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Free hobbing - a rogue method?

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  • David Powell
    replied
    Not quite on topic but----- A few times in my rather" lets get her done" way I have cheated the " proper" ways to make gears and gearing, and ended up with success. My first was making a copy of a proper 70 tooth 10 dp gear by drilling hacksawing and filing a blank, using the geniune gear as the divider. The gear I made is still in use 40 yrs later. Subsequently I have made a few worms to match with gear wheels I already had by rolling the gear wheels by hand on taps till I found one which seemed to match reasonably, cutting that thread on a blank which LOOKED !!! like an appropriate size, meshing the two together and running them in together till they ran reasonably smoothly.
    I well know these methods do not produce gears of merchantable quality, but they have solved problems for me.
    Give it a try, all you have to lose is some time and a bit of material. If you are successful give yourself a pat on the back, if it does not work as expected, well you have learnt something !
    Regards David Powell.

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  • Robint
    replied
    OP has deleted this post





    Last edited by Robint; 10-15-2021, 06:39 AM.

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  • J Tiers
    replied
    Originally posted by davidwdyer View Post
    ...........

    Despite what some who have invested in a specialized hobbing machines have stated, you should NOT approach the work at an angle unless you want an angled gear.

    Keep the head tilt at 0 degrees. DO NOT TILT THE HOB! This may be necessary for a dedicated machine, but will not work with free hobbing.

    Don't bother to argue with this unless you have tried it yourself. I have tried tilting the head and know the results.

    S
    .....................

    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.

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  • J Tiers
    replied
    Free hobbing with an actual hob is nothing radical, new, or even necessarily bad. The big issue with it is that the part does not mechanically get moved in synch with the hob.

    That's all that the "purists" have to hang their hat on. It's not a negligible point, if it were really true, but.....

    Why would the hob not force the part to move correctly? Well, that depends on how much metal is being cut by each side of the hob tooth, and the degree to which that pushes it off-"center".

    There is no significant friction to stop the part rotating, And, once the front of the tooth has started cutting, the balance of the tooth is acting as a regular gear, to force the rotation. Folks who squall and scream about free hobbing tend to neglect that factor.

    Well, the hob does have some back relief, so the "teeth" are not exactly where gear teeth would be. How much does that cause as error? If the teeth are reasonably even in thickness, as is usually the case, not too much error. you can tell at the end of the first rotation, if the last toothspace is not centered.

    If the last toothspace is centered, it can't be that bad on average, and the gear should be quite usable.

    The OP can feel free to quote this to the other lot of "theorists".





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  • davidwdyer
    replied
    Congratulations Robint. You had the courage to try this with some success.

    I hope some others will try it also.

    Sorry I didn't see your message. I am an infrequent visitor on this site.

    This way of hobbing, which I tried to explain in my previous post (but was roundly criticized and ridiculed and therefore did not post again), is workable!

    I have meant to post another video with some further thoughts.

    To obtain good results, you should "gash" your gear blank first.

    One of the biggest problems, if not the only problem, is starting the hob without damaging the blank. My mill (Bridgeport) does not have a very slow speed.

    Therefore, when I entered the blank with the mill on, the hob then started the blank rotating. This sudden jerk of starting often damaged the leading edge of the blank.

    I believe this can be solved!

    The method I believe that will work (but have not had time to try yet), is to put a ratchet and socket on the draw bar "nut" and just apply a little pressure on the blank.

    This amount of "pressure" (how many thousandths) you apply, may be a question of trial and error until a good result is achieved.

    Then turn the mill with the ratchet and make a beginning of a row of teeth by "gashing".

    What this means is just to start the beginning of the row of teeth. Just on the edge of the blank.

    You would not need to work all the way across the face of the blank.

    With a full round of gashing complete, the hob (whether purchased or made) will then follow this line.

    The hob teeth then have a guide and the freewheeling blank will easily follow.

    If you have a mill with a VERY low RPM perhaps this could be done with the motor.

    Despite what some who have invested in a specialized hobbing machines have stated, you should NOT approach the work at an angle unless you want an angled gear.

    Keep the head tilt at 0 degrees. DO NOT TILT THE HOB! This may be necessary for a dedicated machine, but will not work with free hobbing.

    Don't bother to argue with this unless you have tried it yourself. I have tried tilting the head and know the results.

    Since my first post I have added a second support to the shaft holding the blank. More rigidity!

    There is no doubt in my mind that this method can be used to make usable gears for home use.

    If you want to open a gear factory or make gears to sell to others, by all means buy a dedicated machine.

    But for personal use, this method is VERY workable. It just needs someone with some time and patience to work it out.

    It is a way to quickly make a whole set of matching gears.

    Additional thoughts: I think that a key and key-way on the shaft should be used to help the blank not to dis-align or spin under cutting pressure.

    I know it's extra work, but will help the result.

    Rigidity and ease of rotation for the blank are important for good results.

    This probably means having shaft supported on both ends by bearings. The first set up takes some time to make, but from then on, it's ready to go.

    The number of teeth should be predictable if you have the right DP diameter.

    Also, with my new untried method of gashing, you can count the number of teeth before you begin. The hob will NOT jump this gashing as long as

    your setup is really freewheeling.

    There is no reason that, with a hardened steel hob, you could not make gears of ferrous metal.

    Chinese hobs were (who knows now) cheap. One hob makes up for a whole range of gear cutters.

    Using a commercially made hob, there is no reason why you could not produce a very acceptable, or even "correct" gear form.

    Even with purchased cutters the gear form is just close and often not "perfect".

    OK. I've said my peace. You can now bring on the abuse.


    Last edited by davidwdyer; 10-10-2021, 08:27 AM.

