differential dividing (which doesn't incorporate a differential and should be called 'difference dividing&#39?

It seems to me there are 2 gear train ratios, A and B both incorporating a common worm to complicate things that must satisfy the differential equation (Target gear) = 2A-B and probably A-B < 10 to stop the diff from spinning like crazy. The equations have to be exact not the usual 'near enough' to a few seconds of arc as for simple dividing tricks and metric thread calculations.

[Engine BuilderParticipant @enginebuilder]

OK as I see then, with a single start hob, the hob rotates one revolution for each tooth in the wheel being cut.

For a 2 start hob the hob rotates the number of teeth x 2, ie twice the speed.

Am I reading this correctly?

[AjohnwParticipant @ajohnw51620]

The aspect I find odd about home built hobbing machine is why apart from the challenge. My initial interest was down to the cost of gear cutters, not that hobs were cheap really but one does all. Hobs do appear to be cheaper now but the usual cutters can be too. In practical terms as some of the books mention for home use often a reduced set is more than adequate.

I'm curious. The home built one still has to fed etc by hand so I don't see the advantage over the Thomas approach for being able to cut prime tooth counts. Another plate could be used instead of a dial if time is important. A 127T gear would need nearly 19 turns per tooth with his set up. It divides into the low arc second range.

John

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[Gustav ThiesenParticipant @gustavthiesen53240]

To Engine Builder:

If use a 40 teeth gear as index gear the hob will cut 40 teeth in the blank with one revolution. This is because of the coupling by the idler gear. If I change the idler gear by two gears on the same stud, which will deliver a translation of 1,5 you will get 60 teeth by one revolution. But these teeth are not complete, it will start with little notches which will be greater by feeding and get deeper by repeating passes.

[Duncan MunroParticipant @duncanmunro42894]

Hello there, I think this is the first time I've posted on here and is definitely the first day of my retirement so don't beat me up

I've received an email from Gustav asking about the CES hobber and the differential gears and he has pointed me to this thread. When I get back home tonight I will dig out a spreadsheet I did some years ago in conjunction with a chap who had sight of a Jacobs hobber with differential gear. That will maybe help with his question in some way and provide some numbers to show how the primes ratios are formed from normal non-prime gearsets.

Also some great replies on the thread on different ways to do things, and thinking outside of the box. If the 127T gear is for metric cutting on an imperial lathe it may be possible get some really good approximations using standard gears that can exceed the cutting accuracy of the machine without going to primes.

As I'm getting incredibly lazy as I get older, rather than try and concoct some fancy solution to cut a gear I need but will only use once or twice, the 3D printer often gets a workout.

I've attached some pictures, hope they show up. There's a couple of shots of a spare 60T gear on the Myford ML7 printed in ABS and a 127T gear for the same printed in PLA. I'll post again tonight when I've tracked the spreadsheet for the differential down.

Regards,
Duncan

[Michael GilliganParticipant @michaelgilligan61133]

Welcome, Duncan !!

MichaelG.

[Gustav ThiesenParticipant @gustavthiesen53240]

Many thanks Duncan!

My special aim is not to make a 127 teeth gear. This is only one of the possibilities. My main target is to build up a hobbing machine, which should be able to cut prime number gears to. One reason for this is, that I have made a dividing head last year. Because it should also manage prime numbers, i made a compound dividing head. But when it was finished i saw that the handling was prone to errors. So I have transformed the construction to a differential dividing head. I had to cut 12 gears for this. I made them by poor mans hobbing method. To facilitate and to get better results I want to make this machine.

[BazyleParticipant @bazyle]

John, the hobber does have an auto feed option if you look at the gears behind the worm driving the blank.

Duncan, Great to see a practical ME use of printing. Of course a printer is a lot of trouble and expense to make so perhaps on a par with a hobber. But lots of whirling gears – got to be a winner.

