Mandrels, Gearcutter & Gears

[Woodguy]

Apparently those videos were removed, but were compiled into a single movie available here: http://leatherwoodplayground.com/Gears%20and%20Hobs%20Joined.wmv

[arnoldb]

Many thanks for the responses Gents  ; I didn't think there would be this much interest!

Wagner, I only tested one bottle  - the rest will be kept for that day - and some saved for future as well! 

John, I recall your purchase of those gear hobs.  I can't remember if they were straight-cut though     There's also been quite a bit of discussion about this method on the 'net and it seems to work.  I'll try and keep a look-out for the next MEW - its delivery here in Windhoek is totally sporadic and non-guaranteed 
The only free-wheel hobbing I've tried was when I made the worm wheel for my rotary table - that worked a treat, even in the hands of a complete newbie like me.

I first tried this straight-hob method when I made the 0.5 module gears for my Cracker locomotive - and this was the results after the second try (hob, gears and grooving tool shown):

For those who do not know about my Cracker build and are interested in small live-steam locos, you can have a look at this thread on MadModder

Kind regards, Arnold

Edit:
John S & Woodguy, I'm not sure precisely which video Bogs was referring to; videos are a bit scarce in my neck of the woods, and the link Woodguy posted to would take a lot of time downloading; Internet bandwidth here is a bit slow.

[Bogstandard]

Arnold,

It is this sort of post that will help many of our members.

If they can follow your instructions, they too can make their own gears for their little engines. It could save them lots of pennies in the long run.

You keep showing, we'll keep avidly reading.

John

[IanR]

There was an article in ME years ago by the late J A Radford, on cutting gears this way. It saves making several B & S type cutters, but I suspect indexing the blank is slower. Awaiting the next instalment with interest.

[grayone]

OK for a complete idiot like me can some one explain what gear hobbing is as opposed to cutting gears with a cutter which I understand needs a different cutter for different numbers of teeth.   While I wait for an answer 

Graham

[arnoldb]

John, Ian & Graham - many thanks for your input Gents 

Ian, I don't think indexing the blank is  slower; the small gears I showed were done to full tooth depth for each of the index positions - exactly the same as what would have been done with a B&S cutter.  The only slow cut was the very first one, as it had to remove a significant amount of stock on that cut; the rest of the cuts actually became lighter.  I'll try and show that when I cut the gears.   
For this set of change wheels, I'll have to see if I can go in at full tooth depth; that will depend on how sharp I can get the cutter, as well as my mill.  I'll see when I get there 

Cheers Graham  - I hope you enjoy your libation 
The term "hobbing" might be confusing - it's also just a way of cutting a spur gear.  In many industrial gear-cutting operations, they use a helical "hob" that is a cutter that basically resembles an Acme thread, and by synchronised rotation of the gear blank against the turning cutter, presented at the correct angle to it, the gear is cut - hence the term "hobbing". 
The cutters you were referring to is what has already been referred to in this post as B&S cutters - there you need a different cutter for gears with different ranges of tooth counts.  A complete B&S cutter set comprises, if I recall correctly, 8 cutters in total for the whole range of gears from 12 tooth to rack - for one specific DP or Module. 
What I'm showing, while loosely called hobbing, is more akin to cutting the way you thought of it.  The "hob" I'm busy making is basically a multi-point cutter, and will cut an approximate tooth profile with facets on it.  This method produces less accurate tooth profiles than either "hobbing" or using the correct B&S type cutter - but produces entirely satisfactory gears for home/hobby use.  In fact, I think it will produce a better resulting gear than the spur gears that came with my small Cheap 'n Cheerful Asian Import lathe.

Please note; failure IS an option here; If the results are not satisfactory, I'll resort to making gear cutters using the "button" method to get a more accurate tooth profile - these basically work like B&S cutters.  I'm trying the "hob" method first, as I have to make a 60, 35, 30, and two 21 tooth gears.  I haven't checked yet, but that lot would require 3 different B&S type cutters at a quick thumbsuckguesstimate.

At least the weekend's getting closer 

Kind regards, Arnold

[Alan Haisley]

Arnold,


I see how, for 14.5^o you ground your tool to that angle on each side. I assume for a 20^o pressure angle that each side would be ground to that. What is the geometry of how deep to cut with the tool and how far apart to space the individual cuts?


Assuming that this method turns out satisfactorily, a little table of the math/geometry involved would be useful to us aspiring gear cutters.


Alan

[Dan Rowe]

Arnold,
I always like to read and watch gear cutting threads. Gears are just fun.

Alan,
One of the best books on hobby gear cutting is "Gears and Gear Cutting" by Ivan Law. It has the math and explains the set ups with a chapter on grinding cutters.

