"B.B." u wrote in message
news
In article ,
"Harold and Susan Vordos" wrote:

[...]

I'm shaking my head in disbelief as I read you're being taught to use a
negative rake holder with aluminum, however.

It isn't really that we're being taught wrong as much as we're not
being taught about choosing cutting tools. Really, I'm using this class
in somewhat the wrong way--it's designed as a class to tech new kids
where all the knobs are, weed out slackers, and get everyone comfortable
around the equipment. Not so much a general introductory machining
class. Up until now, without knowing much at all about cutting tools
I've been making passable parts--they're just not pretty or fast. So it
would seem that they just chose a bunch of cutters that would survive a
semester of new students at a time and let the teacher cover a wider
range of material. In fact I believe the curriculum has a class
somewhere that's dedicated almost entirely to cutter angles, materials,
shapes, and physics.
Although I see the logic behind this, admittedly, I'd prefer at least
a couple of days on cutting physics. We are all going to make cutters
for threading, but that's not exactly the same thing as making a
generalized cutting tool.

In a sense, once you understand the geometry, it's all closely related.
Threading offers some challenges beyond normal turning because of the rapid
advance of the tool with coarse threads, so your side clearances have to be
modified. I gather you'll get to that eventually. I didn't realize your
class was not an introductory to machining, so I may have spoken out of
turn. Lets keep a watch and see what you're taught. Also, try to keep
in mind I'm an old school machinist, with absolutely no hands on with CNC
machinery. Many of the things that I hold dear and spelled success for me
may very well not be being taught now. However, for the guy with a manual
machine at home, knowing the things I learned spell total freedom.

I saw elsewhere in the thread you were starting to wonder about my
teacher's competence. As far as I can tell he's quite skilled and so
far every question I've brought to him has been answered. I figure if I
start using HSS tooling instead of the inserts he'll hang around me and
point out all the important stuff. It's just that this is a basic class
And as a bit of trivia, he's still got Kenneth McDuff's old toolbox.
Apparently, Mr. Sadist took this same class before he got arrested.

I'm afraid you have me at a disadvantage. Kenneth McDuff? Doesn't ring
a bell.

[...]

Rake angles I think I've got a handle on. At least enough to get
started. As I understand it a chip breaker is just a little groove
some
ways back from the cutting edge, correct?

It can be as little as a line ground parallel to, and slightly back from,
the cutting edge. Anything to disrupt the flow of the chip and cause it to
start winding will usually suffice. Needless to say, it relies on depth of
cut and feed rate for proper performance, so there's no hard and fast rule
on how to grind them. Deep cuts with heavy feed require a fairly wide
groove so the chip doesn't stack up as it makes the turn. Often the
breaker will wind the chip perfectly, but they won't break. I usually will
live with that scenario, especially if the long curls fall of the side of
the carriage peacefully. Anything to avoid the long strings.

So then, with the grind we're talking about, you can see that if you use the
corner of a wheel that has a small radius dressed on it, you can grind a
chip breaker that goes full positive at the cutting edge, then slowly goes
from positive to negative, rolling the chip in the process. This design has
the added benefit of minimizing hogging. Depth and width are related to
feed and depth of cut, so you more or less just have to experiment with the
combinations until you become familiar with them and how they behave under
certain circumstances. Does that make any sense?

Yes, that's correct, but it's a good idea to incorporate a chip breaker
as
your rake angle. It takes a little grinding experience to get good at
it,
but it really pays benefits. A simple rake angle improves machining
ability, but creates horrible chip problems. When you couple the chip
breaker with the rake angle, all that goes away. If nothing else, you
gain
considerable safety, not generating long strings that can wrap around
your
spinning object.

Heh, I know how much of a pain it is to have a wad of razor wire get
snagged by the chuck and spun around at a few hundred RPM!
How would I incorporate a chip breaker into the rake angle? My
wild-assed guess would be to grind a too-shallow rake angle across the
whole top of the tool, then the proper rake angle only about half way
back. That would make a little hump just behind the cutting edge. Any
photos on the web of grinding chip breakers?
And how would I judge the effectiveness of a chipbreaker? Should I
expect to produce a bunch of little uniform chips, or should I just
expect the string to snap off every so often?

Again, many things determine how well they work. You can expect perfect
chip breaking when the design fits the application. When your breaker is
too narrow, the chips may just ignore it, sliding across the top instead of
curling. That's not common. What usually happens is the breaker is too
narrow and deep for the cut, so chips stack up in it, which can lead to a
broken tool if you're not on your toes. When that happens, you widen the
groove, providing a more gentle arc so the chip can roll through without
stacking up. You can also take a lighter feed, but that's not the best way
to get where you're headed. As I said, chip breakers are very reliant on
the cut in order to perform properly. Often you'll start a cut with .008"
feed and the chips won't break, but increase to .012" and they break
perfectly. Without standing over your shoulder, it's pretty hard to
describe. Experience, even bad experience, will help you make sense of it.

Is diesel close enough to kerosene to work? I can get a little
container of diesel easy, but kerosene will be a hassle. I'm in texas,
so our diesel is as sulphury as law allows. (:

I own a couple Dodge diesels, and hate the smell of the fuel. I've
never
tried it, but I can't imagine it wouldn't work. I used Stoddard in
place
of kerosene to get away from the smell, which is similar to diesel. If
you have a (Stoddard) solvent container for cleaning parts, just dip a
little out as needed. I've done that for years. Sulfur is very much a
part
of lubricating when machining. One of the best oils for machining steel
is
sulfur based cutting oil. The stinky stuff.

Ah, my day-job is diesel equipment buffoonery & repair, so I'm quite
accustomed to the smell. Even kind of like it. (: That's why I can
get it easily--have a (cleaned out) mustard bottle full of it already.
I have no idea what's in the bottles of cutting oil in the machine
shop. Some are quite thick, some are thin, all have different smells,
most are the color of fresh motor oil, some are the color of used, one
looks like gear oil, but doesn't smell like it, and another I swear is
water. Probably some good oil somewhere in that collection.
If I can find some known good cutting oil I'll use that.

For aluminum, it's fairly safe to say that you should avoid any of the heavy
oils. Considering you don't mind the smell of diesel, I'd make that my
first attempt, but don't expect it to make a difference if you use the
negative rake inserts. I should have mentioned that, for the most part,
*because* of the false cutting edge that is expected when running negative
rake inserts, the tip, even on new ones, is usually not sharp. It need not
be, and is better off to not be. That's one of the chief reasons to avoid
using them on aluminum. They simply aren't sharp enough to cut well.
Aluminum responds to being "peeled", something not accomplished with
negative rake.

Steel on the lathe was always right on once I quit mixing up the
inch and metric dials, but aluminum (oddly enough!) was pretty much
unpredictable -- always somewhere between -1 and +4 thou.

While I'd be a fool to say I know why, my guess is that the insert you're
using isn't doing the cutting, but the built up edge is. The cut would be
ragged and change according to the amount of buildup present, which would
likely be constantly changing to some degree, affecting the diameter. The
finish would show exactly that, being a torn and ragged surface instead of a
shiny one.

There's nothing like a sharp tool when you're machining aluminum. It's
perfectly willing to machine freely and leave a decent finish, providing you
do your part and supply the proper tool and lubrication. Grade of
aluminum makes a difference too, as does the condition. Soft aluminum is a
bitch.. 7075, 6061 and 2024, all in an artificially aged condition,
(T-6, T651, T351, etc.) are all great to work with. Some grades are not
considered machining grades and cut poorly.

Harold