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Free hobbing - a rogue method?

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  • Robint
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    OP has deleted this post
    Last edited by Robint; 10-15-2021, 06:32 AM.

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  • nickel-city-fab
    replied
    Huh, that's pitiful regarding the fractions. We were required to have that all down by the time we were ~10 yrs old. Calculators didn't exist yet. I use fractions as well as decimal Imperial units daily on the job, doing the math in my head as required.

    But as far as the DP and AGMA, try that one over at Practical Machinist. Some of the posters both there and here, happen to actually own gear manufacturing companies. And hence are well-qualified to speak.

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  • nickel-city-fab
    replied
    Originally posted by lbender View Post

    6 TPI hobs 18.85 DP.
    18 DP would require 5.73 TPI hob.
    So if you cut a 5.75 thread like your SouthBend can, you can make a hob that makes 18.000 DP gears.
    Thanks for the correction, I was a bit suspicious of my numbers. It had been some time since I checked it.

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  • lbender
    replied
    Originally posted by nickel-city-fab View Post
    FWIW, 6TPI Acme screws mesh beautifully with 18 DP, 14.5 PA gears, and should do a nice job of hobbing the same.
    6 TPI hobs 18.85 DP.
    18 DP would require 5.73 TPI hob.
    So if you cut a 5.75 thread like your SouthBend can, you can make a hob that makes 18.000 DP gears.

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  • nickel-city-fab
    replied
    Originally posted by J Tiers View Post
    Gee, to cut worms, you don't need a book, you just need to know the advance per turn for your leadscrew. From that comes the number of LS turns per worm pitch, and from that comes out the gear ratio. There is nothing like understanding WHY something is how it is to allow you to work with it.
    True, but it's a PITA to do module pitch on a SB... some unusual stuff required

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  • Mcgyver
    replied
    Originally posted by Richard P Wilson View Post
    Look guys, if you want a nice clear explanation of how to do free hobbing, without a lot of rhetoric and slagging off of others, you can't do better than the articles I referred to in post 53, Jock Smith's 'Gears from Scratch', Model Engineer, 17th March and 21st April 1995.
    Otherwise, just read on---
    I just went into the archive and pulled that issue. I won't copy it, its under copyright, and we're here on this site by the good graces of a magazine publisher. Model Engineer and other mags are a wealth of info but they are often hard to give away....so if someone wants a copy they are out there to buy. The second part, in the April 4th issue, is the issue Eric Whittle began his great series on the Aero V8 - great stuff! On the gears, I'll describe the basics below and anyone can fill in the details, i.e. common knowledge available through searching

    Its a good article and gives lots of info on making hobs. The hob is held in the lathe and the work mounted on an axle which is bolted to the vertical slide. In his set up, the axis of work end ups parallel to the X axis (cross slide) and is positioned under the hob (on the Z axis). The cross slide is the feed, and the vertical slide the depth. As anyone with any gear knowledge has said repetitively in these threads, the verticle slide assembly (on the cross slide) is rotated (around the Y axis) to match the hob's helix angle (to a good tooth form) and the work is rough gashed in advanced. That's the short synopsis of the article. He gives a detailed description of making a gear and also goes onto helical gears.

    This should create a reasonably good result suitable for lots of purposes, however when making ones own hobs the challenge becomes getting the hobs pitch to that of a standard diametrical pitch or module gear. IT can be done with change gears (or a DSG does it)....or if its a closed system, gears only used with themselves, it won't matter. Of course if you are buying a hob, its moot.
    Last edited by Mcgyver; 10-13-2021, 10:02 AM.

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  • Richard P Wilson
    replied
    Look guys, if you want a nice clear explanation of how to do free hobbing, without a lot of rhetoric and slagging off of others, you can't do better than the articles I referred to in post 53, Jock Smith's 'Gears from Scratch', Model Engineer, 17th March and 21st April 1995.
    Otherwise, just read on---

    Leave a comment:


  • Robint
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    OP has deleted this post
    Last edited by Robint; 10-15-2021, 06:56 AM.

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  • Robint
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    OP has deleted this post

    Last edited by Robint; 10-15-2021, 06:33 AM.

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  • mattthemuppet
    replied
    Originally posted by nickel-city-fab View Post

    Ooo nice! (slightly envious) Being a Narex, I'll bet the gears are Modular pitch, which might make the math a bit easier.
    I wouldn't be too envious, it's currently a very neat paper weight as it's missing most of the bits that allow it to work as a boring head Didn't cost much and it'll be worth it in the end. As far as I can tell the worm driven by the star wheel for feed advance is TR12x1.5, so I would expect the worm gear to match that. I think the max diameter is 16mm, but I'd have to check. That will also be tapped TR12x1.5 for the feed rod, which I also have to make..

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  • john b
    replied
    I think his eyes are crossed from looking down his nose at us mere mortals.

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  • sid pileski
    replied
    Originally posted by Robint View Post
    Q


    OBTW we got 1450 views in 4 days so the silent majority have shown interest?
    I think most people are really waiting to see where your cryptic ramblings and pompous attitude is going to end???

    Yikes!

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  • J Tiers
    replied
    Gee, to cut worms, you don't need a book, you just need to know the advance per turn for your leadscrew. From that comes the number of LS turns per worm pitch, and from that comes out the gear ratio. There is nothing like understanding WHY something is how it is to allow you to work with it.

    Leave a comment:


  • nickel-city-fab
    replied
    Originally posted by mattthemuppet View Post

    I have a small worm gear for a Narex boring and facing head that I need to make, that should be an interesting introduction to gear making
    Ooo nice! (slightly envious) Being a Narex, I'll bet the gears are Modular pitch, which might make the math a bit easier.

    Leave a comment:


  • mattthemuppet
    replied
    Originally posted by nickel-city-fab View Post

    It'll come in handy if you ever decide to do some fancy weird stuff with your lead screw for example. I was considering a SB model B instead of the A that I got, just because of this book. I got the A because it was in better shape and already tooled up.
    I have a small worm gear for a Narex boring and facing head that I need to make, that should be an interesting introduction to gear making

    Leave a comment:

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