Posted by Chuck Awot on August 05, 2000 at 01:17:04:
In Reply to: PROPER HOBB SHIFTING PROCEDURE posted by JASON on November 28, 1999 at 23:17:58:
As you probably know already, the purpose of shifting a hob is to move the worn portion of the
tool away from the active area of the cut, and to move a fresh portion of the tool into it. On newer
hobbing machines,especially CNC, the shifting is controlled automatically, and the shift amount,
and the frequency of the shifting is controlled by commands input to the machine control. Very often,
especially on the newer CNC hobbers, it is very common to shift the hob, a small amount, after
every cycle. However, on an older, manually shifted machine, this is not feasible, and it
makes sense to establish a shifting strategy to establish the amount and frequency of shifting.
There are a number of ways in which you can do this, but I really would recommend that you first
start with your feeds and speeds. One mistake that a lot of people make when trying to establish a
strategy for hob shifting, is that they do not have a good system in place for establishing the
cutting speeds and feeds for the jobs that they are running. Unless you establish consistant speeds
and feeds, for the materials that you are working with, you will never be able to establish a good
system for hob shifting. Once you do that, you are ready to start looking at shifting procedures. The
one important thing that you have to consider is how a hob wears. Initially, when you start cutting
with a hob, the hob will experience a fairly rapid breakdown on the cutting edge within the first few
parts cut, until there is approx. 0.002-0.003" of peel-back on the tool. Once this happens, the "rate"
of wear will kind of even out. The hob will continue to wear at a reduced rate until is reaches a critical
point. For fine pitch finishing hobs, 20DP and finer this is usually when the wear reaches about .006-.008"
of wear. For coarser pitches, it is approximately .010-.012" and can be even a little more, if it is
a pre-grind or pre-shave roughing hob. When it reaches this critical point, the hob will begin to wear at a
much higher rate than it did before reaching that point. This is the point where you would want to shift
the hob. As the hob gets dull, it generates more and more heat. The more dull it gets, the more heat
that it generates. It will finally reach a point where so much heat is being generated, the heat will
actually anneal or soften the tool steel, causing it to wear much faster. Because of the difference in the
machineability of different gear materials, and difference in their hardnesses, the rate of wear on the tool
will change with a change in the material. The same thing applies to the hardness of the gear material
as well. A change in only a few points in the hardness of the workpiece, can really cause major
differences in the way that the tool will wear. If you are cutting with a fine pitch hob, like I described
earlier, you want to shift the hob when you get approximately .006-.008" of wear on the portion of the tool
that is in the cut. If you wait too long, you will waste the hob, unnecessarily. What you have to do is to
establish how many gears, of a certain material, and a certain hardness, that you can cut, before you get that
.006-.008" of wear. What I do is to work with one material,at one hardness, to get the initial data that I need
to do this. For instance, cutting 12L15 steel at 180 Brinnell hardness, compared to 4150 pre-hardened steel at
a Brinnell hardness of 375 is like day and night. Start out with one material and hardness. What I do, is to look at
this one part, and determine the number of teeth in this part. Next, multiply the face width of the gear by the number
of the teeth, and you will get what is called the "lineal" number of teeth. For instance, a gear with 20 teeth, with a
face width of .500" will have 20x.5= 10 lineal inches of teeth. This will be your starting point. This will set a base line that
will allow you to compare this gear to another one with the same material and hardness. For instance, if two weeks later, you cut
a gear out of the same material, with 60 teeth, and a face width of 1.000", this gear would have, 60x.75"=45 lineal inches of teeth.
If both gears are the same pitch, the gear with 60 teeth has 5 times the amount of lineal inches of teeth that the 10 tooth gear
has. That means that you should be able to cut 5 times the number of 10 tooth gears, with the same frequency of hob shifting.
To next thing you need to look at is the hob shift amount. On older manual machines, one good consistant method that I used was to
take the circular pitch of the part that I was cutting, and divide it by the number of gashes on the hob. For instance, if you are
cutting with a 10 pitch hob, divide 10 by PI. (10 / 3.14159) This will give you .314159". Next take the number of gashes in the hob, for
instance, use 12 gashes. Now take .314159 and divide it by 12 gashes, (.314159/12) and you will get .0262". Use this for your shifting
amount. This will at least give you a start point. Now comes the hard part. What you have to do, is to start cutting parts, and after each
part cut, inspect the hob to see how much that the hob has worn. Keep doing this until you have the .006-.008" wear in that portion of the
hob. Once you get that amount of wear, now shift the hob. Start cutting parts again, and do the same thing again. Cut a part, and re-inspect
for wear. You will finally get to a point where you can start to predict how often you have to shift to stay within the .006-.008" of hob wear.
After you have shifted all of the way across the hob, and you have taken a count of the total number of parts that you have cut, you can use
that information to determine the shifting frequency needed for a different workpiece, made out of the same material, with the same hardness, and the
same pitch. For instance, say that you were hobbing the 20 tooth part, and you were able to cut 16 parts, before you had to shift the hob. That would
mean that you could cut a total of 10 lineal inches x 16 parts, or a total of 160 lineal inches of teeth before shifting the hob. When you go to cut the
60 tooth gear, you would already know that since it has 5 times the number of lineal inches of teeth (45 inches), that you would have to shift 5 times
more often, to get the same amount of wear on the hob, or every 3.2 pcs. Since you cannot shift after 3.2 you would round it off to 3, and shift after every three
60 tooth gears.
Many times, when you are cutting very accurate gears, you may start to lose the accuracy of your tooth profile after only about .003-.004" of wear on the hob.
Sometimes, in a case like that, even though the hob may last a lot longer, you have to shift sooner, just because of the accuracy of the tooth profile that you need.
The shifting direction of the hob is important too. Always make sure that you are shifting the sharp portion of the tool towards the gear to insure that you are
bringing fresh teeth into the workpiece.
I have a lot of other information, on hob sharpening and on hobs in general, that I can send to you if you are interested. If you would like me to send this to you,
let me know.
Koepfer America, LLC
Contact me at my e-mail: email@example.com
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