A Micro Radius-Turning Tool and Boring Head
and a boring head essentially consist of a tool slide that rotates
around an axis perpendicular to that of the sliding movement, allowing
the radius of swing of the cutting tool to be adjusted. Initially I had
contemplated to adapt the geared dividing head and an existing boring
head for the purpose of radius turning. However, it would be rather
awkward to change the whole assembly during machining operations. In
addition the overhang of the boring head would be considerable. Hence I
decided to construct a tool from scratch. I was lazy, however, and did
not make any drawings, just worked by eye.
radius-turning tool should be capable to produce spheres with a
diameter as small as 0.3 mm, while the maximum capacity should be
20 mm diameter. In addition, there may be a need to turn arcs into
parts, which requires the rotating axis of the tool to be above or
below the centre line of the lathe. To facilitate complex machining
operations, it should be quickly interchangeable with ordinary turning
tools. Hence, the radius-turning tool should be an accessory to the
quick-change tool post. When I made the quick change tool post, I made
few spare inserts with applications like this in mind.
|(1) Different views of the assembled radius turning tool and the tool bit holders belonging to it.||
(2) Its main components.
(3) The tool on the lathe - before a cut.
|(4) The tool on the lathe - after a cut.||(5) Close-up.||(6) Result viewed through a loupe (actually mm not inches!)|
tool-holder block was
squared in the lathe to be drilled and reamed in situ. For the actual head I used
material from the scrap box. A piece of 15 mm diameter round steel was
faced off on the lathe and
then set up on the mill for milling the central dovetail slot with a
miniature (6 mm) cutter that I had picked up at a flea-market many
years ago (never seen anything that small commercially).
Rather than providing a gib for the slide, with a circular saw I cut a
narrow slot next to the dovetail. This metal wedge is pressed against
the slide with a single screw that also serves as a lock. I used the
same arrangement on the large boring head and it works very well as
movements are only small and in small increments. Before milling the
dovetail, cross-hole was drilled through the body to receive the
movement screw, which is half in the dovetail and half in the body.
hole was milled away half using a slot mill in preparation for milling
the dovetail. The slide thus acts as a half-nut. The
movement screw is retained in place by a ring laid around the head.
This ring of 20 mm OD was turned and bored on the lathe to a tight fit around the body of tool. After
fitting and securing the ring with a sunk head screw, the assembly was
returned to the lathe
and the shank turned down to the required 6 mm diameter.
(7) Turning a 'larger' ball - before the cut.
| (8) Turning a 'larger' ball
- before the cut.
||(9) As boring-head with end-mill holder||(10) The tool mounted in the end-mill holder||(11) with boring tool bit|| (12) Ready to work in the vertical mill.
|| (13) Cleaning out the relief holes in a pulley.
The head of the movement
screw (which is an Allen sunk head screw)
was engraved with five radial marks, indicating a movement of 0.1 mm,
on the 0.5 mm pitch of a M3 screw. It is thus easy to set it in
increments of 0.05 or even less, giving a corresponding diameter change
twice this. I attempted to stamp the face of the screw with the
appropriate numbers, but failed to place the punch with necessary
precision in spite of using the punch guide described elsewhere on this
site. A round slotted nut set with Loctite keeps the screw in place.
types of turning tool holders
where produced to suit the various envisaged tasks. Shown in the
a more conventional one that is bored for a round tool bit fashioned
from a broken twist drill. The other two holders are of a collet type
to hold thin (down to 0.5 mm diameter) bits that cut at their front
face. The idea is to cut with them a double radius whereby the ball
merges into the stem via a S-curve. These collet-type holders where
turned from a piece of 4 mm round steel. The front end received a cone
and they were drilled and reamed to the desired toolbit diameter. An M4
thread was cut on the lathe for the cup nuts that were drilled and
reamed to fit the cone on the holder.