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cutting spur gears on a mill

a rogue method?

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brian jones 1113/09/2021 21:06:20
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Regular hobs have triangular teeth and generate involute forms

I didnt know that?

But look at the method

if i want two gears say 2:1 ratio, stick the blanks of 2:1 OD in your jig and 5 mins later ish you got a result

no fancy hobber linked to spindle ££££, just slice up an HT bolt to suit blank OD/pitch = teeth minus 1

Cheap and cheerful for non ferrous mtl

Andrew Johnston13/09/2021 21:21:44
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Posted by Martin Kyte on 13/09/2021 20:55:31:

So explain to me why it should not be an involute........

It probably is an involute. But since the tap was unified thread form the pressure would nominally be 30°. That leads to a more triangular tooth shape. So while the involute is probably there the divergence from a straight line is too small to be visible.

Andrew

Martin Kyte14/09/2021 09:13:05
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I was commenting on tooth form not on the method brian.

That confirms what I thought Andrew. Which leads to another reason why free hobbing with taps is not terribly suitable for making gears as it limits you to quite high pressure angles. Nothing to stop you making your own hob but that makes the process more involved.

regards Martin

SillyOldDuffer14/09/2021 11:47:46
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Posted by brian jones 11 on 13/09/2021 21:06:20:

...

if i want two gears say 2:1 ratio, stick the blanks of 2:1 OD in your jig and 5 mins later ish you got a result

no fancy hobber linked to spindle ££££, just slice up an HT bolt to suit blank OD/pitch = teeth minus 1

Cheap and cheerful for non ferrous mtl

Cheap and nasty rather than cheap and cheerful! Although it produces teeth, Brian's method is unreliable. It's a good way of producing unreliable gears, which most of us don't want at any price!

Look at Jason's example:

badteeth.jpg

The green circled teeth show the free method 'working', but the nearby red circled teeth highlight a serious problem - malformed teeth. Jason's example wouldn't mesh consistently with itself! Not because Jason is unskilled - it's because the method is flawed.

Having made one plastic gear by this method Brian bravely asserts the number of teeth is OD/pitch = teeth - 1. Maybe, maybe not. The number of teeth produced depends on slip during the cutting process, and it's unpredictable. Half teeth are possible.

The claim that the same cutter will produce 2:1 ratios is untested, and engineers really shouldn't jump to conclusions.

Brian seems keen to sell his idea as a practical man versus Armchair thing. It's not! I suggest others try the method and decide for themselves. Bear in mind the flaws only become obvious when many gears have to fit together. Asking several gears at particular spacings to transmit power or step up with low friction will reveal problems galore. No one has to believe me or Brian, because the method can be tested.

Dave

JasonB14/09/2021 12:59:26
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It's only the corners that were chewed up on a few teeth, OK along the face once the tap had something to mesh with. Starting with a slightly overwidth gear and then skimming each face would possibly cure it though that was the edge I started on in the video and soon found it better to come in half way along.

brian jones 1114/09/2021 17:57:52
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JB you were on track and then spotted this was not the proper way, dont attack from the side as the tap doesnt have space to mesh properly at first with the blank. You quickly retrenched and attacked from the middle inwards (as they do with making a worm). It righted itself and you proceed full sail to finish. The damaged teeth were a periferal result left over from your first attempt - trivially observed by the old SOD not paying attention from his armchair.

Now your first result you reckoned you made a nice knurl but you were only using 1/4 BSF 26tpi

Now I am using 1/2" UNC 13tpi so my pitch is ca 2mm and produces a more credible size gear

BUT spiral flute taps are rare on EB and larger ones get very expensive, so not a practical direction I admit

I had a cunning plan and Ive had more results from Doreen see below

John Haine14/09/2021 18:14:15
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Who or what is Doreen?

John Haine14/09/2021 18:14:20
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Who or what is Doreen?

speelwerk14/09/2021 18:31:24
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Posted by John Haine on 14/09/2021 18:14:20:

Who or what is Doreen?

Reading previous posts by Brian, Doreen seems to be a much loved and well used Myford lathe.

JasonB14/09/2021 18:34:04
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Someone else not paying attention, as I previously said I used 1/4" BSP which is 19tpi and 0.518" OD. That's why there is a 19G thread gauge in one of the photos.

Doreen Is his Dore Westbury mill.

