Free hobbing - a rogue method?

[Robint]

I joined up here cos I was directed to view some work done on this topic last year by https://bbs.homeshopmachinist.net/fo...po-boy-hobbing
and https://bbs.homeshopmachinist.net/me...176-davidwdyer
I dont wish to be seen as hijacking his thread or over topping his work. I sent him a pm but sadly no reply

There were many other commentators on his post and perhaps those with an open mind would support my re-opening of this topic for discussion following my own similar path of work
Hopefully the OP may resurface and help share experiences with you all

Despite the skeptical views from Gear purists I believe this method has a place but I readily admit it has limitations

Pros
a) Cheap and quick to produce vs laborious dividing head method
b) No special $$$ equipment needed
c) DIY hobbing tools for a few dollars

Cons
a) Only for light duty using non ferrous mtl - not intended for power xmission or precision use
b) Does not produce standard DP conforms so may not mesh with std gears
c) Gives a 30deg PA
d) Is thought to be unpredictable re number of teeth vs design (I hope to show this is not necessarily the case)
e) May well be difficult to fit defined centers (but could drill centers afterwards or just adjust profile shift)
f) For the moment limited to 3mm CP but work in hand for larger values possible

The method must be seen as an agricultural approach that can and does produce meshing gears so typical criticisms by gear theorists (as experienced by the OP) are really not appropriate for this method and should be regarded as OT


obtw i forgot

https://bbs.homeshopmachinist.net/fo...-to-long/page5
an intriguing method I havent tried myself

[J Tiers]

As Doozer said, you have to gash the blank or rely on "PFM" to make it work.

And tilt the hob...

It's legitimate, in that B& S mention it as a method, as do others. It's likely to produce a fairly good... "acceptable" ...gear for many purposes.

[Mcgyver]

Despite the skeptical views from Gear purists

Whats a purist?"

imo its just a pejorative term. Like everyone better than you is a purist, everyone worse a hack

[Robint]

OP has deleted this post

[Peter.]

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.

[The Artful Bodger]

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.

[Mcgyver]

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

[Robint]

OP has deleted this post

[Doozer]

Robin the magician !

-D

[J Tiers]

[QUOTE=Robint;n1965133]

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.

[Robint]

OP has deleted this post

[Mcgyver]

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

[djc]

...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?

[DrMike]

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.

[davidwdyer]

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.