Looking for specs on Tooth rounding

[gearman1234]

I am looking for a tooth rounding spec for 12DP, 20 deg Pressure angle gear. Is there any book available on this subject which lists that?

 

[carburized]

Gearman1234

I do not think there is any spec for this. Thjere are some guidelines you can follow. I really think you are asking for tip modification rather than rounding. Look under "Special design considerations" in Gear Handbook by Dudley.

 

[shopguy]

If by "tooth rounding" you are talking about bullet-nose chamfering, I am not aware of any national specification. However, many companies have "in house" specs. It all depends on who the oem of the part is. For the most part, bullet nosing a part is not rocket science. It's just a matter of getting the gears to engage smoothly when they shift so that they don't break off the ends of the teeth.

 

[gearman1234]

I know that it's not a rocket science and by the way rocket science has its roots in simple laws of physics.

I have also developed my spec based on common sense. But I thought there may be some dimensional guide about the radius in both the cross sections of the tooth (radial and axial) depending on the DP and diameter. Bigger tooth would require more curvature than the smaller tooth.

 

[shopguy]

OK, so...
Several major US tractor producers, some of which are no longer in business, have multiple specs based on the size of the tooth for bullet nose chamfering. A typical format tries to control about half dozen dimensions including: tip of tool to part od (Depth of cutter), the angle from the root to the od, the diameter of the tip of the tool, the path of the centerline of the tool, the tool radius and the radius at the bottom of the tooth. The specs are usually tied in with a cutter spec because the cutter has to have certain radius and forms ground into it based on the pitch. A lot of times, the cutter will leave a small flat on the gear face which is often defined and toleranced on the blue print. Another defined parameter is the length of the bullet nose vs the face width of the gear.
The next really interesting part of tooth rounding is inspecting it. You can define complex radii and arcs and flats on paper without any problem. But the guy who puts the radius on only has a few hand tools and radius gages to check it with. Don't get too carried away with your dimensions. Good Luck in developing your spec.

 

[GearmanPE]

I beleve there are standard cutters for common bullet rounding. If you could get your hands on a Cross manual, they may be defined there. The typical tooth rounder has a cam that really controls the rounding radius from one side of the tooth to the other, and the cutter from root to tip.

The cam can be changed, but the standard one has a heart shape with a smooth, not pointed transition oppisite the crotch. The blank is prepaired with a 15 degree angle on the face, starting about 20% up from the root.

All this presumes that you will do this on a Cross machine, the mast common way. (Hey has a simular machine). here are other methods that use hollow end mills etc, but they produce more of a pointed shape, typical of automotive gears operating with syncromesh elements.

If you are really stuck, I can get data on cutters and some copies of info from a contact in gear jobshop. If you need just to describe on a drawing, a note "full round bullet nose rounding required" will suffice for most jobers. ehahlbeck@wi.rr.com

 

[EnglishMuffin]

On a Cross machine you can do pointing of various types as well as rounding, depending on the cam. The cutters are special tapered end mills. Pointing is better if you have all the teeth engaging simultaneously, as in a clutch, since it is more likely to engage on the first try, but produces rather too fragile a tooth for single tooth power shifting. I talked to an engineer once at Cross (I used to work for the parent company) - I think he said that the guy that used to design the cams had retired and there was nobody left who knew anything about them any more. I think I was looking for a special cam for doing an unusual internal rounding application. I have a feeling that Hey is a better bet for info, but I could be wrong.

 

[diamondjim]

Please note that Dudley gives
dimension for tooth rounding
but does not show how to accomplish
this on the part. It could be
produced by lapping or have special
cutters or hobs to create the rounding.
Normally it is not much more than
removing the sharp edge at the tip
but can include tip relief as well.
I would think the speed of the mesh
would determine how severe the rounding
should be and also to assure that you
have the proper contact ratio. Any tip
modification reduces the contact ratio.

 

[EnglishMuffin]

We generally seem to find ourseleves in agreement, Diamondjim, but on this occasion I'm not sure that I would agree with your characterization of rounding as "just removing the sharp edge" - rounding generally involves producing a precise cone shape, but usually with the conical surface intersecting the tooth flanks at a noticeable angle. I have found that having the proper rounding shape can be quite critical in shifting applications, which as far as I know provide the main reason for doing it. Of course, it is always necessary to remove sharp edges, but I wouldn't categorize that as "rounding".

 

[diamondjim]

Tooth rounding implies rounding not
only at the tips of the flanks but
also at the ends of the faces. I
guess this is the trouble in lack
of definition as to "tooth rounding"
and to why it is asked for. Dudley
does show it at three places in his
illustration of rounding. Rounding
at the ends of the flanks can be
provided by a special hob. I am not
sure which type of gear rounding
gearman1234 is asking for.
According to Dudley's illustration
there is tip round, end round, and
edge round is his illustration on
page 5-39 table 5-14.
For the 12 dp he shows values of
.005/.015 for tip and end rounding
and values of .005/.020 and
.012/.030 for edge rounding
depending whether the gears are
medium strength or high strength gears.
He does imply that this is accomplished
by burr-brushing edges. I have
recommended to our shop people to
ensure tip rounding be done prior to
induction hardening. We presently
chamfer the flank ends and chamfer the
faces of the gears if induction
hardening is called out for the gear.
The reason why we do it is to reduce
the hardness at these edges so they
are not brittle as well as to reduce
the chances of developing hardening
cracks during the induction hardening
process. Would like to hear other
opinions on this.

 

[EnglishMuffin]

Unfortunately, my version of "Dudley",(second edition),does not even seem to have a page 5-59. It does seem that we all have a different idea of what tooth rounding is, and Gearman1234 should maybe clarify what he means. I think a number of us have automatically assumed he meant "bullet nose", which is the most common type that you create with a Cross or Hey tooth rounding machine.

 

[gearman1234]

I thank all of you for your inputs.
The tooth rounding which I have mentioned is just the rounding of the ends of teeth which I believe is the "bullet nose", which we produce on tooth rounding machine.

I do not mean the tip or edge rounding.
The edition I have of Handbook of Practical gear design by Dudley does not mention about any rounding at all.

 

[diamondjim]

It is not in the index but does appear in
the first edition page number 5-39 not
5-59. It has the defined values in
Table 5-14. If I get to the technical
library on campus, I will check out the later
revisions. I am surprised that it would not
be in the new editions.
Now you have aroused my curiosity in why you
are specifying only rounding at the faces.
Is this in lieu of crowning?