Gear hobber (mechanical)

[Michael GilliganParticipant @michaelgilligan61133]

Posted by John Haine on 27/02/2016 16:43:15:

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.

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Thank You, John … That's a very useful contribution to the communal knowledge.

If you have the will to do it, I can suggest another test which might give a more definitive 'proof'.

Make two 'identical' plates [any awkward count (maybe 7) would do] with nicely drilled holes … Then check whether a set of dowel pins will fit them:

1. as an in-phase pair
2. at various out-of-phase positions
3. with one plate flipped-over

… Just a suggestion; not a request

MichaelG.

[AjohnwParticipant @ajohnw51620]

I was only pointing out that steppers usually have a 5% step error that also varies with load John.

There are some electronic solutions about. This one seems to have been going for a long time and not changed since 2012. There may be others

**LINK**

Sounds like he had problems with keyboards. One approach Eddistone used on their radio's was to pole and shift into a byte – All 1's is one state and all 0's another, inbetween undefined. I've used a similar technique using state counters. Many moons ago I used edge trigger interrupts with no state times – oh dear but the first job I ever did like that.

There is was another one looking at using a lower step count motor for higher holding torque. Also another that accepted G code for cnc. I haven't noticed any software for either of these.

John

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[John HaineParticipant @johnhaine32865]

Michael, maybe when I have the DH set up in a config where I can drill I may give that a go.

I can recommend the Steve Ward controller. He sells a complete kit, it's easy to build and works well. I could use a lower step count, say 24000, which would mean the motor is only 2x microstepping, so holding torque would be better perhaps, but there is a trade off between inherent resolution and precision here I think, not sure where the optimum is.

[John Stevenson 1Participant @johnstevenson1]

Diff fitted to a CES hobber.

[AjohnwParticipant @ajohnw51620]

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.

The example given on the web isn't a sensible way of doing it Bazyle. If you look at the chart I posted earlier 127T is 13 holes on a 39 plate with a 56 and 44 gear plus 2 idlers. That is very probably with a 40T worm wheel.

There's the rub really. If some one lacks a table and has 3 plates with 100's of holes in them, rather a lot of possible holes between each division and on top of that a selection of gears working out what to use is likely to be a bit tough as there are a LOT of possible combinations. Not an easy thing to handle as numbers like 127 are irrational. Probably the best option is to find a table some one else has produced. I think there is one in machinery's, also probably based on a 40T worm wheel.

Checking that head does use a 40:1 worm wheel and comes with plates with 15,16,17,18,19,20,21,23,27,29,31,33,37,39,41,43,47 and 49 holes. The gears look like typical change wheel values.24,24,28,32,40,44,48,56,64,72,86 and 100 teeth. They give divisions of all numbers up to 500 starting at 51. From a quick look all numbers up to 50 are covered by the plates used the normal way.

Must admit when I first came across model engineering items like these I was a bit bemused by the extra teeth on the worm wheels.

John

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[John HaineParticipant @johnhaine32865]

It's my inner pedant, I can't resist it, but 127 is a prime integer, not irrational.

[Michael GilliganParticipant @michaelgilligan61133]

For info.

This incudes a good introduction to differential indexing:

**LINK**

MichaelG.

[AjohnwParticipant @ajohnw51620]

True John but my brain was locked in 1/127 which is. 1 over this and that is what makes dividing systems work.

Actually I read the table incorrectly. It should be gears with 56 and 24 teeth. Didn't enlarge my pdf view enough.

To be honest as 1/127 is irrational my gut feeling would be that correcting 1/120 with another simple fraction wont be exact. Having caught a cold and it being early Sunday morning not much chance of getting my head round the math to find out.

John

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[BazyleParticipant @bazyle]

AJW. You keep diverting into Differential indexing but this thread is about a Hobber with a differential. The maths is very different. It is also not about the best way to make a one off gear but about how to make any gear with a hobber. Professional hobbers come with not one but two sets of gears from 20 to 100 that's 160 gears not the dozen for a dividing head.

For the benefit of the very few reading around the subject of hobbing be aware that authors often use the term 'feed' both for the movement of the blank across the hob during the cut and for the movement of the hob along its axis which is available on a professional machine.

