Hey guys, Aaron here from Gough Custom! So this
machine you can see behind me is a 1996 Fadal VMC 15 that I've been
fixing up and it's going to take over production of all of my knives from my older machine, Milly.
So most of the work on this machine's already been done but I missed one really important thing,
which is I found some lost motion in the z-axis, and obviously lost motion
is a a real problem when I'm trying to do high-precision machining so I have to fix it.
So I thought you guys might like to
join me as I go through the process of diagnosing and fixing this!
So when we have lost motion in a machine like this, what does that actually mean?
What it means is that the computer thinks that it's moving the table
back and forth a set amount, but if something in here is worn out it has to take up
that worn slop. So you can imagine if I'm trying to move this hand with this hand, and
there's a gap between them, then I have to move it this much before I start moving my other hand.
There are a couple of sources of lost motion. There's the ballscrew and the ball nut itself,
there are the thrust bearings, so the thrust bearings here are what's responsible for
allowing the ball screw to rotate but not to move back and forth and then there's
the coupling here. So the coupling is what joins the servo motor to the ball screw.
Now in my experience the most likely culprit of, you know, large amounts of lost motion, of
backlash, are the thrust bearings and the reason that this is the case is because all of the wear
and tear in the thrust bearing is concentrated in exactly one spot right here. Because they just
turn around in a single location whereas with a ball screw the wear and tear is spread out over
the length of the ball screw, and the reason that this is actually a good thing is because
the replacing the thrust bearings is about 100 bucks whereas the ball screw itself
is about a thousand bucks, so fingers crossed that it's the thrust bearings and not the ball screw!
This is what a ball screw looks like. The reason it's called a ball screw is because there are
half round channels cut into the screw, and also into the nut itself, and then in those channels
inside the nut there are actually ball bearings that act as the interface between the ball nut
and the ball screw. What this does is it allows it to rotate and to follow the threads but with
very little friction because the ball bearings actually roll inside the channel as opposed to
like a traditional nut and bolt where you have metal on metal scraping as it as it turns.
In a standard nut and bolt about the best power efficiency that you can get
is 40 to 50 percent, in a ball screw like this the best power efficiency you can get is
90 plus percent, so it creates a lot less heat creates a lot less friction
it allows you to accelerate loads more quickly and more accurately.
Okay so to measure the lost motion in the y-axis what we're going to do is put an indicator up
against the outside of the spindle housing, and now we're gonna move five thousandths away from
the indicator so we should see the needle move from zero to five on the left there.
One, two, three, four, five. Okay so we can see that it moved
but it didn't quite move exactly to the five mark, so what I'm gonna do is line it back up with zero
add two ten thousandths of an inch of backlash compensation.
And now we're going to move back the other way again. One, two, three, four,
five. And you can see all of a sudden it's lined up and then it's actually lined up
perfectly it's just because of the angle of the camera it doesn't quite look like the
indicator needle is on that five let's look at what the z-axis looks like.
Okay so we've got the indicator on the underneath of the face of the spindle,
let's see what happens when we move that axis by five thousandths of an inch. One,
two, three, four, five. So you can see there we've got a significant amount of lost motion,
we've lost over a thousandth of an inch and then let's line it up with that five
and then we're going to move it in ten thousandths increments left and then right.
That should be moving from five to five and it's not, you can see we're losing quite a lot of
motion there so we need to fix this.
Now to work out where the problem actually is we have to go through and diagnose
the the machine and the way we're going to do that is actually by putting an indicator on the end of
the the screw because, what will happen is, if the thrust bearing is worn and loose then when
the machine tries to move back and forth the screw itself will actually end up moving back and forth.
Now it's time to start disassembling the machine so I can get access to the z-axis ball screw!
To do this I need to remove the way covers, some of the overhead covers, and the head cover.
Oh there's that screw I was looking for!
