Lathe runout

tradertom · Post by **tradertom** » Sat Jan 07, 2012 12:52 pm

While using the ShopSmith as a lathe with a SuperNova2 chuck and I am having a run-out problem. It only seems to be a problem when turning with the chuck.
The example I have pictured is a 5" bowel with a hole that I sanded in one side of the bowel. On the oposing side the wood is so thin you can se light through the wood.
I have a 2 bearing quill I think. it is a 510 SN no. 51-0008581

Turning between centers I have turned many pens without a problem as long as the centers are aligned

Any recommendations?

JPG · Post by **JPG** » Sat Jan 07, 2012 1:10 pm

tradertom wrote:While using the ShopSmith as a lathe with a SuperNova2 chuck and I am having a run-out problem. It only seems to be a problem when turning with the chuck.
The example I have pictured is a 5" bowel with a hole that I sanded in one side of the bowel. On the oposing side the wood is so thin you can se light through the wood.
I have a 2 bearing quill I think. it is a 510 SN no. 51-0008581

Turning between centers I have turned many pens without a problem as long as the centers are aligned

Any recommendations?

I am having trouble understanding how 'runout' could have caused this.

Since runout would have consistent eccentricity when defining the inner and outer walls this would in effect create the same thickness. i.e. the turning would have no runout relative to itself and the actual axis of rotation, but only relative to the chuck runout if it exists.

I be suspicious of work piece slippage between inner and outer wall turning. Especially if not done near simultaneously.

Did you encounter a need to 'true up' the walls at any time?

Spindle 'slop'(varying) is another matter!;)

pennview · Post by **pennview** » Sat Jan 07, 2012 1:40 pm

I'd expect that you've rechucked that bowl blank a few times given the bite marks in the dovetail spigot, and this may be source of your problem. While runout can contribute to out-of-round turnings, it should be minimal, so I surmise that the problem is a matter of technique.

After turning the outside of the bowl and creating the dovetail spigot, you should reverse the bowl in the chuck and clean up or true the outside of the blank before turning the inside of the blank. It's best that you not remove the blank once it has been installed because of the difficulty in getting it back in the same position. But if you need to remove the blank for some reason, you should put witness marks on the chuck and blank so that when you mount it again it will be held where it was before removing it.

tradertom · Post by **tradertom** » Sat Jan 07, 2012 1:51 pm

Finished inside and outside at the same time. I Was finishing sanding the inside when the the hole appeared. I could see light through both opposing sides before the hole. There is a detectable wobble with any wood mounted in the chuck.

robinson46176 · Post by **robinson46176** » Sat Jan 07, 2012 2:08 pm

My first suspicion would be that the bowl sides kept moving as it was turned due to it drying farther as it was exposed to the air and as wood stresses were released.
That seems even more likely since two opposing sides were both thin especially if this was a single block rather than a glued up unit with grain all directions.

.

tradertom · Post by **tradertom** » Sat Jan 07, 2012 2:10 pm

My technique may not be perfect but I have turned other bowels without trouble using another lathe.

algale · Post by **algale** » Sat Jan 07, 2012 2:21 pm

Is this the first time you used this chuck on the Shopsmith? If not, what were the previous results?

JPG · Post by **JPG** » Sat Jan 07, 2012 2:39 pm

tradertom wrote:My technique may not be perfect but I have turned other bowels without trouble using another lathe.

Whose? And they did not object?:D

JPG · Post by **JPG** » Sat Jan 07, 2012 2:59 pm

I gotta agree with Farmer after looking closely at the grain.

I believe this is a crotch cut with the branch knot on one (thin) side and the pith of the branch probably exited out the open top end.

This approximates the same as turning a bowl from a blank with bark on one end and the pith region opposed. Thus the grain is longitudinal (no shrinkage) on a perpendicular to the axis of rotation, and subject to shrinkage at right angles to the 'long' grain.

So as it is shaped opposing sides(non-long grain) will shrink, but the long grain will not. Thus as the shrinking sides are turned, more material is removed from the long grain side. Do that to both walls and ya get what ya got.

I missed the opposing side thinness comment originally.

terrydowning · Post by **terrydowning** » Mon Jan 09, 2012 8:13 pm

Combined with grain issues, the blew out happening during finish sanding indicates a possible overheating condition.

When sanding, I only use my fingers to hold the paper, when it gets, warm, I stop sanding until things cool down. Heat in thin crossing grain situation like you have in this bowl can cause a blow out. It's happened to me before. I like crotchwood bowls, but they can be very tricky.