I finally finished my carbide insert toolholder and tried it out. I
went't very slowly, with minimal toolholder overhang, in a 3/4" billet
of aluminum. There was no chatter or vibration, so I took a moderate
cut. It worked very well for both turning and facing, with a very
smooth finish even with higher than normal carriage feeds. The
toolholder is very simple, consisting of a bar of 6061-T6 to fit the
lathe toolpost, with a section on the end filed down to hold the insert
at the correctt angle. It is drilled and tapped for the 4-40 screw
that holds the insert (included with the insert). I built the entire
thing with hand tools and draw-filed all the surfaces smooth and flat.
I could feel no vibration at the end of the toolholder during use, so I
believe this is a solid tool. My next project is to make a carbide
insert boring bar/inside turning tool for the same kind of inserts.
Let me know if you have any comments.
Woodworker88
Los Altos High School "Eagle Strike" Robotics Team #114
www.lahsrobotics.org
Congratulations on a job well done.
I'd like to give you something to think about. Carbide is often put to
serious use, with considerable tool pressure. While aluminum will work
for a tool holder, it wouldn't hold up to the rigors of roughing steel or
stainless, not under the under the considerable compression of the cut, nor
the abrasion of the chips on the shank. If you intend to make any
further tools, I'd encourage you to consider using some pre-heat treated
4140 instead of aluminum. It machines well enough, and has considerably
more resiliency. It would serve you far longer than would the aluminum
variety you've been making.
Harold
Steel is also 3X as stiff as aluminum. This probably won't
make a lot of difference as long as the toolholder isn't
hanging out too far, but aluminum would be a very poor
choice for a boring bar.
Ned Simmons
Yeah, I realize these shortcomings. 99.9% of the stuff we machine is
aluminum itself, the toughest being the occasional bolt or mild steel
bushing. I used the aluminum because I had a piece lying around that
happened to be the perfect size for the toolpost. It was longer than I
needed so I cut it in half and had two scraps to experiment with. I
didn't want to spend loads of time preparing a piece only to destroy
it. I am familiar with drill rod for round tools. Is there a square
version? It sounds like this is the 4140 you speak of. I'll look on
mcmaster, but this is a low budget operation, so I'll probably still
prototype in the aluminum.
I also used the aluminum specifically because I knew that if the
carbide were to grab or catch on the piece, the aluminum will give
slightly before the carbide shatters.
Thanks again for the advice and the encouragement.
The equivalent would be flat ground tool steel, which is available in a wide
variety of shapes, squares through rectangles. It's available in various
alloys, although the most common is likely 02. You can get even precision
ground low carbon steel if necessary. These are often found at dealerships
that sell measuring tools, such as Starrett. Starrett, in fact, has a line
of such materials. My personal favorite is Graph-mo, however. It's an oil
hardening (02) flat ground tool steel that is a pleasure to machine. Most
tool steels are not fun to work.
It sounds like this is the 4140 you speak of. I'll look on
4140 is not considered a tool steel. It's one of the "chrome moly"
materials you may have heard of, and has a high enough carbon content to
achieve a serious hardness when necessary. The pre-hardened stuff is
drawn back for machineability and runs in the range of 28/32 Rc. I
suggested it because it avoids the heat treat process, which may, or may not
be, a problem for you. Point is, it would far better resist deterioration
of your holder, although even it would offer some premature failure as it is
used.
While I appreciate your idea, it's a false one. Carbide, in order to be
successful in use, must have complete support, for it lacks sufficient
tensile strength. It's terrific in compression, so as long as it's
properly supported, it will withstand serious punishment. It's for that
reason that many of the insert type holders (made from heat treated steel)
have a carbide anvil, which presents more surface area than does the insert
bearing directly on the softer-than-the-insert seat. While they're the same
size, the insert often has a reduced surface area because of the chip
breaker. The anvil prevents the insert from wallowing in the holder,
creating a matching pattern of the insert at the point of pressure. When
that happens, the insert often breaks in use because it lacks proper
support. That's what's going to happen to your aluminum holders in good
time, depending on how hard they're used.
My pleasure, and please understand I'm not being critical of your
accomplishment. Just a tip that might be useful for you in the future. You
won't always be machining soft materials.
Good luck.
Harold
You do not need fancy hardened or hardenable steel for an insert tool
holder. It has no need to hold an edge. The important property is the
modulus of elasticity ans this is, to a good approximation, the same for
all steels. Just get yourself a length of suitable cross section mild
steel and make your holders. I have been using the ones described at
< http://www.metalwebnews.com/howto/toolholder/toolholder.html
for over ten years now and have yet to need a replacement holder.
Ted
While that may be true of an insert tool that takes moderate cuts, a holder
that accommodates negative rake inserts that is put to serious use would
prove you wrong. As I suggested, carbide anvils are provided for such
tools to prevent the seat from deforming-----even with heat treated seats.
That, of course, is highly unlikely to occur on a small machine with limited
rigidity and horse power.
Harold
I believe the carbide anvil goes between the insert and the holder.
Since tungsten carbide has a higher elastic modulus than steel, this
will certainly do as you suggest independently of the type of steel used
for the tool holder. Unless sufficient cutting forces are being
developed to _permanently_ bend the tool holder, a hardened steel tool
holder provides no advantage. Hard steels are stronger but not stiffer
than mild steel.
