Methods of Mounting the Work Piece in a Lathe for Best Precision

The methods of mounting a work piece, in descending order of precision, along with some comments are:

Between Centres. This is the "uber-precise" method. Work can be repeatedly removed and re-mounted with guaranteed

absolute accuracy. Treat the centre drill cones with great respect. Everything will depend on them being clean and not bruised.

If a tap in or press fit needs to be tried, use a hollow point pusher or drift to avoid distorting the centre holes.

Damaged center cones may be re-machined in a four jaw chuck, with steady if required.

Face Plate. This is a method not suited to many classes of work. When used, repeatability of truth is as assured as with

between centres, so long as the mating faces remain clean and free of blemish, and your dial gauge is accurate.

For machining flat surfaces on irregular shaped objects, this method should often be the first choice of set up, as it offers

much more rigidity than the mill.

Collets. These are almost, but not quite as good as between centres. Removing and replacing work will introduce a degree

of run-out, unless the work piece can be replaced exactly in the same position.

The degree of run-out created on replacing work will depend on how well the collets were ground in the first instance.

Tapered mandrel. This is a good method for cylindrical work that has to be returned to the machine after trying a fit etc. Essentially the mandrel remains in the machine, is very slightly tapered near the chuck, and is turned to a neat parallel fit

at the outer end. A keeper nut and washer may be required for left hand turning. For longer work the outer end should be

supported on a centre. The work returns to a very close tolerance. Very good for batch work too.

Four Jaw Chuck. Work in a four jaw chuck can be returned to the machine and trued up repeatedly. The run-out is as small

as your patience is long. (Practice makes truing up pretty quick.) The main problem I find with four jaw chucks is that the axis

of the jaws never points along the axis of the machine spindle. Thus work trued near the chuck face will almost always run out further down the bed ways. There are work-arounds for this problem though. Don't force the work true with a centre. The work

will spring back when released, and may well "walk" out of the chuck jaws (ouch!).
A fixed steady will also produce repeatable accuracy in conjunction with a four jaw chuck. Make sure the steady is on the

same axis as the spindle or it's walkies again. A strip of well oiled linen backed abrasive tape interposed between the steady

points and the work, will not wear like the brass steady points can. (Abrasive side out of course.)

Travelling steadies- umm, rather not mention these. Travellers are an art to work successfully with, and it seems I'm not an

artist of the required type. Without great care, these steadies will produce all manner of irregularities at small diameter.

The points wear quickly, and one can find they are turning tapers where they thought parallel was happening.

Three Jaw Chuck. These chucks will never run really true. There are just too many variables to maintain any decent sort

of control. Even the super true expensive types will gradually wear and become less reliable. Grinding the jaws will only reap benefits at one diameter of work, and then only temporarily, although results may be “good enough” for much work and can

salvage an otherwise useless chuck. These chucks are good for work that is to be machined all over in one set-up, or where

run out on portions of the work doesn't matter. Returning work to a three jaw chuck will always introduce a degree of error.

In some classes of work, turning the work in relation to the chuck and trying the work in various positions will reveal a position where the run-out is tolerable.

There are other pretty accurate methods, such as temporary fixings as used by watch and clock makers. These usually involve machining something fixed to the nose of the spindle that will provide a snug fit to the work, or be slotted to work like a special collet in a three or four jaw chuck.

Clay.

Clayton Bonser