From: "Ed Huntress" <huntres2@optonline.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Flycutter from hell?
Date: Thu, 17 May 2001 01:28:40 GMT

>>I remember spending several hours (over the course of a few days) grinding
an HSS bit downto a .064" wide 'cutoff tool' 3/8" long for a particularly
nasty groove that had to be cut.<<

Most people are too cautious grinding hss. You can grind it until it starts
to glow red. It actually cuts pretty fast when you apply enough pressure.

So does the grinding wheel. <g>

Ed Huntress

From: "Ed Huntress" <huntres2@optonline.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Flycutter from hell?
Date: Thu, 17 May 2001 15:37:47 GMT

>>I have always used water (usually room temperature or so) to cool HSS when
grinding tools.<<

Never, ever, ever. Change your thinking on this quick. High speed steel is
tough stuff, but it can't tolerate that kind of thermal shock. Neither can
most other high-alloy tool steels -- and the low-alloy and straight-carbon
types (O-1, W-1) can't take the heat to begin with.

I use water to cool mild steel when I'm grinding it, and I'll do it on
woodworking chisels and plane-iron blades, which I never let get hot enough
to show color, anyway. My good ones are never allowed to get too hot to
touch. At those low temperatures it's not much of a risk and water dips
speed the operation a lot.

But you can't grind HSS very well without getting it hot. It can take the
heat, right up to a low red glow. What it can't take is thermal shock.

>>I hold HSS barehanded and fairly close to the area being ground and I give
it a few swishes when I start feeling some warmth.<<

That's fine for grinding woodworking chisels, but it's too dainty for
grinding high-speed steel. If you can handle it you aren't applying enough
pressure to grind it efficiently. HSS requires some pressure, which is hell
on grinding wheels but it beats taking an hour to grind a threading bit from
a blank.

I hold HSS bits with Vise-Grip pliers when I'm grinding them. Sometimes I
clamp them in a toolholder and hang onto that. I can grind a threading bit
from a 3/8" blank in about five minutes.

Ed Huntress

From: "Ed Huntress" <huntres2@optonline.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Flycutter from hell?
Date: Fri, 18 May 2001 02:51:53 GMT

OK, Jack, let's take a closer look at this. You're grinding a material
that's designed to *cut* at temperatures as high as 1,000 - 1,100 deg. F
(higher for cobalt HSS). It won't be damaged, annealed, or otherwise
affected by temperatures below that. But it will develop surface cracks like
crazy if it's cooled too quickly, even from temperatures well below its
annealing temperature.

The material contains 10 - 15% carbides: chromium carbide, iron carbide, and
tungsten carbide. If you grind it slowly, those carbide particles stand
proud of the matrix and grind your grinding wheel. It's a tug-of-war. Your
absolute rate of wheel wear will be low if you grind gently, but your
relative rate of wear will be higher. In other words, you waste grinding
wheel by grinding HSS slowly.

To grind HSS effectively you have to use enough pressure to work on the
matrix that holds the carbide particles. That's what it takes to grind HSS
quickly and efficiently. Because you're applying a lot of local pressure
when you do that, it's hard to avoid gouging the wheel locally. Its relative
wear rate will be better, but it will lose shape, and the net effect is a
toss-up, in terms of how much HSS you can grind with a given amount of
wheel.

If someone has told you to grind HSS this way, he or she never learned about
the properties of the material or of the results of 100 years of shop
experience. I see absurd recommendations on this and many other subjects on
the web and sometimes in print, by people who have done things some odd way
for 20 years and who claim to be "experts". They were never taught right,
either.

So, grind HSS slowly if you want to. As you say, one of the pleasures of
doing this for a hobby is that we don't have to worry about production
rates. But be careful about the implications of what you're saying. To
water-dip HSS between grinding passes is not a wise thing to do. If you're
grinding so slowly that you're not heating the steel beyond the temperature
at which you can hold it, you probably won't have any problem. But that's
not the way HSS is usually ground. If someone heats it up by grinding with
firm pressure, and then dips it in water a few times between passes, he'll
wind up with a lot of chipped and broken tools. And he'll never know why,
because someone told him that the books say to dip it in water between
passes.

