Flycutter for gear hobbing.

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timo_gross

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A while ago I saw some "magic" single tooth flycutter used on a hobbing machine. (cannot find the youtube anymore)

Now it is a little hit and miss, because I do not really understand what I am doing :)
I ground a tip that should be matching the gap in a gear rack. ( I used a commercial hob to check the width.

K1600_IMG_5410.JPG
I tried to adjust the radius of the flycutter tip to 11 mm, for a module 0.5 I calculate a helix angle of 1.3° If I understood the few hints, I found here and there correct, the flycutter has to be rotated by that amound, and the head of the milling machine needs to be tilted by the same value.

With that I hobbed the first gear yesterday and it seemed to be O.K. it was meshing reasonable well with other gears I made with a propper commercial hob.
K1600_IMG_5409.JPG
The upper gear was hobbed with a hob some time ago, the lower one was done with the described flycutter. They seem to mesh reasonable well for what I can expect.
The gear has a rel. high number of teeth, so I am not sure if the tooth profiles would be formed more or less accurate for smaller gears.

So I tried to cut a smaller gear and apparently in the larger gear I just had good luck centering the cutter height by coincidence. The cutter height is important for the outcome. (for the commercial hob it is more or less irrelevant, as long as not changed during cutting).
K1600_IMG_5406.JPG
So with the wrong height of the flycutter the tooth profile is way out. ( funny enough it seems still to roll somehow on the bigger gear even with that weird looking teeth).
K1600_IMG_5407.JPG
Things got a little better after trying to adjust the heigt on center.

I think I will have to keep experimenting.



Greetings Timo
 

pete

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For the very best information about cutting gears at our level I'd highly recommend this book. https://www.teepublishing.co.uk/books/workshop-practice-series/gears-and-gear-cutting-1/ I've even seen it on the book shelf in a few professional machine shops. But unless your somehow slaving the cutting tools rotation to the gear blanks rotation by mechanical or electronic methods, it's not hobbing a gear. Since you didn't show or mention anything like that, I suspect that's not what your describing and are cutting each individual tooth by more standard indexing methods. There is at least one pretty decent small mechanical gear hobber available if you build it from a set of castings. Gear Hobber Casting Set and quite a few bits of information around if you use the search terms Jacobson Gear Hobber on Google or YouTube. The more common single row multi tooth blade type gear cutters, or more accurately the design originally invented by B&S, will have a total of 8 individual cutting tools to cover the full range of gear teeth that can be cut at that pitch. However they can only cut a less accurate approximation of the correct tooth profile against what the proper single hob can cut for the same full range of tooth counts on module (metric) or diametral pitch (imperial) gears. For most of us and what we might be needing gears for, those B&S cutters or even your single tooth fly cutter can usually work well enough. An incorrect tool C/L gets you that incorrect profile you mentioned, and yes it would still work, but high rates of wear and permanent damage to the other correctly cut gear would be the likely result. Gear teeth are shaped the way they are to minimise as much as possible the amount of sliding wear and frictional losses as each tooth rolls into engagement and then rolls out as they rotate against each other. For something as simple as they seem, I'm not sure it's even possible to fully understand the complexity's of gearing given all the different types by even the best gear experts. I understand just enough to realize I don't know very much and probably never will. :)
 

SmithDoor

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A while ago I saw some "magic" single tooth flycutter used on a hobbing machine. (cannot find the youtube anymore)

Now it is a little hit and miss, because I do not really understand what I am doing :)
I ground a tip that should be matching the gap in a gear rack. ( I used a commercial hob to check the width.

View attachment 138800
I tried to adjust the radius of the flycutter tip to 11 mm, for a module 0.5 I calculate a helix angle of 1.3° If I understood the few hints, I found here and there correct, the flycutter has to be rotated by that amound, and the head of the milling machine needs to be tilted by the same value.

With that I hobbed the first gear yesterday and it seemed to be O.K. it was meshing reasonable well with other gears I made with a propper commercial hob.
View attachment 138801
The upper gear was hobbed with a hob some time ago, the lower one was done with the described flycutter. They seem to mesh reasonable well for what I can expect.
The gear has a rel. high number of teeth, so I am not sure if the tooth profiles would be formed more or less accurate for smaller gears.

So I tried to cut a smaller gear and apparently in the larger gear I just had good luck centering the cutter height by coincidence. The cutter height is important for the outcome. (for the commercial hob it is more or less irrelevant, as long as not changed during cutting).
View attachment 138803
So with the wrong height of the flycutter the tooth profile is way out. ( funny enough it seems still to roll somehow on the bigger gear even with that weird looking teeth).
View attachment 138804
Things got a little better after trying to adjust the heigt on center.

