Large Segmented Bowl - #7 Final Turning & Finishing

After quite a bit of time spent removing very little material I got the shape to 95% done.  Most of the time went into refining the shape and making very sure all the curves were smooth as you move along the outer face with no flat or high spots.  It’s what’s called a “faired” curving surface.  Fairly easy to see or feel one that isn’t but it can be hard to cut one when turning.  To make sure I was there I took the piece off the lathe and stood it upright.  Here is what that looks like and I am happy with the result.  I can now do the final 5% of the turning then finish sand to 320 grit both inside and out.

Next is to part off or cut the bowl from the particle board mounting blank.  I start by using a parting tool that is a little over 3/16” thick, really I think it’s actually 5mm.  I go as far in as I comfortably can then quit.  Here the parting is in progress.

From there on I lock the spindle then use a hand saw to cut down toward center.  When I get close the piece wobbles a bit and at that point a little twist and it pops off.  Here it what the bottom of the bowl looks like at that point.  The piece of MDF I had put in long ago in post 3 is now visible and will be used to center the tail stock in a bit.

I could mount the bowl back in the lathe and turn the particle board remnants off but I hate to turn particle board.  It’s hard and dulls the tools.  So, I turned to a pneumatic tool, a die grinder with a carbide burr.  It spins at about 20,000 RPM’s and makes quick work of the removal.

Mounting the bowl back on the lathe is next.  No screwed-on face plate here as the top rim is finished and there is no place to screw into.  Two methods come to mind.  First, is a vacuum chuck that uses a vacuum pump and some air tight fittings to hold the piece in place.  Unfortunately, I do not have one.  What I did use is a large set of quarter circle aluminum plates that attach to my scroll chuck and have rubber knobs screwed into them.  They can hold onto a piece from the outside or the inside.  Here due to the curved outer surface I will put them inside the bowl then open the chuck expanding the plates and holding the bowl in place.  However, I do not want to put too much outward force on the bowl because I could split it apart and that would not be a good thing!  Here is the plate and the bowl mounted on it.  The tailstock has been run up and set in the little piece of MDF I talked about earlier perfectly centering the piece.

To rough out the bottom I decided to turn the bowl at the very low speed of 49 RPM’s and use the die grinder to get a flat albeit fairly rough bottom.  It’s pretty simple to do, I set the tool rest parallel to what I want my finished angle to be, clamped a guide board to its back and run the grinder back and forth.

From there I can do the final turning of including the last inch of the outside, smooth the bottom and turn the center recess smooth then do the final sanding.  Here you can see one of the glue blobs from when I glued the bottom ring together.  Because I had put a finish on the center insert before assembling the ring the glue just popped right off with no problem.

To get from the rough stair stepped blank to a finished form requires a fair amount of material removal.  When I was doing the inside, I cleaned the shop up at the end of each day.  When I did the outside, I did not.  The result is a big pile of shavings that would make some really, really good tinder for starting a fire.

With the turning completed I just had to remove the MDF center block that was held on with double face tape and do some finish sanding.  Also, as I was turning the bottom I decided to add a little rim and recess. 

Here is the bowl sanded and all ready to be finished.

As I said before I debated on the finish going back a forth between my favorite finish, matte sprayed lacquer and Danish Oil Finish.  I finally decided to use Danish Oil Finish as it will give me a nice low-luster finish bringing out the grain of the cherry and walnut.  It does take longer than lacquer where I can do two or three coats a day the Danish Oil Finish will probably take me the better part of a week to do.  That’s because I want to put at least three coats on.  I will put one on the inside plus about a third of the outside and wait overnight for it to dry then the next day do the rest of the outside and wait overnight for it to dry, then repeat.  I probably could push it a bit and put one coat on in the morning and one late in the afternoon but it’s late summer here in the desert and the afternoon’s get hot.  Besides, I am not up against a deadline so I can take my time.  The photos below show how I set the bowl for the inside (left) and outside (right) application process.

The bowl ended up being about 18 ½” in diameter and right at 12” tall.  That’s a little smaller diameter that I had initially intended but the final shape was determined by what looked right.  To give you an idea of what it looks like here is the bowl with a 2-liter bottle of soda next to it.

After a week of applying the three coats of finish and letting it set to cure for a couple of days here is what the completed bowl looks like.

I enjoyed making the piece but right now am not sure where it will end up.  It’s too big to go most anywhere so I will have to give it some thought.  I had thought I would run into more problems than I did due to the size so that was a good surprise.  The one thing that was a little surprising was when I got the top and bottom halves glued together how heavy the whole assembly was.  However, after turning it to final size and thinning the walls down it is not heavy at all.