Ed Huntress

From: "Ed Huntress" <huntres2@optonline.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Flycutter from hell?
Date: Fri, 18 May 2001 13:30:18 GMT

>>Let me guess ... this isn't a 1/3hp grinder?<<

Actually, it is. Sears' finest, ca. 1965, which I bought new. Norton 60-grit
aluminum oxide wheel, fairly open, medium-hard.

I'm not talking about stabbing the tool into the wheel until chunks fly off.
<g> Just apply a moderately firm pressure, until grit is flying off the
wheel, without being too dramatic. Short passes. I don't let the tool get
red hot, although it could tolerate it if it has to.

Ed Huntress

From: "Ed Huntress" <huntres2@optonline.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Flycutter from hell?
Date: Mon, 21 May 2001 18:58:28 GMT

I'm glad you've joined the Grinders from Hell, and that you'll have hours
more to spend with your butterfly collection, or whatever.

But, if you've heard the same thing about grinding HSS cool, I'm really
perplexed. I checked many of my books and found nothing about grinding HSS.
The last I read about it must have been 20 years ago, and it wasn't in the
books that I own, or at least the ones I have handy. I have some old
machinists' textbooks stored away and I'm tempted to dig them out.

Grinding tools until they're just warm to the touch and then water-dipping
them is the time-tested way to grind woodworking tools, made of water- or
oil-hardening steel. But every machinist I know grinds HSS with some real
authority. It's worked for me for nearly 30 years.

I may get a chance to call someone from the steel makers today, but I'm
leaving in a few hours for the Eastec metalworking trade show, and I may not
get to it until I return on Friday. In either case, I have to get this
cleared up with some current information.

Methinks this is a case of my brother metalworking journalists and authors
feeding on each other's b.s., and passing it around until it's part of the
catechism. It's happened before. That's why I *always* go to the original
sources on things like this.

Ed Huntress

From: "Ed Huntress" <huntres2@optonline.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Flycutter from hell?
Date: Mon, 21 May 2001 21:14:38 GMT

Whoo-ee. I think you've uncovered a subject to deal with in my next Tooling
column.

I talked to a young metallurgist at one of the two big HSS makers (to remain
nameless), and he first said, "Oh, yeah, keep it cool." When I pressed him,
he said, "Oh, yeah, you can grind it up to red heat. But nobody can control
the temperature that closely." Sheesh. Well, I can, sonny. I asked him about
water-dipping. "Yes," he said. Then, "Oh, no, you'll crack it." I am not
reassured about the knowledge of today's degreed metallurgists.

Then I called the head of a machine shop training course at a major
university. It, too, shall remain nameless. This guy has been teaching
machine shop for 30 years. "Yes, I hand-hold them, and I keep them cool," he
said. "I don't let them get above a middle brown." Middle brown is 500
degrees F. I figured this guy's shop must smell real good, like a barbecue
pit. There's burning meat on the spit every day. <g>

It appears that the b.s. has gotten quite deep. Perhaps everyone has
forgotten how to do it. In any case, I'll dig into it deeper when I get
back. It sounds like a fun item for my column.

Ed Huntress

From: "Ed Huntress" <huntres2@optonline.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Flycutter from hell?
Date: Thu, 24 May 2001 23:59:30 GMT

I'm back from Eastec. I asked about 30 people, mostly applications guys from
the tool companies, about grinding HSS lathe bits. Those under 40 looked at
me like I was crazy to grind tool bits at all. Those over 60 looked at me
like I'd just crawled out from under a rock, but at least they knew what I
was talking about.

The surprise was that very few had a clue about how to do it, or why. Those
who did know divided into two camps: those who were totally clueless about
how HSS behaves and why, and who did it (rightly or wrongly) on the basis of
incorrect understanding, and those who knew. Among the latter, and
especially among the old shop guys, most said they had learned to grind
tools to shape aggressively. A few didn't even know you could grind HSS with
aluminum oxide wheels. They were brought up with Norton green wheels
(silicon carbide), and they cut fast enough that you don't have to worry
about aggressiveness much one way or the other.

