An alternative method is to bring the saddle up to the stop but this time without
any packing, and again the tip of the boring tool is advanced until it contacts the
part being machined. In this case, the saddle is then moved away and the top slide
advanced by 18mm. The machining process will then take place in the same way. The
method is simpler than using packing but is open to errors due to mistakes in calculating
the number of turns and part turns of the top slide leadscrew whilst the packing
method is all but foolproof, probably the reason why I prefer this method. As I am
invariably, working to metric dimensions on an imperial lathe this prods me in the
direction of using packing.
If you prefer the packing method then a combination of the two methods can be used
for more precise dimensions. Assume a depth of 18.5mm is required, then the lathe
could initially be set up for 18mm as above but before commencing machining traversing
the top slide by just 0.5mm to give the required result of 18.5mm.
Protecting the Faceplate
A frequent operation is to bore a through hole in a part mounted on the faceplate
in which case the problem is not getting the depth of the hole just right but to
avoid the cutter contacting the faceplate surface. Normally, a thin piece of hard
card would be placed between the workpiece and the faceplate surface and the part
bored taking care as the tool breaks through, one lapse of concentration though and
one has a damaged faceplate, not that serious but it does not look good.
In this case, with the saddle against the stop the tip of the boring tool is brought
up to touch the faceplate, Photograph 3, after which the top slide will be set back
say 0.1mm ensuring that the boring tool stops just short of the faceplate's surface.
It would also be advisable to lock the top slide to prevent it being accidentally
moved which may result in a damaged faceplate, the very thing that one is attempting
Machining to a step
A very frequent requirement is to machine a workpiece to a step but having a depth
necessitating this being done with a number of passes. The problem is virtually the
same as boring a blind hole though the operation is fully visible and therefor much
easier. It does though need a degree of concentration at the end of each pass, perhaps
having to work to the leadscrew's dial readings. Setting up the saddle stop to set
the step's position will therefore make it less demanding.