[AjohnwParticipant @ajohnw51620]

You mentioned the differential dividing head Bazyle followed by a criticism that may or may not be correct. I just responded.

As I understand it the hob feed is for feeding the cut in tangentially with a tapered hob rather than plunge cutting.

What I suspect the OP needs is details of the math behind a differential hobber or the design that some one has produced and lacking the math a table. There are some forums about on the web where maths type questions are sometimes answered. Might be a solution for the OP as they appear to like tougher ones.

Good link Michael. There is another type with 2 dividing plates but I believe that is mostly used for pure angular divisions. Then there was that optical one on ebay for a song.

John

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[Gustav ThiesenParticipant @gustavthiesen53240]

To John Stevenson: I'm looking for this differential addon of the CES hobber. Can you give more information about the construction and the use of this addon or otherwise do you know whether the plans of collegeengineering will give this information?

[Duncan MunroParticipant @duncanmunro42894]

Hi Gustav, I've found some emails from 2003 to a chap called Michael Durkin where we tried to work out the hobber workings and some of the numbers around it. He kindly dug out some of the following pictures for me which he took at the Harrogate show. He also gave me the following explanation of how the differential worked. I'll have a deeper look for the spreadsheet as well, but with the picture and the description it should hopefully be enough to make your own differential. I'll try and dig out the calculations as well as that will get you to a point where you can cut the primes from non-prime gear sets.

"…I have figured out the differential. I noticed that the gear being cut has 36 teeth, so I tried a teeth counting exercise on the photos and tried to add everything up to arrive at 36 teeth on the blank.

I did so in this manner (you need to look at the photos at the same time!):-

Input from the motor drives the big gear behind the one with the differential, (I think they are on co-axial spindles, but not coupled), this has 60 teeth.

I think this gear is coupled to one of the differential wheels, the rear one, facing the one visible on the hob drive shaft. It also drives an idler (don't care about teeth on this) which you can just see 4 teeth of, and then onto a 60 tooth Acetal gear. This latter drives the worm and wheel, which seems to be a 36 tooth job.

The 60 tooth Acetal gear is coupled to a 30 tooth Aluminium gear and via another (Acetal 60 tooth) idler, drives the 30 tooth gear which also forms the differential cage.

I did the "mind game" involved with this differential and if the rear gear is driven at, for example, 100 rpm c.w., then this would drive the visible differential gear at 100 rpm a.c.w. if the cage was static. With the fitted gears, the cage is also driven at 100 rpm c.w., which would drive the visible gear at 200 rpm c.w. if the rear gear were static. The differential acts as a summer and the final speed of the visible gear (and the hob) is the sum of the two resultant input motions, 100 rpm c.w.

Of course, by playing with the ratios of the train between the big gear and the differential cage, you can play with the speed of the hob and the speed of the worm.

The worm acts as a 36 to 1 reducer and would have an input of 100 rpm, so the blank would rotate at 100/36 rpm. This is exactly right for the gear we look to be cutting.

I set this up in an Excel spreadsheet and played with the available change gears in a Myford set (maybe 2 sets for some ratios). It will generate lots of different outputs. If you change the worm gear, this adds even more.

I intend to analyse it further, because I think some interesting possibilities arise. This is because the differential is used to sum two speeds and end up with a third speed which can be prime. For example, if you input 200 rpm and 500 rpm (actually 250 rpm multiplied by 2 in the differential) you get 300 rpm…."

The above quoted bit was from Michael Durkin, following is the images that he used to go with the description above. More soon when I dig out the spreadsheets… Regards, Duncan.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Duncan Munro on 29/02/2016 09:47:42:

... I intend to analyse it further, because I think some interesting possibilities arise. This is because the differential is used to sum two speeds and end up with a third speed which can be prime. For example, if you input 200 rpm and 500 rpm (actually 250 rpm multiplied by 2 in the differential) you get 300 rpm…."

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Many thanks or sharing this, Duncan

I think that brief quote ^^^ shows why this mechanism should be of wide interest.

MichaelG.

[AjohnwParticipant @ajohnw51620]

The only thing I have managed to find on this subject so far is here

**LINK**

It seems that a helical gear cutting action is removed by using the none differential method against it to set the gear teeth angle back to zero or close to it.

