# Centering the work piece?



## Alex (Sep 2, 2007)

Hello group,

Got my lathe the other day. How I try I cannot get the work piece to line properly in the chuck- there's always some minor wobble. Anyone knows how to solve this?


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## IanN (Sep 2, 2007)

How much wobble do you get? - There will always be some error in a three jaw self-centre chuck.

Ian


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## Alex (Sep 2, 2007)

I have nothing to measure with so it's hard to say. But it's certainly not more than 1 mm (.04") at the end of a 50 mm (2") long, 20 mm (3/4") dia piece.


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## Bogstandard (Sep 2, 2007)

Hi Alex,
As Ian says, there is always some runout in a 3 jaw.
Supposedley there is a master key socket that you should always use for tightening. This is the one used when they grind the chuck jaws in situ, and is usually marked with a '0'. That is how it is marked on my chucks.
If you haven't got one on your chuck, try slackening and tightening each one in turn and see which one gives the less runout, then mark the best one with a pop mark so you can always recognise it.
Two or three thou runout is normal, even for a new 3 jaw, unless you pay an arm and two legs for a precision one.
If you want to have no runout you will have to set it up in the 4 jaw or get yourself a precision collet chuck.

John


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## m_kilde (Sep 2, 2007)

Hi Alex

Sound really bad !

Try to place you cuttingtool near the outer diameter of the chuck, and test if the whole chuck is running out, if so check if the chuck hasn't been munted correctly, maybe something is trapped between the chuck and the mounting flange.

Try also to take the chuck off, and place the cuttingtool near the flange recces and check if this is running out, if so I believe the lathe if very badly made.

If the flange is running true the chuck is badly made and you might have to get another from your supplyer, if they are serious the will supply you a new one without charge

Hope this made sense to you


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## Cedge (Sep 2, 2007)

Alex
All the advice given so far is spot on, but there are a couple of factors that come into play.  Raw bar stock is not usually perfectly round, having a somewhat rough surface and some amount of runout all its own.  The lathe, once the material is chucked will begin truing the surface with the first few cuts. It's something that becomes very normal very quickly.  

Once you begin cutting, try to perform all the planned operations that you possibly can, without ever taking the material out of the chuck.  This will assure everything is kept concentric.  It sometimes takes a bit of thought before cutting to get the proper sequence of events in your head, so that you don't get cornered by one of the steps you'll want to do.  

You might need to face, turn,, contour chamfer, round  fillet, drill groove and cutoff a work piece to get the profile you'd need.  Doing as many of the needed operations in one go makes them all run on the same center axis.

Once you unchuck the piece, uless you are very lucky, rechucking it can introduce some runout due to chuck innaccuracy.  One trick that sometimes helps is to mark a particular jaw (3 jaw chuck)  and a corresponding spot on the work piece, before removal.  These are called "witness marks" I usually do this with a permanent marker like a "Sharpie".  Then if you replace the piece back into the chuck, it can be positioned very close to the same contact points as it was originally and the runout problems tend to be minimized. 

Basically your lathe is there to true a piece of metal.  The fact that it lets you make a cut to any desired depth is a secondary function.

Don't sweat it too much unless it becomes obvious the 3 jaw chuck has a large enough problem to cause real problems.

I lucked out when I installed my 5 inch chuck and mananged to get one from Little Machine Shop that never seems to be off by much more than a couple of thousandths runout when you have to rechuck. It often rechucks with none at all. 

Steve


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## mklotz (Sep 2, 2007)

Three jaws are convenient and will produce accurate products if all machining can be done in a single chucking.  Nevertheless, they will always have some runout and it will only worsen as the chuck is used.  Grinding the jaws is a worthless bodge since it only makes the chuck accurate for a single diameter.

If you want to eliminate runout, you need to get a four jaw chuck.  You'll need one anyway to chuck oddly shaped pieces, e.g., boring a hole in the middle of a square plate.

Many new lathe users are put off on the four jaw because they think it's difficult to center work in one.  While admittedly a bit harder than using a three jaw, the process need not be daunting.  I've written a treatise on the process for our metalworking club.  You can read it here:

http://www.schsm.com/html/centering_work_in_the_four_jaw.html

I hope this helps.


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## Alex (Sep 2, 2007)

Thanks a lot guys!

I retighted it a few times and eventually I got it right. not perfect but almost. Tomorrow I'll check the chuck for wobble.

Again, thanks.


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## Rog02 (Dec 23, 2007)

Another site with a good tutorial on centering a 4-jaw. This one has pictures for those of us with AADD. 

http://www.cartertools.com/fourjaw.html


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## Tin Falcon (Dec 23, 2007)

there was an article in Home shop machinist by Forrest Addy that suggested a beginner set the 3 jaw aside for at least several months and use the 4 jaw just to learn how and get used to it. It is more accurate in the long run and with practice does not take long. 
I worked in a shop doing specialized polygon grinding for a while. each part was indicated in on the face then for runout to within a tenth or two do that for several days strait you get used to it. We used 6 jaw set true chucks. 
Tin


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## mklotz (Dec 23, 2007)

FWIW, here's a short treatise I wrote on the subject for the members of our club.


Centering Work in the Four Jaw Chuck

Probably every machinist has his own pet way of doing it. My technique works
for me. Take whatever is useful and modify as you see fit.

To easily center work in the 4J, you'll need to make yourself two tools. 
First, make a dedicated holder of some sort so you can mount a dial indicator
(DI) on the tool post (or directly to the compound) with its axis
perpendicular to the spindle axis. Adjust the DI so its plunger is vertically
aligned with the spindle axis. An easy way to do this is to put a pointed tip
on the DI plunger and align the point to a dead center in the headstock. The
idea is to make something that you can drop into place, already aligned, and
lock down in ten seconds or so. Leave the DI permanently mounted to this
holder. A cheap import DI (<$15) is fine since we'll be using this only for
comparative, not absolute, measurement.

