Dividing head and gear cutting questions

I have been learning some about what it would take to cut gears with my Bridgeport. What exactly is the differance between a universal and a semi universal dividing head? I dont really need the cream of the crop but I dont want to rebuy after discovering I have purchased junk. Just hobby and hopefully some job shop stuff. I have been looking at the Dividing heads in the Enco catalog. Are they worth the money? What should I look for for features?

Also I found a video of some one hobbing brass or bronze spur gears much like you would with gear cutting saws. That is to say the gear was not spun while cutting such as some folks are doing when cutting gears for southbend threading dials. Is hobbing practical for one off or small production runs in harder materials? Is hobbing possible for harder materials? And are there any good links to information about cutting hobbs? Looks to be a very affordable way to get started cutting gears once a dividing head is added to ones arsenal. I will admit I only have a very basic understanding of gear cutting thus far. Thanks in advance for any info!

The simplest/cheapest way to get in the gear-making game is with a horizontal/vertical rotary table with indexing plates and tailstock. You won't be able to make bevel gears though. As for dividing heads, I don't know what differentiates "universal" from "semi-universal". Semi-universal seems like a self-contradiction to me .

Hobbing is typically thought to be a generating process, whereby the toothform is a result of synchronized rotary motion of the cutter AND the workpiece. Using a profiled form tool, either a multi-tooth milling cutter or a single-tooth flycutter, is not considered "hobbing".

There are 2 kinds of hobbing. "Free" hobbing relies on the natural tendency for the workpiece to rotate as the hob does its work. This may be what you saw someone doing when making a thread dial gear -- Acme-form cutter spins, and as it advances screw-like, it induces rotary motion of the gear blank. The other kind of hobbing is where the workpiece is rotated at an exact rate, in sync with the cutting tool, via a gear train or computer-driven servo.

Either hobbing or form cutting can be done on harder materials, you just need the right cutter. I've cut gears from 4340HT, with HSS involute milling cutters. Works fine, just have to mind the SFPM. For truly hard gears, the rough toothform is cut into an annealed gear blank which is then hardened, and the final toothform is ground.

Regards.

Finegrain

A universal DH can be driven via the table screw of a milling machine.
A "semi" cannot - it is just a divider.

The only time a universal would be needed in gear cutting would be in milling a helical gear type part in a universal milling machine with a form relieved gear cutter.

True hobbing can only function with blank rotating. This is the only thing that enables the straight side hob teeth to "generate" involute profiles on the teeth being cut.

John Oder

For some inspiration have a gander through this thread ... it's worth reading the whole thread which is currently still ongoing

http://madmodder.net/index.php?topic=2624.0

They are called Plain-and Differential Indexing Heads. Best advise I can give you: buy a Machinists Handbook -even a used one. Plain dividing is by use of indexing plates on the dividing head only.
Differential indexing makes use of a set of gears between the indexing plate and the spindle on the dividing head. Also can be geared to a universal mill spindle to cut spirals etc.. If you don't have a universal mill - forget the gears, unless you need to cut gears with (prime) numbers like 51, 53, 57,.......250, 251 etc.. Than you need a universal (differential) dividing head. Buy the book first.

This book by Ivan Law provides a pragmatic approach to gear cutting in the home shop.
Gears and Gear Cutting (Workshop Practice): Amazon.co.uk: Ivan R. Law: Books
It covers all us men in sheds are likely to to need in an easy read format. Yes, he is a friend of mine, but I bought the book!

Good luck

Steve

johnoder said:

A universal DH can be driven via the table screw of a milling machine.
A "semi" cannot - it is just a divider.

The only time a universal would be needed in gear cutting would be in milling a helical gear type part in a universal milling machine with a form relieved gear cutter.

True hobbing can only function with blank rotating. This is the only thing that enables the straight side hob teeth to "generate" involute profiles on the teeth being cut.