John

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Edited By Ajohnw on 29/02/2016 12:07:21

[Gustav ThiesenParticipant @gustavthiesen53240]

to Duncan Munro:

Many thanks for this long explanation. I must confess that my competence in English language sometimes comes to a border because I learned it fifty to sixty years ago. So I must have more time to understand what's about your chaps explanation. I will repeat reading what you have posted for some times until I will be back.

[Duncan MunroParticipant @duncanmunro42894]

Hi Gustav, I now have the diagrams and spreadsheet which may be easier for you than the English text? There is a link at **LINK**

This now has a spreadsheet hobber_differential_2016_02_29.xlsx which has diagrams and text explaining how the gears interact, and also the calculations to allow the non-prime gears to be made. Hope this is of some help to you.

[Gustav ThiesenParticipant @gustavthiesen53240]

Hello, Duncan, Your spreedsheet seems to give a very good explanation. I will study it this evening. Another source with basic knowledge is this:

**LINK**

**LINK**

Many thanks!

[Michael GilliganParticipant @michaelgilligan61133]

Duncan & Gustav

There's some remarkably good reference material coming from what first looked like a simple practical question.

Thank you both.

MichaelG.

[BazyleParticipant @bazyle]

Thanks for the spreadsheet Duncan and a splendid bit of work by Michael Durkin.
Please can you correct the 'what if' page cell B11 to "3 Output driven Gear" instead of "4 Output….." to align with the diagram.

Translating the formula I gave earlier Target = ( 2 * differential train ratio) – (input train ratio) becomes

Target =( 2 * worm * output driver / cage) – (worm * output driven/input)

example 127 = (2 * 60 * 41 / 30 ) – (60 * 37 / 60 )

Clever choice of worm = input = twice cage means it reduces easily to depend on the two other gears. It looks like the 'Lathes.co' version uses cage = 40 so an adjustment can be made.

[Duncan MunroParticipant @duncanmunro42894]

Posted by Bazyle on 29/02/2016 19:23:11:

Thanks for the spreadsheet Duncan and a splendid bit of work by Michael Durkin.
Please can you correct the 'what if' page cell B11 to "3 Output driven Gear" instead of "4 Output….." to align with the diagram.

Thanks for spotting that, I've update the website page and the spreadsheet.

Regards,
Duncan

[BazyleParticipant @bazyle]

Duncan, another glitch. In 'what if' B10 and B11 need swapping over. driveR and driveN are the wrong way round. When changed the numbers for row 41 & column 37 which meet in 127 then agree with the calculations page. Difficult thing to have checked if you haven't got the machine but if left wrong people might not think to swap the gears and lose total confidence in what is a very clever spreadsheet.

[Duncan MunroParticipant @duncanmunro42894]

Posted by Bazyle on 01/03/2016 18:00:53:

Duncan, another glitch. In 'what if' B10 and B11 need swapping over. driveR and driveN are the wrong way round.

Thanks again for spotting that. I made sure that bit had been double checked, so not sure how I managed to make a mess of it… Spreadsheet and web page should now be corrected.

[Gustav ThiesenParticipant @gustavthiesen53240]

I bought the CES Plans a couple of months ago and now I try to manufacture the different components of the machine. There is one problem with the feeding of the working head carrying the gear blank which I don't under-stand. The video "little hobbing machine" doesn't show this action.

My question is: How can I bring back the working head to the start position after the first feeding of the gear blank, to begin the next feeding with the condition to find the same cutting groove.

[Chris Robinson 2Participant @chrisrobinson2]

Hello. I have just seen this thread and it may have gone cold. The Lathes UK page on the Jacobs Hobber was from my input and the photos are of my machine. The purpose of the differential add-on is purely to allow the hobbing of spur gears with prime numbers of teeth without first having to have one in the index train. A 127T gear can be made in this way. The maths id fairly simple. If anyone would like to PM me I can send you details as I have written it up.

Edited By John Stevenson on 25/10/2016 11:17:06

[Ian S CParticipant @iansc]

Probably best not to post your e-mail address for every one(not in the forum membership) to see, and send you junk mail. Could a moderator sort Ian S C

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