basic rack tip radius 1

[roseda]

Does anyone know what the standard value is for the tip radius on the basic rack for hobbing gears?

The gear that I'm working on is metric (European), has a module of 3 and a pressure angle of 25degrees.

I'm hoping to construct it using kinematics in CAD.

Is the radius size related to the module?

Thanks in advance, Dan

 

[Philrock]

0.25 x module is a widely used value for tool tip radius.

 

[roseda]

Thanks,

I found this page which specifies 0.38 x module but it could be an american spec:

http://www.mech.uwa.edu.au/DANotes/gears/generation/generation.html#top

 

[israelkk]

roseda

Danotes refers to FILLET RADIUS and you asked about TIP RADIUS. As far as AGMA there is no requirement for tip radius for the basic rack but, only for fillet radius. Please refer to Machinery's handbook 26th edition where this issue is explained.

 

[dimjim]

A .38 times the module is not possible
for a 25 degree pressure angle. If you
use .25 times the module you will have
a .086 times the module flat portion
at the bottom. I think I would back off
to .20 times the module for the radius
for a 25 degree pressure angle rack or
hob.

 

[gearcutter]

Because you’re asking about the tip radius of the basic rack, I’m assuming you are talking about the tool. I’m also assuming you don’t require tool data for profiles that include protuberance.
Here are the most common profiles you’re likely to encounter in Europe; take your pick!
Most of the DIN & ISO profiles are not “full fillet”. The standards talk about the cutter’s tips as having a “radius with line”.

Cutter Addendum x M / Cutter Tip radius x M / Cutter Dedendum x M

ISO 53.2 Profile A: 1.25 / 0.38 / 1.0
ISO 53.2 Profile B: 1.25 / 0.30 / 1.0
ISO 53.2 Profile C: 1.25 / 0.25 / 1.0
ISO 53.2 Profile D: 1.40 / 0.39 / 1.0

DIN 867: 1.25 / 0.30 / 1.0
DIN 867: 1.25 / 0.25 / 1.0
DIN 867: 1.20 / 0.20 / 1.0
DIN 867: 1.16 / 0.16 / 1.0
DIN 867: 1.30 / 0.45 / 1.0
DIN 867: 1.40 / 0.39 / 1.0

DIN 3972 I: 1.167/ 0.20 / 1.0
DIN 3972 II: 1.25 / 0.20 / 1.0

 

[dimjim]

Gearcutter,
Those standards are for 20 degree
pressure angles only?

 

[gearcutter]

Woops!!!
Yes dimjim you are correct; they are all 20deg, thankyou for pointing out my mistake.
I should have read roseda's post more carefully, I assumed it was 20deg.
I'll see if I have standards for 25deg.

 

[roseda]

Thanks for all the answers, I didn't realise there was so many different standards.

For clarification, I was asking about the tip rad on the tool.

In the past, I have never specified the radius on the drawing so which one of the ISO 53 profiles would you generally get if you don't specify?

gearcutter, I am not familiar with protuberance.

 

[dimjim]

You can always layout the rack and use the
max radius that you can use and still make
certain that you have a straight profile
thru the 1.00 times the module on the
addendum of the cutter plus some
tolerance for wear. This would at least
give you a maximum radius to put on your
drawing.
A protuberance cutter essentially cuts an
undercut at the bottom of the tooth to
allow for a grinding operation if required.

 

[gearcutter]

I was surprised to find that there is very little info in regards to any thing other than 20 deg PA.
After contacting a tooling supplier in England, I was told that there is no mention of any of this in any standards that they are aware of so they use 0.28 x module for their 25 deg PA hobs.
I did find this though; Maximum Hob Tip Radius = (0.785398 cosPA - b sinPA) / (1 - sin PA) where PA is the pressure angle and b is the gear dedendum.

 

[dimjim]

Gearcutter,
That formula checks out with a layout
of a max radius for a 1.25 addendum on the cutter.

 

[Spurs]

The basic rack is really a Virtual shape that can be used with modifications to define both the tool and the gear. There is enough information about the basic rack that you can define both.

It sounds like what you are asking about is the basic rack fillet radius. On the basic rack, the fillet radius is a simple circular arc of some size r. The value of r will correspond to the size of the radius on the tip of the cutter. On the actual gear, the fillet is a trochoid shape which is a special shape created by the sweeping path of the cutter tip that has been radiused. The trochoid has a varying radius throught its non circular arc.

When we specify fillet radii on gear teeth, what we are really specifying is the fillet radius on the "Virtual" basic rack.

The most commonly used North American standards for 20 degree pressure angles are.

For fine pitch gears - a 0 basic rack fillet radius - although it is customary to give it some unspecified value - I know it really sucks doesnt it

For course pitch gears .38 x the module

And generally available for all gears
a full fillet radius - this gives the lowest stress conditon

I would suggest using a full fillet radius whereever possible for the lowest bending stress. The only disadvantage is that this tends to make the form diameter larger - which usually isnt a problem, but on designs which are marginal in contact ratio, this could be a challenge.

http://www.webgearservcies.com