John Oder

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Thank you that makes sense. I understand that anything is possible but would it be feasible to set up a universal dividing head on an old Bridgeport knee mill? Again any pictures of this set up? I cant tell you what the machine is really. Its a bastard with a Becker base retro fitted with what looks like a Bridgeport knee table and a Bridgeport head with no quill movement (as in quill drop) gotta run the knee up for everything. Im sure I will just end up with a semi universal. But I am just wondering what my options are. The thought of learning to cut helical gears is really enticing. At this point there is no particular need. But the more I can get under my belt the better. Thanks for the help.

It does seem to me that this guy is hobbing involute gears with out free turning. Im sure true free hobbing where the hob turns the gear or true hobbing machine where the gear is turned by power relative to the hob rotation the gear may run smoother due to more precise forming of the (for lack of better terms) involution? This seems like using hobbs of this sort would be a really cheap first gear cutting set up for the fellow that owns a lathe. Cnc is not really needed for any of this. Heres the link. Any thoughts or feed back?

YouTube - Sherline CNC Gear Cutting Part5

juergenwt said:

They are called Plain-and Differential Indexing Heads. Best advise I can give you: buy a Machinists Handbook -even a used one. Plain dividing is by use of indexing plates on the dividing head only.
Differential indexing makes use of a set of gears between the indexing plate and the spindle on the dividing head. Also can be geared to a universal mill spindle to cut spirals etc.. If you don't have a universal mill - forget the gears, unless you need to cut gears with (prime) numbers like 51, 53, 57,.......250, 251 etc.. Than you need a universal (differential) dividing head. Buy the book first.

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I have noticed that every one seems to reference the Machinery's hand book. At this time I only have the pocket companion, also Machine Shop Basics, Machine Shop Tools & Operations, Some for of basic machinist text book from 1975 (sitting on the kitchen table 5 miles away right now) and a copy of of the American Machinist handbook by Colvin and Stanley (on loan from a friend. Its dated 1941 in pen inside the cover). And yeah I have alot more reading and studying before I fire up a mill to try to cut or hobb a gear. Thank you for the help.

baldwin said:

This book by Ivan Law provides a pragmatic approach to gear cutting in the home shop.
Gears and Gear Cutting (Workshop Practice): Amazon.co.uk: Ivan R. Law: Books
It covers all us men in sheds are likely to to need in an easy read format. Yes, he is a friend of mine, but I bought the book!

Good luck

Steve

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Never too many books!! Thanks!

I have watched some of his videos and they are hilarious.
He does NOT use hobs.
He actually does not even cut a true involute gear in my opinion.
He talks about the fact that true machinists have given him a bad time on his methods and he does not care because it works for him. I am sure for low speed and low load use his gears do work.
I would not want to contract any gear work out to him. He enjoys what he is doing and that is great. I would read some books and watch his vids only after I knew enough to enjoy them as entertainment only. On second thought read a couple books and cut a couple gears and forget about his videos entirely.
Use a hob on a hobber and use involute cutters on the mill. Hobs cost lots more money. They are more versatile in that they will cut any gear for the pitch and pressure angle the are made for. They also cut a better gear. There are 8 cutters in a set of involute cutters for the same pitch and pressure angle. You probably do not need the set.
Stock gears can also be purchased that may save both the time and tooling cost. Not as much fun though.
Cutting spur gears for a lot of applications is not rocket science. That video is proof.
I am not a gear guru but I do know a couple. I have done some gear work for a gear shop in my spare time.
My $.02

I have use the process in the video to cut spur gears as shown. Any arrangement with indexing plates and with the spindle mounted horizontally can be used. The hob (its not really a hob it's a form cutter) has the tooth form of a rack which means the tooth flanks on the hob are straight, this in turn means you can easily make one on a lathe. The down side is it cuts the gear tooth flanks as a series of flats, so depth of cut per pass needs to be low and thus time to finish is high. It works fine for spur gears for use in not critical applications. This process is not covered in Ivan Laws book (at least not my version), I was surprise.