You can see the protruding end of the z-axis ballscrew and when I move that axis,
even by just a ten thousandth of an inch, you can see it rotating. And what we need to do is to
measure the end of the ball screw for movement up or down, unfortunately we can't just do that
by putting an indicator on the face of the ball screw because if the face of the ball screw is not
perfectly flat then as we rotate it the indicator will go up or down depending on on that surface.
So the trick is that pretty much all ball screws have a hole in the end of them, and the reason
for that is so that they can hang on to the end of the ball screw while they're machining it.
So we're going to make use of that hole by sticking a ball bearing in
the end of the hole and then trapping it in place with the needle of the indicator,
then that gives us a perfect surface to indicate off even though the ball screw is rotating.
So I've just put a little tiny bit of grease in there
you can see that that ball bearing is actually sticking onto the end of the ballscrew.
I've got an indicator in there pressed up against the ball bearing that's on the bottom of that
ball screw, and now I'm going to jog the z axis, and theoretically if the
thrust bearings don't have any play in them at all then there should be zero change on the indicator.
But we can see straight away
that whenever I reverse the direction the ball screw itself is moving a significant amount!
We've just identified that our problem is in fact the thrust bearings which is great news!
The next step is actually to take apart the entire z-axis. The z-axis has a counterweight,
and when the head goes up the counterweight goes down. The only problem is if there's any
mismatch in the balance, which there always is, then it's going to be pulling in one direction or
the other, and that means that when we take the ball screw out the whole system is going to
want to move in one direction or the other. The first thing we have to do is actually to block
the z-axis in place so that we can safely take the ball screw out.
This is the thrust bearing mounting block, and then this is the ball screw itself,
and then down here you see the ballscrew mounted onto the z-axis head casting.
Victory!
Wow so we can actually see as soon as I've got this out we can see how
badly fucked this thing is. If you listen and watch really closely you can actually
see the ball screw moving inside the assembly!
I can't believe how bad that is.
So now we have a nut here that holds the thrust bearings onto the ball screw. And
when I first started working on these machines I was told that the correct way to remove this nut
was to grind flats onto this part of the ball screw, and then to use a pair of wrenches but
that is absolutely not the way.
This is the way!
That's it!
So now we need to take off the thrust bearing retainer here.
Oh these are very loose... Is that the problem?
Fuck. Okay these two nuts, these two bolts rather, were totally loose and tightening them up
has completely taken the wobble out of this assembly. I think that some clown worked on this
and forgot to put it back together properly!
This outer bearing retainer is actually heat shrunk onto that bearing,
so to remove it we need to heat it up, but I don't want to melt the seal here in case this
stack is still good and we can reuse it... Wrap this with a damp rag just to try and protect it.
The inside of this is spotless, so I'm gonna put her back together and
tighten her up properly, like actually put these bolts back in the way they're supposed to be
and see how it comes together!
If that's actually the problem, like if those two bolts were the problem, like really???
Goddamn this thing's fun!!
All right!
There we go!
Okay so I've got the indicator set up on the bottom of the spindle nose I'm just going
to get it to zero, and then moment of truth. We're gonna count five thousandths out. One,
two, three, four, five! That is fucking beautiful! That is very very close to perfect,
so let's comp out one tenth worth of backlash and see what happens!
So the camera is not quite lined up with the dial
but looking at it from perfectly lined up with the dial I now see zero backlash!
All right guys so let's have a quick look at that blade we just machined! So this is the
one we just machined, and then this is one that I machined before I fixed the z-axis.
And if you have a look at this one you can see straight away that this
is not good. So comparing that to the one we just machined, this looks really good!
So it's awesome that the machine is fixed it seems to be running really nicely now which is
fantastic! I've done quite a bit of work on this machine apart from what you just saw,
um, lots of cleaning and a bunch of electrical work and so on. So
I think it's a worthwhile investment. I got these machines very inexpensively and spending some
money on fixing them up isn't a problem, in this case we didn't have to spend any
money which is amazing! I can't believe that the problem was just those two bolts, it's so silly.
Hopefully you guys enjoyed
coming to work with me, and working on this machine, and i'll see you guys again soon!
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