FWIW, Dapra, who sells little grinding machines for small-shop tool
grinding, have dropped even the green wheels. They're now using a
steel-shell design, with plated-on cubic boron nitride. The young lady I
talked to at Dapra, who was quite sharp, who knew all about using CBN, and
who had heard of silicon carbide, was amazed that anyone could grind HSS
with AlOx. <g>

I reached a senior metallurgist at Carpenter Technology today, and here's
what he said:

Most HSS today is tempered at 1025 deg. F (triple tempered, and held at
tempering temperature the first time for 4 hours -- don't try this with your
propane forge...). As long as you stay below that temperature, you won't
hurt the steel. Don't dip it in water if you grind it hot enough to show any
oxidizing colors. (That occurs at around 275 deg. F, I think, for light
straw.) It can't take the thermal shock. Also, if you raise it to 1400 deg.
F too quickly, it can crack without water-dipping it. But the tool is
wrecked by then, anyway, so it doesn't matter much.

If you get it a little bit red at the edges, a low-pressure dressing pass
will take off enough steel to clean up any marginal material. As long as you
can't see anything above dull red, you're OK.

The metallurgist said that the youngish machinist in their shop grinds HSS
cool. He doesn't know why. My guess is that he read Moltrecht's book...

He reminded me that surface grinding of HSS is done cool, often with
continuous coolant flow. This has nothing to do with damaging the metal.
It's done to avoid warping, which is a killer in most surface-grinding
applications. It doesn't apply to off-hand grinding of tool bits, but some
people may have picked up the idea there.

One of the old guys reminded me to turn off the work light on your pedestal
grinder when you're grinding HSS, so you can see the dull red. I forgot this
because the bulb on mine burned out around 15 years ago and I haven't gotten
around to replacing it yet.  8-)

All in all, it confirms what I thought, with some new information that I
found interesting. I'm amazed at how few people know how to do it at all. It
wasn't that long ago that this was part of a basic machine-shop course.
There is some discussion among machine-shop instructors about whether they
should drop the teaching of grinding lathe bits entirely.

One of my co-editors, Ed Hoffman, may know Moltrecht. They're both authors
for Industrial Press. I'll see if I can reach him and find out where his
info came from.

BTW, I don't think I'm going to use this in my tooling column. Nobody under
50 is going to know what I'm talking about.

Ed Huntress

From: "Ed Huntress" <huntres2@optonline.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Flycutter from hell?
Date: Fri, 25 May 2001 09:15:28 GMT

>>That's about the fourth time you've implied I'm an idiot for not doing
it your way...<<

Aha. Well, something is not coming across, because I certainly don't think
you're an idiot, and that has never been on my mind in any of the posts I've
made here. If you're reading it that way, I think you're assuming that I'm
attributing the original claims to you. I'm not. I'm attributing them to
people who have published them, or who have passed them on with the claim
that they're an authority and know what they're talking about.

Earlier in this thread someone quoted Moltrecht saying that getting HSS hot
will damage it. I don't remember the wording, and I can't find it now. That
was a dead giveaway that he's mistaken. HSS will cut at low red heat, as any
basic metallurgy book, including _Tool Steel Simplified_, will tell you.
Carpenter Steel certified that _Tool Steel Simplified_ is accurate. They
provided the information and they back it up. The experience of thousands of
machinists supports it. Moltrecht is simply wrong about this issue.

The fact that he's in error here doesn't make him an idiot, either. It just
means that he's wrong in this instance. And the information is getting
passed around and accepted as gospel. That's bad.

He's not alone. Even when they get it right, a lot of writers get it wrong.
Here, for example, is what Sherline has to say about it:

"Unless you push the tool into the wheel with enough pressure, the tool will
bounce around and you'll never get a good flat cutting surface. It isn't
necessary to worry about getting the tool too hot. Modern day tool steels
don't anneal and a little discoloration doesn't effect the tool life in tool
room use. What you should worry about is not burning yourself or grinding
the tips of your fingers off!"

