cutting spur gears on a mill

[brian jones 11Participant @brianjones11]

Don't confuse quantity with quality.

Sounds like the motto of Tesco

or perhaps

Millionaire Alan Sugar and his egregious Amstrad rubbish

IMHO there have been numerous high quality insightful postings on this thread

May the freedom to rattle cages and shake the trees be long fruitful and may we be tolerant enough to sweep up the dummies spat out on the floor

Now Maureen needs my attention – Wimin

OBTW have we got any members of the err opposite gender on this board? They were good at making bullets in WWII

[Howard LewisParticipant @howardlewis46836]

There are threads on here that have been posted by those not of male gender.

And just look in awe at the work produced by Cherry Hill! Puts many men.to shame.

A while ago there were articles showing one of her workshops.

Howard

[John PParticipant @johnp77052]

Posted by brian jones 11 19/09/2021 23:06:16

Why was the concept of DP Diametral Pitch introduced when the CP
Circular pitch addresses the same size issue and is physically
realisable ie you can measure it with calipers

———————————————————-

I expect it was more to do with the folk around at the time of his
avatar setting out gears with calipers and circles of whole inches.
Maybe it goes back further than this even to the time
of wooden gears in wind mills.
Although this does raise an interesting point for model makers ,
just like using stock size threads mostly all gearing is
Dp or module and made to a standard, for the production of
small gearing and rack type rotary cutters this does not make
much sense.For model engineering applications the only considerations
for gears would be to fit in the space available the required tooth count
and to look in the correct proportions it does not matter whether
they are mod or dp or cp standard.
Pitching out a 20 dp circular rack type cutter at .1571 inch is just
plain daft, since a cutter is being made it would seem to be
more sensible to use a CP system either imperial or metric
depending if that you favor one or the other units.
Laying out the spacing between each tooth can be so much easier
using this system .The page here

shows the near equivalent DP,CP ,MOD ,providing there is no intention
to link up to a commercial gear train there is little point in doing it from
a model engineering perspective.
Dividing also becomes somewhat easier from the example above of 20 dp.
eg for 20 tooth gear pitched out now at .150" CP instead of .1571 "
moving the cutter down by . 030 inch and setting the dividing head
to cut 100 divisions would ensure accurately timed cuts leaving just a
few facets on each tooth.Similar combinations of other size gears
just need to be worked out.

The chart here

shows the basic rack dimensions
and there is enough information there to calculate the tip width
and from that a form tool can be ground to make the cutter.
There is some benefit to having cutters properly form
relieved to avoid the type of finishes noted by
Roger Best in his posting 21/09/2021 21:57:52

"I did see some very good imitations of severely worn traction engine
gears. Modelling to such realistic detail is wonderful."

I made one of these circular rack cutters around 1985,because the
cutter had no form relief the cutting action was so poor i never
pursued the idea further,seen here a pair of
timing gears and a cast crank case it was never completed.

A few years later the Eureka device was published and provided
an easy solution to the making of form relived cutters.

Look in at Lathes .uk the latest submission there is a
Micron gear hobber 102 a very interesting machine,
apparently will cut gears down to 800 dp.
In the section Brief General Catalogue 1960s-70s
shows the smallest gear in the world the profile of the teeth
looks remarkedly similar to BJ 11's gears.

Finally with interest shown on here with horology and armchairs
maybe a combination project would be of interest.

John

[brian jones 11Participant @brianjones11]

Posted by John P on 18/09/2021 11:40:12:

Somehow doubt if BJ 11 would be interested in cutting a rack.
This one here took over 5 hours to cut the 159 teeth.

Was definitely a "OMG what a painfully slow process as you mill each one"
John

You see I knew you were doing it wrong

you need a cycloid profile for rack and pinion

https://www.geartechnology.com/articles/0115/Non-Involute_Gearing,_Function_and_Manufacturing_Compared_to_Established_Gear_Designs/

That why I suggested a motor bike chain

fascinating stuff

Whack a Mole

[John PParticipant @johnp77052]

Posted by brian jones 11 23/09/2021 11:58:05

Posted by John P on 18/09/2021 11:40:12:
Somehow doubt if BJ 11 would be interested in cutting a rack.
This one here took over 5 hours to cut the 159 teeth.

