I have a Jet 1442 that is about 4-5 years old with modest use. Yesterday, I was finish-turning a shallow maple bowl about 11 inches in diameter. It was dry, and securely attached with a 4" supernova jaws expanded into a recess. Everything was locked down tightly and the rest was close to the workpiece.
While making a final passes on the interior I was getting sympathetic vibration when making a scraping cut. Even with light (axial) pressure, both at the axis and 3 inches out I would get a vibration that would go away as soon I removed all pressure from the cut.
There also seems to be a low-pitched hum when the motor is running that I'm not sure has always been there.
So, I'm wondering if my bearings are going. My questions are this:
Do (can) bearings fail with axial rather than radial slop?
How do I know that a bearing is bad/going?
Has anyone over done a bearing replacement on a Jet1442... and how big of a deal was it?
This vibration can be caused by a couple of things. If the bearings are going bad, there is usually some noise as well. Grab the spindle, and try to move it. If there is any play, then that can be one cause of the vibration. Most probable cause is the bowl. On larger bowls, especailly if they are getting thin (1/2 inch or less) there is going to be some flex in the rim of the bowl when you apply any pressure. Was there any vibration when turning the outside? Was the vibration only when cutting on the walls, and not on the bottom of the bowl? Mass makes the wood solid, thin walls are not solid and will move no matter how light of a cut you do. This can vary depending on the wood as well, and can happen on very small pieces if they are very thin. Think of a soap bubble, left to itself, it is round, but slight pressure can make it wiggle like jello. You can deal with this a couple of ways.
One, is to gently apply hand pressure to the outside of the bowl. If you are pressing too hard, your hand will get hot, and you can deform the bowl into your scraper causing more vibration and even bowl destroying catches as the walls of the bowl vibrate. Do round off the edges of the rim before doing this, as they can be very sharp. Take very light cuts.
You can use a steady rest which will also put gentle pressure on the walls, but not deform the bowl.
Make the clean up cuts in stages. Turn to final thickness about 1 1/2 inches down, then do your clean up cuts for than part. Then go down some more, and clean up again, only to where you left off the last time. Do not try to go all the way back up to the rim.
For me, when I make this type of clean up cut, I will cut with the scraper at a 45 degree angle, not flat on the tool rest. This doesn't put as much pressure on the bowl sides, and gives a cleaner cut. I start towards the center of the bowl, and gently pull the scraper up towards the rim. This is not a stock removal cut, but a 'whisper' cut where you get tiny whispy shavings. This also is a cleaner cut than the flat scraping cut.
If your bearings do need to be replaced, you may be able to do it yourself. You can probably buy the bearings locally cheaper than you can through Jet. I took my PM to a tool repair company to have them do it. For me it is like working on a car, I leave that to some one who knows what they are doing.
robo hippy pretty much gave you the answer, but let me add a note
it is almost for sure NOT your bearings - think of the bowl as a bell - when you stimulate it, it rings - you aren't smacking it with a clapper, but you are stimulating it with the cutting tool. this gets increasingly pronounced as the walls get thinner. That is why, when turning thin, you turn a little bit at a time starting from the outer edge - maybe 1/2 to 1 inch at a time max, turn to near final thickness leaving the mass in the center to keep the bowl from resonating.
A scraper presents a much larger surface to the resonator than a gouge, so it puts more vibrational energy into it. Angling the scraper as Robo said reduces the cross sectional area that is excited and also makes it more of a shear cutter and both help.
I have had good luck putting a large rubber band around the outside to deaden the vibration, but this works only when you are pretty thin (say well below 1/4, pushing 1/8 inch) - varying speed also helps. But the main thing is to be sure you are cutting not scraping.
Being what they are, Chinese bearings are not from the highest quality rung, but I also would suspect that it is probably not the bearings.
Turning bigger bowl and especially with steep walls, the wood does like to get away from the cutting edge, scraping is even worse, and it's really not a good idea to use them on thin walled bowls, disastrous results do happen often if using the scraper on flexing walls.
If I want to make thinner walled bowls, I'd go the route Bill was telling you about, thin a narrow piece and go down in steps to the bottom.
The first thing I will do when there's chatter, is to first sharpen my gouge, next if that does not eliminate the chatter is to use a smaller gouge, (less pressure with smaller cutting edge) and steadying with my hand, or to set up the bowl steady if all else fails.
After al of the above, there are just two more options, fist is the 40 grit gouge, last one is the wood stove ;-))
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