Had a look at the gear trains for making 127 and one option is 125 & 126, but for 126 you need a 63 prime. So you can make a 63 using 60 and 57 – oh dear another prime and so it goes on.

[BazyleParticipant @bazyle]

Looking around the professional gear hobbers it appears they often don't have a differential. Instead they may knock up a quick prime as required on a dividing head then use it to give the exact ratio. They are of course into mass production so worth the effort.
Their next option is to find a close, very close, approximation perhaps using 6 or 8 change gears to get an error with 7 or 8 zeros after the decimal point. This will give a slight progressive 'creep' out of sync. However this can be compensated by moving the hob ever so slightly along its axis using another set of change gears.
Even with a differential it may not be spot on. The errors in the two trains can be arranged with one + and one – to partly cancel out.

It is much easier with computers now but in the past you could get tables with eg 14000 ratios from gear sets listed.

[John HaineParticipant @johnhaine32865]

Umm… 9×7=63?

Posted by Bazyle on 26/02/2016 17:39:50:

John, the hobber does have an auto feed option if you look at the gears behind the worm driving the blank.

Duncan, Great to see a practical ME use of printing. Of course a printer is a lot of trouble and expense to make so perhaps on a par with a hobber. But lots of whirling gears – got to be a winner.

Had a look at the gear trains for making 127 and one option is 125 & 126, but for 126 you need a 63 prime. So you can make a 63 using 60 and 57 – oh dear another prime and so it goes on.

[BazyleParticipant @bazyle]

[AjohnwParticipant @ajohnw51620]

I seem to recollect that on Myfords the 63T means no need for the 127. Pitches with very very tiny errors.

More for others that might be reading the thread. This is G Thomas's solution

As is often the case with these sort of designs all can be made on a Myford but the worms and wheels are usually bought. His BIG thick book goes through making it pretty thoroughly. Some people would buy the plates too. It's interesting to note that who ever made this one may have stuck to a long plain hole through the spindle of a size that also accepts the collet chuck. The short bar at the side is for a centre. One could be made based around a suitable small tailstock or fabricated.

My one by Dore or Westbury has a 2 morse socket and a myford spindle nose on it. Fine but if I want to use the hole I need something with 2 morse on it – a pain to make. A plain diameter mandrel is easy.

John

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Edited By Ajohnw on 27/02/2016 15:48:44

[AjohnwParticipant @ajohnw51620]

63 teeth isn't a problem with the plates Brown and Sharp came up with which gives these with 60T worm wheel. They were intended for 40T. I would have thought any design of plate would need the same base primes.

I mentioned B&S because I noticed some Vertex stuff with parts I could use that also had plates but I wondered why they had departed from the usual hole counts so gave them a miss. I think the Cincinati design had yet more holes. Dore or Westbury went for fewer holes and no 63.

John

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[John HaineParticipant @johnhaine32865]

Since the question of the accuracy of digital dividing came up earlier in this thread I thought that I would see if I could measure how well my Ward / Myford divider does.

The only way I have to measure angles conveniently is a Wixey gauge, so first I bolted an angle plate to my VH6 rotary table, set vertical on the mill table, and rotated the VH6 by various amounts, initially in 1 degree, then 10, then 45 degree increments. During this the mill drive was running as a little bit of vibration seems to make the Wixey behave better – I think it may have a very small amount of stiction.

The Wixey indication was never more than 0.1 degree out, and happily came back to zero after a complete rotation.

Then I set up the digital DH on the table, with a 4J holding a steel plate horizontal to take the Wixey, and repeated the measurements – with exactly the same results. Setting the DH up for 127 divisions, the angle readings agreed exactly with theory for each division at least to the resolution of the angle gauge (which is only 0.1 degree of course). My controller is set up for 4x microstepping, or 48000 steps per rev with the 60:1 worm drive in the Myford DH. Minimum theoretical angular resolution is 0.0075 degrees (with all the caveats about microstepping).

I don't think I'm going to worry about the accuracy of digital dividing for what I do.

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