Phil

PS: My understanding is that true hobbing is where the rotation of the cutter is geared to the rotation of the work-piece. Not easy to do on a BP, although I'm sure somebody will prove me wrong.

Tuff Luck Tom said:

Thank you that makes sense. I understand that anything is possible but would it be feasible to set up a universal dividing head on an old Bridgeport knee mill? Again any pictures of this set up? I cant tell you what the machine is really. Its a bastard with a Becker base retro fitted with what looks like a Bridgeport knee table and a Bridgeport head with no quill movement (as in quill drop) gotta run the knee up for everything. Im sure I will just end up with a semi universal. But I am just wondering what my options are. The thought of learning to cut helical gears is really enticing. At this point there is no particular need. But the more I can get under my belt the better. Thanks for the help.

It does seem to me that this guy is hobbing involute gears with out free turning. Im sure true free hobbing where the hob turns the gear or true hobbing machine where the gear is turned by power relative to the hob rotation the gear may run smoother due to more precise forming of the (for lack of better terms) involution? This seems like using hobbs of this sort would be a really cheap first gear cutting set up for the fellow that owns a lathe. Cnc is not really needed for any of this. Heres the link. Any thoughts or feed back?

YouTube - Sherline CNC Gear Cutting Part5

Click to expand...

Tuff Luck Tom said:

would it be feasible to set up a universal dividing head on an old Bridgeport knee mill?

Click to expand...

I purchased my Edgewick (Cincinnati clone) dividing head from a closing down mould shop who claimed to have used it in true universal mode on a Bridgeport by linking the drive to the table feed via a chain drive (!?). On general grab first think later principles I took the chain and two of the sprockets which they claimed to have used with it but the rest of the kit "had been lost". One sprocket is bored to replace a Bridgeport table feed handle and the other fits the dividing head input shaft so the claim is probably true. How well it worked is another matter. Much of what I saw had a fairly desperate Heath Robinson air about it. Naturally the proper gear-set and banjo were long gone as were two of the dividing plates. I didn't realise until I'd got it home that the head had one pair of the 3 plate pairs extended range set not, as I thought the standard double sided A & B plate. £25 for a "Cincinnati" and tail stock was far too good to miss even with about 10 lb of grunge thrown in.

Clive

Thanks guys! I do have more reading, study/ research. But for some reason I feel like I should be learning to cut at least basic spur gears to some day consider my self a machinist. Just some thing I feel like I need to have under my belt sooner or later. But to be honest I have alot more than that to learn. Thank you again for all the help!

Tom

I'm with Kevin on this and with Phil's PS. A hob for involute gears has rack tooth form with the tooth faces inclined at the pressure angle. The whole point of proper hobbing with the blank revolving by being geared to the hob spindle is that the rotation of the rack form hob generates the involute in the process. As Kevin said, thus one cutter will cut any number of teeth for its particular DP because of the way it works.

I can only aspire to using the Brown & Sharpe type form cutters (8 in a full set as mentioned above) to mill gear teeth using a dividing head to index between each tooth. These cutters are apparently most accurate in form for the smallest number of teeth in the range covered. e.g. No.4 does 26 - 34 but is best for 26 teeth. I think this is best illustrated by No.1 which does for 135 to a rack, but clearly if it has the form for a 135 it cannot cut a proper straight sided rack.

My copy of Ivan's book does mention trying free hobbing where the blank is free to rotate rather than being driven, but warns of the difficulty due to drag and the cut being heavier on one side. In one trial he ended up with one tooth less than planned!

I watched the video - Kevin is right, but it did make me smile.

Steve

Oh you can cut prime gears using just a rotory table, non of the dividing plate nonsense either, just a list of degree off sets. Gets a bit analy retentive - parinoid doing 127 Tooth gear though! Had night mares about stuffing up the 126 tooth!