Basically OK, and the author understands the practicalities, but he probably
doesn't have a scientific understanding of what he's saying. For example,
"modern day tool steels" most certainly *do* anneal, and you can soften and
destroy HSS by getting it too hot with aggressive grinding. Above 1100 deg.
F, you're causing damage. Strictly speaking, it takes hours to "anneal" HSS,
but he isn't talking about annealing in the metallurgical sense. He means,
from context, softening the steel by overheating it. It's not a big issue,
but it tells me to be cautious about what the Sherline author says regarding
material properties. He's seriously wrong about not being able to soften and
destroy "modern day tool steels" with a grinder.

This evening I pulled out my uncle's old machine-shop training books, dating
from Burghardt's old text, last updated in 1936, to shop textbooks from the
'60s and '70s. I could write a book about how contradictory they all are.
<g> Most of the old guys were opinionated as hell -- some being opinionated
idiots. I especially like the one that says "Don't overheat the steel. You
will soften it." Nowhere does he say how hot "overheating" is supposed to
be. Maybe no one told him that HSS is designed to take the heat. Maybe he
means don't melt it.

One of the books says not to push hard when grinding HSS, because it will
expand near the cutting edge while the metal behind the edge remains cooler,
and the differential heating will cause it to crack. He then says to dip it
in water. (!) Of course, water won't cause it to contract...and he must have
forgotten that it's the edge of the tool that cuts, and that it gets hot
when you cut steel with it. BTW, this same idiot, later in the book, makes
the point that you should turn on the coolant before starting a cut in the
lathe, because starting coolant after the cut has started will subject the
tool to shock and possibly crack it. Perhaps he didn't read his earlier
chapter on dipping the damned things in water.

I hope you see my point. People who write machining books are only
occasionally thoughtful writers. Often they're ambiguous, contradictory, and
generally thoughtless about what they pass on. I doubt if many of them
really have the experience they imply they have. I read them with caution,
and I think we all should.

But I don't attribute those stupidities to you, nor to anyone else who read
them and believed them. As for my authority, I have only the experience of
having ground HSS this way for 30 years, having been taught to do it that
way by two of the best machinists I've known. And I know other machinists
who do it the same way. I do know how a heat-wrecked tool behaves, and mine
don't behave that way.

Having talked with an experienced steel metallurgist about it today, who
works for what is probably the most knowledgeable company in the field, and
having him confirm the principles if not the details of practice, I think
we're at least on firm ground. What I'd need to hear now to contradict it is
the experience of someone who has ground many tools aggressively, without
water-dipping and without overheating them, to the effect that his tools
have broken or have been too soft when he ground them that way. So far, I've
not heard a single such voice, only people who say they've heard that's what
can happen.

Ed Huntress

From: "Ed Huntress" <huntres2@optonline.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Flycutter from hell?
Date: Fri, 25 May 2001 16:04:55 GMT

>>The throw-away society, eh.  How do these folk get special shape tools
(not just form tools)?<<

With CNC you don't need many. You use simultaneous computer-controlled moves
to do a lot of what we do with special tools. If the job is production, you
have cutters custom made if you need them. We sold a couple of grinding
machines at Wasino that were built to do that work. One sold for $150,000.
The other was $350,000. They, too, are computer-controlled.

When I was part owner of a job shop we ground a lot of special HSS tools. My
guess is that a lot of small shops still do it.

I'm not done with this subject, BTW, at least for my own purposes. One
metallurgist is not enough, and I have shop owners to talk to. But that will
take a while.

Ed Huntress

From: "Ed Huntress" <huntres2@optonline.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Huntress/Moltrecht was Flycutter from hell?
Date: Fri, 25 May 2001 17:53:13 GMT

>>Hmmm. I'd say different, not wrong. Ed says you can grind HSS fast and
hot, if you don't heat to color or cool it too quickly.<<

OK on the cooling, but "heating to color" should be no problem. Oxidizing
colors peak at around 750 deg. F, with light blue. You can grind HSS up to
at least 1000 deg. F without producing any ill effect in the steel. At that
temp it's faintly glowing red. The red glow first appears at around 900 deg.
F, in dim light. In bright light you may miss it and overheat the metal. But
you can go all the way through the oxidizing color series without a problem,
unless you shock the steel by heating it too quickly (don't stab the tool
into the wheel, in other words) or cooling it too quickly (don't water-dip
if you're getting the steel too hot to hold by hand).