Edited By JasonB on 14/09/2021 18:34:35

speelwerk14/09/2021 18:38:09
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Posted by JasonB on 14/09/2021 18:34:04:

Someone else not paying attention, as I previously said I used 1/4" BSP which is 19tpi and 0.518" OD. That's why there is a 19G thread gauge in one of the photos.

Doreen Is his Dore Westbury mill.

Edited By JasonB on 14/09/2021 18:34:35

Very sorry for the mistaken gender of Doreen. Niko.

brian jones 1114/09/2021 18:54:25
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When I woke up from my pantograph AO drawing board this am, I was seized with a new direction . Why not try

straight slits down a bolt thread. My angle grinder disc is 1mm thick, so I made 5 axial slits coinciding with the hex head - in a heartbeat. If the gear blank is say minimum 3mm - mine was 6mm, then the cutter shouldnt come un meshed.

So this is the result

20210914_165243_001.jpg

This is during the rolling and polishing process

20210914_165234_001.jpg

This was just a quick try with my grinder, slitting along 6 edges to match the Hex head, done by hand so a bit naff, could do better

img_0760.jpg

Not a chip in sight all round and SHOCK 74 teeth (not 73 predicted) -well it wouldnt suit a clock maker

img_0761.jpg

Im not skilled in taking close ups. the gear is mucky from the brasso fluid which I applied to polish it up to please the Grommets

As you seen its a presentable form, even with rounded tips and roots?

I polished it up by using a plain bolt without cutting edges and some Brasso. I think the applied rolling pressure helped (after all they make quality precision threads by rolling process)

So even if it fails on soft metal, it can produce respectable plastic gears quickly and cheaply without complicated attachments, universal dividing head and loads of patience etc

I have some AL and brass blanks on order

Maybe look up some larger thread - 2" bsw 4.5 tpidevil that should knock the stuffing out of sofa dwellers

brian jones 1114/09/2021 18:57:37
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Who or what is Doreen?

A Dore Westbury vertical mill, as classic as the Myford (Maureen) is to a turner

they are both temperamental wimin who dont like being neglected

Edited By brian jones 11 on 14/09/2021 19:00:48

http://www.lathes.co.uk/dore-westbury/

Edited By brian jones 11 on 14/09/2021 19:21:48

brian jones 1115/09/2021 20:02:07
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As I am sure you are all on the edge of your seats (not possible for sunken armchairs) here's my first attempt at a brass gear

20210915_180229.jpg

I keep forgetting to landscape my phone, notice a cheapo brass blank - 17p whats not to like

20210915_193434.jpg

before you all leap from your electrically assisted orthopaedic memory padded comfort

YES its not perfect, there are flaws but I put that down to the rough way I chivved up my spirally sliced bolt on my vice with an angle grinder, lots wrong with that method

I have a cunning plan to make better cutting tools from bolts the proper way

More work needed

brian jones 1115/09/2021 20:09:52
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And for the record

this is what real hobbers use, who has those attachments?

701223.jpg

And Im not the only one with a perverted interest in rough gears

A senior luminary on this VERY board was caught doing this a few years ago

675400.jpg

Michael Gilligan15/09/2021 20:45:54
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Posted by brian jones 11 on 15/09/2021 20:09:52:

.

A senior luminary on this VERY board was caught doing this a few years ago

675400.jpg

.

But that’s a worm-wheel is it not ?

MichaelG.

JasonB15/09/2021 20:53:37
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And the other a helical gear if I remember rightly

brian jones 1115/09/2021 20:54:05
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Well MG - think out of the box

if that cog was 10x dia then its a spur gear, just shunt the tap across the z axis, not so easy on a lathe without a vertical slide - but doable

innit?cheeky

 

obtw you would have to tilt the axis to match the helix angle to get orthogonal teeth as JB has pointed out before

Edited By brian jones 11 on 15/09/2021 21:02:56

Michael Gilligan15/09/2021 21:13:20
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Posted by brian jones 11 on 15/09/2021 20:54:05:

Well MG - think out of the box

.

.

I’m leaving that to you, Brian

I’m happy just observing … and chipping-in with the occasional fact-check

MichaelG.

Pete Rimmer15/09/2021 21:40:16
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Posted by brian jones 11 on 15/09/2021 20:09:52:

And for the record

this is what real hobbers use, who has those attachments?

701223.jpg

That ain't hobbing. Not sure what it is from this angle some kind of spiral gear setup but it's using a form cutter to cut the tooth directly not a hob to generate the involute.

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