Gear hobbing

Any maths wizards out there?

I am trying to design a method of hobbing gear teeth using a lathe. I have a number of commercial hobs the right size. So a mandril in the lathe with a gear one end and the hob the other. The gear blank will be driven via a 40:1 worm and wheel which in turn is geared to the lathe mandril. So far so good.

one rotation of the mandril will put 40 teeth on the blank as long as the drive ratio in front of the worm is 1:1. A 60 tooth will require a 1.5:1 ratio all other teeth numbers I am lost, ha ha. surely there must be a table someplace does anyone have one or can point me at a page in the Engineers handbook. I have always had a gearbox on a lathe so never needed to calculate gear trains and how do TPI actually relate to teeth on a gear (if they do).

There is a factor of pi involved. When going from tpi to DP if one value is an integer, then the other must be irrational. For instance if the tpi (of a rack) is an integer then the DP of any mating gear will be irrational. Conversely if a gear has an integer value of DP then the tpi of the mating rack will be irrational.

Andrew

No definitely not but if it helps over the years I’ve had my hobber I have slowly worked out a change wheel chart based on available change wheels (converted the machine to 20dp Myford type which were cheap as chips then. As you say your fixed ratio is 40 to 1, mine is 16 to 1 so to cut a 10 tooth gear it’s 10 div 16 = 0.625 which I achieve by using a 48 and 30 tooth wheel setup. For some tooth counts compound trains are necessary but I shy away from them unless really necessary. Hope this is some help

Big difference between a mandrill and a mandrel

Martin C

oops well am monkeying around

If it may serve to provoke some ideas, could I suggest that you take a look at the Radford worm-wheel hobbing attachment for the Myford lathe. As its name suggests this device as built will not hob anything but worm-wheels but maybe coupled up with a rising and falling slide to carry the gear blank it may provide a suitable drive.

Edited By Chris Crew on 24/02/2022 21:17:08

ok I get that much for me that would be 10 divided by 40 it is the next part that I have problems with how do you get from .625 (5/8) to using a 48 and 30 tooth for that ratio. I tried using the log method in machinerys handbook but the gear numbers that it comes up with seldom include gears in the set 30, 35, 40, 45, etc and has all sorts of odd gears required and whilst that could be done with a stepper motor that is not a route I wish to go down I need to be able to work out compound gear trains. Once I have the basic ratio using gears that I have, even if three trains are needed at times how to break a ratio down into the various compound units.

Roger divide by 6 then 48/6= 8 and 30/6=5

I'll certainly have a look at that. In the mean time design changes already before I start building, instead of 40:1 worm drive I will make it possible to change the driven gear so it could be any ratio I have a gear for, that will give me a lot more scope. The mandrel (see spelt correctly, ha ha) will be made to take both one inch hobs and inch and a quarter. I have several of both and don't think it worth making two mandrels. Everything still in my head at present but will start sketching to find what I need to make

got it thanks

I am sure many of you have tried gear hobbing one way or another, having seen the Jacobs (CES) machine and fighting old age crap memory etc I am going ahead to build one to fit on the lathe. What do I have, I have a number of inch and a quarter bore commercial hobs (including 16dp 14.5pa as used by both of my lathes). I have a couple of lathe top slides, a Boxford dividing head to fit the topslide (when mounted as a vertical slide) A large piece of angle plate, assorted 16dp gears from many different lathes. I have ordered a couple of UJ's around £11 the pair could not make them for that if I had the time. Lots of bits to make, an arbor to hold the hob and a drive gear to drive the blank, a suitable banjo to hold the required gearing, numerous connecting structures and a chance to make a vertical slide for the lathe. (the angle plate will swivel and also hold a tiny rotary table I made many moons ago, or one of the topslides. So lots of bits to make or cobble together. One day I will actually make something other than a machine to make something, ha ha

Roger, It’ll be interesting to see that it could spark a bit of interest as very little out there except cues and small mikron machines like mine ,best of luck..

Which hobbing machine do you have Bernard?

I am sure there not only is a lot of interest but a lot of people who have experimented one way or another. The forces involved only limit the size of the gear or the profile I suppose though most people will need either 20dp or 16dp only the odd person will use 14dp for older machines. Mmm not meaning the person who uses 14dp is odd, ha ha. My problem will always be a non mathematical mind. Plus my workshop is such a mess photos will not be easy and I seldom work to a drawing these days its all in my head and developed as I go so one problem will follow another, ha ha. I will take a series of photos as I go and to hell with the mess and if it fails then maybe others can suggest how to rescue the project. Though basically I will not be spending a great deal of money on it, thats for sure.

As a make do and mend engineer my approach is a function rather than a pretty thing so I will not spend time on unimportant finishes. Whilst I am purportedly retired I do run a a campsite so my time is limited, most years to the winter months, going out now I could be some time, ha ha.

PS why is there not a facility to "like" a post as I hate to ignore other peoples input.

Pete, It’s a mikron 92 with a standard head. Bought it for £100 many years ago and overhauled it that was almost a waste of time as couldn’t find anything wrong with it and it’s been working ever since. I mostly cut gears for mechanical instruments (speedometers and rev counters) with it but have got a couple of 20dp hobs so I can generate odd changewheels for the machine itself which I converted to use Myford style wheels.

Indeed!

Don't make a monkey out of me

Does that have a 8mm or a 10mm spindle? I have a bunch of surplus fine pitch hobs I'll never use.

Hi peter, it has both 8 and 10 mm spindles.

Hi Roger,

Have you thought about electronically hobbing on your lathe.

Article in MEW 75 for a lathe.

Also articles in MEW 108 with correction in 110 for a milling machine. I have cut 25 gears on my emco F1 mill

using this.

John

I remember several articles of Gear hobbing in ME and MEW. in one they use the DP thread cutting feature of the machine to drive the hob. And then the building of a complete one purpose hobber in MEW iirc from the numbers 245. At the moment I try to make a similar project with my little Emco C5. I want a life toolholder from a CNC lathe with a motor and an encoder. The measure of the encoder will divide with an arduino or simpler an oldschool ttl-ic circuit to the desired tooth number.

If I use an 500 cpr encoder so I get 2000 peeks/rev with the decoupled spindelmotor. I want to drive the spindle with a 10/1 geared stepper motor, so I get 2000 steps/rev of the hob. The divider give 1 peek out per tooth of the gear. the feed of the machine can be made with the normal feed of the machine from the spindle.

The biggest problem for me, is the electronic part. It were good, if I can start with the index peek from spindle and hob encoder, so I can go a bit deeper, if the geartooth isn't deep enough, but I haven't any idea, how I can realise this.

Edited By uli Schnelzer on 15/04/2022 10:13:36