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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, 06:36 AM.

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  • DrMike
    replied
    Originally posted by Peter. View Post
    They won't mesh if you generate them both using the same hob because they need to be opposite handed.
    Yikes... I knew that, brain fart. Sorry.

    Still looking for the answers to my other questions

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  • Peter.
    replied
    Originally posted by DrMike View Post
    Generating helical gears isn't necessarily a bad thing, especially if they are only to mesh with gears generated using the same process. However but I'm a bit confused why they won't mesh. They will try to push each other away both radially and axially, but that's what helical gears do.
    They won't mesh if you generate them both using the same hob because they need to be opposite handed.

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  • DrMike
    replied
    Some of us are still waiting with open minds for a clear and detailed description of this process and its results.

    So far we know that Robint takes a standard bolt, mills axial grooves into it to form cutting edges, then uses it to mill tooth-like shapes into the face of a soft (aluminum, plastic) blank. We don't yet know the precise shape of these teeth. nor do we know everything about his process in order to try it for ourselves.

    A couple of posts above we learned also to "tilt the work piece in the vice jaws - in my case 2deg up from horizon if you dont tilt then you get a slight helical gear which will show up when you mesh two together and they dont want to rotate in a coplanar manner."

    Is that 2° a precise number, and is it related to the M24-3 bolt you used? The helix angle of a M24-3 bolt should be close to 7°, just wondering where the 2° came from.

    Generating helical gears isn't necessarily a bad thing, especially if they are only to mesh with gears generated using the same process. However but I'm a bit confused why they won't mesh. They will try to push each other away both radially and axially, but that's what helical gears do.

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  • Robint
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    OP has deleted this post
    Last edited by Robint; 10-15-2021, 06:42 AM.

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  • Robint
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    OP has deleted this post



    Last edited by Robint; 10-15-2021, 06:42 AM.

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  • J Tiers
    replied
    Originally posted by The Artful Bodger View Post
    Ummm..........

    Using the pitch of the hob/bolt to gash the circumference of the blank would produce an error as the circumference of the blank is more than the circumference of the Pitch Circle. However the hob/bolt is not aligned with the blank but does this correct the error?

    The hob/bolt has an angle of 30 degrees but the cutting edge does not meet the work at 90 degrees therefore the final angle will be less than 30 degrees.

    John
    "Spose you just make the OD what it should be, and gash the number of teeth you will have? No pitch involved, and it surely does not seem it can go far wrong. One often gashes that way doing form cutting. It always works out.

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  • Richard P Wilson
    replied
    [QUOTE=Robint;n19652
    FWIW why did BSW choose 55deg - the answer lies in geometrical drawing methods of the time - compass and set sq - try it out for yourselves, draw the triangle with 55deg, lop off the top and bottom 1/6th of the total height, now you will see how to draw the top and bottom curve taken from the width of the cut off as the radius drawn on the centre line PFM
    Very few sources bother to explain this



    Robins in Top Gear[/QUOTE]

    BSW didn't choose anything. BSW is British Standard Whitworth, a UK British Standard range of threads but based on a system originally devised by Joseph (later Sir Joseph) Whitworth..

    The problem JW was trying to solve in the early 19th century, was to establish a standard range of screw threads instead of each manufacturer having his own standards, so that a bolt from company'A' wouldn't fit a nut from company 'C'. He took samples from a large number of companies, and averaged them to create his well known table of screw threads. By his own account, he did the same thing for the thread profile, the well known 55 degree profile being nothing more scientific than the mean of a number of different profiles, rounded off top and bottom to prevent accidental injury to the screw.
    He presented this in a paper to the Institution of Civil Engineers in London in 1841, and by 1860, his screw thread standards were pretty much accepted throughout the UK

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  • The Artful Bodger
    replied
    Ummm..........

    Using the pitch of the hob/bolt to gash the circumference of the blank would produce an error as the circumference of the blank is more than the circumference of the Pitch Circle. However the hob/bolt is not aligned with the blank but does this correct the error?

    The hob/bolt has an angle of 30 degrees but the cutting edge does not meet the work at 90 degrees therefore the final angle will be less than 30 degrees.

    John

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  • J Tiers
    replied
    We got one "don't do it, or everything will be screwed up" And we got one "you need to do it or everything will be screwed up".

    Too bad I don't like popcorn.............

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  • Robint
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    OP has deleted this post
    Last edited by Robint; 10-15-2021, 06:41 AM.

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  • J Tiers
    replied
    Originally posted by davidwdyer View Post


    I'm not entirely sure of the reason. My guess is that on a real hobbing machine, the slight angle provides something (maybe a slight change in cutting angle) which helps

    form the proper involute tooth form. In a true hobbing machine, the blank is being rigidly held to a certain rotation, in relation to the hob, so that it cannot actually follow

    this angle and so it just impacts the form of the cut...................
    The angle just compensates for the angle of the helical hob, as I understand it

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  • Robint
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    OP has deleted this post
    Last edited by Robint; 10-15-2021, 06:40 AM.

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  • davidwdyer
    replied
    Originally posted by J Tiers View Post

    Not arguing..... just trying to understand it, as I cannot see the reason for it. You have tried it, so.......

    Did your gashing affect that?

    The OP did not gash, would that affect it?

    What do you think actually causes the problem if the hob is set at the "proper" angle?

    I've not done this with a real hob. Did try it with the "tap" type hob, to see if it could work.

    I'm not entirely sure of the reason. My guess is that on a real hobbing machine, the slight angle provides something (maybe a slight change in cutting angle) which helps

    form the proper involute tooth form. In a true hobbing machine, the blank is being rigidly held to a certain rotation, in relation to the hob, so that it cannot actually follow

    this angle and so it just impacts the form of the cut.

    But in free hobbing, there is no resistance to this angle change and so the hob just follows the straight line, which in this case is at a slight angle.

    I tried this with a "less than slight" angle to test the concept. Sure enough, the teeth all ran along the face of the gear at the angle to which I set the head.

    About the OP not gashing: He may be able to run his mill at a very low speed and therefore did not have damage when beginning the cut.

    I have cut a worm gear with a tap (many others have done this) using a freewheeling blank and it worked out quite well.

    I have purchased four hobs on the internet (sizes I thought I might need, from not too fine to not too coarse) for around $80 apiece.

    (Nothing metric). But with these four hobs, totaling around $320, I believe I can cut almost any gears I could want. (DP 12 16, 18, 22. 20 PA if I remember correctly).

    The truth is I have too many other projects demanding my attention and I haven't been able to get back to this challenge.

    But with my limited success, I am sure it can be done for anyone who would like to make a bunch of gears.

    Besides the ease of this method (once a good fixture is made) there is the huge advantage of speed compared to using a typical single gear cutter.



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  • Robint
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    OP has deleted this post
    Last edited by Robint; 10-15-2021, 06:40 AM.

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