With the complexity of setting up a independent 4-jaw chuck, why use them?
- You can dial-in more accuracy than is attainable than any auto centering chuck.
- You can dial in offsets for drilling or turning eccentric holes or edges.
There are more than one way to get your stock centered in a 4-jaw chuck. I'm going to show you how I do it. It's supper easy so lets get started.
Start by setting all the jaws to an equal distance from the outer edge. Here I just set the outside of each jaw roughly flush with the outer edge of the chuck.
Next, place your stock in the chuck and tighten the jaws against it. At this point you can just eye ball it.
Only snug the jaws at this point.
Place a dial indicator on your lathe with the tip centered on the stock as shown here.
You can use a magnetic base or dedicated quick change tool holder as I did here:
This allows me to add and remove a a dial indicator quickly and consistently.
Rotate the chuck until you find the high and low swings. Move the chuck to the lowest and zero the chuck as shown here.
Rotate the chuck to find the highest swing. In my case its just under 20.
Divide the highest number by two and rotate the chuck back to that point. In my case that is just under 10.
Zero the dial indicator on this half way point as shown here.
Move the closest jaw to the dial indicator parallel to the indicator as shown here. This will be the axis we will adjust first.
Loosen the the two jaws and adjust them so that the dial indicator reads zero on both ends of the rotation.
Now repeat the process on the other two axis.
Once the second axis has been zeroed, rotate the chuck slowly around and verify that the total run-out is less than .001". If its not, repeat the process to get it closer.
At this point your stock is centered in your chuck. You can move to the next phase of your job. This may included simple turning or adding an offset.
If you are moving to another operation, such as the drilling an eccentric bushing, follow the following steps to remove and add the new part.
Step 12 (Optional)
If you have not done so, mark two adjacent jaws with a permanent marker, as shown here.
These jaws will be used to loosen your grip on any part that needs to be removed.
Step 13 (Optional)
Loosen the grip on the stock by slightly loosening the indicated jaw.
Remove the stock.
Step 14 (Optional)
Insert your new part and tighten the two indicated jaws.
Note that this procedure only works when replacing the same diameter parts. If the two parts are not exactly the same diameter, you will loose some accuracy.
It is generally used when the part you need to machine does not have an exposed surface to center on in the first place. Much like the eccentric bushing shown here. A little lost accuracy does not matter much since we are going to offset the bushing anyway.
I created a couple tools to make adjusting the chuck a little easier.
The first tools are these little thumb chucks.
They slip into the two chucks being adjusted.
They are small enough to fit behind the chuck so I can adjust both jaws as needed when parallel to the dial indicator.
The second was this extended chuck.
This allows me to adjust a jaw from the front of the lathe as well as the top.
Here is a video I found useful when I first started using a 4-jaw chuck.