cutting spur gears on a mill

[John HaineParticipant @johnhaine32865]

I wish we could stop using these weird acronyms (AGMA, SIAS?)…

I think that for an involute the OD is PCD+m and BCD is PCD-m. Then the root diameter is a bit less than BCD, there is a standard fraction IIRC but can't recall what it is. So from these if you know the OD and TC you can calculate modulus, PCD and PCD.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by John Haine on 24/09/2021 17:41:25:

I wish we could stop using these weird acronyms (AGMA, SIAS?)…

[…]

IIRC

[brian jones 11Participant @brianjones11]

Calling AGMA weird????????????

Wow its like insulting the Grand Wizard of the gear hobbers masonry

Moving the future through power transmission innovation

SIAS suck it and see what gear snobs do as a last resort

You see you are given one small gear lots of teeth

What do you know

you measure the OD, count the number of teeth and then start guessing

For the benefit of the Armchairs hereinotwithstanding , todays youth have their own lingo used on phones.  Here is a list of some 1500 acronyms for you to learn while you've got nothing on (in this hot weather)

https://www.webopedia.com/reference/text-abbreviations/#Huge_List_of_1559_Text_and_Chat_Abbreviations

My great grand kids try and catch me out

 

 

 

 

 

I got caught out on Maureen's toolpost, I thought it was 7/16 BSF

I had a suitable nut but it wouldnt got on more than 4 turns, so i thought the thread was damaged and started using a die on it. Didnt feel right so I checked my tables

bsf 18 tpi .438 od

unf 20 tpi .438 od

the naked eye did not see the difference

So imagine that issue x 100 in complexity – welcome to Gear World

Obtw a wheel nut off a mini was repurposed

 

 

Edited By brian jones 11 on 24/09/2021 18:30:45

[duncan webster 1Participant @duncanwebster1]

Posted by brian jones 11 on 24/09/2021 16:13:28:

I think the article by Jannick referring to Base Pitch is meant to be what we call the CP

His Base Circle is what we call the Pitch circle or defined as PCD

……….

 

Not according to the interweb. The base circle is the circle from which the involute is generated, and the base pitch is the circumference of this circle divided by the number of teeth

picture 

Edited By duncan webster on 24/09/2021 19:18:25

[Anonymous]

The PCD is fundamental to the design of spur gears. If the gears had no teeth, but were simply cylinders, then the speed increase, or decrease, will be set by the diameters of the PCD. The DP system is defined in imperial while Module is defined in metric. The PCD is the circle from which the gear is designed. In the DP system the addendum (part of the tooth above the PCD) is 1/DP while the dedendum (part of the tooth below the PCD) is also 1/DP, in theory. In practice the bottom of the tooth space needs clearance so the dedendum is increased. Common factors are 1.125, 1,157 and for gears where one has a small tooth count 1.25, although any value can be used as seen fit by the designer. The common values give a total tooth depth of 2.125, 2.157 or 2.25 respectively all divided by DP. The tooth depth (in inches) aka cut depth is usually marked on DP cutters as it saves knowing which clearance value was used.

In the DP system the OD of the gear blank is (N+2)/DP where N is the number of teeth. Simple algebraic manipulation will show that the equation is equivalent to the PCD plus twice the addendum.

Spur gears are simple compared to other types of gear. Gawd help us if we ever get onto other types of gear!

Andrew

[brian jones 11Participant @brianjones11]

Many thanx AJ for your enlightenment

But can you show me algebraically why Addendum = 1/DP

I cant find any explanation except Google sources say thats what it is

I have even proved it to 6 places on the Dr's gear calculator

But why????????????????? What have I missed

Stupid boy Pike (ready for punishment)

[Michael GilliganParticipant @michaelgilligan61133]

Posted by brian jones 11 on 25/09/2021 01:15:05:

Many thanx AJ for your enlightenment

But can you show me algebraically why Addendum = 1/DP

I cant find any explanation except Google sources say thats what it is

[…]

.

