We recently needed to turn down the ends of some ballscrews, and if you do some research
online, everybody points to fancy inserts. This one has a cubic-boron-nitride tip on
it, and has a fancy price to go along with it. We wanted to see if one of the inserts
that we already had could get the job done. Before going straight for the ballscrew, we
wanted to do some practice. This is a high speed steel drill rod, and it's pretty tough:
it's hardened to about 65 Rockwell-C and the file can't even touch it. The insert that
ended up working the best was this CNMG432 from Iscar. We tried a few other inserts.
They didn't really work that well. This was the only one that really could touch the drill
rod. We'll talk about those in a little bit, but, this really comes down to just as much
technique as it does the particular insert you use. You might think that if you're cutting
really hard material, that you would want to take a very shallow depth of cut, but that
doesn't really work that well, and I'll show you that in a little experiment. Here I'm
taking only ten thousandths off the diameter with each pass, so that's a five thousandths
of an inch depth of cut. I'm using the power feed on the lathe. It's set to the lowest
feed that the lathe can do which is two thousandths per revolution, and I'm turning at about fifty
surface feet per minute, I think it's about four hundred RPM's. I took four passes, on
that drill rod, and I'm going to speed them up here. Between each pass, we measured the
diamter in three places with mic's, and we wanted to quantify if we were cutting on-size,
and whether or not there was any taper. These are the results of that test. You can see
by the fourth pass, there's some pretty significant taper, and it get's progressively worse with
each successive pass. If we look at the cutting insert under a microscope, we can start to
get an idea of what's happening here. You can see that the tip of the insert has a radius.
This is called the nose radius, and the last number of that CNMG432 insert size, so the
"2", corresponds to the actual radius of the nose, which is thirty one thousandths. In
this illustration, let's compare what the cutting forces look like between a normal
depth of cut, which is about equal to the nose radius, and a very shallow depth of cut,
like what we did in our experiment. With a normal, proper, depth of cut, the cutting
forces are distributed with an axial component and a radial component. If you take a very
shallow depth of cut, the cutting forces are almost exclusively in the radial direction.
This is significant because those radial cutting forces tend to deflect the part away from
the cutting insert, and that doesn't do it any favors in terms of actually removing material,
and it also tends to create a big taper on your finished part. So, knowing that, let's
repeat the experiment but increase the depth of cut to the nose radius, or thirty thousandths.
You can see this is going great. It leaves behind a great surface finish, makes a nice
chip that breaks easily, and those chips are that straw yellow color that everybody strives
for. I'm pretty satisfied with this cut, and this goes pretty well. When we check it with
mic's, just like before, it cuts pretty much on-size, and there's no taper. This is a picture
of the insert before cutting, so this is fresh out of the box... and then after cutting,
and you can see there's some wear right there at the tip, but overall, it's pretty much
intact. We tried some other inserts. This is a cheap one from McMaster, and it seemed
to go kind-of okay, but then the cut quality started to rapidly degrade and the chips,
if you notice, change color (they get a lot hotter) and that suggests maybe a lot more
friction and higher cutting forces. This is what that insert looks like fresh out of the
box, and then I think we can see why the insert starts to degrade pretty quickly. That very
sharp tip just breaks completely off, and if you keep going... this ends up happening.
We tried a Sandvik insert too, and I wish I could show you what the cutting looks like...
but... still getting a hang of this YouTube thing. This is what it looks like fresh out
of the box, and it's a similar story as with that McMaster insert. After one pass, it's
pretty heavily wearing, and then eventually the tip just totally shears off. Now, I'm
not a carbide whisperer by any means, but I do think there are a couple of aspects of
the design of the Iscar insert on the right that make it better suited for this job compared
to the economy insert on the left. For one thing, the actual cutting edge on the Iscar
insert isn't nearly as thin, which makes it quite a bit stronger. In addition, the economy
insert doesn't have any coating, and I can't find any information about the actual carbide
substrate. The deal with carbides is that they're actually made of little pellets, or
grains, of tungsten carbide, and then they're held together with a binder. Cobalt is common,
nickel is also used, and the smaller those grains are, the tougher the material is actually
going to be. So, sub-micron is the toughest that you can have, and then, on top of that
stronger carbide substrate, there's also several layers of very hard coating. This is a TiAlN
coating, and it's much harder than the carbide, and I think just overall makes for a lot more
durable insert. Since we've got a good recipe of insert, feed, and speed figured out, we
can machine the actual ballscrew now. To protect the threads while the ballscrew is held in
the chuck, we made an aluminum bushing with a slot in it. Ballscrews are only case hardened,
which means they're extremely hard on the surface, but much softer towards the middle
of the material. Sort of a nugget-center sort of situation. So, we're taking a thirty-five
thou depth of cut here, which is larger than the nose radius of the insert, and it's also
going to cut deeper than the case depth, so we'll get right to that middle, softer, metal
in one pass. I was really worried about turning this part, and put it off for quite a while,
but, in the end, it ended up not being an issue. We tackled this like we do for most
engineering problems: we identified the key risks, proposed a few possible solutions,
and used a simple experiment to quickly test and select the best option. I put a link to
the exact insert we used in the description. This ballscrew is for a project we're really
excited about, but we're not quite ready to share yet, so be sure to subscribe so you
don't miss it. Thanks for watching!