Lathe – centering a workpiece on a four jaw chuck
With a three jaw chuck the chuck determines the centering of the workpiece. It is seldom perfect but is often good enough. When it isn’t good enough the solution is to use a four jaw chuck. But, of course, the user has to align the workpiece in the chuck.
There are three basic situations where this might happen:
the workpiece is round
the workpiece is rectangular or square
a punch mark on the workpiece has to be aligned with a center in the tailstock.
The key tools for centering a workpiece in a four jaw chuck
All centering on the lathe is made much easier with the aid of the right tooling. This is not a DTI on a magnetic stand. It is a DTI fitted to a bar so it can be held in a quick change toolholder. This is because this tool is frequently used and it is worth a quickchange toolholder so that it is always ready for use.
The DTI can be either metric or imperial. But it must not be too sensitive and it must have a sufficinet range. For example it can be seen that a DTI with a sensitivity of 0.01mm per division and a range of 10mm is used here. It is assumed that there is a secondary scale showing whole millimetres.
Notice that the dti is clamped it is not held with a grub screw.
The DTI is fitted with an extension bit so that it can touch the surface of the workpiece not only when has a small diameter but also when the workpiece is short and the dti has to work right up against the chuck.
It is also necessary that the tip of the DTI is at the center height of the lathe.
The other tool needed is second chuck key. This key is different in that it can be used on a socket when it is farthest away from the user. Because of the distance and because the space is limited this key is shorter and narrower than the main key. It is not suitable for tightening up a screw but is used when adjusting an opposing pair of screws.
Both of these are always needed but there is another tool that is needed in the case of centering a punch mark or a center in the workpiece. This is a piece of round metal rod, steel is fine. This is about 150mm long. on one end it has a male 60º center and at the other it has a female 60º center. Needless to say both centers must be concentric with the rod.
In all cases when centering the workpiece it is moved by means of the screws on the chuck. However when measuring any deviation this can only be done when all four screws are reasonably tight. For example, a round bar might be centering in the horizontal plane but if the vertical screws are loose it may be tilted in the vertical plane.
In all cases, in general when one screw is loosened the one opposite will have to be tightened to move the workpiece and to hold it securely. But for the last hundreth or so the movement required can often be achieved simply by tightening one screw though very tightly.
Centering a round workpiece in a four jaw chuck
There are several videos on YouTube showing how to do this in an apparently effortless manner. What the viewer might not realise is that the key to this is to align the workpiece quite accurately before using a DTI.
Usually when centering a round part, the DTI would be near the chuck end of of the workpiece.
The difference here is that in real life when a round workpiece is fitted in the chuck it will be off center. Sometimes it might be quite accurate but it is quicker to align it as accurately as possible by sight and then to not assume it is accurate enough. What usually happens is as the workpiece is rotated the needle on the DTI wizzes all over the place.
The method here is to ignore the big needle and to use the small needle on the small dial.
With one pair a screws horizontal, just touch the surface with the DTI. Move the cross slide till the small needle touches the 5 on the small dial.
Rotate the chuck by 180º. Usually the reading will now be between 0 and 10 on the small dial. The workpiece is moved horizontally to halfway between the new reading and the previous ie 5.
If the workpiece is out in both the vertical and horizontal directions it might not be possible to make a big adjustment in the horizontal plane because the workpiece will catch on the vertical jaws. One of these might need to be loosened to let the workpiece through. However at all time when a reading is being made all four jaws must be tightened though not to their final tightness.
This will align the workpiece in this direction to within the normal range of the DTI.
The chuck is rotated by 90º and the workpiece is again centered using the small dial.
The whole process is repeated again but this time using the large needle and large dial.
This should get the workpiece centered to within a few hundredths. The final adjustments can be made simply be the tightness of the screws.
Centering a square/rectangular workpiece
In these cases the problem is that the DTI cannot touch the workpiece continuously as it rotates. Each time the surface changes, the DTI has to be moved to the right, the workpiece rotates and then the DTI can be moved back. Very often, as the DTI moves back, the probe as the be pulled back using your fingers. If the DTI is on a magnetic stand this can often alter the readings. This is one reason why the clamp as described above is recommended.
In the case of a square piece unless it is square attempting to center it accurately is probably pointless. Square means alle angles are at 90º and all four sides are the same length across. In the case of a rectangular piece, all four corner are 90º and both opposite pairs of sides are parallel.
There are at least two distinct ways of doing this:
Method 1 – using the DTI
Method 2 – using a height gauge
Centering a workpiece with a punch mark of center in it
The simple way of doing this is by fitting a center in the tailstock and moving the workpiece till the punch mark or center fits the center in the tailstock. In this case the workpiece can be any shape that can be held in a four jaw chuck so long as the mark can be aligned with the tailstock.
Alternatively, it can also be aligned by using the special rod and the DTI. The workpiece is fitted in the chuck. It is align as well as possible by eye. The rod is then fitted into the mark or center in the workpiece at the workpiece end and onto a center mounted in the tailstock.
The DTI is set so it touches the rod near the workpiece.
fig aligning a punch mark or center
Any error in alignment shows up on the DTI as the workpiece is rotated. Alignment is achieved by adjusting the chuck until the error on the DTI is zero.