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  • DrMike
    replied
    Robint - Please describe your process in detail, and with a minimum of attitude. At the moment all we know is that you can generate some tooth-like forms on a blank. I am willing to do the modeling and math to determine the shape of your gear teeth, and how they differ from a standard involute profile. But this will only happen if you lose the attitude; this discussion will not devolve into an argument.

    It appears that you take a standard bolt, mill axial slots into it to form cutting edges, then use it as a hob to generate teeth on a blank. The bolt/hob both cuts and turns the blank at the same time. In doing so you generate teeth in your blank, but you do not know the details of the tooth form or even (correct me if I'm wrong) if two such gears mesh smoothly with each other.

    You already seem aware that you are not generating standard involute profiles, and that your gears will not have the advantages of conjugate action. However, for the limitations you have already listed (soft gear materials, no precision movement or power transmission applications, not compatible with standard involute gears) it may have some applications that simply require relative rotary motion without much precision or power transmission to justify an inexpensive hack, even if it's to quickly develop gears for an experimental prototype before spending the money on gears or involute cutters.

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  • djc
    replied
    Originally posted by Mcgyver View Post
    ...it could be movement (like a clock movement)
    But each part of the movement will go in an opposite direction to its neighbour.

    I reckon his ideas are too advanced for the community here.

    They might better be explored in a forum where new concepts are welcomed by the far-seeing, receptive members, particularly if he can show how they might be realised on the excellent machine tools coming from Asian sources. Does anyone know of such a forum?

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  • Mcgyver
    replied
    Good for you campaigning on gear social justice, a Robin Hood of sorts bring gears to the common man. (like they were purview of only the elite before lol)., Give the people gears (or the gears), it could be a movement (like a clock movement)

    Few of us will have the equipment to generate gears - however technology does hold the promise of freeing the masses and bringing good gears to all. Simply tilt the mill head to the hobs helix and electronically couple the spindle to a 4th axis. Whats so hard about that? Any peasant could do it. If you wanted to contribute something, lots of possibilities there.....or how about coming up with electronically controlled relieving tool for the lathe. A modern version of the Eureka so the common made could make & relieve his own hobs. Or take it to the next level and including grinding capabilities? There are lots of ways someone wanting to could really help with the democratization of gears, could make a real difference, and contribution.

    There are also ways to make gears that are less than perfect (generation) but are good enough for the task at hand. Form cutters is one, what I called faceting is another - see here: http://www.helicron.net/workshop/gearcutting/ . Talk about a gears for the proletariat, comrade. The second one works and is very simple and cheap - uses a home made cutter. That's the cheap charlie method that gives something workable. If you want to experiment - work on ways to create more facets (e.g. move the Z), i.e. more facets gets close to a curve.

    ok, put the hyperbole aside. If you expect people to listen and hear you out on a new idea, great, its only reasonable that you to also listen and are willing to learn, right? In addition, to have a sense on whether a method will work, you do need to understand what is going on with a gear, I right? I mean its no more intelligent to scoff at theory while proffering something you think is new, than it is to scoff at the theory of gravity in the hope it doesn't affect your day to day life. You'll have no way to assess if you do not understand the theory. This is not art, it is mechanics and engineering.

    And lose the chip, just because someone who knows what they are talking about says you've got something wrong, doesn't make them a purist, naysayers, theorists etc. View it as positive that people are willing take the time to share their knowledge.

    Maybe start by describing in detail exactly what you see as the process/technique; the devils in the detail and I'd bet not all are necessarily on the same page as to exactly what you are proposing
    Last edited by Mcgyver; 10-12-2021, 04:21 PM.

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  • Robint
    replied
    OP has deleted this post
    Last edited by Robint; 10-15-2021, 06:47 AM.

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  • J Tiers
    replied
    [QUOTE=Robint;n1965133]
    Originally posted by J Tiers View Post

    It's legitimate, in that B& S mention it as a method, as do others.

    UQ

    Have you found any sources? Ive not has any success

    Will post some pix and vids shortly

    Robin
    I'm not where I can look for them. But it has been in textbooks etc. Mostly with gashing first, so no chance to get away from a dividing head.

    The "PFM" comes in if doing the "hobbing" (hobbying?) without gashing, but it can work.

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  • Doozer
    replied
    Robin the magician !

    -D

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  • Robint
    replied
    OP has deleted this post









    ‚Äč

    Last edited by Robint; 10-15-2021, 06:47 AM.

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  • Mcgyver
    replied
    Originally posted by Peter. View Post

    And then delete them all no doubt.

    Just so the membership know this guy has run a recent 14-page thread on another forum on the exact same topic, didn't like some of the the critical comments he received, dismissed the more knowlegable members who attempted to point out the fundamental errors in his thinking as 'thoeretical gear purists' then promptly trashed the entire thread by deleting all of his own content.

    Indulge him at your peril, or if you have time to waste.
    at least the guy here didn't delete all his content, otherwise sounds like deja vu all over again

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  • The Artful Bodger
    replied
    Originally posted by Robint View Post


    https://bbs.homeshopmachinist.net/fo...-to-long/page5
    an intriguing method I havent tried myself
    I was fortunate in that the first piece of plastic I tried was ABS and it worked well, other types of plastic have not been so successful for me.

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  • Peter.
    replied
    Originally posted by Robint View Post

    Have you any sources? Ive without success

    Will post some pix and vids shortly

    Robin
    And then delete them all no doubt.

    Just so the membership know this guy has run a recent 14-page thread on another forum on the exact same topic, didn't like some of the the critical comments he received, dismissed the more knowlegable members who attempted to point out the fundamental errors in his thinking as 'thoeretical gear purists' then promptly trashed the entire thread by deleting all of his own content.

    Indulge him at your peril, or if you have time to waste.

    Leave a comment:

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