One of these days I hope to cut true scale bevel gears for a Shay.

Dan

[RMO]

This is fasinating, one of these days I want to try this.  Until then I watch and hopefully learn.

Mike O

[arnoldb]

Alan, Dan & Mike, Thanks for looking in.

Alan, yes, for a 20^o pressure angle, it would be exactly the same method; just set at the correct angle.  I cheated and used Marv's GEARSPUR program - it gives all the necessary figures.
Like Dan mentioned, Ivan Law's book is very good about explaining all the theory involved.
This method does work - at least for small gears; the brass gears I posted earlier runs a treat in the little locomotive they are installed in  .  It works best for higher tooth-count gears.  I'll try and post close-up photos of the results on these bigger gear tooth profiles once done.

Dan, if I recall correctly, Chuck (cfellows) showed an excellent method to cut bevel gears a while ago.  That might make interesting reading if you haven't read that yet.  I seem to recall reading somewhere that the Shay's bevel gears are kind of "offset" as well...  - I do hope you'll show when you cut them .

Mike, I hope there's something useful for you 


On to today's shop session; not as much as I wanted, but progress at least.

First order of the day was to form the teeth on the cutter.  I used a 6mm mill to mill them out - I put the bottom of the milling bit about 1mm below the centerline of the workpiece, to try and get some rake on the teeth.  It's minimal, but it's there.  By a quick eyeball after cutting the first profile, and cranking the dividing head while counting turns, I saw that I could form six teeth on the circumference:


As a last step, I used a file to add a bit of rake to the short "round" sections on the perimeter behind the tooth tops.  Just a shallow angle to nearly meet up with the tooth tips. 
After removing the workpiece from the mandrel:

You can click on the photo to see a bigger image.

It looks a bit of a dog's breakfast as I didn't de-burr it, and overshot some of the milling cuts  .  The no-de-burring is on purpose - and based purely on one of my own pet theories:
If the cutter is de-burred before hardening, the cutting edges can already be slightly blunted.  During the hardening process, the high temperature needed and the resulting oxidation of the metal will also blunt the cutting edges further.  By leaving the burrs attached, these protect the cutting edges during hardening, and it is thus easier to sharpen the cutter after hardening. 

I don't have photos of the hardening process - I'm not yet comfortable about wielding around a camera while playing with a torch and extremely hot metal 
Before hardening it, I coated it lavishly with dish-washing soap; this also protects it from oxidation during hardening.  Then I heated it to orange-red with a butane torch, grabbed it with a pair of pliers and dunked it vertically into a bucket of water (the workpiece is made of water-hardening silver steel) 

Proper hardening was just tested with an old file on one of the edges; it skidded right off without removing any metal.  I gave the workpiece a quick brush-off with a small stainless steel wire brush - that removed a couple of the bigger burrs - as they were very hard they just broke off, and then started removing the rest of the burrs with some diamond files.  Here the front bottom set of teeth has been cleaned up a bit, while you can see burrs left on the rest:


After more time spent carefully de-burring it further, it's pretty sharp already, and just needs a light hone.  In the past, I've mostly worked with cutters hardened up like this as-is, but that was in soft metals like brass and aluminium.  As the gears will be made from cast iron, I decided to rather temper the hob, as I could feel that it was extremely hard and thus very brittle while de-burring it.  I didn't want to loose the hardness on the teeth edges though; just toughen up the "inside" of the hob.  That meant raising the inside of the hob to a high enough temperature to turn it blue again, while not allowing the flame to touch the outside...  My solution - slip it over a 10mm bolt, and heat the bolt alternately on both sides of the workpiece while checking for the colour to flow:


The result - I took the photo on a piece of paper to show up the colour contrast a bit better.  You'll notice it's darker on either side and in the middle, and what I'd like to think of resembles the "straw" colour described in tempering guides towards the tips of the teeth:

All it needs now is a final hone 

As I'm supposed to be making gears here, I set the bandsaw to work:


While waiting for the saw to do its business, a thought broached my mind...  I'll need to cut keyways in the gears!
A quick grinding job on some virgin 4mm round HSS stock, and I have a match to the keyway in one of my Myford's gears:


It was easier to use the new piece of HSS, as it is 80mm long and gives some better control while holding it for grinding.  It's still a bit tight in the keyway in the gear, but that's fine, as it also needs a final hone on the oilstone.  After cutting off the necessary length, it was mounted on an arbor I've had for a couple or three years (actually it's an inside turning tool!).  A bit light-weight, but I'll be cheating a bit while broaching the keyways 


The last thing I did in the shop today was to face the first gear blank to thickness.  The Myford gear specifications are for a 3/8" (9.53mm) thickness, but testing the gear from my lathe, they range from 9.4mm to 9.6mm - so, as I thought, not a critical thickness:

I must say, this is very nice cast iron indeed; there's a small chilled spot close to the skin that runs in towards the center for about one mm, but I did expect that and specified the material slightly-oversize.

