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« on: January 24, 2009, 06:59:54 PM »
Wow, this is turning in to quite a thread . . . yuck, yuck.
There are two ways to single point threads; a straight plunge, or cutting only one side of the 'V', which I would imagine would be far better for the 'puny' lathes. In either case, backlash is irrelevant as you always cut in the same direction. Your error far exceeds the backlash so it would be an academic discussion even if you were improperly cutting the 'near' side of the 'V' where backlash might come into play.
Having seen the picture of your pulse trigger setup, I believe that is your suspect. The color is not as relevant as the surface, I would imagine. You have shiny tape on a small radius. It is probably only slightly less reflective than the shaft itself. Perhaps even more so to IR light. You can see the highlight on the shiny black tape even in your photo. That's also probably what the sensor is seeing.
To support the consensus, I'll add that I also use a slotted disk. Single round hole of about 1/4" dia. Mach seems extremely accurate picking up deviations of a few RPM at speeds over 6,000 and also reading steady at slow speeds.
My sense is that at very low speeds like 100RPM, more than one slot would be beneficial, but it appears from several people who have posted their successful threading experience, that one pulse is sufficient.
There is enough information now to suggest that moving your black area to the face of the disk instead of the shaft as suggested by RC (Overloaded) would probably solve your problem. You also should use flat black, and not something shiny. Even better, as has also been suggested, go with a slot or blade arrangement for the most positive trigger.
As a minimum, strip off that shiny tape and spray some flat back paint in it's place,( and you might consider making it significantly narrower as well) - or - (for diagnostics and not as a permanent solution) take some 600 or finer grit wet/dry and at least scuff off that shine!
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