I think I will have to keep experimenting.



Greetings Timo
I use that type too
If look the down load section I put set drawings for my holder.
Being a small diameter the cutter can turn at higher speed.
Handy in small mills and speed up cutting time.

Dave
 

timo_gross

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For the very best information about cutting gears at our level I'd highly recommend this book. https://www.teepublishing.co.uk/books/workshop-practice-series/gears-and-gear-cutting-1/ I've even seen it on the book shelf in a few professional machine shops. But unless your somehow slaving the cutting tools rotation to the gear blanks rotation by mechanical or electronic methods, it's not hobbing a gear. Since you didn't show or mention anything like that, I suspect that's not what your describing and are cutting each individual tooth by more standard indexing methods. There is at least one pretty decent small mechanical gear hobber available if you build it from a set of castings. Gear Hobber Casting Set and quite a few bits of information around if you use the search terms Jacobson Gear Hobber on Google or YouTube. The more common single row multi tooth blade type gear cutters, or more accurately the design originally invented by B&S, will have a total of 8 individual cutting tools to cover the full range of gear teeth that can be cut at that pitch. However they can only cut a less accurate approximation of the correct tooth profile against what the proper single hob can cut for the same full range of tooth counts on module (metric) or diametral pitch (imperial) gears. For most of us and what we might be needing gears for, those B&S cutters or even your single tooth fly cutter can usually work well enough. An incorrect tool C/L gets you that incorrect profile you mentioned, and yes it would still work, but high rates of wear and permanent damage to the other correctly cut gear would be the likely result. Gear teeth are shaped the way they are to minimise as much as possible the amount of sliding wear and frictional losses as each tooth rolls into engagement and then rolls out as they rotate against each other. For something as simple as they seem, I'm not sure it's even possible to fully understand the complexity's of gearing given all the different types by even the best gear experts. I understand just enough to realize I don't know very much and probably never will. :)
Hello Pete,

yes the mentioned book is for sure the starting point with lots of explanations. It does not cover a whole lot of hobbing. The "free" hobbing of the wormwheel comes closest to what I am trying to do, but with only one tooth free hobbing will not work out. I like the small mechanical hobber.
The machine is indeed "slaving" the gear blank to the spindle.
The startpoint was a simple low end cnc mill that I bought 2nd hand. Quite some modifications done to it so far.
K1600_IMG_3561[1].JPG.JPG

Number of workpiece tooth I change in the software. The Idea is that the shown flycutter is only ground with straigt flanks no curves. Hopefully it will produce the desired involute gear form somewhat automatic, but with the rather high tooth counts I did so far, it is not yet very clear if I got it all wrong in theory, or is it just just poor execution.
Yes complicated and I probably will never get a high speed high performance quality, but I am already getting some usable gears.

Greetings Timo
 
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A while ago I saw some "magic" single tooth flycutter used on a hobbing machine. (cannot find the youtube anymore)

Now it is a little hit and miss, because I do not really understand what I am doing :)
I ground a tip that should be matching the gap in a gear rack. ( I used a commercial hob to check the width.
Hi Timo,

The gear with the offset teeth reminds me of "wolfs tooth" gears used in some high end large watches for winding :) Don't tell people you screwed up, tell them you are experimenting with a buttressed form enhanced strength power transmission device. Funny thing is you will actually be telling the truth, even if you don't know it!

In all seriousness, it is quite surprising just how effectively a fly cutter can generate gear spaces. Antique clocks had many difference tooth forms and commercial cutters seldom matched up very well. I made a lot of fly cutters to either cut entire new clock wheels or to cut perfectly matching teeth after inserting a new piece of brass where badly damaged teeth used to live.

Cheers,
Stan
 

timo_gross

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Hi Timo,

The gear with the offset teeth reminds me of "wolfs tooth" gears used in some high end large watches for winding :) Don't tell people you screwed up, tell them you are experimenting with a buttressed form enhanced strength power transmission device. Funny thing is you will actually be telling the truth, even if you don't know it!

In all seriousness, it is quite surprising just how effectively a fly cutter can generate gear spaces. Antique clocks had many difference tooth forms and commercial cutters seldom matched up very well. I made a lot of fly cutters to either cut entire new clock wheels or to cut perfectly matching teeth after inserting a new piece of brass where badly damaged teeth used to live.

