I don't turn many hollow forms but I have some nice wood that I am turning into small vases (5 inch diameter by 8 inches long). I'm hollowing with a typical 3/16 tool bit mounted in an articulated arm device followed by "smoothing" with a tear drop scraper. My problem is at the bottom and I can guarantee a gigantic catch. I've tried turning with the scraper at center, slightly above center and slightly below center but no matter, BANG! If I readily admit my ignorance on the subject, can some kind person give me some advice other than go back to bowls? Thanks
You need to be dead center and don't go to the right of center. If you go to right it lifts the cutter up and then slams it back down on the tool rest even with an articulated arm. If you stay on center it should be very easy to clean up the bottom as you go to the left. Dead center you just have to take really light cuts back and forth until you've cut it down level all the while trying not to go right of center
Are you getting the catches with the cutter or the scraper, or both?
With the scraper, the problem is most likely a combination of taking too wide a cut and having an excessive burr on the cutting edge. Remove the burr by honing the top surface. You can take this further by giving the cutting edge a very slight negative rake.
For the "too wide a cut" issue, options are to re-shape the scraper or to dispense with it altogether. An alternative is to grind a 3/16" cutter with a rounded cutting edge to form a small round nose scraper. Again, remove the burr if it turns out to be catchy. Take it further by giving it a slight negative rake.
What is the shape of the cutting edge of the cutter? Round nose or V-shaped? I've had some success with a V-shape (for wood removal as opposed to smoothing), but do not plunge the cutter straight ahead. The cut will very quickly widen and make a catch more likely. Advance it sort of sideways, parallel to one side of the V so that cutting occurs only on one side. Again, pay attention to the burr.
Is the articulating arm level (parallel to the ways of the lathe)? It's possible to have the cutting edge exactly on center but at the same time have it pointed slightly upward if the arm is not level. This would cause it to be a bit grabby.
It might be instructive to mount a test sample of the wood on a faceplate or chuck (end grain orientation) so that you can see what is happening. That is, pretend you are turning a shallow, end-grain bowl and try the cutter and the scraper. When you are able to get satisfactory cuts on the test piece, go back to the hollow form.
How much does the cutting edge overhang the tool rest? You mentioned that the workpiece is 8 inches long. If you are hollowing to a depth of 8 inches, the bar supporting the tool will need to be at least 5/8 inches in diameter and preferably 3/4". Otherwise, the bar will flex up and down with a severe vibration and be out of control.
You can practice on doing smaller box forms. Mostly with a box, the form is more shallow and it is easier to see what you are doing. There is an exercise that Dave Ellsworth did, and I think it is in one of his videos. Using a standard scraper, he 'fishes' for the dead center, so he sweeps up and down as well as side to side. This is easier with hand held tools. Being dead on center to cut that really helps. The problem is that as soon as the cutter goes into the wood, this pushes it down slightly, so then you are below center. For me, I prefer a scraper with a ) nose, so shallow curve, and I cut with it at a 45 or more degree angle rather than flat like a scraper. Again, easier to practice this cut on box forms, and Doug Thompson's fluteless gouge is ideal for this cut. A negative rake scraper can also be used the same way, and maybe your tear drop scraper. So, dig out a piece of scrap wood, end grain and try a few cuts that way where you can see what you are doing, even to the point of having it 8 inches off the tool rest like your hollow forms.
The only thing I would add is use the heaviest shaft you can. The farther you get over a tool rest, however slight, there is the possibility of some flexing. The flexing can also cause a catch.
If all else fails you could get a tool rest that inserts into the neck of the vase and supports a hand held tool. A sharp round nose scrapper usually does a good job on the bottom to smooth it out. I think John's response is most likely the problem you are incurring, if your tool engages the bottom of the vase to the right of center you will get a big catch. It might be hard to tell where the tool is going with the articulating arm going into the vase at an odd angle. A laser pointer or video system would help guide you with this task.
You have already had a lot of good suggestions. The only thing I might add is regarding to being on center. If you setup your hollowing rig with it being on center at the top of the vase and the hollowing rig is not level, it will not be on center at the inside bottom of the vase. So you might want to consider that. Hope you get it figured out.
Gentlemen, I really, really appreciate all the responses to my question-I now have several things to try. I really like the idea of a small "open" turning where I can easily see what is happening as I do the cutting. I'm off to the lathe. Thanks so much!
I saw video of a form turned with x-ray vision. (Lucas video?)
Actually, the piece had a few large holes cut in the sides which made the walls invisible while turning - so easy to see exactly what the tool was doing on the bottom, the transition, and the sides.
I thought that should be a required exercise for anyone learning to hollow or using a new tool or rig.