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FORUM This section is for frequently (or non-frequently) asked questions which may be of help to readers or which might stimulate some help from readers.
Recent questions received and answered: How to make a Spiral Dividing Chuck The
Spiral Dividing Chuck and how to make it. Type 1. Front Mounting: This was the earlier design used mainly for lathes without traversing mandrels. Disadvantage: when they are mounted on the front you can't also mount the Ellipse Chuck because there is nowhere to fix the cam-ring. Type 2. Rear Mounting: Later development; mounted on the tail of the mandrel after removal of the sleeve that prevents to mandrel from traversing. The Holtz design has a cam-lock and a click-wheel adjustment which is inferior because the cam can shake loose with vibration and the handle can get snapped off (as has happened with the two
as the worm-wheel can. The Evans design
is superior in that it has a locking screw and a worm & wheel adjustment;
this can be adjusted to any position, it can't come
Subjects of other questions received and answered: Notes for Beginners -click here- Cutting Frame Shank Sizes -click here- Cam-ring for Ellipse Chuck -click here- How to do Basket Patterns -click here- How to do Interrupted ECF Patterns -click here- Polycord belting -click here- Recommended Timbers for O.T. -click here- Slender Turning Guides -click here- Types of Steel etc., for Cutters -click here- What is 'Grailing' or 'Graining' -click here-
The Heart-shaped Cam for generating Rosettes Heart-shaped cams are used on the Geometric
Slide-rest to generate attractive rose-shaped patterns; rotating the cam several
times to one rotation of the lathe spindle produces the pattern designated by
Holtzapffel as 'F'. Holtzapffel & Co., provided rosettes of several counts, such as: F3, F4, F5, F6, F8. The picture opposite shows a pattern cut by a multiple of 3 hearts, or 'F3'; the effect of spreading out the heart shape in this way gives the appearance of a series of 'Cupid's bows'. This type of cam may also be used on the Geometric Slide-rest to generate rosettes for 'Rocking Headstock' type rose engines. The writer has made heart-shaped cams in plastic by mounting the blank disc on the Eccentric Chuck, cutting 3 curves of different radius on each side, then smoothing the joins by filing and finishing with fine abrasive papers. However, this method is not truly satisfactory. The curves are determined by trial and error after marking out the blank in the manner shown in the drawing below:
The curve on one side of the heart shape is like a logarithmic curve or a Fibonacci spiral. Surely, there must be a simple mechanical means of generating one side of the heart shape, then reversing the process to generate the other side. Here are two theories that might give a clue to how this could be done. If any reader knows the answer or if any reader would care to experiment with these and any other theories, please send an email of your findings to ornamental.turning[at]talktalk.net.
Theory #1. If a long rod or beam was fixed across the face of a gear wheel such that, as the lathe spindle rotates the gear also rotates and the rod, starting vertical, swings round towards horizontal, or vice versa. If the top end of a second rod was joined by a swivel joint to the top of the first rod and the bottom end of the second rod was joined by a swivel joint to a horizontal slide, surely the slide would accelerate and decelerate as the relative angles of the rods changed? If this works it should be possible, by changing the relative lengths of the rods and the ratio of the gear to the lathe spindle, to generate a curve suitable for one side of a heart? Theory #2. If, instead of a rod, one were to fix an oval cam to the gear wheel and a rubber to the slide, then as the cam rotates its eccentricity would cause the slide to accelerate then decelerate relative to the rotation of the lathe spindle? If this works it should be possible, by changing the relative sizes of the major and minor axes of the oval and the ratio of the gear to the lathe spindle, to generate a curve suitable for one side of a heart? Or, is there a better way? - If you know - please share that knowledge. |
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Last modified: November 19, 2008 |
loose
and the locking screw can't easily be broken off.
Construction:
Make a sleeve in bronze which is about ¾" longer than the steel one on the
tail of the mandrel but with the outer surface very slightly tapered. Make
a bronze back-plate and cut into it a recess and silver-solder the
thickest end of the sleeve into it; and mill a small channel to take a steel
index point which should also be silver-soldered in and filed level with the
surface of the back-plate. Mount the sleeve in the lathe and bore through
the back-plate a hole sufficient for the end bolt of the mandrel to pass
through. Cut two keyways in the back-plate at 180°, then make a
steel bush the same diameter as the mandrel tail; cut a key-way across one end
to fit exactly over the two keys on the tail of the mandrel and cut a pair of
keys into the other end of the bush so they locate precisely in the two key-ways
you have cut in the back-plate. Fit this assembly over the tail of the
mandrel and shorten either the sleeve or the bush until the combination, when
bolted to the mandrel tail, allows the mandrel to rotate freely but with no end
float. Make another sleeve in bronze (or steel) large enough to fit over
the first sleeve; the inside bore must be slightly too small to fit at this
stage. Make a worm and wheel of 96 divisions; a worm of 8 t.p.i. will
fit on a wormwheel of around 2.24" outer diameter. Cut the wormwheel
blank slightly oversize then cut into it a recess and silver-solder the end
of the outer sleeve into it. Turn the outside of the outer sleeve to
2" diameter to fit the standard Holtz (or Evans) gears and cut a
screw-thread (about 26 t.p.i) onto which a steel washer and a steel clamping
ring may be screwed. The clamping ring should be made with 6 holes and a
spanner made to tighten the ring against washer and the gear. Bore out the
centre of this combination slightly taper to match that of the taper on the
inner sleeve and a little oversize so that they may be lapped together.
Lap them together until the wormwheel fits snugly against the backplate and the
sleeves may just move freely. When the lapping compound is washed out and
the two sleeves oiled, they should turn easily but with absolutely no shake.
Mount the assembly on the lathe and cut 96 slash-cuts with a Universal Cutting
Frame set to the helix angle of the worm; then hob these cuts with a tapping
spindle. Make a cage for the worm of the thickness needed for the
worm to mesh precisely with the wormwheel when the cage is screwed to the
back-plate. Fix one side with a temporary screw which can be fully
tightened, then drill and tap for the locking screw. Then elongate the
hole through which the locking screw passes so that the worm may be lifted off
the wormwheel or engaged with it.
Replace
the temporary screw with another having a small shoulder so that, when it is
fully tightened, the cage can swing freely when the locking screw is loosened. Scribe
division lines around the edge of the wormwheel, engrave numerals for every 12th
division, long lines for every 6th and dots for every 4th.
