I just finished making a 15" spalted sugar maple bowl
I was sanding the inside at a slow speed and it started to wobble so I grabbed the bowl, it was then that I noticed it was unscrewing from the shaft.
Have this happened to you? I need to understand what went on, so it does not happen again. The chuck is the Vicmarc 100, new last winter, it does not have a proviso to tighten it to the shaft. The lathe is my trusty Powermatic 35-20b, 10 years old that gets used day in day out.
Sounds like a clear case of sanding in reverse with too much sanding pressure. Vicmarc uses a different retention system than everybody else. They expect you to buy a Vicmarc lathe as an accessory to your Vicmarc chucks. If that sounds a little too extreme then you could consider drilling and tapping holes for a couple setscrews. The ideal location is in the groove that Vicmarc uses for the retaining clip. Another option is to tighten the chuck to the spindle with a snap of the wrist to firmly seat it against the shoulder ... but not so tight that it gets permanently stuck.
No I never use the reverse, i thought the chuck would come out if I did, It was turning forward at a slow pace, around 90 rpm. I will follow your suggestion of drilling and tapping, and yes I would love a Vicmarc 3000, with a 3 HP motor, time to start saving.
With the lathe running only in the forward direction, the chuck will tighten up on the drive spindle. Now if you have a large bowl, then the mass of the bowl will want to unscrew the chuck, if the lathe slows down faster than the mass of the bowl. Usually seating the chuck properly against the headstock threads, creates enough friction to eliminate this problem. I don't know if you use a nylon/plastic washer between the chuck and the drive spindle threads, but you might want to consider not using it. Just some thoughts for you to consider.
When you put the chuck on the lathe spin the chuck fast for the last revulsion the chuck will come to a sudden stop this locks the chuck to the spindle and registers it so the chuck will turn true. If you do not set your chuck in this way it can unscrew easily when you press the stop button your lathe stops in a programmed number of seconds. Generally this works fine for light weight work but with heavier item the spindle stops faster than the chuck and can unscrew the chuck.
If you decide to add the locking screw you will need to be careful to remove the locking screw before trying to remove the chuck. Failure so can do major damage to the spindle threads.
I haven't read the other posts but what I do is put the chuck on finger tight. Then I take the chuck key and snug it up. It comes off with about the same force on the chuck key. Ever since I started doing that I have not had the chuck come loose or had trouble getting it off. I did have one come loose one time on a really thick 16" bowl. It actually came loose due to the electronic brake on the Powermatic. I used to just sort of snap the chuck closed against the spindle and apparently got away with that for years.
If memory serves, the Vic 100 comes with an adapter for different spindle threads which is locked on by a hex head set screw. If this works loose, the chuck will wobble and eventually come off. If the chuck is off, but the adapter is still on the spindle, you might try to drag a magnet through all of those shavings looking for that screw. Just a thought.
>>>I was sanding the inside at a slow speed and it started to wobble so I grabbed the bowl, it was then that I noticed it was unscrewing from the shaft.
I remember someone on another forum reporting a similar incident. I can imagine the harmonics of a vibration at the right frequency working to unscrew the chuck in the opposite direction of rotation.
(Not exactly the same thing but just yesterday I had vibration from a polishing operation loosen a tight thumb screw and spin it off the thread and throw it across the room!)
Do you have locking set screws for your chuck? That should prevent a recurrence.
I tighten chucks by hand, giving the chuck a spin with a "calibrated" flick of the wrist for the last 1/8th turn or so. I have been doing this for years and never has one come loose, even when turning in reverse. (I don't use the locking set screws)
I make sure the seating surfaces are clean before installing the chuck. The chuck is "locked" on so tight it takes a hefty tug with the chuck wrench to remove it. (I always use a chuck wrench.) My wrist is calibrated to apply more flick for a larger piece!
As for sanding, I've given up power sanding with a rotating disk and mostly quit any sanding while the piece is rotating. I use NRS and hand scrapers to remove gouge marks and small (1", 2", and 3") pneumatic random orbital sanders to remove the scraper marks. Little dust, better surface.
Any more. Did it once and came close to screwing up the threads. I just give the Chuck a little snap when I mount it. If you don't put a lot of pressure while there is on problem with it unscrewing.
I am close to hit bowl #1900 and this never happened before, the spindle threads have quite a lot of play until the last half turn when it gets tight. I"ll try your method. Thanks
Were you power sanding? I can imagine, depending on the orientation of the sander, that the sanding force could spin the piece off if the chuck was not snug.
I rub air tool.oil on my spindle and then wipe it all.off with a rag. Of course it does t get every trace of and it's just enough to keep it lubricated withough making the chuck come loose. Oh by the way I learned the chuck tightening trick from John Jordan.,