Cheers,
Stan
Did you use the flycutters for cutting and indexing?

Have lots of not so good trials :cool: If I cannot make them smaller and try again, they become spacers. (at least they are reamed holes with something around them)
With the right spring it will make a good ratchet wheel.
Tried to make apinion with the flycutter and it was totally off, so there is something fundamentally wrong the way I tried it.

Maybe someone will give me a hint what I am doing wrong.

Greetings Timo
 
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Hi Timo,

OH BOY! Gear cutting! Books, no make that Tomes, have been written on gear forms, cutter creation, it's a lovely can or worms. But - like screw cutting, if you just ignore all the drama it falls into place well enough to get perfectly usable results without undue stress. Perfectly usable is defined as gears that run well with others you make, or others purchased from commercial gear makers. It does not mean the gears are so utterly perfect that they will run silently at 500 RPM or be perfectly formed with no variance from the theoretical ideal form.

My usual fly cutters were used for cutting clock wheels, which are typically cycloidal rather than involute in form. I machined the cutters from A2 tool steel, then hardened after they were correctly shaped and relieved. An end mill the same or slightly smaller in radius than the radius of the tooth tip is used to rough out the cutter, with the piece of A2 inclined 7 degrees in the mill vise. Remember to do a cut across the end of the cutter to be, inclining the cutter blank generates all the relief needed for clock work in thin stock. If you need side clearance as well it can be added with a needle file or a small grinding point as you wish.

KEY POINT: People talk about cutting gear teeth. I never cut a gear tooth unless I was removing it to repair a gear. We cut gear spaces. We all know this, but thinking teeth while making gear space cutters has caused more than one person to get turned around and make a perfect single tooth on the end of the cutter, rather than a perfect copy of the space to cut.

For involute gears you can always use a gear of the desired pitch and number of teeth as a template and just layout the cutter by scribing from the gear space. There are quite a few online guides to making gear fly cutters, or for making form tools to create more conventional gear cutters. I believe Ivan Law's excellent book in the Workshop Series (number 17 I think) has been mentioned. If you start with hardened lathe tool blanks, grinding is about the only way to get the cutter made. I'd rather cut than grind, it's quicker and when done in mill a more controlled process for me, as I tend to have a hard time getting dead on the line of concave curves with a grinding point. Use the largest diameter end mill or grinding point you can that still fits within the curve. Let the tool generate the form as much as possible, leaving only refining the small detail work to hand work. In smaller module or higher pitch gears, you can usually find an end mill that is close enough to the theoretical size to work just fine. Let it be easy when you can.

Here are a couple of videos that might be of interest:





Both of these are using grinding and hardened cutter blanks. Lots of work and lots of mess in my book, but if you imagine the grinding tool being an end mill of close to desired diameter it will all fall into place. Depending on metal availability grinding may be the only way to go. Lathe bits are all over, good tool steel in annealed state may not be.

Here's a time consuming way to generate gears with minimal equipment. A perfect time vs money trade off option:



Here's a better way to create multi tooth involute approximating cutter that looks like a gear hob but does not require synchronized cutter and work spindles:
The interesting part starts around 2 and half minutes in, the beginning is a recap of traditional methods. There are several videos in this series, gears are actually made in the follow on video.



If you find it easier and the material you are making gears from is a wee bit less demanding, perhaps aluminum, brass, or leaded steel, you may find it easier to cut the tooth space profile on the end of round stock, then mill away half the diameter, just as when making D bits. Sounds iffy, but I used this method a lot for the nasty little stubby odd form teeth on some French round movement spring barrels.

Here is a pic of a cutter made this way, the cutter on the right is a fly cutter, the others are piloted oil sink and bushing cutters. These are probably oil hardening drill rod rather than A2. Quickie stuff knocked out to do a job, it isn't fancy. The bushing and oil sink cutters are hand turned in use, no effort to make them perfectly symmetrical as they will never spin fast.

Cutters.jpg


You asked about indexing? However you can hold and index the blank in your shop is the way to do it. I made dividing plates that can be mounted on one of my spare Taig (Peatol) lathe headstocks that are great for quick jobs. For larger work there is a dividing head. A rotary table with dividing plates works well, as does the cute little automated rotary table Sherline in the US sells. I believe on your side of the Atlantic it's knows as the Division Master. A four axis CNC mill is lovely, lets you do other stuff while the machine does the work. Once again, if you take away the "complicated gear cutting" terms, and I asked you how cut 24 reasonably equally spaced slots in the edge of a disk, I bet you would have an easy answer for your shop and tooling. Just replace the slitting saw with a form tool and you're in the gear business.

