Unfortunately there are several factors that combine to make this a rather more difficult task than it at first appears. Because the diameter and pitch of a spindle-nose thread is often not to a “standard” (and combined with wear in use), machining a thread inside a backplate to match it can be tricky – unless the spindle is removed from the lathe and is used to check the job as it progresses. You can’t remove the job from the lathe and try it on the spindle – once removed, it can never be replaced with sufficient accuracy for a cut to start where it left off.
if spindle thread is a Whitworth or metric standard you may be able to locate a tap of the appropriate size and find that a thread cut with is perfectly satisfactory; however, large-diameter taps are hard to find, and expensive.
There is one well-known technique that will enable an accurate measurement of the thread diameter to be made and the problem of making a backplate with accurate threads solved.
The “3-wire method”
his makes use of freely-available brazing rods. A diameter of rod needs to be chosen so that, once in position it protrudes just above thread crests. The illustration below should make it clear as to the shape to be used – this cleverly allowing the wire to stay in place whilst readings are being taken. Using a micrometer measure the diameter over the wires at six points along the length of the thread and average the result.
- the length of the thread
- the length and diameter of any plain “register” inboard of the thread
- the number of pitches per inch (or mm pitch)
- the thread angle (usually 55 or 60 degree)
- the average diameter over the two “3-wires”
the spindle can be removed from the lathe and tried into the backplate as work progresses, the whole process is made so much easier.
A note on the “register” of the spindle end might also appropriate – the register being that small length of finely-finished plain shaft between the thread and the abutment face. Although by its name one might assume this to be a critical part of the assembly it is not, and has no bearing on the accuracy of the backplate-to-spindle fitting.
This machine turned with perfect accuracy – even deep cuts failing to have any detrimental effect. As something of a discussion has developed around this point, as an experiment a brand-new Burnerd chuck with a close-fit register was fitted and tested – and absolutely difference in performance could be detected.
thread is a poor way of mounting a chuck and the several alternatives are a great improvement.