Free hobbing - a rogue method?

[J Tiers]

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".

[J Tiers]

...........

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.

[Robint]

OP has deleted this post

[David Powell]

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.

[Robint]

OP has deleted this post

[davidwdyer]

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.

[Robint]

OP has deleted this post

[J Tiers]

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

[Robint]

OP has deleted this post

[J Tiers]

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.............

[The Artful Bodger]

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

[Richard P Wilson]

[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

[J Tiers]

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.

[Robint]

OP has deleted this post

[Robint]

OP has deleted this post