This may be common knowledge to some, but being able now to centre a 4-jaw chuck in a couple of minutes instead of spending 20 minutes cussing and swearing is an absolute joy!
I do not claim originality for this, I was shown it in my turn.
The essence of the trick is that you need _TWO_ keys for your
4-jaw chuck. With the extra key, you can effect a smooth linear transition of the workpiece across the chuck by simultaneously releasing one and tightening the other, at all times retaining the workpiece in a tight grip. No more will small rods fall out and disappear in the interval between loosening one jaw and tightening the other! (I actually made up two extra keys, using 1 and 1/4 inch round bar for the handles, and squared-off 10 mm round rod for the working end because the Tommy bar on my original as-supplied chuck key fouls on the lathe body and it is not possible to do the simultaneous loosening/tightening. Also, a piece of 1 and 1/4 inch bar sits nicely in the palm of the hand and is a lot easier to twirl than a Tommy bar)The Technique.....
- You need a dial gauge set to centre height on the cross slide (I have mine in a Quick-Change tool holder)
- Place the workpiece in the chuck and approximately centre by eye. This stage is important because if really badly off-centre, you won't be able to transit with one pair of jaws because the workpiece will foul on the other pair being closed up too far below the diameter. (Thereby hangs a tale, was demonstrating last night!)
- Put one set of jaws horizontal.
- Bring dial gauge up, in line with the horizontal jaws, and using the cross-slide, wind it on a couple of turns so that it will give both positive and negative readings.
- Set the dial gauge to zero, either by its movable cursor, or with further cross-sliding.
- Rotate the chuck 180 degrees, ignoring what may appear to be a large error of eccentricity on the way round, so that the other jaw of the pair is now horizontal.
- Read the dial gauge and note the value
- Without moving the chuck, use the cross-slide to adjust the dial gauge by the cross-slide so that it now shows HALF the difference between the two 180 degree positions.
- Use the two chuck keys, one in each hand simultaneously, and transit the workpiece so as to bring the dial gauge to zero.
- One set of jaws is now centred! Repeat steps 3 to 9 for the other pair of jaws and, Voila!! Zat is Cointreau!
There you are, dead easy, and it takes a lot longer to explain than to do!