That’s what it is “for standard gears” … it’s a nice practical/expedient ‘engineering’ choice

This page is written for Module gears, but the underlying logic is the same for DP

**LINK**

Gear terminology and teeth calculation formulas easy guide

MichaelG.

.

Edit: ___ Scary sums, and friendly illustrations here:

https://en.wikipedia.org/wiki/Involute and https://en.wikipedia.org/wiki/Involute_gear

Edited By Michael Gilligan on 25/09/2021 08:18:39

[Martin ConnellyParticipant @martinconnelly55370]

It's not an algebraic thing, it's a chosen thing. You could add height to the teeth and end up with a pointed tooth but that is both pointless (pun not intended) and poor practice as there is no strength in the pointed tip. It is also a redundant feature if the next tooth has already started to take the load before the pointed tooth has disengaged from its mating part. That then fixes the dedendum as a minimum of the addendum and as pointed out above by Andrew a small clearance is then added to the dedendum. Aiming for pointed teeth would require a larger blank and would therefore cost more in materials that were just being turned into waste and take more power to cut which costs in terms of energy use and then there is more wear and tear on the tools.

I've not done any clock making but from general reading I think clock makers have to apply their own rules as they may have small tooth counts on a gear. That means that the part above about the following tooth being engaged to take the load before the point disengages is less likely. This requirement for longer teeth would then become a requirement for deeper dedendums and undercutting of the tooth forms.

Martin C

[brian jones 11Participant @brianjones11]

Yes MG a nice description

BUT why oh why does the industry seek to cover up the fact that algebraically mod (or 1/DP) is NOT equal to Addendum – its a fudge factor and indeed it can be 0.8*mod for stub gears)

Why dont they just say "for standards we make mod equal to addendum"

We can live with that – its honest

But no, the carpetbaggers driven by AGMA have sought to skate over the issue of geometrical correctness

This deceit gummed up my gear box for some time as I couldnt make the connection in strict algebraic terms yet all the texts state Addendum = mod never once qualifying it with "by convention"

Its quite scandalous really and I hate being lied to. Its as bad as Clinton denying he had ykw with Lewinski

Could not pass GO and collect £200

WaM

OBTW

Spare a thought for the gear boxes used in wind turbines, These a predictably unreliable

look at the specs

sun and planet gearing up ratio 100:1 from 15rpm to ca 1500rpm

xmit 2000 hp

weight 20t

Shaft torque 700,000 ft lbs

Large sudden variations in loading, gusts, start up, emergency stop

Must be changed every 4-5 years

Thats something the green lobby dont like to talk about

AWS Wind energy is a long way from being a cheap renewable and is heavily subsidised by the taxpayer (levy on energy providers)

As for offshore conditions OMG

Thought for the day

from the desk of

SBP

Edited By brian jones 11 on 25/09/2021 08:57:13

[Neil WyattModerator @neilwyatt]

Posted by brian jones 11 on 25/09/2021 01:15:05:

why Addendum = 1/DP

Convention. There's no absolute physical law that says it should be any value, but this just happens to give well proportioned teeth. Much longer and undercutting affects even large gears, much smaller and teeth don't engage enough to run smoothly.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Andrew Johnston on 24/09/2021 20:15:58:

…

Spur gears are simple compared to other types of gear. Gawd help us if we ever get onto other types of gear!

Andrew

These examples are from FreeCAD's 'Gear' Workbench, which may be downloaded from Tools->Addon manager.

1. Involute and Cycloid side by side: although the difference isn't obvious these two gears don't mesh properly:

2. Bevel Gear:

3. Crown Gear

4. Lantern Gear:

5. Timing Gear

6. Worm:

7. Hypocycloid gear.

The hypocycloid form, with or without pins, is a new one on me. Looks hard to make – anyone know what they are used for?

Dave

[brian jones 11Participant @brianjones11]

Posted by Neil Wyatt on 25/09/2021 09:22:16:

Posted by brian jones 11 on 25/09/2021 01:15:05:

why Addendum = 1/DP

Convention. There's no absolute physical law that says it should be any value, but this just happens to give well proportioned teeth. Much longer and undercutting affects even large gears, much smaller and teeth don't engage enough to run smoothly.