My objection to some of the recommendations recounted here is that the
authors claim that you will damage HSS by heating it to color, or to red.
That simply isn't true, and it's misinformation that leads to some wrong
conclusions. If you want to grind slowly, it won't hurt anything. That's the
way you do a final dressing, or re-dressing, anyway.

And it probably makes little difference when you're roughing a tool that
involves little metal removal, such as a sharp left-hand or right-hand
roughing tool. It makes a big difference when you're grinding a finishing
tool, a threading tool, a grooving tool, or other special shape that
involves a lot of metal removal. Pete's example of a tool that took him 3
hours to grind to shape, and that now takes him 15 minutes, is a good
example. It's frustrating to find out that your esteemed author was wrong
about what will damage the tool, after you've cumulatively spent a week of
your life unnecessarily standing in front of a pedestal grinder.

This is what drives me nuts, as a writer. I've spent at least 15 hours on
this subject since the thread started, if you count my time talking to
people at Eastec this week. And I'm still not confident enough to commit
something to print. On a NG, we have a conversation, and the facts get
sorted out over time. In print, you're it. Any author who costs his readers
tens of hours of wasted time because he won't spend the time to test his
facts in the flames for a few hours makes me angry. Before I write anything
about this, I'll check with at least two or three more solid sources. That's
the way it's supposed to be done.

Ed Huntress

From: "Ed Huntress" <huntres2@optonline.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Huntress/Moltrecht was Flycutter from hell?
Date: Sat, 26 May 2001 01:42:04 GMT

>>Of course -- it would be interesting to know *exactly* what the author
thought he was saying by "heating to color".<<

An old term of art, it meant heating until oxidizing colors begin to show.
But the jargon in this field is in such disrepair that it could mean
anything these days.

In any case, it beats the statement by one of the old, respected book
authors (Burghardt), not to "overheat" it. Hot enough to fry eggs? Hot
enough to light up the room? Hot enough to melt?...

Ed Huntress

From: "Carl J." <carlj@jcn1.com>
Newsgroups: rec.crafts.metalworking
Subject: Re: Huntress/Moltrecht was Flycutter from hell?
Date: Sun, 27 May 2001 00:45:17 -0500

  Hi Ed and RCM,
    I appreciate your efforts to research this topic for the group. I
grind tools for a living and until this thread started I just did as I
was trained (DON'T BURN IT!)
    I dusted off a couple of books:

Metal Cutting Tool Handbook, United States Cutting Tool Institute c.1989
 Page 503: "They do not lose hardness permanently unless exposed to
temperatures in excess of 1000F.
The book also states in another section that insufficient coolant can
cause cracking in the surface due to localized heating/cooling (this also
applies to the tools in use)

Another reference,
Heat Treatment, Selection, And Application Of Tool Steels, Bill Bryson,
c.1997
(This is pretty windy and I'm not very fast at keyboarding)
P. 106:
"The temperatures reached at the cutting point of the grinding wheel and
the surface will range from 2000F to 3000F....The effect of grinding can
cause high internal stress on the outer skin of the part, which will crack
the surface if the stress exceeds the strength of the material....Grinding
cracks are usually very shallow.. (.002)..they are typically parallel
cracks at 90degrees from the grinding direction... Heavier cracks will
form a checkerboard surface...

(regarding grinder scorching):
    "As the grinding process takes place, and the 2000F to 3000F
temperatures are generated...it will cause discoloration of the surface.
If you observe..straw..you have reached..400 to 450F. There is a 30%
chance of some surface cracking...If you see a bluish purple color...you
have exceeded 450F and you can expect better than a 70% chance of cracked
surfaces..
(paragraph skipped)
....Every time you grind over the area, the crack will heat up and run
deeper...shallow passes .00025" with a soft wheel and cooling time between
passes may remove the cracks...

My observation on this topic:
    I believe lathe tools are more forgiving of heat abuse because the
cutting edge is supported better than say-the cutting edge on the o.d. of
an endmill. Also, with all of the metal surrounding the grinding point on
a lathe tool, heat transfer is better throughout the tool. It is far
easier to get a localized burn at the tip of a flute (nowhere for the heat
to go).
    This may be where the observations of the machinists and engineers
diverge.
  For anyone interested in these books, I believe they are still available
from MSC.
  Thanks for making me question my training Ed. I learned something

Carl J.

