Vacuum chuck question

[Hilel Salomon]

Finally, Finally, I got my vacuum chuck set up and working. I hate making jigs, so I bought two sets of a holdfast chuck, and of oneway's 3.5 and 5" vacuum chucks. Here in Columbia, I noticed that I couldn't pull out a bowl from the holdfast, but I could w/the 3.5 oneway. Is that as it should be? The latter does hold, but I'm wondering if I should try anything when with a medium hard pull I can take the bowl out.
Thanks, Hilel.

[Steve Schlumpf]

Hilel - couple of questions for you first: what size bowl and what was the vacuum setting at?

[Bob Hamilton]

Hilel:
Was it an open segmented bowl? :D :D :D Sorry, couldn't resist. If the chuck is 3.5" in diameter then that is only about 9.5 sq. inches of area that the atmosphere is pushing against to hold it against the chuck. If you were able to achieve a "perfect" vacuum (-30" of mercury) that would mean a force of about 135 to 140 lbs. pushing the bowl on to the chuck. That is not a lot of grab, but should be adequate for very light cuts and sanding.

Increasing the diameter of the chuck increases the force by quite a bit, since the area of the circle increases by the square of the radius. A 6" diameter chuck with the same "perfect" vacuum would be holding on to the piece with a force of about 415 lbs.

Take care
Bob

[Hilel Salomon]

Steve,

I actually didn't look carefully at the pressure reading, but I will this morning. For my experminent I was using a small, shallow bowl which I got w/my corer. It seemed small enough to try the 3.5 Oneway, but the 6" holdfast chuck held it so I couldn't pull it off. There's a structural difference in the two as the little aluminum threaded #2 morse taper tap holds the holdfast tightly while it only achieves holding the threaded rod tightly to the spindle head and the oneway chuck can go in and out. I'm wondering if there are leaks in the oneway?
Now Bob, as the resident expert on using a vacuum on segmented bowls, of course I did it on a segmented piece. I did think that the openings were too small so I used a chisel on them to expand them.... Ha, Ha. No dumb move goes unremembered. Normally, only my grandchildren are encouraged to make fun of me, but you did it so lightly that I've been smiling since I read it. Thanks, Hilel.