# Centring in a 4-Jaw Chuck

### Help Support HMEM:

#### stackerjack

##### Well-Known Member
Does anyone know an easy method of centring a rectangular bar in a 4-jaw chuck please. After spending about a quarter of an hour, it's still out by more than 5 thou.?
Jack

#### goldstar31

##### Well-Known Member
Does anyone know an easy method of centring a rectangular bar in a 4-jaw chuck please. After spending about a quarter of an hour, it's still out by more than 5 thou.?
Jack
Make another chuck key and both opposing each other.

#### BaronJ

##### Grumpy Old Git.
There is no guarantee that the faces are actually flat all the way across. Even so a slight rotational difference in measurement position will alter the readings. Depending upon what you want to do with the workpiece, 5 thou is probably not worth bothering about.

#### ShopShoe

##### Senior Member
stackerjack,

A good place to start is this piece written by the folks at LittleMachineShop:

https://littlemachineshop.com/images/gallery/PDF/centering4-jawchuck.pdf

This video is also pretty good:

And that is not to say that many of the other videos by other people are not worth watching as well.

The main thing is that with practice it will make sense to you and you'll get the hang of it.

--ShopShoe

#### GRAYHIL

##### Well-Known Member
Hi All
This is how I do it.
With a magnetic base dial gauge mounted on the saddle and the point at roughly center height wind the saddle up to the square stock.
With one side of the square stock vertical move the saddle forward more so that the point of the dial gauge just touches the stock by moving the mag base.
Turn the chuck a small amount forward and reverse to find where the dial gauge changes direction and set the gauge to zero at this point.
Wind the saddle away to clear square complete with mag base.
Turn the chuck approx 180 degrees.
Wind saddle back and repeat first steps but do not zero clock as the reading will show how much it is off center
the square is.
Move the job half the reading in the required direction.
Repeat till all reading are the same an all faces.
Difficult to describe but it works for me!!
Graham

#### kwoodhands

##### Well-Known Member
Does anyone know an easy method of centring a rectangular bar in a 4-jaw chuck please. After spending about a quarter of an hour, it's still out by more than 5 thou.?
Jack
Start by marking the chuck, #1 opposite side#3 , then #2 and#4 opposite sides again.
Two chuck keys as mentioned is a very big help. Layout centers on the end of bar, both ends if needed. I use layout fluid and a height gauge to scribe the centers
If the work is short enough to stand in a drill press or mill vertically, then spot drill, center drill and install in the 4 jaw. Now turn chuck so #1 is topside and #3 is opposite on the bottom. Bring tail stock and center up to the end of the bar. With two wrenches move the jaws so the center is dead on the layout. Now turn the chuck so #2 or #4 is top side. Repeat above. Snug up the jaws , check that the center matches the centered layout.
Spot drill, center drill and then install a center in the tail stock.
This method so far will get you close ,usually within .010
I forgot to mention at first you will need 3/8" or so round bar about 8" long. Turn a 60° point on one end and spot drill, and then center drill the other end. The pointed end enters the face with the center layout. The other end enters the live or dead center.
I use a dial indicator mounted on the tool post , lots of ways to do this. Turn the chuck so #1 shows top side.
Move compound til the indicator moves about. .010 Turn the serrated band ( bezel) til it shows zero.
Turn chuck so #3 is top side and record the difference from zero. Example , #1 reads zero / #3 reads 16
Divide this # in half, = 8. Now slowly turn the chuck til the DI reads zero. Snug up the #2 and #4 jaws because these were the jaws you adjusted when the #1 & #3 were top side. Often you won't get a reading when the chuck is turned, Just move the compound in til you get a reading. Use this as zero.
Both readings should be the same, you can tighten one or the other more if you were .002 or less out after checking.
Now repeat with #2 & #4 top side .
Make sure all 4 jaws are tight.
That's it, your part is centered. When you do this several times you will be able to center a part in 3 minutes or less.
I just looked at the Little Machine Shop Video, very good. My method as well.

#### stackerjack

##### Well-Known Member
Thanks everybody, I'll go and get some practice in. "Cheers"
Jack

#### jcreasey

##### Well-Known Member
This is a good tip for using the 4 jaw

#### stackerjack

##### Well-Known Member
This is a good tip for using the 4 jaw
Excellent idea. Gota try that.
Jack

#### tornitore45

##### Well-Known Member
I made a holder for the dial indicator to be mounted on the QCTP.
It also have a long rough adjustment slide to position the Indicator radially without moving the cross slide.
This feature is a time saver avoiding to wind the cross slide to adapt to the part diameter.

Eyeball the part into center by looking at the chuck jaws relation to the chuck groves AND looking for the jaw to touch about the high point.