Was definitely a "OMG what a painfully slow process as you mill each one"
John
—————————————————————————-

You see I knew you were doing it wrong
you need a cycloid profile for rack and pinion
https://www.geartechnology.com/articles/0115/Non-Involute_Gearing,
_Function_and_Manufacturing_Compared_to_Established_Gear_Designs/

That why I suggested a motor bike chain
fascinating stuff
Whack a Mole

————————————————————

I suppose 5 hours + for cutting rack teeth passes into
insignificance in comparison 14 + years and still
counting in making the parts the rack fits to.

It's called making things
John

[brian jones 11Participant @brianjones11]

Tremendous magnum opus, 14 years longer than most marriages then.

Is it the sort of m/c to make special gear hobs?

WaM

[Anonymous]

Posted by brian jones 11 on 23/09/2021 14:22:16:

Is it the sort of m/c to make special gear hobs?

Looks more like a cylindrical grinder.

Andrew

[John PParticipant @johnp77052]

I'm being generous here , Brian 5/10 but Andrew gets 10/10.

John

[Anonymous]

Posted by brian jones 11 on 19/09/2021 23:06:16:

….CP Circular pitch addresses the same size issue and is physically realisable ie you can measure it with calipers…

You can't measure CP with calipers. Circular pitch is defined as the length of an arc on the pitch circle diameter. A caliper measures between two points on a line, which is also not likely to intersect the PCD at the point of measurement. Using a caliper to measure across multiple teeth doesn't give CP either. There are published tables that give theoretical values measured across multiple teeth to get pitch and backlash, but they don't give CP.

Andrew

[brian jones 11Participant @brianjones11]

Agreed, you are measuring chordal pitch slightly less than CP

So how did they actually measure gears in late victorian times with simple tools

I think someone had some actually gear design books from that or edwardian period

[Martin KyteParticipant @martinkyte99762]

Could I suggest that circular pitch is actually more sensible when you think of someone cutting gears with a saw and a file. or inserting teeth into a wooden gear. Once you have your pitch circle you can then mark the positions of the gears by dividers. Draw your lines and then start cutting filing or whatever.

regards Martin

[JasonBModerator @jasonb]

But would stepping out with dividers not have the same problem as measuring with callipers in that you are stepping out a cord not an arc?

[Pete RimmerParticipant @peterimmer30576]

Posted by JasonB on 23/09/2021 17:09:19:

But would stepping out with dividers not have the same problem as measuring with callipers in that you are stepping out a cord not an arc?

If you're stepping out with calipers, marking, spotting then drilling the tolerance probably means that it's neither here nor there.

Compounding errors by stepping out isn't the best way to produce a gear anyway, dividing is much more reliable.

[Martin KyteParticipant @martinkyte99762]

My point was the a suggestion as to the origins of circular pitch. So historically if it was about planting teeth around a circular disk or shaft by using hand tools and simple measuring tackle then it seems to me to be the origins of CP as a way of describing gears as it was essentially how you made them. The Anticythera machine was probably hand filed gear wise and certainly had some very unusual tooth counts. Gearing would be slow moving and sliding would definitely occur. Low friction gearing for clocks and rolling action for high speed power tramission would have been far into the future. Module and DP are better ways of doing things in the machinary age and as said dividing has long superceeded marking out.

regards Martin

[John PParticipant @johnp77052]

Posted by Andrew Johnston 23/09/2021 15:22:18

Posted by brian jones 11 on 19/09/2021 23:06:16:
….CP Circular pitch addresses the same size issue and is
physically realisable ie you can measure it with calipers…

You can't measure CP with calipers. Circular pitch is defined
as the length of an arc on the pitch circle diameter. A caliper
measures between two points on a line, which is also not likely
to intersect the PCD at the point of measurement. Using a caliper
to measure across multiple teeth doesn't give CP either. There
are published tables that give theoretical values measured across
multiple teeth to get pitch and backlash, but they don't give CP.

Andrew
————————————————————

Whilst this is true and you can calculate the sizes of gears in
circular pitch, you can check the size of the made gear by
measuring across a number of teeth using this calculator from Ondrives

https://www.mesys.ch/calc/ondrives.fcgi

The inputs are module gear sizes for example the bull gear
here on the Myford at 16 DP

Input on the top left column 16 Dp = 1.5875 module
second line down pressure angle 20 deg
9th line down if you put in a tooth count any number
eg 60 tooth the diagram at the bottom of the page
will show the tooth profile of the main and mating gear.
The 6th column down is the normal tooth allowance
DIN 3967 adjusting the number in the check box will adjust the
Base tangent span at the bottom of the right hand column
(Left box)
and display the gear profile and backlash at the bottom of the
page.The two examples here also show how you can check
the PA of a gear ,something that crops up fairly often on the
forum ,as can be seen numbers of teeth in the span are different,
at around 30 tooth the differences are quite small but still enough
to be a reliable indicator.