From: "Ed Huntress" <huntres2@optonline.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Huntress/Moltrecht was Flycutter from hell?
Date: Sun, 27 May 2001 06:38:34 GMT

Interesting info, Carl, and it sounds like it's based on real science.

Grinding does produce a locally high temperature (that's where the sparks
come from) and microcracks, which is why the metallurgist from Carpenter
brought up the suggestion of taking a slow finishing pass. A HSS tool bit
roughed with high grinding forces and left that way, without any dressing,
could potentially be prone to crack. But we naturally dress them after
roughing, so common practice takes care of it.

When we get into the study of grinding cracks we find that it's very serious
business, and the reason, for example, that grinding crankshafts is now
frowned upon by automakers. Research shows that even light cuts with
cylindrical and surface grinders causes some microcracks. Crankshaft makers
are now more likely to use precision machining (crank-milling is the
preferred method at present) and then microfinishing. And hard turning is
replacing production cylindrical grinding in a lot of applications just for
this reason alone, although turning's lower production cost is the bigger
issue for them.

Much more serious is the propagation of microcracks, the opening of these
into deeper cracks, from thermal stress and/or phase change. That's why
water-dipping is risky business at anything above the lowest temperatures.
It's also why the Carpenter metallurgist said that heating HSS quickly to
1400 deg. F may crack it even without water-dipping, because you're getting
some phase change at that temperature and cycling back and forth between
austenite and martensite phases creates a lot of stress.

Note that flood coolant prevents the cyclical shocks, but it may actually
exacerbate the temperature differential in high-pressure grinding. This
comes up sometimes in creep-feed production grinding, and can actually
increase the seriousness of microcracks in certain applications. Without
flood coolant, though, thermal growth and loss of dimensional accuracy
becomes an issue, so it's a balancing act.

All in all, offhand grinding of lathe bits doesn't encounter many of these
problems as long as you don't overdue it and heat the steel above 1000 deg.
F. The higher temperatures occur only at the very surface, and you can't
avoid some of that even with slow grinding -- again, or you'd have no
sparks. Still, you should dress any part of the tool that you've ground, and
that will be highly stressed when it's put to use.

Ed Huntress

Newsgroups: rec.crafts.metalworking
Subject: Re: Huntress/Moltrecht was Flycutter from hell?
From: Norman Yarvin <norman.yarvin@snet.net>

In article <Kh1Q6.82264$MR1.12292686@news02.optonline.net>,
Ed Huntress <huntres2@optonline.net> wrote:

>Much more serious is the propagation of microcracks, the opening of these
>into deeper cracks, from thermal stress and/or phase change. That's why
>water-dipping is risky business at anything above the lowest temperatures.

One thing to notice about this is that basic physical law strongly
suggests (indeed, almost dictates) that the cracking happens when the
surface is colder than the interior.  If the surface is hotter, it has
expanded more, and is in a state of compression, and the interior is in a
state of tension.  The reverse is true if the surface is cooler: the
surface is in tension and the interior in compression.  And cracking is
produced by tension, not compression.

From: "Ed Huntress" <huntres2@optonline.net>
Newsgroups: rec.crafts.metalworking
Subject: Grinding HSS update
Date: Fri, 01 Jun 2001 14:48:04 GMT

For those who were following the thread on grinding HSS lathe bits, here's
an update, based on thorough discussions I had with the metallurgy staff at
Carpenter Technology, the company that's made HSS from the early days, and
that co-produced _Tool Steel Simplified_.

You can grind HSS hot, but nobody today will recommend it, because they
don't want to get into trouble. You have to do it right. When it was a
productivity issue, HSS bits often were ground aggressively and it generally
works just fine. Today, few people are using plain HSS bits in production,
so the issue has become moot -- unless you're taking hours to grind form
tools, like Pete Somebody was.