  - there's nothing like the smell and taste of machining cast iron for me - I love the stuff !

More tomorrow 

Regards, Arnold

[vcutajar]

Making gears is something I need to learn, so Arnrold, I will be looking over your shoulder.

Vince

[zeeprogrammer]

Making gears is something I need to learn, so Arnrold, I will be looking over your shoulder.

Vince

Same here.

[arnoldb]

Thanks Vince & Carl 

The next installment...

I drilled out the center of the gear blank to 13mm, and bored it out to the required 15.86mm.  Seeing as I'd be making 5 gears, I'd turned up a plug gauge beforehand to test the hole size.  Then I moved the chuck over to the rotary table on the mill (purely because it's convenient to mount it like that), and drilled two 1mm holes on the inside of where the keyway corners will be:


Next I used  a 2mm drill to drill out most of the excess stock - the drill broke through slightly into the hub hole, but stayed nicely on course:


Next I used a 2mm end mill to clean out most of the rest of the stock - to leave a minimum for broaching out:


Then I mounted the chuck back on the lathe, honed up the broach I made yesterday, and mounted it on center on the toolpost.  Then I had at it using the apron to feed the broach longitudinally, and after each in-feed and retraction, the cross-slide was moved out about 5 thou before the next pass.  After it was done, the Myford's change-wheel mounting bush easily, but accurately fits into the gear blank hub:


The broaching went so easy, that for some subsequent gear blanks, I didn't bother to drill & mill the excess stock out first.  I just jumped in and slotted the keyways right from the start; much quicker than the palaver explained above...  Just lighter feeds on the cross slide required; .0025" (0.05mm) per pass worked a treat - making for light cuts but it goes quite quickly once one gets the sequence right of feeding the apron and the cross-slide 
I forgot to take a photo of the broaching setup; so just before I stopped shop today, I took a photo where I'd broached the keyway slot in the 60 tooth gear's blank:


The last photo was out of sequence; the rest of the photos to come are the continued work on the first lot. 
As you may have noticed, that gear blank (it's the 30 tooth one) was just faced and bored, but not turned down to size yet.  For that, I mounted it on the mandrel I'd turned up for the gears, and turned it down:

Lazy me  - the mandrel was still centered up in the four-jaw, so it was quick to mount!

A final hone on the gear cutter, and it was sharp enough to shave some grooves into a bit of brass plate by hand:


Back to the mill, and with the dividing head mounted and the 4-jaw chuck screwed on, I used my height gauge to find the top of the gear blank:


Then it was easy to deduct half of the blank diameter and mark the center line:

The black ink mark on the front was made to "remember" the starting position for the machining to come.

I mounted the cutter, and aligned one of the teeth on center with the mark made:


The first cut was made in three passes; two of 1mm deep each, and the last the additional 0.85mm required to reach full depth.  I kept spindle speed well down; just 280rpm as the cutter will be doomed the moment it heats up too much, and a gentle rate of feed:


If you like to watch paint dry, here's a video clip  :
<a href="https://www.youtube.com/watch?v=ZpE60v1RvPE" target="_blank">http://www.youtube.com/watch?v=ZpE60v1RvPE</a>

From the first cut to the last pass took about 45 minutes - and that includes time in between monkeying around to set up for the video and so on.  Not fast, but OK considering I was really taking my time  :


So, how does it measure up to the real thing ? - well, the tooth profiles are slightly thinner than the real deal.  If you did look at the video clip, you'll see that the cutter is running slightly eccentric... I'd grabbed my "wonky" 16mm collet, and the wobble on the cutter will cause it to remove more metal.  Overall it's a result - far from perfect, but serviceable:




Nobody wants to count teeth every time a gear is picked up - so I swung the hammer a bit:


I've already turned up the blank for the 35 tooth gear; I'll switch the rummy collet out for the good one, and see if the result improves.  If so, I'll re-make this gear as well. 
The cutter really stood up well, and still feels as sharp as when I last honed it, so that's some good news at least 

Kind regards, Arnold

[chucketn]

Beautiful work, Arnold. And, great photos and explanaton. I do have one question. After the first cut is to full depth, how far do you index for the next cut. Is the 2nd and subsequent cuts to full depth?

Chuck

[arnoldb]

Thanks Chuck.

Indexing is to the next required tooth position each time.  I did all the subsequent cuts to full depth, as there is less material to be removed after the initial cut.  That will depend on your milling machine though; if its a bit light, you might have to make two or three passes as needed.

Kind regards, Arnold