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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.
Questions received and answered: Q: When is an Oval not an Ellipse? - an old Victorian argument. [click here] Q: Were these bowls made using a Spiral-Spherical Slide-rest? [click here] Q: - MYSTERY ROSE CHUCK? - Information Wanted . . . . . for more information and more pictures [click here] Q: Do you know anything about a Carving Machine called the Dentifractor? It is a machine described in Holtzapffel Volume 2, page 955, Appendix L. [click here] Q: As a newcomer to ornamental turning, how do I sand and polish my work without destroying the facets of the patterns? [click here] Q: As a newcomer to ornamental turning, how do I choose what shape rosettes to make for my rose engine? (this is a revised answer to a previous question) [click here] Q: How can I make a spiral effect on my rose engine? [click here] Q: How can I make a Holtzapffel style Dividing Chuck? - [click here] [see also Spiral Dividing Chuck] Q: Could early turners cut spiral twists on a Pole Lathe - [click here] Q: How can I raise the Centre-height of my 5" Spherical Slide-rest? [Answer] Q: What size is the Triangular Bar on an Evans Overhead? [Answer] Q: What is the best size and type of Cam-ring for an Ellipse Chuck? - expanded answer to a recurring question. Subjects of other questions received and answered: What are the best ways of Chucking workpieces in the OT lathe? Is it possible to cut Basket-weave pattern on a Rose Engine? - and what about the Barleycorn pattern? -click here- What Rosette profiles should I choose for my new Rose Engine? -click here- How fast should I run the Ellipse Cutting Frame? -click here- Are there any plans for making a frame for a Holtzapffel Lathe? -click here- How to make a Spiral Dividing Chuck -click here- 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: February 25, 2013 |
