Need gear hobs for the following :

[Boo]

Hi,

I have several plans and casting sets for some Chenery designed model IC engines and would like to find the following hobs for making the gears :

32dp x 12t, 32dp x 24t, 32dp x 20t, 32dp x 16t,  40dp x 15t, 40dp x 12t, 40dp x 20t, 40dp x 10t

Anyone know where these can be purchased in the UK at reasonable prices ?

Thanks,

Boo

[Don1966]

Make your own Hobbs they are easy to make. I have a calculation sheet that figures it out for you here http://www.modelenginemaker.com/index.php/topic,6012.0.html


Don

[pgp001]

Hi Boo

I think you mean involute gear cutters as opposed to hobs, anyway these are listed on the RDG tools ebay listings
https://www.ebay.co.uk/itm/RDGTOOLS-INVOLUTE-VARIOUS-DP-GEAR-CUTTERS-ALL-VARIATIONS-SIZES-TO-CUT-ALL-TEETH/370710110759?hash=item56500b6227:m:my9tv0-bRgr6ObSQY8Jh1-w

Just for info, a hob is used to generate the teeth, and for what you have asked for you would only need two hobs as each one will cut any number of teeth within normal limits, you would need a special machine to use these though.
The individual cutters are in sets of eight to do the same range of teeth, and at best are an approximation of the tooth form required, most hobbyists use this kind of cutter, and as already stated they can be home made and can be used on your milling machine.

I am very lucky in that I have a gear hobbing machine so I use the hob type cutters for most of my gear cutting needs.

Hope this helps
Phil

[Jo]

  Sounds like Aeronca and Anzani - You can't buy a cutter for the 10 tooth gear

Jo

[b.lindsey]

Jo, would the #8 cutter approximate it close enough?

Bill

[Jo]

If it is the gear I think it is it is the middle gear inside the dummy magento driving the distributor. Even with all my gear cutters I was considering buying three of those from HPC gears 

Jo

[Jasonb]

maybe that is why Boo wants a hob as I don't think they give the problem that using involute cutters on a small tooth count gear do

[pgp001]

That's right Jason.

The gear blank diameter can be adjusted to modify the tooth shape to minimise undercutting of the teeth.

If you are at a loose end for a few hours, its all in here.

https://books.google.co.uk/books?id=p0q-h9fEw-MC&pg=SA8-PA8&lpg=SA8-PA8&dq=modified+diameter+to+reduce+undercutting&source=bl&ots=xbgGxJR4Xp&sig=dXmurdPdXmAohJA3cKFEZNP2LkE&hl=en&sa=X&ved=0ahUKEwiw152N1ZrYAhUiDsAKHUHODyIQ6AEILDAA#v=onepage&q=modified%20diameter%20to%20reduce%20undercutting&f=false

Phil

[Doc]

Make your own Hobbs they are easy to make. I have a calculation sheet that figures it out for you here http://www.modelenginemaker.com/index.php/topic,6012.0.html


Don

Yes I have to agree Don I will never go back I like the hob approach simple to make and simple to use.
Thanks again Don!

[Boo]

Sounds like Aeronca and Anzani - You can't buy a cutter for the 10 tooth gear

Yes Jo,  those are the engines I need gears for.  I'm quite keen to make my own rather than buy some,  does the fact that you recognise the gears mean you have made thoise engines ?

I do also need to know the pressure angle Mr C. relied on ?

Make your own Hobbs they are easy to make. I have a calculation sheet that figures it out for you here http://www.modelenginemaker.com/index.php/topic,6012.0.html

Yes I have to agree Don I will never go back I like the hob approach simple to make and simple to use.

Thanks for the (link to) the spreadsheet,  that will save a pile of time..

The type of hobs I want to obtain is the rack-form type with circular pitched teeth rather than helical pitched teeth,  does anyone here know if/where these can be bought ?

Boo

[Jo]

Yes Jo,  those are the engines I need gears for.  I'm quite keen to make my own rather than buy some,  does the fact that you recognise the gears mean you have made thoise engines ?

I do also need to know the pressure angle Mr C. relied on ?

I have not made them yet. I have the necessary castings for both (and two other Anzani engine casting sets    ) in the collection for building 

They are box standard normal 20 degree PA gears. For their purpose in the engine as long as they are all of the same PA it will not really matter you just need them to go round and round and maintain the right ratio 

Jo

[Jasonb]

Most of the ones you can buy have the helical teeth as they are made for gear hobbing machines where the blank rotates. The straight type that are used with the gear blank on a dividing head or rotary table tend to be home made as it's not really a commercial method though the gear cutting machines that use a straight rack and planning action use the same principal.

If Jo was buying from HPC I would assume that 20pa is OK as that is the standard angle they do.

[Boo]

I do also need to know the pressure angle Mr C. relied on ?

They are box standard normal 20 degree PA gears. For their purpose in the engine as long as they are all of the same PA it will not really matter you just need them to go round and round and maintain the right ratio 

If Jo was buying from HPC I would assume that 20pa is OK as that is the standard angle they do.

Well, the RDG Tools involute cutters are for 14.5 degree pressure angle, does that affect the spacing of the centres (spindles) for gears compared w 20 degree pa gears ?

What are the reasons for prefering one of these common pas vs the other ?

Thanks,

Boo

[Jo]

What are the reasons for prefering one of these common pas vs the other ?

The pressure angle of the tooth alters both the shape of the teeth and the length of the contact path: The larger pressure angle makes the teeth more stubby and gives a longer the line of contact = it can provide more force. The pressure angle does not alter the pitch or outside diameter of the gear but only gears of the same pressure angle should be used together.

If you look at an involute gear with many teeth and compare the shape of the teeth with those of a small gear with few teeth they seem very different but when the two gears come together you will find the magic of the involute shape means they mesh perfectly. Because we need different shape teeth gear cutters come in different tooth forms which they have given numbers to make it easier for us to find the right cutter shape, a hob works differently and can cut any number of teeth

RDG are probably selling 14.5 degree cutters as that is the pressure angle for some of the early lathe change wheels. 20DP are the norm when buying gears these days so 20 degree cutters would be a better long term investment  .

Jo

[Bertie_Bassett]

14.5 PA is/was the standard Pressure angle for the change gears on southbend (and its derivative) lathes so there is a reasonable demand for 14.5 hobs as plenty of people need to make few gears to get a complete set.

20PA is what my Denford lathe has (on the few gears i have) and is supposed to be more robust. i think it now more common than 14.5 but i could be wrong.

you could either go with a set of 8? cutters that give you nice rounded profiles or you can make up your own straight sided hob and use a method similar to planing on a mill with a dividing head.
http://www.helicron.net/workshop/gearcutting/ shows how to do your own although the final tooth form is not rounded but made up of facets. supposedly they run smooth enough for most needs but i haven't tried making any yet so cant say for sure.
main advantage is you would only need on hob to make any number of toothed gears you wanted so might be worth an experiment?