While you could use a conventional adjustable magnetic DI holder, I strongly
recommend that you make a dedicated mount that is easily installed and removed.
A general maxim of machining is that you'll be much more likely to do something
'the right way' if setting up to do it is quick and simple. If it isn't you're
much more likely to try some half-a$$ed setup that doesn't work and ends up
damaging the tool, the work, or, worst of all, you.

The second tool to make is a clone of your 4J chuck wrench. We're going to be
adjusting two jaws at a time and it's infinitely easier to do if you can move
both jaws in and out in concert without having to swap the wrench from hole to
hole. It's another example of the maxim I mentioned above. The clone wrench
doesn't have to be anything fancy. Machine a square tenon to match the
existing wrench on the end of some suitable stock, and drill for a press-fit
cross bar. Use your existing wrench as a guide for dimensions. I've found
that, if there's not a lot of room on the back side of the lathe, making the
clone somewhat shorter than the supplied wrench is a good idea.

Ok, now for the procedure. Mount the work in the 4J and roughly center -
either by eye or by using the concentric circles scribed into the face of most
4Js. Snug the jaws down so the work is held securely. Turn the chuck so one
jaw is at the nine o'clock position as seen looking from the tailstock down
the spindle axis. Use the cross-slide to bring the DI up against the work and
reading about the middle of its range (e.g., about 0.5" on a 1" DI). Turn the
scale on the DI so its needle indicates zero. Now swing the chuck through 180
degrees. Unless you've got an impossibly good eye, the DI will now read
something other than zero. (For an example, let's say it reads 0.038.) Turn
the DI scale so the zero is halfway to this reading. (Move the scale so the
needle points to 0.019.)

Now, insert both chuck wrenches and adjust the jaws so the DI needle points to
zero. Swing the chuck 180 degrees and check the reading - it should be close
to zero.

[Aside: If the part you're centering has the same dimension in both jaw axes
(i.e., it's not rectangular), the DI zero you established above will also be
the zero for adjusting the other two jaws below - another advantage of this
technique.]

Repeat this entire process for the two other jaws. [What we're doing here is
treating the 4J as two two-jaw chucks. We can do this because the jaw pairs
are orthogonal and, to first order, adjustments of one pair will have very
little effect on the setting of the other pair.]

If you've been careful, the total runout on the part should now be only a few
thou. Depending on your esthetics and the part requirements, this may be good
enough. If not, repeat the entire process until the runout meets your needs.
After centering, check to ensure that all the jaws are clamped down tightly. 
It's easy to leave one loose. If you have (left one loose), you may need to
rerun the centering procedure after you've tightened it.

With this procedure, you should be able to center something to +/-0.001" in
ten minutes on your first try. With not much practice, you can get that
number down to one or two minutes. Soon your three-jaw will be gathering
dust.

One of the most common uses of the 4J is for drilling/boring offset holes in
eccentrics (i.e. cam drivers for model engines). In this case, you aren't
centering the stock itself (as we were above) but rather need to center the
location of the hole.

First centerdrill the location of the hole in the milling machine. Mount the
stock in the 4J and roughly center this centerdrilled hole. [A fast way to do
this is to use the pointy end of an edge finder held in the tailstock drill
chuck.] Now you need a PUMP CENTER. This is a longish rod (mine is ~10"
long). At the tailstock end is a spring-loaded female center. At the
headstock end is a rigid male center. The male center goes in your
centerdrilled hole. The female center is supported by a dead center in the
tailstock and the tailstock is adjusted to lightly compress the spring. The
DI is made to bear on the rod near the male center. Using the procedure
outlined above, adjust the jaws until the DI shows little or no runout. 
Voila, the location of the offset hole is now centered.


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## wareagle (Dec 23, 2007)

Marv, thanks for posting the tutorial. I think I will put an indicator mount on my list (at the top) so this procedure may be adopted in my shop. It sounds like a very simple way of performing the operation.   Simple = Best! At least in my shop!


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## shred (Dec 23, 2007)

LMS has an interesting variation that I like for round stock: http://www.littlemachineshop.com/Reference/Centering4-JawChuck.pdf


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## mklotz (Dec 23, 2007)

shred  said:
			
		

> LMS has an interesting variation that I like for round stock: http://www.littlemachineshop.com/Reference/Centering4-JawChuck.pdf



Shred,

If you compare the LMS method with mine closely, you'll see that we're both doing the same thing.


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## shred (Dec 23, 2007)

mklotz  said:
			
		

> Shred,
> 
> If you compare the LMS method with mine closely, you'll see that we're both doing the same thing.



Yeah, pretty much the same-- they explicitly figure the global desired figure first and set the indicator zero there, then go move the jaws around, versus figuring it on one pair first and then applying it to the other. For some reason my brain groks "give the 4-jaw a spin version" better, though it doesn't work for non-round stock.


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## greenie (Dec 23, 2007)

Here's another way of achieving the result that your after.
Sometimes you have a washer that just needs trimming that fraction more, or a shaft that you have to turn off that bit more and there is NO centre hole in the shaft.
Well, this gizmo fixes those problems, a photo is worth a thousand words, so have a look, it is self explanatory.
















regards greenie


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## GeorgeGreek (Jan 7, 2008)

Hi Marv,

very smart though I am, I never thought that "it's infinitely easier to do if you can move both jaws in and out in concert".

Thanks for the idea, simple though it seems.

George


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