Results with Homemade Carbide Insert Lathe Toolholder

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

I get it. 4140 is very similar to the 4130 chrome-moly I am familiar. The alternate alloy caught me off guard.

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 < > 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

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: F1{is}Solve '50E3-1 6[]''''(30E6,I_sqr 1 .5*.5)Beam 0(1.5 w)'
F2{is}Solve '50E3-1 6[]''''(30E6,I_sqr 1 .5*.625)Beam 0(1.5 w)'
F3{is}Solve '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

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

Stick with your experience, Harold, it beats simplistic maths every day.
Tom

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

Although you're talking about last century, though...
Tom

As if I'm not feeling old enough?
Thanks, Tom!
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

Well, Ted, that took some time working out! Harold was streets ahead of you! He was also able to figure out the context too!
Tom

You do realize that five figures is between 10,000 and 99,000, don't you? Kind of hard to count that high on one hand :)
Tillman

The other thing this doesn't consider is the insert's desire to twist around the pivot point of the bolt or clamp that holds it in place. This can cause the tool holder to deform on the side where there is no carbide to back it up. This problem is compounded during an interrupted cut.