"B.B." u wrote in message
news
In article ,
jim rozen wrote:

In article ,
B.B.
says...


Rather than measure everything out and go for exact grinding I just
eyeballed it this first time around. I was also operating under the
assumption I could get the bit as hot as I could stand holding without
doing any damage. Got some blue and brown on it, but otherwise looks as
much like http://www.sherline.com/images/grndfg10.gif as I could
remember at the time.

That one would work fine. You don't really need a chipbreaker for
what you are doing. I respect harold a lot but he's pretty
compulsive about doing things the 100% right way. For small depths
of cut you won't see any real detriment to not having a chipbreaker
except maybe the chip coming off will mark the finish to some
slight degree.

True on all accounts. The problem here is if you wait until you actually
need a proper chip breaker, you've missed a golden opportunity to get it
straight, from the start.

Chip breakers, once again, are load sensitive. Jim is right on the money.
A chip breaker that works beautifully for a roughing cut is likely to fall
flat on its face when taking finish cuts. At best, you'd profit by the
positive rake, but not much else. That's not a problem when you're running
parts in volume, because you generally go through a roughing sequence before
any of the parts get finished. That, too, is a part of learning to machine
properly. Your tools are ground and setup appropriately. My point in
getting you to try chip breakers is to learn about them and have them firmly
entrenched in your mind, so when the need arises you don't have to go
through the learning curve.


Good point. But somewhere along the line I'm going to try to force
the issue and find a cut that'll require a chip breaker, just so I have
a chance to play with the idea. I think perhaps steel would do it. So
far even with the carbides it's been more prone to looooong curls of
chip.
If you know a way to force long chips that would mandate a breaker
I'd like to know it too.

Simple. Proper positive rake. Light feed. No chip breaker. You can
generate all the strings you desire. That's the point of my information.
The use of chip breakers is a package deal. Lots of things have to come
together.

Understand that some aluminums cut without making strings. It's the nature
of the material. Amongst them are 2024 and 7075, each of which will gladly
cut string free under the right conditions, including the absence of a
proper chip breaker. 6061 is another story, and is generally more than
happy to generate strings, although generally very easy to control.


snip-

While I'm thinking of it...
As I understand things, if I do change the height of the cutting edge
the only real drawback is that I need to compensate and move the tool up
until it's level with the centerline. Correct?

Regardless of the holding system, that's an ongoing problem. I use a
square tool block (my choice) so I shim all my tools. You get used to
having a small box of shims handy and it takes almost no time to set up your
tools. You get to the point where you can pick up a tool and know what shim
is required to hit center. I use anything for shims, including strapping
material, which comes in a myriad of thicknesses. Old feeler gages are
fine, too, just expensive. And then there's always shim stock!

I've found that back rake has never been essential for the stuff
I do, at work or at home. So I just leave it out.

You can go full circle on this issue and stick to making finishing cuts with
chip welded negative rake inserts. I strongly recommend you *don't* avoid
the learning curve. Learn to do it right, even when it takes more time.
You can always back off when it's not important, but without the experience,
you'll have nothing to use as a reference when you're facing problems.
The real benefit of learning the little things is when a tough job comes
along, you may be the only one equipped to deal with it. I gained the
respect of my peers because of my anal approach to machining. No one
wanted to emulate what I did, but they sure as hell understood why I had
success when they did not. Remember, often the difference between one who
can and one who can't is nothing more than how they apply what they know.
You see that on a daily basis in a commercial shop.


If you *wanted* to form a chipbreaker in that kind of tool,
the idea would be to run a narrow groove alongside the cutting
edge, on the top surface. This allows the chips to flow off
at a larger angle than the real rake angle, and they eventually
strike the grooves far edge and snap.

That's too simplified, and may or may not work. If you look at chip
breakers that are formed on negative rake inserts, you'll notice they are a
shallow radius which begins at the theoretical false cutting edge of the
insert. It's assumed you're going to take a particular depth of cut and a
given amount of feed, so they factor that in when creating the inserts.
The whole idea is to encourage the chip to start rolling, not to stub into a
wall. That raises cutting pressure, often breaking your tool from chip
stalk up.

A chip breaker that doesn't function has no value. Be certain to learn to
incorporate rake when you attempt chip breakers. It's particularly
important when using HSS, so you can keep temperatures low at the cut, and
reduce cutting pressure as well.

There's no shortcut, you have to start at the bottom and experiment with
each material, slowly accumulating enough information in your head that it
will become useful. You'll know when you've hit a winner. The chips curl
and break nicely, and the tool cuts without much noise. Watch the chip
color (when machining steel) so you don't produce anything above the
slightest hint of yellow. That usually spells an early death for your tool
unless you have very high cobalt, or are lucky enough to be using Stellite.
Tools of this nature generally respond very well to lubrication, too. Helps
keep the temperature down and prolong the tool's life.

Is there a standard distance back I should go, or just look and see
where the chips are hitting the top of the tool and put it there?

My policy is to grind the breaker and take a trial cut. Again, the cut
required determines everything. Maybe start short and shallow and see how
it behaves. You can always go wider and deeper, but can't reverse the
process without wasting a good amount of tool. Once you see what the chip's
doing, you can make the appropriate adjustment. This process can really
pay benefits when you're boring deep holes. With the proper tool
configuration, you can go deep and not generate any strings, which generally
spell tool death when they wrap around the bar. If your bar has enough
clearance, your chips will come out as short crumbles and drop neatly at the
mouth of your part. Lots depends on bar clearance, but I think you get
the idea.

When I took a shop class for the first time, there was an entire
four hour class period devoted to trying to get HSS tools ground
properly. Apparently in some classes the instructor simply
hands out 1/4 CRS blanks for the students to practice on.

To start, that's not a bad idea. The only real problem is the CRS doesn't
grind worth a damn. The concept remains unchanged, though.

One thing I don't use is a tool rest. Never. Not for grinding tool bits.
They get in the way and restrict your ability. You likely won't be able to
do that at school, but keep it in mind for home use. Have a special grinder
that you use exclusively for sharpening toolbits, and have it mounted such
that you can stand erect and grind at a comfortable level. It takes some
getting used to, but once you master it, you'll never go back to a
conventional pedestal grinder for grinding toolbits. Knowing how to
properly dress the wheel becomes quite important when you grind this way.
It must run smooth and true, for it becomes your reference point.

Harold