I am running a big lathe ( 36inch diameter ). We are roughing out a concave shape on the end of a 4 inch diameter piece. I rough it out with power feed then run the power feed cross slide while advancing the compound. Has anyone tried swinging the compound using an arm reaching back to the tailstock while operating the cross slide? I was thinking of slightly loosening the angle adjustment nuts to get a radius action.. I have to get a dish of about 3/8 inch deep at the center of the 4 inch bar. Randy
facing off concave
Apr 29, 2005
7 Replies
The radius of swing would be r = (c^2+4h^2)/(8h) = 5.52, where c=chord length=4 and h=chord to arc height=3/8. (See
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) With the radius this short, there is a possibility of the compound or tool holder interfering with the work toward the ends of the arc, unless the tool sticks out far enough. I don't know whether the setup would be rigid enough, or would have other problems - someone else can answer that.
What will this item be good for?
-jiw
Thanks James, We are doing a lot of theses things, hundreds at a time. They are Oslo points. ( tips for pilings) I could easily adjust the tool bit to give me a five inch radius. It's a big crude old lathe Randy
And if you have a lot of this to do over time, and little time to build accessories, there's always the Holdridge Radii Cutter attachement.
LLoyd
I made some chopsticks by turning a bobbin to replace the cross slide handle. I put a few turns of wire around the bobbin and clamped the end near the tailstock. As the lead screw moved the saddle along the chopsticks the wire advanced the cross slide. I turned the compound to a sharp angle so I could use it to adjust the depth of cut.
If you're satisfied with the result of hand cranking the compound maybe you could use a similar trick to advance it as the power feed moves the cross slide.
Hey, that's a neat idea. Wouldn't be very precise, but for objects like chopsticks it would be fine. The taper for such a device would be:
cross feed pitch / bobbin diam X PI
For example, a 1" diam bobbin and a .1 pitch cross slide would give a .03 taper (*radius* change of .03" per axial inch).
A cylindrical bobbin will give a linear taper, but a bobbin with a taper will give a curved taper on the work. As the bobbin diameter changes, the wire feeds off a different rate and advances the tool at a different rate. So, if the bobbin were tapered with increasing diameter as the wire fed off, a concave cut would be made (faster advance at first, reduced advance later). With careful attention to the bobbin profile, the concave shape could be spherical.
A complication with an increasing-diameter bobbin is keeping the wire from slipping "downhill". This could be addressed with a spiral cut bobbin where the wire had a grove to sit in. Or by arranging the setup such that the wire always fed off from large to small diameter.
Bob
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