My lathe has a second set of mounting holes on the legs that are offset from the main bed.  Doing some measuring I find that if I really wanted to I could remount one of the bed extensions to the alternate mounting holes and turn a piece almost 38” in diameter.  That would be a whopper and probably so heavy I probably would need a hoist just to move the blank.  Remember that just because you can does not mean you should.  On the other hand, it’s like a mountain that just has to be climbed.

Large Segmented Bowl - #6 Gluing the Halves Together & Turning the Outside

Gluing the two halves together follows the same process as adding layers.  I need to align the offset between the mating rings then clamp it all together.  The one difference is the centering alignment is more critical here as I want that interior joint to align as close to perfectly as I can get it.

Once the glue cured I removed the clamps and the small face place from the top stack.  Then it is time to mount the whole stack on the lathe.  That’s easier said than done as the blank is fairly heavy and pretty awkward.  Once mounted I put a live center with a flat plate end in the tailstock and ran it up against the MDF disk.  The live center applying pressure against the MDF disk will help kill the vibration while I am doing the turning.  The flat plate is because as much as I tried the top stack was not exactly aligned with the bottom stack.  It was probably less than 1/32” off but the existing center point of the MDF disk did not exactly line up with a pointed live center.  Here is a photo of what the tailstock looks like. 

To get a feel for how the full blank turns I started out at a slow speed, like 50 RPM’s and gradually increased the speed up to about 350 RPM’s.  That may not sound like a lot but remember the blank is about 19” in diameter so that outer edge is whizzing by at over 1,700 feet per minute.  Surprisingly the blank ran very smooth and I had no problem in truing the outside of the upper stack to match the bottom stack.

With that initial work done I started bringing the rim and the top 3 layers into shape.  After a little turning I took the MDF disk off and did a check with a pair of calipers to see how the wall thickness was progressing.  Having just started the outside shaping I had plenty of room and so far, no surprises.  Here the left photo shows the whole bowl and the right one is a closer look at the initial shaping.  Note the sacrificial ring is still attached although there is only about ¼” material left holding it to the top ring of the bowl.

As I turn the bowl to its final shape the process is to cut away some material, take off the MDF disk, measure the wall thickness, mark the outside with reference marks showing areas still too thick, put the MDF disk back on, run the tailstock up against the MDF disk then turn away more of the wood and repeat.  Not really hard but just a little tedious and time-consuming.  Below is where I am not quite half way done working on the outside. 

There are two things to keep in mind while doing the final turning.  One is keeping an eye on maintaining an even wall thickness which I do with calipers.  The other is making sure the outer surface is not only smooth but that the curved surface is just that a continuous curve with no flat spots, hills or valleys.  That is a lot harder than it sounds as there are no templates, no jigs to hold the cutting tools and no line to cut to.  It is all done freehand by eye.  That said I do use a couple of aids – one is my fingertips.  Lightly running your fingers from top to bottom will let you know a lot about surface irregularities even when you can’t see them.  Another method is to take something like a credit card or some sort of straight edge and hold it along the top to bottom axis then rock it back and forth, it should rock smoothly.  If there is an inconsistency in the curve the rocking motion will not be smooth.  Here I am using a card scraper to check. 

When I find a spot that is just a hair off and needs help I turn the lathe to its lowest speed, in my case 49 RPM’s and shave the high spot off or if it is a low spot work both sides until I get a smooth transition.  I use the card scraper because it is flexible and I can put a little curve in it which helps even things out.

With the outside of the top half of the bowl finish turned I am now close enough to the bottom of the bowl I do not need to keep the MDF disk attached to the top to kill the vibration while I am turning.  As such I can part off the sacrificial top ring.  Here in the left photo you can see where I have very carefully started the cut.  I continued cutting until I could start to see some gaps then quit and just pulled the ring off.  The right photo shows the thin sliver, barely 1/32” thick, that was all that was left holding the ring on. 

Getting ready to turn the leftover ridge off and down flush with the bowl surface it occurred to me that a little raised lip at the edge might help define the rim.  A little delicate work and I had the transition both inside and outside nearly done.  Here is a closeup of the lip and the entire rim.  I think I will keep it. 

Once I had the top done I went back and started working my way down the last half or so of the bowl.  It’s the same process as the top half except I don’t have to mess with taking the MDF disk on and off so it goes faster.  Here I have the shape roughed out but the walls are still about an eighth of an inch or so too thick.  From here I keep turning and measuring to get the wall thickness and the shape right until I get to about the last three bottom layers. 

Next up – Final Turning & Finishing