Lathe – use of soft jaws

The jaws on a three jaw chuck are usually made of hardened steel. However, it is sometimes possible to get jaws that will fit the chuck but are made of “soft” steel. These jaws have teeth on them and the grooves that let them fit the t-slots on the chuck but the rest of the jaw is just a plain slab of steel.

2385 soft jaws teeth

fig soft jaw

These jaws are just like the hard jaws and so are numbered one to three and have to be fitted in exactly the same way. This is where the similarity ends.

Like this they are useless. To use them they have to be machined to fit the workpiece.

There are similarites between the problem of grinding hardened jaws and turning sort jaws. The common factor is that, in both cases, the jaws have to be moved by the scroll so they as hard against the workpiece whilst they are either being ground or turned.


Holding the jaws in position whilst they are being machined

A using a plug

The simplest method is to fit a round piece of material in the chuck. This is the “plug”. When the jaws are tightened up they will be pushing inwards on the plug. The plug is fitted right at the back of the jaws. It is only long enough so when it is fitted it will be “straight”, ie not at an angle to the jaws. This leaves the front of the inside surfaces of the jaws free.

The accuracy of the plug in not important. All that matters is that the three jaws must be pushed inwards. If the plug is not perfect it will shift off center till all three jaws are under equal pressure.

2384 soft jaws with plug

fig chuck with soft jaws and plug

Like this it is possible to use an ordinary cutter to cut a shape out of the jaws that could take a round shape. The diameter of the round shape has to fit within the diameter of the jaws.

The problem with soft jaws is that they get used up over time. The key to making them last is always to cut the minimum off them for each new job. The key to this is to have a set of plugs of varying diameters so the best one for the current job can be used.

Once this has been done any round shape cut in the jaws will be concentric with the spindle but only for this one diameter.

fig shape cut out of jaws

Whenever the jaws are turned there is not only the side that holds the workpiece but there is also a back. The ¬†advantage of this is that the back of the cut produces a “stop”. This is very useful if many parts are being made.

Often the part being machined might have a sharp corner so it is very useful to cut a groove in the corner between the side holding the workpiece and the back so the workpiece will fit in in an accurate and consistent manner.

fig groove cut in corner

The workpiece in this case would either be accurately made round stock material or it might have already be turned to fit the diameter in the jaws.

Since the diameter cut in the jaws is concentric, if the workpiece has been machined to fit this space, it can be machined on the end sticking out to the same diameter and can be turned round and will still be concentric.


Cutting the jaws to a particular diameter

Where soft jaws are being used on a three jaw chuck then setting the diameter of the jaws is finally determined by machining them to the required diameter. Measuring the diameter that will fit three jaws is not always easy.

If the lathe is fitted with a dro it is possible to turn the diameter to that required is the zero for the cutting edge is known.

It is also possible to measure the diameter using a bore gauge with three arms.

It might seem that it would be possible to make a gauge that is the diameter of the part to be held in the chuck. But, of course, if this size is right it will be slightly too big to fit in the jaws. It is probably good enough given the difficulty of measuring the diameter without a three probe bore gauge.


Tightening the chuck

When using a three jaw chuck with soft jaws every time the chuck is tightened up it is essential that the same socket is used for the chuck key when tightening the chuck up.


Use of this when grinding hard jaws

Using the plug method means the inside of the jaws is less at the back that at the front. This is unacceptable for hard jaws because they will be used for many different jobs whereas soft jaws are only used for the current job.

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