Thats me! :-) But I get the impression that the OP is not into heavy
duty stuff.
Ted
holder
would
such
seats.
limited
Chuckle!
Oh, yeah! He made it clear that he is machining aluminum and a little mild
steel. I was just looking down the road, when he might want to put the
tools to some serious use. It would be a shame to spend the time building
a nice tool, only to have it let you down when you needed it under trying
circumstances. I can envision the day when he may have a lathe at his
disposal with some serious horse power, and perhaps a piece of chrome moly
or stainless that needs to be reduced substantially in size. Quarter inch
depth of cut isn't beyond reason, assuming the tool can withstand the
pressures involved. I've broken commercial insert tooling at the
pin/screw before from heavy cuts. Maybe I should say from "too heavy"
cuts? :-)
Harold
All the above makes sense but I thought it might be fun to put some
numbers to it. And, yes, I have seen photos of lathes where, when you
look carefully, you can see that there is a man standing on the
carriage. Nevertheless, let's play.
Assume we are cutting steel at 200 feet/minute, the lathe tool holder is
sticking out 1.5" from a "perfectly" rigid tool post and toolholder
clamping system, we have a carbide cutter on a proper carbide anvil all
clamped to the end of the tool with the best of the best insert clamping
systems. All I'm interested in for this game is, "What will it take to
bend or break the toolholder?"
The tool holder is a cantilever beam loaded at the end by the cutting
force and supported at the other end by the tool post. The maximum
bending moment is at the tool post end. A decent mild steel has a yield
strength of about 50,000 psi. The power going into the cutting is, as
always, force times speed and horsepower is pounds time feet per minute
divided by 33,000. That and the cross section of the toolholder should
be all we need to know to calculate the required horsepower neglecting
power transmission losses in the lathe - probably fairly small.
For 1/2, 5/8 and 3/4" square toolholders, maximum applied force is:
F1Solve '50E3-1 6[]''''(30E6,I_sqr 1 .5*.5)Beam 0(1.5 w)'
F2Solve '50E3-1 6[]''''(30E6,I_sqr 1 .5*.625)Beam 0(1.5 w)'
F3Solve '50E3-1 6[]''''(30E6,I_sqr 1 .5*.75)Beam 0(1.5 w)'
Therefore horsepower required is:
0.1 Rnd 200*F1 F2 F3÷33000
4.2 8.2 14.2
I'm sure you (Harold) have used lathes with far more than 14hp so
hardened and therefore stronger toolholders would make sense even with
3/4" square holders. But I can see why I've never had any problem with
my 1/2hp Smithy and I doubt the OP will for any machine he's likely to
use in the foreseeable future. If he ever gets that 50hp CNC machining
center, he'll have to replace the 1/2" mild steel holders. :-)
Ted
mild
the
building
trying
moly
inch
Impressive! I envy you your math skills!
I've broken insert holders on lathes with much smaller HP, and have broken
more than my share of inserts as well.
My argument isn't with the holder breaking so much as the seat deforming.
That's a reality, even on small machines (relatively speaking), with only a
few horse power. It's a different world than that with which you may be
familiar, Ted. A roughing cut on some 3" chrome moly or stainless @
.012"/.015" feed @ 500 RPM, 1/4 depth of cut per side, is quite challenging
(and not uncommon). On larger machines, I've taken half inch deep cuts.
Under these circumstances, it's highly unlikely a mild steel holder will
resist failing at the point of the insert---although not from breaking.
Seat deformation is my concern, as it has been right along, and this is why:
Cutting pressure at the point of the tool forces the insert downwards until
the seat is deformed into a radius of sorts. That allows inserts to break
under load because they are no longer supported where it's needed most.
Again, carbide anvils are provided for many insert holders to prevent that
from happening, even when hardened seats are the norm. It's a real problem,
not one of my imagination.
Remember, in this instance, this young man has built his holders from
aluminum. Unless his choice was 7075-T6, they will have far less tensile
than mild steel, so the chance of failure under load is quite real. On
the other hand, if they are always to be used with a fractional horse power
machine, they'd likely hold up fine----especially if they're positive rake
tools, where serious cuts are rare. His biggest concern then would be the
deformation of the area where it is held in his holder with screws.
I realize that most of us measure things by the world with which we're
familiar. It might be hard for you, or others, that have never seen an
industrial rated machine removing metal as they're able, to fathom how it's
done. It can be an awesome sight for the uninitiated. I can put on
quite a show even on my humble 12 Graziano. Feel free to stop by for a
demo.
Harold
why:
until
break
that
problem,
tensile
On
power
rake
the
it's
on
Chuckle! Don't really have much choice, Tom, considering I still have the
tip from the broken holder to remind me.
In fairness, it broke at the pin, so it wasn't all that strong, but it
should have withstood what I threw at it----with only 3 horse at the
spindle. As you know, Tom, negative rake inserts with chip breakers lack
support where they need it most. It was that configuration I was thinking
of-------and that exact configuration is what I was using when the tool
broke. Years ago, when I was actually a machinist! :-)
Harold
I realize you have problems with math that involves more than five
fingers (you'd have to put the beer down to use ten) but last century
was only six years ago. ;-)
Ted
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