Once you have setup and cut some gears. It is no longer much fun, Rather tedious. For starting out get a set of cutters, or close to a set as you can. Nothing worse than finding you need a #4 cutter to actually make a mating gear set. Go with a fine 24+ dp, makes less chips on smaller stock. Make a few gears different sizes, same DP,then follow the formula for axle spacing, stick them on the axles, are they jammed, loose, or just right. Now thats Fun.

Figuring out how and actually making a few gears is a lot of fun in IMO. Once you get over the personal satisfaction over doing something difficult and have patted yourself raw.... it gets old quick.

As far as the video is concerned... That was not a hob he was using to cut the gear. He was not "hobbing" a gear. He was merely cutting a gear. He could have gotten better results with a simple involute profiled cutter of the correct specs.

There are lots of quick-and-dirty methods on youtube and on the hobby sites for producing gears on the mill and even on the lathe. Many use real hobs and sometimes even make their own. For the most part they are simple spur gears for very low speed and low load telescope drives. Some are worm gears such as on threading indicators (low speed and low load.) I do not know enough about gear making to pass judgment on their methods. If they succeed with the desired results and quality... who cares.

No one mentioned gear shapers. There was a thread a little while back on that topic. 10 fingers (the only gear shaper guru I know) posted a link to some old Fellows videos on Youtube about how gears are designed and generated. Here is the link to the series (four parts):

Fellows Gear Shaper. The Art of Generating Gears

As far as books for making gears and how to use dividing heads and universal dividing heads see: Machine Shop Practice (Volume 2) by K.H. Moltrecht (available on Amazon.) Chapters: 7 Indexing, 8 Dividing Head Work and 9 Helical and Cam Milling for lots of details and examples of setups.

Whenever possible, you are way ahead of the game if you can just buy an off the shelf gear to do what you want.

If you plan on producing any quantity of gears then you will need specialized equipment (gear hobbers, universal heads, gear shapers) which cost a LOT of money... then buy cutters which can easily cost in the thousands for any serious production work.

If you are doing it as a fun challenge with available equipment or restoring something with an unobtainable gear... go for it! I wish you the best of luck.

-DU-

Gear cutting is pretty easy. Once you calculate the size of the blank, the plunge depth is printed on the gear cutter. They're about $30 if you shop around, so I only buy as needed. I have a lot of them, but not a complete set of any size.

You can get the small B&S copy import dividing head, but remember that most of that import stuff can be considered a kit. Mine was about $260, but required much disassembly, deburring and re-machining to make it decent. You can also get a small rotary table that can stand on end, and has dividing plates. I don't see any reason that wouldn't work as well.

I make most of my gears out of Delrin. It's plenty strong for change gears in the lathe and your gear cutters will last lots longer.

Cut, crank, cut, crank, cut crank... repeat until gear is finished or you make a mistake.

Best,
Conrad

Thanks for all the info, The references, books, links and what have you. I can see why it is more practical to just buy a gear if you can. But as I stated I want to learn pretty much just for the challenge. I feel all in all it would be a good project to hone and develop my ability. And no I dont want to develop any more bad habits than I already have. So thanks for setting it straight on the gear whole deal in the youtube vid. There are still so many basic machinist tools I need it may be a minute before I can set up to make some scrap. But a dividing head may be the next big purchase when I can swing it. I have just gone to bed with my head all wrapped up in the little I have been learning about cutting gears the last 3 nights. I also had an accomplished machinist claim he had no idea even where to begin with cutting gears. The desire was already in the back of my head. But that really made me determined to become some fairly competent at it when I can invest time and dough. There are alot of other things I should master on a milling machine but its nice to have a project to force me to learn. Thank you very much and any thing any one has to add Im all ears! Have a great one!

....bad habits....

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Good for you Tom.

It is one thing to cut sort of right grooves in a blank and call it a "gear".

Jolly if only for you and your application.

But it is quite another to first learn about and then MAKE gears that can travel 2000 miles from your shop and still work fine for your customer in a machine you will never see.

John Oder