JG

One for John Stevenson I suspect.

I cannot reconcile the helix angle engraved on a 0.7Mod Hob with my
calulations.

The hob in question is 25mm O/D (ex ArcEuro) and it states a helix angle
of 1° 46'.

If I use the PC Circumference as the base of the triangle (as I think I
ought) () and assume that the dedendum is 1.4*MOD (for mod <1.25) then
my figures (72.382mm base and 2.199 perpendicular) give 1° 44' 25". If I
assume that the PCD is 'upside down' and therefore the addendum has to
be deducted from the OD to give PCD then I calculate 1° 41' 56"
B=74.142, P=2.199).

If I take the OD as the base then I get 1° 36' 14" (B=78.54)

If I take the root then I get 1° 51' 10" (B=67.984)

Can anyone suggest where I'm going wrong?

JG

JG

from JG

> One for John Stevenson I suspect.

> I cannot reconcile the helix angle engraved on a 0.7Mod Hob with my
> calulations.

> The hob in question is 25mm O/D (ex ArcEuro) and it states a helix angle
> of 1° 46'.

> If I use the PC Circumference as the base of the triangle (as I think I
> ought) () and assume that the dedendum is 1.4*MOD (for mod <1.25) then
> my figures (72.382mm base and 2.199 perpendicular) give 1° 44' 25". If I
> assume that the PCD is 'upside down' and therefore the addendum has to
> be deducted from the OD to give PCD then I calculate 1° 41' 56"
> B=74.142, P=2.199).

> If I take the OD as the base then I get 1° 36' 14" (B=78.54)

> If I take the root then I get 1° 51' 10" (B=67.984)

> Can anyone suggest where I'm going wrong?

It appears not )

This morning I spoke with ArcEuro but found that no-one there had any
idea what I was talking about so I can only assume that my figures are
correct and the informaion on the hob is in error.

JG

Ketan Swali

Giles just called to say that he had a healthy discussion with you,
and that it is probably a printing error - 46, instead or 45
(rounded). This would make sense as the person doing the printing is
different from the person who would have done the writing, and to a
Chinese, 5 or 6 is just a picture, and sometimes mistaken. I apologise
for any inconvenience this may cause, but I stand by the working
quality and results of this factories hobs, so I am happy to suggest
that you use it with confidence, should you wish to do so.

Thanks, Ketan.

JG

from Ketan Swali

> Giles just called to say that he had a healthy discussion with you,
> and that it is probably a printing error - 46, instead or 45
> (rounded). This would make sense as the person doing the printing is
> different from the person who would have done the writing, and to a
> Chinese, 5 or 6 is just a picture, and sometimes mistaken. I apologise
> for any inconvenience this may cause, but I stand by the working
> quality and results of this factories hobs, so I am happy to suggest
> that you use it with confidence, should you wish to do so.

Thanks for your input Ketan.

As you will no doubt already realize I never had any doubt about the
quality of the hob and will certainly purchase more as and when I need
them. I simply wanted to be sure in my own mind that my calculations
were correct since I am in the process of modifying the CES Hobber to
make it easier to set the correct helix angle - and ultimately the
correct angles for producing left and right hand helical gears.

It was interesting to discover that the 20DP Giles has (from your other
Chinese factory) also appeared to have the wrong helix angle engraved on
it but using my logic to calculate the PCD from the OD, I arrived at the
same figure as the Chinese so we all seem to have learned some useful
lessons today.

