Hi All,
As a follow up to the above.
Turning down the long thin stem on the part held in the 4 jaw. This would seem to be a straightforward operation but to do it right, there is much more to it than meets the eye.
To follow best practice:
The rectangular part in HR steel (from my scrap bin) was first rough formed by milling and left approx.0.025" oversize. There were good reasons to do this to accommodate how the part will be finished and not have problems with other subsequent machining operations . One always has to think ahead when multiple machining operations are involved.
The roughed out part was also left longer than needed for the same reason as above. As well, when the tool bit has to machine right up next to the chuck jaws, clearance issues with some of the older forms of tool holder such as I have dictate this.
When turning long thin shafts like this, tailstock support is mandatory. And if your part can't be left with the recessed centre which subsequently has to be machined away, this extra waste length has to be taken into account also.
When turning thin shafts like this (5/32" finished OD), the use of a half centre is necessary, as shown, so the cutting tool has clearance to be able to start the cut at the minimum diameter.
No seen, but the cutting tool itself has a generous nose radius ground into it on its leading edge. The part drawing itself shows a sharp 90 degree transition between the round stem and the rectangular head of the part. This is not good engineering practice as a sharp transition induces stress risers at the transition point which should be avoided (sometimes it can't) if at all possible. In this case, it was quite possible to have a generous transition radius.
For the best surface finish possible which is needed in this case:
The cut is being undertaken using the finest longitudinal automatic feed rate available on my lathe.
The cutting tool is set exactly square to and a touch below the centre line of the work so the trailing edge of the cutting tool scrapes and polishes as well as cutting through shear. Of course, the tool should be very sharp.
Keep any and all tool/slide overhang to a minimum.
Use cutting oil to improve the surface finish.
When approaching the end of the cut, the transition radius, I release the belt drive on the lathe and then turn the spindle by hand, feeding very slowly into the radius with the leadscrew still engaged. This way the cutting tool does not slam into the transition radius under power and leave chatter/gouge marks.
I do sometimes feed only by hand to have a even finer finish but have had lots of practice doing this over the years.
I am sure I have left things out which others can add. All of this is much longer to describe in words than is done automatically in practice.
