Bowl Alignment

[Steve Doerr]

I was reading the discussion on lathe alignment and it makes me think of my struggle with alignment. I have a Grizzly G0462 (I know, not the best but do plan to upgrade at some time.). When I turn the outside of the bowl using a face plate or centers, and then turn it around on my chuck, to hollow out the inside, I notice that it is no longer running true. (Yes, the tailstock is helping to hold the wood in place.) It is very noticeable, and as a result I can have a difference in wall thickness. This has caused me to redo the outside of the bowl once it is put in the chuck. To me, this does not seem normal. Is there something I can do to improve the situation, or am I doing something wrong.

Any suggestions would be greatly appreciated.

 

[john lucas]

It is somewhat normal. Wood is a fickle thing. There are things you can do to minimize the problem. First of course is to use a good quality self centering 4 jaw wood turning chuck. Not one of the skinny jaw metal lathe looking things that Grizzly sells for wood.
Next is to use the proper size jaws. The jaws should be closed as small as possible. If you open them very wide to grip the tenon they crush the wood more because they only grip with the very end. Wood has varying density and will often crush unevenly from these points which throws the bowl off.
Now here's a trick to try and get the wood to crush more evenly as you tighten the chuck. If you imagine the grain running from 12 to 6 oclock, mount the bowl so the jaws grip at 2,4,8, and 10. Well not exactly but you get the idea. Since the jaws are pushing into the grain in the same direction on all 4 sides you don't get as much movement.
One more factor. Wood often moves when you remove a lot of wood to shape the bowl. The stresses in the wood will often change it so it might be very slightly out or round when you reverse it. If it's really critical such as turning a piece with a 1/8" or less wall then I true up the outside after I reverse turn it.
Hopefully some other people will have ideas. My bowls are often off but usually only by a millimeter or so. Depends a lot on the wood species and how wet or dry it is.

 

[Wayne Spence]

Steve-Having had the same problem with several varieties of wood and checking out my lathe time and time again I did the following. To me it showed the wood was moving as mentioned by John Lucas in the above reply. After shaping the outside leave the bowl on the faceplate and both on the lathe for a half hour. Turn the lathe back on and check to see if the bowl is running true or is out of round. I was surprised to see how much out of round the bowl could get in that short of a time. You could expect this with very wet wood but the so called dry wood can often have much more moisture that you think, especially if you do not weight it.

The best idea I have heard in a long time is to hold the wood at the 2-4-8-10 positions. It may not be a new idea but it is to me.

 

[MichaelMouse]

Go to a non-destructive hold rather than a wood-crusher. Dovetailed holds do not distort the tenon, and used in a mortise, provide better depth to a piece than a tenon turned away.

The optimum self-centering hold is the one which keeps the most metal on the wood by matching the mortise or tenon dimension to first point of circularity on the jaws. http://i35.photobucket.com/albums/d160/GoodOnesGone/Chuck-marks.jpg You can see that here, and you can see that what keeps the bowl rotating true is seating it against the face of the jaws. Holds true (pun intended) even with crusher holds. Make a shoulder for your tenon and use the tail to make sure you keep it in contact. Little more difficult with crush designs because a lot of them have ridging which is symmetrical, so when it bites the wood it's a 50-50 proposition whether it will wedge away from it or in to the shoulder. The dovetail, of course, wedges into the shoulder only.

If you get a bit oversize with your mortise or tenon when using a dovetail, you will want to pay special attention not to crank the chuck in the mistaken belief that tighter is better. The arcs or points in contact will exert more force per unit of contact area than an optimum size, and you can actually crush or split the tenon or mortise by exceeding the compression strength of the wood. Snug and seated is all you need. The wedge will do the rest.

 

[KellyDunn]

Steve, I also had never had anyone tell me the best way to chuck the wood. Really good advise. For me I do not start finishing cuts on the outside untill the bowl is put on the chuck. I tend to double turn. Green then dry then rechuck. I prefer little to no movement when finishing the outside. Depending on the wood, where is was in the log, etc. A lot of stress can be released when taking down the inside. If it goes really wonky I really slow the lathe speed down to sand the inside so the paper does not dig into the high points.