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Extending the Blue Bowl process to irregular green turned bowls with Bondo
This article was published in the Spring 2010 edition of Woodturning Design.
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Blue Bowl Reversing
Introduction
I know—you’ll read down a little further and think “hot-melt & tape? That will never work for bowl reversing work”. I was rather skeptical myself when I thought of it. But since I believe if all the new things you try work then you’re not trying enough new things I tried it anyway. I was led up to it in small steps. First I tried using masking tape & hot-melt to hold small flat Christmas Ornaments to a backup plate where I had just been using the hot-melt glue. It held wonderfully, was about the same to remove, and much easier to clean up. Then I had to clean up the bandsawn back surface of an ornament variation which was tear drop shaped. I wanted to hand plane it but it wasn’t easy to clamp—so I put some masking tape on the front of the ornament blank and my work bench, and tacked it down with some hot-melt dots. It easily held on during planning and came off easily and cleanly.
I tried turning a domed pad out of construction lumber and tried it on a medium sized bowl. To my surprise it worked wonderfully. I tried remounting and removing another bowl with a few variations and the only improvement I found was turning a semi-donut made it easier to guess the contact points. Briefly, the process is this: Prepare a semi-donut of construction lumber that’s sized to fit inside the bowl. Turn your bowl normally. Cover both the inside of the bowl and the donut blank with blue (or other extended release) masking tape and lightly sand the tape. Put some dots of hot-melt glue on the semi-donut and glue it roughly centered in the bowl. Then turn a tenon for your chuck on the back of the semi-donut. Reverse mount the bowl and turn the foot. Then pull the bowl off.
Consider these benefits:
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There are no centering problems as the reverse mounting tenon is turned while the bowl is still in its original mounting. This is especially helpful when turning outboard when no tailstock center is available to help center the reversed bowl.
-
You can work on the bowl reversed as long as you like or take it off to work on something else without incurring centering problems. Nor do you need to worry about power failure causing the bowl to fall off its mounting as with a vacuum chuck.
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The semi-donut cannot slip and burn rings in the inside of your bowl.
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Because the bowl is held close in to the middle of the bowl instead of by the probably thin and floppy rim it is more solidly mounted and heavier cuts are possible. If your last cuts on the rim (or potential last cuts if you finished the rim while the rest of the bowl was still thick) were vibration prone so too will be the cuts on the foot if you hold by the rim.
Thinking tape and hot-melt would be too insecure? Consider these points:
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The P in PSA abrasive disc, for instance, stands for Pressure. Pretty much all tape requires pressure to adhere securely and to a certain extent the bond is proportional to the pressure applied. If you apply masking tape to a surface and burnish it with a half inch diameter finger tip with say 20 pounds of force you’ll be applying about 100 psi. If you use double stick tape and a 4” diameter block even if you stand on the block 200 pounds worth you’ll only get about 16 psi.
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Hot-Melt glue isn’t pressure sensitive and fills gaps nicely so you don’t need a completely matched fit.
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Tape resists shear force well, but gives in relatively easily to tension. This fits the need for bowl reversing well. Vacuum chucks, on the other hand depend on friction for resisting shear.
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You’ll be making heavier cuts forming the tenon on the semi-donut than you will on the foot of the bowl. So if the tape/hot-melt bond survives turning the tenon it mostly likely do fine turning the foot.
Can I guarantee the glue/hot-melt will never fail? Nope. But neither can I guarantee the tenon you turned the bowl with in the first place will survive either.
Semi-Donut
Begin by making a Semi-Donut before you start the bowl—unless you have two lathes—or need an excuse for one. Construction lumber will do fine. If the future bowl is deep, remember that you need to be able get the key in the chuck so you may need to glue up a thicker blank. Pick a size for the Semi-Donut that will fit in the bowl well—mostly on the flatter bottom if possible. Draw the diameter you want on construction lumber or other cheap wood and knock off the corners with your band saw. Pin the blank to a faceplate or chuck face with your tailstock as in Fig01.
Fig01:
The Semi-Donut blank pinned to a 4 jaw chuck for turning.
You could also drill a shallow hole with a big Forstner bit and grip the
blank directly with the chuck.
Turn the rim of the blank more or less true and the round over the edge. Then reduce the thickness of the center area as in Fig02—this will make where the semi-donut will contact the bowl more predictable. Either turn away the nub or get rid of it with a chisel after removing it from the lathe. The completed Semi-Donut is shown in Fig03.
Fig02:
After turning the Semi-Donut to shape.
The rim is rounded and the middle dished out.
Fig03:
The completed Semi-Donut.
Mount & Tenon
Now go ahead and turn your bowl. You can apply a film finish such as lacquer if you like to finish on the lathe. Leave the bowl mounted by the original chuck/tenon or faceplate as in Fig04. Cover the inside of the bowl to a larger diameter than the semi-donut with blue masking tape (or other extended release masking tape). Wide tape makes it easier. Burnish the tape with your fingers so that it adheres well. Likewise cover and burnish tape on the curved face of the Semi-Donut. Then sand the tape surface lightly with medium abrasive. The sanding step significantly increases the hold of the glue to the tape, so don’t omit this step. The result is shown in Fig05. If you’re unsure where the Semi-Donut will contact the bowl, use the side of a pencil lead to heavily mark a radial area of the bowl. Hold the Semi-Donut in place in the bowl and twist it back and forth some. Some of the pencil will transfer to the Semi-Donut at the area of contact.
