Lathe – workholding – steadies – intro and links
When a workpiece is being machined on the lathe it is usually being held in some way at the end of the spindle. This is often by means of a chuck. It can also be held at the right hand end of the workpiece by some means by the tailstock.
When the workpiece is long there can be problems because the workpiece will not be supported adequately somewhere along its length. In some cases it might not be possible to use the tailstock to support the workpiece. This occurs not only when machining the workpiece but if it is thin it may try to thrash about.
Steadies are used to hold a workpiece when it needs supporting somewhere other than at the chuck or tailstock positions.
The fixed steady
This is the most commonly used steady. It is fixed onto the bed of the lathe usually near the right hand end of the workpiece. There are two reasons for doing this. Firstly when the end of the workpiece is “missing”. This can occur when the end has a taper in it. Secondly, when a large number of short pieces are to be machined out of a long bar and have to be parted off it.
The travelling steady
The travelling steady is a steady that is fixed to the saddle and moves along either just in front or just behind the cutting tool. It function is to stop long thin workpieces bending under the pressure from the cutter.
the travelling steady
The spindle steady
This fits the back end of the spindle. Is function is to hold a long workpiece sticking out of the back end of the spindle. This is not because there is any means of machining it here but to hold it straight and/or if it is very thin from thrashing about.
the spindle steady
The toolpost steady
This fits the toolpost. This can be used to hold long thin workpieces when they are being drilled. One might ask whats wrong with the travelling steady. The answer it that it only works when it is used in conjunction with a cutting edge. When drilling there is no cutting edge. The benefit of using the toolholding system it that is already contains two essential requirements – the ability to go up and down and in and out.
In the example shown a tapered stanchion had to be held in the chuck. This was necessary because it was only after the tapers had been made it was found necessary to put threads into the end of the stanchions. One way to do this was to grip the bottom of them in the grooves in the chucks jaws. This, though only held them at a point so they could still rotate about this point. The solution was to use a toolpost steady.
fig the toolpost steady
Points relevant to most steadies most of the time
These points apply particularly to the fixed and travelling steadies and especially so if they are being used for the most exacting work.
The fixed steady has three jaws that define a circle the workpiece rotates in. The travelling steady has two jaws but the circle is defined by the two jaws and the cutting edge. If the circle is off center then the workpiece will wiggle as the chuck goes round in such a way that it can work its way out of the chuck.
In either case this circle must always be concentric with the axis of rotation of the spindle.
If the workpiece is made from accurately rolled bar then this must be centered in the chuck, ie., a three jaw chuck is not good enough. A four jaw or collets are needed. Alternatively a three jaw chuck can be used and the outside diameter of the bar has to be turned so it is concentric with the spindle.