It takes practice to grind it hot without overheating or thermal-shocking
the steel. Definitely don't water-dip HSS if you're heating it until it's
too hot to hold by hand. Don't plunge the tool sharply into the wheel -- not
that any sensible person would, anyway. Also, because grinding is such an
inefficient process that causes a lot of frictional heating, the very edge
of the tool is going to be hotter than the mass of the steel and it will be
overheated and softened at the very surface. I can't get a fix on how deep
that overheated region is, but the story has always been that dressing a
tool after rough-grinding will cut that region away, so I'm guessing it's
0.002" - 0.005" at most.

Old-time machinists rough-ground HSS bits in dim light so they could see the
onset of the dull red glow. When you see that, you're at the limit. This is
confirmed by the metallurgists I talked to.

All of this assumes that you're using good HSS, M2 or better, and not the
"M50" crap that's coming from overseas. HSS lathe bits are a poor place to
try to economize. In Europe, the newer M35 is popular for many applications.
It's pretty good stuff and generally can be handled like the traditional HSS
alloys. Otherwise, what we're saying here applies to quality HSS in
traditional alloys, from major suppliers.

It's interesting that Sherline, in their tool-grinding tutorial, says to
grind HSS hot. I suspect the author was taught by an old-timer, because the
basic texts I've seen don't recommend it today. One old machining instructor
I talked to says that he always ground HSS hot when he worked in a shop but
that he doesn't teach it that way today. He doesn't know enough about the
properties of HSS to contradict the textbooks, he says.

This probably is the typical situation. In my discussions with metallurgists
we concluded that the textbook authors probably are reading each others'
material and copying the recommendations, or they're taking the cautious
road that steel suppliers promote. It seems unlikely that they've actually
tested what they're saying, grinding HSS hot and then testing it on a lathe.
Those of us who do it that way know from real experience that it works. Some
of the instructors today are aware that it wasn't done slowly when HSS was
the tool material used for general production machining. But no one seems to
care, because, today, they teach students how to grind HSS bits and then
they forget it in a hurry, since it has nothing to do with what students
need to know in order to get a job in a commercial shop today. It's lost, or
nearly lost, knowledge.

So much for that. Happy tool grinding.

Ed Huntress

From: "Ed Huntress" <huntres2@optonline.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Grinding HSS update
Date: Sun, 03 Jun 2001 03:09:48 GMT

Good point about the soft wheel. The safest way to grind any steel is with a
relatively soft wheel, and it's more important with HSS.

For the record, no one recommends using aluminum oxide wheels for grinding
HSS today. It just happens to be the material that 95% of us use. <g>

Silicon carbide (green wheels) are better for HSS. Borazon is better yet,
but very pricey.

Ed Huntress

From: "Ed Huntress" <huntres2@optonline.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Grinding HSS update
Date: Sun, 03 Jun 2001 21:20:15 GMT

SiC actually is sharper than AlOx, and cuts cleanly. However, grinding
carbide tools with SiC will leave a poor edge, as you say. Finish grinding
or honing with a diamond abrasive will give a much sharper edge.

If you want to open a controversy just as large as the one over how hot to
grind HSS, you can open up the one over whether to use SiC on high-speed
steel. <g> Keep in mind that a lot of "grinding" recommendations actually
apply to surface grinding. For example, soft white AlOx is a preferred
material for surface grinding of HSS. For grinding HSS tools offhand, with a
bench grinder, most people feel it's too soft -- as you apparently have
concluded, too. The dynamics are different and the goals are different.

Ed Huntress

From: "Ed Huntress" <huntres2@optonline.net>
Newsgroups: rec.crafts.metalworking
Subject: Re: Grinding HSS update
Date: Sun, 03 Jun 2001 22:10:14 GMT

Well, I see that saying "no one" recommends them was premature. It sounds
like another quagmire. The people I talked to at Carpenter say not to use
AlOx, the people at Dapra I talked to say not to use AlOx, and I recall that
SiC was preferred for grinding HSS when I worked in a shop, when one had it
on hand and it wasn't all grooved up from grinding carbide. But Norton and
other wheel manufacturers say to use it.

I also just read one source that said SiC is harder and tougher than AlOx;
another source says it's harder but not tougher.

Someone else may have the ambition to track this one down. I'm out of
steam -- and time. <g>

Ed Huntress