Fig05:
After applying tape to the bowl and Semi-Donut.
The tape was burnished in by hand and lightly sanded with medium
abrasive.
Fig05.2 Pencil applied to the blue tape in the bowl.
Fig05.3 After twisting the semi-donut in the bowl some of the pencil has transferred to the donut, indicating where the area of contact will be. The red line points to the transferred pencil.
Heat up your hot-melt glue gun and make sure you have an extra stick of glue. Apply short radial lines of glue crossing the anticipated area of contact at intervals around the Semi-Donut as in Fig06. If you’re too paranoid and apply glue all over the Semi-Donut you may have trouble getting the blank off when you’re done. Quickly place the Semi-Donut into the bowl as in Fig07. You do want it more or less in the center, but it only has to be approximately centered.
Fig06:
Hot-melt glue applied to the anticipated area of contact of the
Semi-Donut.
Fig07:
After gluing the Semi-Donut to the bowl.
Put your tool rest in position and turn on the lathe at a moderate speed. Use a bowl gouge to form a tenon for your chuck as in Fig08. You can start out with gentle cuts to get a feel for how solidly mounted the Semi-Donut is to gain confidence. Hopefully you can tell from Fig08 that I’m taking a fairly heavy cut. If you use a faceplate instead of a chuck, turn a flat recess the size of your faceplate (this is easier to do if you have two faceplates). Fig09 shows the completed tenon.
Fig08:
Cutting the tenon with a bowl gouge.
You should be able to see I’m taking a heavier cut than will be required
to turn the foot.
Fig09:
The completed tenon.
Don Lindsley made the following suggestions:
"Before applying the blue tape to the inside of a bowl, I fold over about 1/4" at
one end, making a pull tab that helps when removing the tape later.
The second one - I note
that you turn the spigot (tenon) after the waste-block donut has been mounted in
the bowl which allows for perfect alignment, as you point out. If you have a
Oneway live center and optional revolving center chuck adaptor (I wouldn't be
without mine), you can turn the tenon and chuck the donut ahead of time, apply
the hot-melt glue, and advance the tailstock with donut into the bowl, thereby
getting a perfect alignment. That way the donut can be used again and again."
Form Foot
Turn
After you’ve formed the tenon on the Semi-Donut, remove the bowl from the lathe and remount it reversed as in Fig10. Turn the lathe on at a moderate speed and turn the foot as in Fig11. I think you’ll like doing it this way—certainly it feels more solid than any other way that I’ve tried. As there is neither tailstock nor tailstock nub to get in the way you can fully sand and finish the bowl foot at this time as in Fig12 and Fig13.
Fig10:
The bowl reverse mounted.
Fig11:
Turning the foot. It felt
very solid.
Fig12:
After turning the foot.
Fig13:
After sanding and finishing the foot.
Even Carve
You can even carve on the bottom of the bowl if you like. Fig14 shows carving with an Automach carver. Fig15 shows carving with a more impact prone chisel and mallet. There’s no picture of the finished carved feet because I never do that and they didn’t turn out. The point is you can.
Fig14:
Carving the foot with an Automach (reciprocal carver).
The 4-jaw chuck is just clamped crudely in my bench vise.
Fig15:
Carving the foot with mallet and chisel.
The Semi-donut still held well even under mallet blows.
Remove
Normal
It’s probably easiest to remove the bowl from the Semi-Donut while it’s still firm mounted in the chuck. Grab the bowl by the rim with two hands at 10 and 2 o’clock as in Fig16 and pull slowly but firmly. It should come right off. Peel off any remaining tape and remove any tape residue with mineral spirits. Your finish should be unharmed.
Fig16:
Removing the bowl by slowly pulling it off the Semi-donut.
Hard Cases
You may occasionally have trouble removing the Semi-Donut particularly if you used gobs and gobs of glue. If the tenon breaks you can screw a new one on and try again. Just don’t use screws that are longer than the Semi-Donut is deep. You could also loop some small (1/4”) rope or even Weed Eater line around the Semi-Donut and twist the loop with a stick until it pulls through.
You can also remove accessible tape and then soften the remaining tape’s hold with mineral spirits. As a last resort, protect the bowl with a small waste block and pry the Semi-Donut out with a large screwdriver. And then resolve to use less glue next time. Remember that if the tape/glue bond survives turning the tenon it most likely will survive turning the foot. And if it fails when turning the tenon it doesn’t hurt anything so there isn’t really much risk to being parsimonious with the glue application.
Materials & Tools
Blue Masking Tape (or other extended release)
Hot-Melt glue and glue-gun
Construction lumber
Author
David Reed Smith is a Basement Woodturner and Meme-Slave
who turns, tinkers, and writes in