Well Neil point taken and I can live with that

Its just the industry and indeed teaching texts present this as and algebraic certainty without any qualifier ( like this relationship is chosen as a convenience for standardisation)

Whats so hard about that, why try and bluff about it.

FCOL

WaM

[brian jones 11Participant @brianjones11]

Here is a typical example of how industry and teachers present these gear calcs to us

Just slipped in amongst other credible bits of algebra – 2nd from the bottom

No explanation or qualifier – btw its a fudge factor we use for convenience so that OD is (N+2)/DP

It falls down when Profile shifting is used to reduce undercut and if peaks are truncated as in stub gears

No mention of this

and the whole industry and educators are perpetuating this algebraically illiterate myth

Addendum = Module

and furthermore the word "pitch" should never been attached to "Diametral" as a descriptor for a gear size factor

Its wholly misleading – call it Diametral Factor" great no problem

Europe opted for Module equivalent to 1/DP and set Addendum = Mod in mm. Thats the teutonic mind for you

Thats sorted then

Another first for MEW

WaM

[Neil WyattModerator @neilwyatt]

Posted by SillyOldDuffer on 25/09/2021 09:52:15:

7. Hypocycloid gear.

The hypocycloid form, with or without pins, is a new one on me. Looks hard to make – anyone know what they are used for?

Dave

That's two gears side by side within a pin 'cage' They differ in number of teeth by a very small amount .

[Anonymous]

As previously stated the value for the addendum is arbitrary. Gear teeth can have a multitude of forms depending upon arbitrary parameters. The Victorians attempted to standardise gear teeth so that gears from different manufacturers would fit together and also allow for standardised machines and tooling. It's no different than Whitworth threads – there is no theoretical basis, it was simply an amalgam of common practice at the time. Similar to threads there are preferred values of DP and Mod for which cutters are readily available.

Much of the design of gears in Victorian times was done graphically, with supporting equations, rather than algebraically. Search for odontics, which substitutes a circular arc for the involute. The "button" method of making a cutter is nothing new. Much of the original work on standardising gears was done by Brown and Sharpe in the US.

As mentioned, it only gets worse for other gear types. A lot of the parameters for straight bevel gears were specified by the Gleason company in the 19th century. It's no surprise they sold machines to cut straight bevel gears. But there are competing standards, such as the octoidal tooth form. Spiral bevel gears have at least three different tooth forms created by Gleason, Klingelnberg and Oerlikon.

Some years ago, when I was exhibiting at a ME show, I got talking to someone who worked for the Mercedes high-performance transmission division. I started quizzing him on gears, but he simply said, we specify what we need and leave the details to the gear manufacturer. We're only scratching the surface of gear design in this thread.

Andrew

[John HaineParticipant @johnhaine32865]

Brian, the table that you copied above is a summary for the practising designer, not a presentation of the theory. No one wants to have to go back into the algebra when they want to do a quick calc and there's no reason to present any underlying theory in such a table. If you want to see it there are mechanical engineering textbooks that go into as much detail as you want. When starting to learn a new and complex subject it's common to get frustrated by the way details are presented and blame the messenger, you just have to work through it.

I don't think anyone else here has expressed any disquiet at the use of the term DP or module, you'll just need to get used to it. I don't think the gear industry or their users are going to change the way they express things just for you.

[brian jones 11Participant @brianjones11]

Ah the reactionary armchairs of protectionist AGMA

Another jaundised view on how blinkered stupidity is maintained cast in concrete

link courtesy Duncan Webster above

**LINK**

[Michael GilliganParticipant @michaelgilligan61133]

Posted by brian jones 11 on 25/09/2021 13:25:30:

[…]

link courtesy Duncan Webster above

**LINK**

 

.

I think it safe to say, Brian, that most of the contributors to this thread are familiar with that page … and I would offer this sentence as the most important one in the whole text:

”They know that the optical projector is very forgiving, and are well aware that competition forces everybody to exploit the situation. Competitive mediocrity is the name of the game.”