Touch the dial indicator by moving the rough adjustment slide (Not the cross slide, unless is on the way or totally off range)

Find the minimum. That is the location farther away from the operator. (indicator mover CCW)

The Indicator bezel Zero is usually toward the operator and does not move.

Move the needle to Zero using the cross slide as fine adjustment.

At this point the dial read Zero and the part is farthest away.

Turn the part until you read Maximum. Needle moves CW

Back away the cross slide 1/2 the reading.

You are now on the spot closest to you. The jaws have a random orientation.

Turn the chuck to have one set of jaw horizontal and the dial reading Positive (to the left)

Working both jaws snug up to overshoot the Zero (negative) about 0.001.
This will give you some spring to tighten up good the opposite jaw.

Repeat with the set of perpendicular jaws.

With this method there is no guessing no iteration except one pass to snug up precisely when you really want zero movement on the needle.
The left hand always tighten, the right hand back off to let the front jaw move in and that snug up.

The description is detailed and long, the operation may take about 20 seconds.
The Indicator slips on and off the QCTP and is already on center.
It pays to have a cylindrical tip on the indicator to allow for small height errors.
Building the holder and the rough adjustment slide is an investment in time that pay back many times over.

#### lemelman

##### Well-Known Member
This PDF shows how I do it. It's very quick, with very little repetition of jaw adjustments. As others have stated, 2 chuck keys are much better that one.
The method works because, after step 3 the bar is centered when the DTI shows zero all round.

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#### ALEX1952

##### Well-Known Member
Start by using the rings on the face of the chuck you should easily get to within .05mm, then use a clock to measure the total throw, move the work over by half the throw, do one pair of jaws first then the other gradually coming to center. When very close just crack the jaw lose and squeeze over the other jaw. As stated difficult to describe but not difficult at all to do. More of a challenge is a face plate and having to "knock" the work into position, First step is to always use the same "knocker" I use 20mm dia brass by about 150mm this gives plenty of weight to move the work, keep this just for that! no pinching it to make something that way you will develop a feel for it. Then its just like clocking in a chuck to a position.

#### stackerjack

##### Well-Known Member
I just watched a video on U-Tube which shows a very uncomplicated method of setting up a long SQUARE bar in a 4-jaw chuck.
Place a piece of shim on the end of each of the faces of the bar, so that about half of it overhangs the end...... say 1/2", and hold them with a rubber band. You now have a sort of hollow box on the end of your bar.
Now using a DTI, the type with a lever (Last Word) etc. position it so that it's lever is touching the inner face of one of the shims. You can now rotate the chuck, by hand, and get a reading from all 4 surfaces.
This means you never have to move the DTI during the whole procedure.
Cheers,
Jack

#### gt1951

##### New Member
Use a pointed tool in the tool holder with a flat bar in the Chuck turn Chuck so that bar is flat or upright. wind in the tool on cross slide until it contact bar note the reading turn Chuck 180 degrees turn in the tool note the reading move the bar by half of the difference toward tool check and repeat and turn 90 degrees and repeat as above easey to get to within a thou

#### Richard Carlstedt

##### Well-Known Member
Too much DTI use for setup
"IF" the stock is square or rectangular, try this
Set the lathe for about 100 -150 RPM and use a magic marker
Bring the marker upto the work piece and the high point will be marked immediately
Tighten the jaws on either side of the high mark. (loosen others )
Repeat a few times, it is rather fast to do
When all four corners get marked , then use a DTI for the last few tests.
You can actually get it close with this method, but be safe .
If your hands shake, support the back of your hand with the tool post
Rich

#### john_reese

##### Well-Known Member
Joe Pie rocks.
If you don't want to mess with indicators make a sleeve with an ID equal to the diagonal of the bar. Split the sleeve. Put the bar in the sleeve and stick it in the 3 jaw.

#### Noel Gordon

##### Member
Hi Guys,
Ive been a toolmaker for over 55 years and I think I have had a 3 jaw chuck on my lathe in 20+ years.. If a 3 jaw is not 100% true its simply just quicker to use a 4 jaw and dti..Its just a case guys of practice and practice .

#### 57mm_M18

##### Member
I agree with Noel Gordon. If you want down and dirty then use a 3 jaw chuck. I personally love my 4 jaw chucks. You can dial in just about anything on them. Self centering chucks are more production based in my opinion.

#### steveastrouk

##### Member
HMEM Supporter
I'm surprised (or I missed it) no-one has mentioned the adjustable three jaw. I have one on a Myford, and very nice one on my big SouthWestern. Right now my big one typically holds anything from 1/4 to 6" or so within about 3 tenths of a thou. The four jaw takes a small crane to lift on, something I care to avoid unless I really need it