Indexing and gear cutting are in the category of things that can be done with time or money. If you are doing it as part of a business, you throw money at it. Doing two gears a year at home, get a nice cup of tea and spend a few hours per gear playing in the shop. Or buy the gears when you can, even if they need some rework for your application.

Last : PINIONS. Pinions can be a pain to deal with. As the number of teeth decreases, the form of the space gets more and more extreme. As you have probably found, using a higher count cutter to cut a pinion results in very skinny tooth bottoms, and big fat teeth at the top. Nothing quite fits, but each tooth in isolation looks just fine until you notice how top heavy it is. The cutter becomes more and more "V" shaped, and also becomes a fatter "V" as the tooth count gets lower. If you are forming the cutter on a lathe, you need to swing the compound to an increasingly more shallow angle with respect to the spindle centerline. On a mill, you need to move the vise to an angle with respect to the axis, and do it twice, once in each direction for the two flanks of the cutter. Yup, pinions are a pain. For clock work I either used commercial cutters to get them almost right then refined the form as needed or cut them with multiple passes of a slitting saw and refined the form with files. Funny, cutting a 9 tooth pinion could take longer than letting my little cnc mill knock out a 96 tooth wheel!

Should you have the misfortune of becoming a gear cutting addict, particularly for clocks or watches, J Malcolm Wild's book "Wheel and Pinion Cutting in Horology" is a classic. For lots of gear cutter manufacturing tooling set ups and such, Robert Porters "The Clock & Watch Makers Guide to Gear Making" is wonderful. Once more, the Ivan Law book is just about a necessity for the home shop person getting started in gear cutting. Just don't get bogged down in all the gadgetry in some chapters, it's the forms and dimensions that are everything in gear cutting.

Hope this gives you some help and not a headache,
Stan
 

timo_gross

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Hello Stan,That is quite a post to digest. :cool: A good video collection with some nice alternative to hardening. For some reason hardening steel was not (yet) working out, without having tried very persistent. I must admit. That is the next worm can sitting in the rabbits storage room.
Today, i was trying to find something about hobbing with a flycutter.
Using a rack profile, not a tooth gap shape is the precondition here. (until an hour ago I was convinced that a form tool is too much work to grind)
I had the thought that the problem might be that the hob has many teeth at different heights and that not only one tooth is cutting.

Conclusion:
  • single point cutter needs to be moved in z-direction
    • very much like trying to use only one of the "discs" of the tool from the 4th video.
  • gear blank needs to be shifted according to the z-shift.
    • like shown in the "time consuming" slitting saw approach. I am too lazy to do those calculations, so the machine needs to take care of it.
      Here's a time consuming way to generate gears with minimal equipment. A perfect time vs money trade off option:



      Here's a better way to create multi tooth involute approximating cutter that looks like a gear hob but does not require synchronized cutter and work spindles:
      The interesting part starts around 2 and half minutes in, the beginning is a recap of traditional methods. There are several videos in this series, gears are actually made in the follow on video.


Busy with trying to "teach" my machine that I want to rotate the gear blank when moving the z-axis. ( older mechanical hobbers do that with change gears and some sort of a differential gear train under the term "hob shifting").

So the trick I tried now is to start approx. 1 tooth length (tip+gap) below center line of the gear blank.
  • move full cutting depth in y
    • move full gear width in x from 0
    • back to 0
    • tiny bit up
    • move full gear width in x from 0
    • back to 0
    • tiny bit up
  • stop when about 1 tooth length above center line.
This looks tedious, but the only accurate dimensions are the infeed of the cutter into the blank, and the diameter of the blank.
Everything is electronically geared and adjusted before starting the spindle.

K1600_IMG_5420.JPG
The result of the exercise was the small brass pinion, which does meet Stans definition. (it will not hold up to the "it is not as good as formula one car gears" criticism :cool: )
Perfectly usable is defined as gears that run well with others you make, or others purchased from commercial gear makers. It does not mean the gears are so utterly perfect that they will run silently at 500 RPM or be perfectly formed with no variance from the theoretical ideal form.
Specially when it comes to gears the "this is not accurate" comment is thrown into the discussion. Gears show me the limits all the time, that is the whole point of the journey.
Gears are often, if not always, very cheap compared to the effort it would take to create the same quality as a "one off" part.

Greetings Timo
 
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