That’s not a criticism … it’s just ‘Production Engineering’

MichaelG.

Edited By Michael Gilligan on 25/09/2021 13:49:43

[egaParticipant @ega]

Most of the little I have read of this thread has been over my head or incomprehensible to me but, noting the OP's avatar, I wondered whether the great man had any particular views about gearing.

I would not be surprised to be told that he had designed his own gear form.

[Michael GilliganParticipant @michaelgilligan61133]

As an aside [or rather, a relevant question not yet posed] … Does anyone know the duration of the aforementioned FM45 [with or without hyphen] Fuse ?

My Google-fu has failed me

MichaelG.

[duncan webster 1Participant @duncanwebster1]

It sounds as tho' the OP thinks the AGMA is run by shape shifting lizards intent on confusing everyone, perhaps to achieve world domination. I've managed perfectly well with DP, PCD etc in my working life, as has the rest of industry for well over 100 years, but because the OP struggles to understand he reckons it's all rubbish.

[brian jones 11Participant @brianjones11]

Read their history

About AGMA

A protectionist cartel to dominate the Gear technology and its direction since 1916

Its not a government organisation like ANSI or BSI/ISO

So read between the lines and form your own impression

and remember – just cos Im paranoid doesnt make me wrong

Like I said, we can live with DP so long as we recognise that its a size factor and not to be confused with CP

ISO got it right with Lets make Addendum = Mod as a convenient design preference

and DP = 1/Mod in their relevant units (profiling shifting and truncating excluded of course)

Its not related to geometry just an international agreement

IMHO its rubbish not to come out and be frank about this – thats the bit I dont understand

One thing that has come out of this huff puff is how hugely complex making power xmission gearing can be

Its seems industry, as has been said, treats gears as components and hands the requirements over to specialist suppliers

That didnt work too well initially for wind turbines as the technology was carried over from Marine gearing

How many designers have you encountered in your career who were what we used to call Catalog Engineers

Maureen has my secret weapon in her jaws I must get on

WaM

 

OBTW SOD  thanx for the heads up on FreeCad, got it loaded with FCgears from Workshop and exported a dxf of and involute, useful bit of kit even a SBP can manage

Looking at the Crown and bevel pinion, without the benefit of CNC how does one make those pieces

 

Edited By brian jones 11 on 25/09/2021 15:53:17

[JasonBModerator @jasonb]

Posted by brian jones 11 on 25/09/2021 15:48:26:

and DP = 1/Mod in their relevant units (profiling shifting and truncating excluded of course)

Where did that one come from?

If you want to convert between DP and mod the calculation is DP = 25.4/MOD

So if you had a 2Mod gear and wanted to know the equivalent DP you would use DP = 25.4 / 2 = 12.7DP as confirmed buy my usual go to conversion chart here

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Michael Gilligan on 25/09/2021 14:35:24:

As an aside [or rather, a relevant question not yet posed] … Does anyone know the duration of the aforementioned FM45 [with or without hyphen] Fuse ?

My Google-fu has failed me

MichaelG.

.

Silly me … I should have remembered that it is probably a fuze, not a fuse

An earlier version of this document may have some details: **LINK**

https://bulletpicker.com/pdf/MIL-HDBK-145,%20Fuze%20Catalog.pdf

MichaelG.

.

Edit: __ So it might look something like this:

.

Ref: https://bulletpicker.com/pdf/MIL-HDBK-146,%20Fuze%20Catalog.pdf

Edited By Michael Gilligan on 25/09/2021 17:20:10

[brian jones 11Participant @brianjones11]

Come on JB keep awake I said

DP = 1/Mod in their relevant units

Its either N/PcD Imp or PcD/N metric

If I had an equivalent sign on Qwerty I would use it

OBTW finished machining secret weapon

tomorrow is bench trials day

Heh Heh

WaM

Edited By brian jones 11 on 25/09/2021 17:49:50

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