JG

Mark Rand

On Wed, 25 Jul 2007 05:36:19 +0100, JG <jg@crescentcomputing.co.uk> wrote:

>One for John Stevenson I suspect.
>
>I cannot reconcile the helix angle engraved on a 0.7Mod Hob with my
>calulations.
>
>The hob in question is 25mm O/D (ex ArcEuro) and it states a helix angle
>of 1° 46'.
>
>If I use the PC Circumference as the base of the triangle (as I think I
>ought) () and assume that the dedendum is 1.4*MOD (for mod <1.25) then
>my figures (72.382mm base and 2.199 perpendicular) give 1° 44' 25". If I
>assume that the PCD is 'upside down' and therefore the addendum has to
>be deducted from the OD to give PCD then I calculate 1° 41' 56"
>B=74.142, P=2.199).
>
>If I take the OD as the base then I get 1° 36' 14" (B=78.54)
>
>If I take the root then I get 1° 51' 10" (B=67.984)
>
>Can anyone suggest where I'm going wrong?
>
>JG


Surely the PCD of the hob should be OD -2*(dedendm + clearance)

clearance will be between 0.1*module and 0.3*module depending on
specification.

Assume a mid point of 0.2*module for clearance.

That would give PCD= PI*(25-2*(1.4*.7 + .2*.7))== 71.5

With 2.199 perpendicular, that gives an angle of 1° 45' 41''.

Half a second is acceptable as a rounding error!

regards
Mark Rand
RTFM

Mark Rand

On Fri, 27 Jul 2007 00:21:09 +0100, Mark Rand <randm@internettie.co.uk> wrote:

>On Wed, 25 Jul 2007 05:36:19 +0100, JG <jg@crescentcomputing.co.uk> wrote:
>
>>One for John Stevenson I suspect.
>>
>>I cannot reconcile the helix angle engraved on a 0.7Mod Hob with my
>>calulations.
>>
>>The hob in question is 25mm O/D (ex ArcEuro) and it states a helix angle
>>of 1° 46'.
>>
>>If I use the PC Circumference as the base of the triangle (as I think I
>>ought) () and assume that the dedendum is 1.4*MOD (for mod <1.25) then
>>my figures (72.382mm base and 2.199 perpendicular) give 1° 44' 25". If I
>>assume that the PCD is 'upside down' and therefore the addendum has to
>>be deducted from the OD to give PCD then I calculate 1° 41' 56"
>>B=74.142, P=2.199).
>>
>>If I take the OD as the base then I get 1° 36' 14" (B=78.54)
>>
>>If I take the root then I get 1° 51' 10" (B=67.984)
>>
>>Can anyone suggest where I'm going wrong?
>>
>>JG
>
>
>Surely the PCD of the hob should be OD -2*(dedendm + clearance)
>
>clearance will be between 0.1*module and 0.3*module depending on
>specification.
>
>Assume a mid point of 0.2*module for clearance.
>
>That would give PCD= PI*(25-2*(1.4*.7 + .2*.7))== 71.5
>
>With 2.199 perpendicular, that gives an angle of 1° 45' 41''.
>
>Half a second is acceptable as a rounding error!
>
>regards
>Mark Rand
>RTFM


I meant half a minute!!! (actually a third of one)


Mark Rand
RTFM

John Stevenson

On Fri, 27 Jul 2007 00:44:50 +0100, Mark Rand
<randm@internettie.co.uk> wrote:

>On Fri, 27 Jul 2007 00:21:09 +0100, Mark Rand <randm@internettie.co.uk> wrote:
>
>>On Wed, 25 Jul 2007 05:36:19 +0100, JG <jg@crescentcomputing.co.uk> wrote:
>>
>>>One for John Stevenson I suspect.
>>>
>>>I cannot reconcile the helix angle engraved on a 0.7Mod Hob with my
>>>calulations.
>>>
>>>The hob in question is 25mm O/D (ex ArcEuro) and it states a helix angle
>>>of 1° 46'.
>>>
>>>If I use the PC Circumference as the base of the triangle (as I think I
>>>ought) () and assume that the dedendum is 1.4*MOD (for mod <1.25) then
>>>my figures (72.382mm base and 2.199 perpendicular) give 1° 44' 25". If I
>>>assume that the PCD is 'upside down' and therefore the addendum has to
>>>be deducted from the OD to give PCD then I calculate 1° 41' 56"
>>>B=74.142, P=2.199).
>>>
>>>If I take the OD as the base then I get 1° 36' 14" (B=78.54)
>>>
>>>If I take the root then I get 1° 51' 10" (B=67.984)
>>>
>>>Can anyone suggest where I'm going wrong?
>>>
>>>JG
>>
>>
>>Surely the PCD of the hob should be OD -2*(dedendm + clearance)
>>
>>clearance will be between 0.1*module and 0.3*module depending on
>>specification.
>>
>>Assume a mid point of 0.2*module for clearance.
>>
>>That would give PCD= PI*(25-2*(1.4*.7 + .2*.7))== 71.5
>>
>>With 2.199 perpendicular, that gives an angle of 1° 45' 41''.
>>
>>Half a second is acceptable as a rounding error!
>>
>>regards
>>Mark Rand
>>RTFM
>
>
>I meant half a minute!!! (actually a third of one)
>
>
>Mark Rand
>RTFM

