Subject: Re: Machining Gears

From: [email-masked] (Mike Rehmus)
Date: 15 Dec 1994 18:16:05 GMT

In <[email-masked]> [email-masked] (Thomas Clarke) 
writes: 

>
>Since the you say just some teeth are chipped, maybe you
>can have someone build up metal in the area of the broken,
>teeth.  Then your problem just beomes one of grinding/machining
>the proper profile into the repaired areas.  
>
>I forget the name of the process, but it is common to build up
>a hard wear resistance layer from welding rod on the edges of
>earth moving equipment.
>
>Tom Clarke
>
>
>

Common techniques to repair broken teeth are to sink a few screws 
vertically into the tooth root and then build up the tooth with brazing 
or to pin & braze a new tooth into the cleaned out area of the old tooth 
and then file/machine to contour.

Get an index to some of the metalworking hobby mags.  I'd bet I've seen 
this info in some detail in one of them.  

Best,

Mike

---

From: [email-masked] (Mike Rehmus)
Date: 12 Dec 1994 04:57:09 GMT

In <[email-masked]> [email-masked] (Erik 
Frank) writes: 

>
>Howdy all!
>
>I've got a question this group concerning the machining of gears.   
First,
>a little backround on my problem.   I have a close ratio five speed 
racing
>gearbox for an old BMW that is somewhat rare.   Both 3rd and 4th gears 
are
>missing teeth.  A synchro failed and went through both meshes.
>
>Because this tranny is rare, replacement gears are no longer available 
from
>BMW.  Thus, my only course of action would be to find another used 
tranny,
>or make replacement gears.   I've been searching for months now for 
another 
>unit, with no luck, so now I'm looking at making some new gears.
>
>The gears themselves are a helical pattern and utilize the Porsche 
style baulk-
>ring synchromesh.  How difficult would it be to machine a helical gear 
pattern?
>What fixturing would I need?  I am a mechanical engineering student, 
and feel
>competant enough to attack this problem.  I have access to Bridgeport 
Series II
>manual mills, with DRO's; a Hurco 3 axis CNC bed mill, and an old Van 
Norman
>horizontal mill.  
>
>One other possibilty would be to scrap the entire gearset and machine 
straight
>cut gears and use dog-type racing synchros (probably easier to 
machine).
>
>Thoughts?  Opinions?   I'd love to hear from people who have actually 
machined
>gears for critical applications like this.   I most concerned about 
maintaining
>close tolerances. 
>
>Thanks,
>Erik Frank
>
>[email-masked]
>Dept. of Mechanical Engineering,
>The University of Texas, Austin
>

And then you get to heat-treat the metal.

Why not adapt another gear box?  With the large number of great gear 
boxes now available, machining an adaptor and modifying the rear mount 
would seem to be much easier.  You can probably find other close-ratio 
boxes that are similar to the bimmer's.

Best,
Mike

---

From: [email-masked] (gary preckshot)
Date: 15 Dec 1994 21:56:10 GMT

> I've got a question this group concerning the machining of gears.

[followed by ambitious plans....]

 You got a real !problem!, starting with metallurgy.  Once you've got that
solved, the usual way they make gears nowadays is with a hobb running
against blanks mounted between centers on a dividing head on a mill.  Or an
alternative is a blank mounted between centers in a dividing that is geared
to table motion on a mill.  You cut single teeth that way in a helix.  In
either case, your major  cost, assuming you get the machines for free, is
cutters.  Form-relieved gear cutters are really expensive (like $100 for
starters) and may take months lead time for metric sizes, (assuming you can
get the specs out of BMW).  Hobbs are astronomical, and you might as well
have the gears made by a pro outfit, since a hobb is at least as hard to
make as the gear it cuts.  Finally, when you get the gear blank cut, you
have heat treatment and final grinding (or did you think that those fine
finishes came from machining operations?)  Let us know how it comes out.