Probably the main reason why soft jaws for a three jaw chuck are rarely seen being
used in magazine articles is that they do not come as standard with the chuck as
purchased, or with those supplied with a new lathe. At this time, limited understanding
of their use, and or a desire to keep costs down invariably results in the chuck
being obtained without this useful accessory. Also, suppliers do not list soft jaws
for many of the chucks they supply and in many cases those on offer are for the more
costly ones, as a result, being another deterrent from purchasing them for some.
When a task arises that would benefit by their use, in their absence other methods
are found resulting in an ongoing delay in them being obtained. Eventually, especially
after some considerable time, there will be some concern that the jaws being offered
in the catalogue will in fact fit the aged chuck possessed. Making ones own, whilst
not impossible, is a time consuming project but if you like a challenge, or have
an aged chuck of unknown make, then I have given my method of producing these in
another page on the site.
The lesson from the above is to purchase the jaws when purchasing the chuck, an approach
that sadly most (myself included) will not have taken.
What then is the purpose for a set soft jaws? Well, their uses are many and varied
but in a nutshell they are to hold parts that would be difficult, or even impossible,
in the chuck as purchased.
A frequent use is to hold parts that are thin compared to their diameter, typically,
a disk 25mm diameter by 3mm thick. If the disks being considered had to be bored
to produce a ring, say only 5mm less than its outer diameter, then the ring would
tend to collapse as the bore increased in size. This being due to the ring being
unable to withstand the pressure from the three point contact of the jaws, how then
can this situation be avoided? If the chuck is fitted with soft jaws and shallow
bored to grip the outer diameter then the jaws would wrap around the component giving
a better grip and at a reduced clamping force, Sk. 1a. Of course, at larger diameters
the improvement would be reduced as the degree of wrap around lessens, see Sk. 1b.
This situation can though be overcome by adding auxiliary jaws to the standard soft
jaws, a subject that will be discussed later in the article.
All pictures can be clicked on to provide a larger view