Try 19 seconds <g>

Seriously though what home shop equipment can get to seconds of a
degree. Even with a vernier scale you need a lot of room to get this
in.
Looking at a Gleason bevel gear cutting machine today and the scale
took up about 8" on a large diameter casting and even that could only
get to 5 second divisions with the vernier

JG

from Mark Rand

> JG wrote:

> >One for John Stevenson I suspect.
> >
> >I cannot reconcile the helix angle engraved on a 0.7Mod Hob with my
> >calulations.
> >
> >The hob in question is 25mm O/D (ex ArcEuro) and it states a helix angle
> >of 1° 46'.
> >
> >If I use the PC Circumference as the base of the triangle (as I think I
> >ought) () and assume that the dedendum is 1.4*MOD (for mod <1.25) then
> >my figures (72.382mm base and 2.199 perpendicular) give 1° 44' 25". If I
> >assume that the PCD is 'upside down' and therefore the addendum has to
> >be deducted from the OD to give PCD then I calculate 1° 41' 56"
> >B=74.142, P=2.199).

> Surely the PCD of the hob should be OD -2*(dedendm + clearance)

That could well be the case if there was extra clearance. The
calculation of the dedendum at 1.4*MOD takes account of the clearance.
Adding .2*MOD for clearance would make the dedendum 1.6*MOD

> clearance will be between 0.1*module and 0.3*module depending on
> specification.

> Assume a mid point of 0.2*module for clearance.

> That would give PCD= PI*(25-2*(1.4*.7 + .2*.7))= 71.5

Adding .2*MOD for clearance would make the dedendum 1.6*MOD and the
total tooth depth would be 2.6*MOD -- 1.82mm (for 0.7MOD) --
and the root dia. 21.36mm rather than the calculated and measured root
dia. of 21.64mm.

> With 2.199 perpendicular, that gives an angle of 1° 45' 41''.

> Half a second is acceptable as a rounding error!

As you correctly point out in your second posting 1/3 minute would be
acceptable.

You will see from Ketan's post that we now think that it could well be a
typing error by the engraving operator who would be working from a hand
written Chinese original.

The important point as far as I am concerned is that I am now confident
in my calculations. Whether I can set my hobbing machine to this
accuracy is another matter entirely !!

JG

JG

from John Stevenson contains these words:

> Mark Rand wrote:

> >>Half a second is acceptable as a rounding error!
> >
> >I meant half a minute!!! (actually a third of one)
> >

> Try 19 seconds <g>

> Seriously though what home shop equipment can get to seconds of a
> degree. Even with a vernier scale you need a lot of room to get this
> in.
> Looking at a Gleason bevel gear cutting machine today and the scale
> took up about 8" on a large diameter casting and even that could only
> get to 5 second divisions with the vernier

I fully agree John but the question was never about setting the angle,
it was purely to get my head around the method of calculating what the
helix angle should be.

As it happens I won't be using a scale, I'll be taking two measurements
at 100mm centers and the difference will be 100 times the tangent of the
angle -- similar to a Sine Bar but with a fixed Base rather than a
fixed Hypotenuse -- and I'm sure I can measure 0.05mm with a dial
indicator.

Whether that will be maintained after tightening the clamps is another
matter but if you don't aim for perfection you will never achieve it!

JG