Gear hobbing 101 required. - Page 2
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  1. #21
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    Pics are still there

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    Search for 'hob indexer'

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    Quote Originally Posted by andrewmawson View Post
    The late John Stevenson did exactly that using a universal (ie swivle table) horizontal, an encoder on the shaft and some 'gearing' electronics between the encoder and a 4th axis for the work blank
    His gear stuff was very hobby-shop cool but for day-in, day-out work that kind of setup is just not rigid enough. He'd have been better off to pick up an old blown-up gear hobber to convert, imo. There's a reason an old 188 Cleveland will blast a 4 DP cutter through 8620 at .125" feed and get good teeth : it's got the beef. There's a lot of metal removed cutting teeth, and it takes a bunch of cast iron to hold everything in place..

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    He was very much a jobbing shop having to make oddball gears in very small quantities repairing things. But I do seem to remember that he and his first wife (Gert) under the nom-de-guerre 'Mary Poppins Bag' sold various gear sets for Myfords - especially tumbler reverse sets - on eBay.

    I think it's horses for courses - his set up was extremely versatile and allowed him to make peculiarities such as a 21 tooth gear on a 20 tooth blank to adjust gear spacing or slightly adjust ratios.

    He was using a pretty solid universal mill weighing probably ten times what the Mikron hobber Pete bought from me does- I don't think ridgidity was an issue.

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    John put a thread in Madmodder covering his hobber including a YouTube link of it running

    http://madmodder.net/index.php/topic,11283.0.html

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    Quote Originally Posted by andrewmawson View Post
    ... allowed him to make peculiarities such as a 21 tooth gear on a 20 tooth blank to adjust gear spacing or slightly adjust ratios.
    That's not a peculiarity, it's a commonality There's even a slang term for it, "drop-tooth gear" although what you described is the opposite. But the same. A nice thing about involute teeth is that kind of thing is simple.

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    That's an interesting feature but I think I'll just stick to standard gears until I get my hand in

    A question - is it true that when cutting spur gears I can disengage the feed, bring the table back and re-engage for another pass without losing index? I've given myself a headache thinking about that.

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    Pete,

    Very nice. I've got a scan of a Mikron 79 model English language manual, and it may be that it is very similar to the Mikron 112 in terms of operation. At first glance the 79 looks just like a baby brother of the 112.

    If your interested its 54MB, so I don't think I can email it to you but I can put it (and the others) on a USB stick for you.


    For what it's worth I've also got a scan of the following which may help you:

    Koepfer-Type-110 (14MB)
    Wyssbrod-125-IIa (39MB)

    and Dowding & Doll v8 (2MB) and v10 (3MB).

    At the risk of hijacking the thread if anyone has a manual for my Dowding and Doll v4 I would be most grateful, PM me.


    Thanks,
    Steve

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  12. #28
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    Quote Originally Posted by SeaMoss View Post
    certain things become automatic - like kicking the feed out at the end of travel or even any time you feel like it. When cutting a helical without a diff, you cannot ever disengage the feed after you have once engaged it. That means at the end of a pass you must turn off the machine, put the motor in reverse, and back the thing up to the beginning to take a second cut.
    SeaMoss has given a really excellent overview of hobbing, except that reversing the motor and feeding back thru as above not only doubles the process time but runs a real risk of climb-cutting a wider tooth space due to the sheer number of change gears in the two trains and the absolute necessity to have some backlash.

    Better to convert the clamp nuts on the vertical slide to handles (like a lathe tailstock) so you can lift the hob off the work and crank back to the load/unload position. If you mount an indicator with a two-inch travel on the vertical movement it will take all the worry out no matter how ancient the machine.

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    Mine has a vertical spindle and a quick-retract lever with a built-in micrometer stop so backing the hob out would pose no problem.

    Ok so cutting a 90 degree spur gear I can just release the travel clutch, back off the hob, wind back the slide and re-engage everything for another pass - regardless of the tooth count or gear pitch - but for helicals I must back the motor up? Is that right?

    I'm just about done with fitting the machine up. I've stripped and cleaned it, fitted a new modern inverter-spec motor, a VFD (because I have no 3 phase. I've disconnected the old mechanical trip and fitted a limit switch as an enable switch for the VFD. All I have to do now is fit a speed pot, set the spindle speed on the VFD and pipe up the old belt-drive oil pump and I can go about cutting some gears.

    Stvy I would love to get those scans off you. Also looking to buy or borrow a 22DP 20PA hob if you know of one so I can cut some gears for my Hardinge. Mine uses 10mm bore but I could make a hub adapter.

    Pete.

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    Pete,

    Ash gear in the usa do the correct hob for Hardinge 22/29 DP 20° PA gears.

    http://www.ashgear.com/pdfs/hm1-20_hmt1-20.pdf

    and with the 10mm bore you want. Not inexpensive though. Ash gear are the only quality supplier I have found so far that also do the smaller size hobs as standard products. I plan on ordering up a 22/29 DP 20° PA hob with a 1/2" shank for my D&D v4 one and a few others I need one day. You will already know this but your 5" limit is too small to make a 22/29 DP 20° PA 127 tooth gear. The 127 might be worthwhile sourcing elsewhere.

    P.S. I have a cupboard full of Hardinge 22/29 DP 20° gears. So am quite happy to loan you some examples to get you started. By the way H&W Machinery near you have some gear verniers for sale. I picked up a DP and MOD vernier from them at the right price when I got the D&SD v4.

    Steve

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    Quote Originally Posted by Oldwrench View Post
    SeaMoss has given a really excellent overview of hobbing, except that reversing the motor and feeding back thru as above not only doubles the process time but runs a real risk of climb-cutting a wider tooth space due to the sheer number of change gears in the two trains and the absolute necessity to have some backlash.
    Wrench - sorry, wasn't clear : that's only when you are cutting a helical without a differential. In that case, you can't disengage the feed or you lose the timing. It's because you are not cutting an integer number of teeth per revolution, so the feed becomes part of the where-you-are-when equation.

    Spur gears are timing in two dimensions. Non-differential helicals are timing in three.

    In the case of spur gears or helicals with a diff, yes, you can and should just kick the feed out. And no, Peter, you won't lose time because the hob revolutions divide evenly into the part revolutions. In general you don't even need to back out. As long as you are indexing in the same direction all the lash in the gear train is taken up in the same way so it's not usually a problem. Maybe for something that work-hardens by even being touched I'd back away but most of the time, not. Looks scary at first but it's okay.

    I don't think you will need to two-cut the fine pitch parts you'll be making, tho. Just the first one to get size, after that a single pass will do the job. And you won't be using gear tooth verniers much on those small teeth : you'll be doing mostly measure-over-wires, so watch out for nice gear wires. Even 0-1" flange mics can be cumbersome with little teeth. Fine pitch is kind of a different world from coarse pitch.

    Quote Originally Posted by Stvy View Post
    At the risk of hijacking the thread if anyone has a manual for my Dowding and Doll v4 I would be most grateful, PM me.
    I was once told that the Dowding & Doll was modelled on the Pfauter RS-00. Take that with a grain of salt but maybe ...

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  18. #32
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    I see that Ash gear do various 'grades' of hob right up to aaa. What do we suppose would be an appropriate grade for cutting lathe change gears? I can't think that a grade A would be necessary for a gear that is neither turning fast nor carrying much load or for very long.

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    By the way, you don't need a 22/29 hob- use a standard 22 DP hob and you will be just fine. Make your blank to the 22/29 OD and don't worry about the extra depth- the gears don't know the root is deep and don't care.
    A class C hob is just fine for change gears in this application.

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