Getting back to the CP measurement to convert cp to mod

eg .150" = 3.81 mm 3.81/ Pi = 1.212760666 module

feed this into the Ondrives calculator here.
and use this to check the gear.

.

Going back to the Myford gear.
From the previous table for the Myford bull wheel
here is the gear as measured appears to be about
.006 mm over the size indicated on the table.

A disc type micrometer is a useful thing to have
to measure these gears a normal caliper may fit in
providing the jaws will get past the pitch circle.

John

[Roger BestParticipant @rogerbest89007]

Posted by Howard Lewis on 23/09/2021 11:34:31:

There are threads on here that have been posted by those not of male gender.

And just look in awe at the work produced by Cherry Hill! Puts many men.to shame.

A while ago there were articles showing one of her workshops.

Howard

Only "many"?

All would be better, surely there are very few men barely worth considering in the same league and 3.5 billion not worth considering.

[Anonymous]

Posted by John P on 23/09/2021 19:11:52:

A disc type micrometer is a useful thing to have to measure these gears……….

Darn it, summat else to put on the list of things I don't need, but that would be nice to have. After all if you don't have it you certainly can't use it.

When I started making gears I spent some time faffing about with gear verniers:

I didn't find them easy to use and the results were indifferent. Now I just calculate the theoretical values and machine to those.

Andrew

[Neil WyattModerator @neilwyatt]

Someone mentioned plastic gears.

Confession – I didn't design the differential gears, just the wheels and outer casing.

Neil

[John HaineParticipant @johnhaine32865]

Surely on the CP/DP question, the reason is that in most industries gears are regarded as a component, ordered in from an outside specialist or made in an internal specialist department. The machine designer wants to work in simple things like gear axis spacing and (especially in the days before CAD or even pocket calculators) not want to have to keep inserting factors of pi into his dimensions. So he doesn't, and calls up a gear of such and such a diameter and number of teeth. The gear maker can then apply the pi factor, though actually that only has to be done once to design the tooling.

[Martin KyteParticipant @martinkyte99762]

Exactly John but thats not how it started.

regards Martin

[Martin ConnellyParticipant @martinconnelly55370]

At work we bought in planetary gearboxes from the likes of Allen. When you looked at all the criteria they took into account to avoid harmonics causing issues it was very complex and best left to the experts/specialists.

Martin C

[brian jones 11Participant @brianjones11]

All you needed to know about CP and the Base Bitch

https://www.geartechnology.com/articles/0992/Base_Pitch_Tables/

this past post went into the problems of measuring a gear (as opposed to calcs + suck it and see)

You cant just use calipers / disc mic and measure PCD just like that

https://www.model-engineer.co.uk/forums/postings.asp?th=166048

If you have two gears meshed together you can deduce the pcd from distance between centres

then there's the eyeball method

[Anonymous]

Posted by brian jones 11 on 24/09/2021 14:41:58:

All you needed to know about CP and the Base Bitch……..

That's a weird way of looking at gears. The base circle is the circle from which the involute is developed. Depending upon the gear specification the base circle can be larger than the root circle. Which leads to the question of what should the tooth form be between the base circle and root circle where the involute does not exist?

I don't need to suck it and see, my granny already taught me to suck eggs.

Andrew

[brian jones 11Participant @brianjones11]

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

I think this was before AGMA got involved

SIAS means you cant actually measure the PCD once youve cut a gear and need another one to mesh before you can deduce PCD with confidence – all the rest is gear snobbery

but thats my junior opinion, what do I know – not a lot as has been amply shown

WaM

Edited By brian jones 11 on 24/09/2021 16:15:39

Edited By brian jones 11 on 24/09/2021 16:16:27

[Dave SParticipant @daves59043]

Pretty sure you can calculate PCD from Mod and tooth count, and can do Mod from tooth count and outer diameter.

From that you can layout the centre distances of a pair of gears.

PCD for a single gear is not a very useful number anyway

Dave

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