Inside of bowl sanding problem

[Brice Rogers]

Bob, thanks for posting the Sorby video. It has given me a bunch of ideas for making a bunch of insert cutters.

[John K Jordan]

...
Like this (you don't need their tool to do it...):
[Sorby multi-tip tool video]
After that you just gotta break out the power sanding.

That's the tool I've used for years, with the teardrop cutter - works very well. Lately I've also been using Thompson scrapers ground with a negative rake like this:

scrapers_neg_rake.jpg

If I got sloppy with the gouge and left a ripple I might use a hand-held scraper first. Between the bowl gouge and the scrapers I seldom have to use sandpaper coarser than 220. I haven't used 100 or 80 grit paper for a long time. Much of my sanding is by hand with the lathe off. When I do power sand I use the Grex pneumatic random orbital sander. Very controllable, very nice, needs big air compressor.

If the "scratches" are really tearout, start over with a sharper gouge and cut the tearout away. Sanding tearout away is not what I want to do with my life.

JKJ

[Bill Jobe]

I used a negative rake I picked up a an estate sale. Having trouble getting it just right. Sometimes it wants to grab, so maybe I'm not sharpening it with enough rake.
It's too late for this bowl. It's down to about .200 already. Perhaps if I told folks I was shooting for rustic......

[robert baccus]

I went through the transition zone thing years ago--used air grinders with 2" discs with slow sucess. Then I tried grinding up a 1/2" bowl gouge until the bevel would touch in the zone and across the bowl bottom. Problem fixed--it cut perfect curls with no bruising--others named it a bottom feeder. This ends up being a70-75 deg bevel, don't be shy here. The "transition zone in my humble opinion is simply where the bevel comes off the bowl side when turning toward the bottom. Try it. Industrial plywood knives--8' long-- peeling round bolts cut finer than this--hard to believe.

[Bill Jobe]

Thanks for all the advice and links. I'm maybe going about it wrong. There's a turning club fairly close. I should join up. Been going to but my calendar is all over the place. Can't seem to get leveled out.

[John K Jordan]

I went through the transition zone thing years ago--used air grinders with 2" discs with slow sucess. Then I tried grinding up a 1/2" bowl gouge until the bevel would touch in the zone and across the bowl bottom. Problem fixed--it cut perfect curls with no bruising--others named it a bottom feeder. This ends up being a70-75 deg bevel, don't be shy here. The "transition zone in my humble opinion is simply where the bevel comes off the bowl side when turning toward the bottom. Try it. Industrial plywood knives--8' long-- peeling round bolts cut finer than this--hard to believe.

I posted this recently, perhaps on this forum: Chris Ramsey showed us how to grind what he uses on the bottom of a cowboy hat - a bowl gouge ground by hand (no jig) to remove almost all the bevel. The only bevel is the tiny bit I outlined in red:

Ramsey_gouge_comp.jpg

I tried grinding one like this and it really works.

JKJ

[Bill Jobe]

A half inch is the size I used, Robert. Just didn't get it right.

John, that's about what I roughed it with, then used a half inch negative rake gouge.
I'll give that profile a try.

The many, many years as a lathe operater machining all kinds of metals did not exactly slide me into turning wood as smoothly as hoped.

[John K Jordan]

... that's about what I roughed it with, then used a half inch negative rake gouge.
I'll give that profile a try.

Ok, I'm confused now. I've never heard of a negative rake gouge. Do you mean a negative rake scraper?

A NRS will in fact clean up a lot of problems but there are some cases, probably related more to the specific wood than anything, where a scraper makes things worse. Consider that tearout is where fibers have been pulled out (ripped out) of the wood. The voids can go far (relatively) below the surface, far deeper than the amount of wood that can be removed with a light finishing pass with a tool used as a shear scraper. As a scraper eventually takes off enough to get down towards the bottom of the voids, more fibers can be torn out and you might NEVER get it clean! (Scraping with a heavy hand or at a bad too presentation can also make tearout worse.) Some woods and areas can be far worse than others about this. (I know of no analogy in metal working except for sub-surface deformation which can cause pitting during polishing.)

To "fight" back, there several strategies I know of, some which work better than others with specific woods and for certain people. Caveat: These are not necessarily universally accepted practices - just what works for me.

- The best is always to cut the wood cleanly with a very sharp tool - not scraping but cutting, in the "bevel rubbing" mode. This very much depends on the turner's skill. Depending on one's skill at, as Clay Foster puts it, "guiding a gouge," cutting cleanly can leave a surface that needs only very fine sandpaper to clean up, if that. This is FAR more difficult as the size of the work increases since guiding the gouge perfectly over longer distances requires incredible tool control. It's also more difficult as the tool is extended more over the rest to reach a deeper area. Far easier for small pieces. Last night I turned a form from a small piece of ebony (2x2x3") and was able to get it smooth enough on the outside with the gouge to start with 600 grit paper. I can't usually do this with a bowl or platter.

- Cutting cleanly with a sharp gouge after reinforcing the fibers with sanding sealer as described below. This can help with problem woods or problem areas prone to tearing. I brush on a coat, let dry, make a cut, repeat if needed.

- Use a sharp scraping tool with a very light touch while the work is spinning. As Chris Ramsey put it, just "whispers" of shavings should come off the edge. Any more and you are risking tearing more fibers. Ideally the scraper should have a burr on the edge. If done right, this burr appears to make a micro cutting edge almost like a gouge, one you can feel with your finger. Some people rely on the grinder to make the burr but I like to remove the grinder burr and raise an edge with a carbide burnisher, just as done with a traditional cabinet scraper. A freshly sharpened gouge also works for this but not easily on the bottom of the bowl or in the transition, but often useful on the sides and the outside.

- Scrape by hand with the lathe off using special scrapers made for holding in the hand. This is often my method of choice, especially for the bottom of the bowl and for "spot" treatment of tearout, less so for the curved transition to the sides unless it is just in one small area. Amazing for large gently curved surfaces inside a platter or shallow bowl.

- Scraping with any method after putting something on or in the wood to either reinforce or soften the fibers.
---My favorite methods are to apply a coat of thinned shellac or lacquer or pre-packaged sanding sealer (which is thinned shellac or lacquer). Let this dry and try scraping, repeat if needed. Great for mildly punky wood which can tear out horribly. Also wonderful for the very fine type of tearout that looks like is on your bowl.
---If the finish will be oil, applying oil before scraping can often fix things.
---Some people use paste wax to soften the fibers.
---For horrible tearout, say in very punky wood (I'm talking deep holes) I have found nothing better than dousing it with very thin CA glue. There are many issues with this method but it can really, really work. I've had wood so soft and punky that chunks would come out from my thumbnail that after the CA treatment turned and finished to look like glass.

- Filling in the tearout with sanding dust. This can work wonders and be an invisible fix in certain situations, especially in wood with uniform color and contrast instead of figure with strong contrast.
---My favorite method if finishing with oil: wet sanding with oil (by hand). The oil makes a slurry with fine sawdust which can fill in very small tearout voids. When allowed to dry the dust is bonded into the holes and can be invisible. Which sandpaper to use is a judgement call, sometimes finer works better than coarse. Some people wet sand with other liquids. I have on occasion used water or alcohol.
---A method I learned recently from expert turner and good friend John Lucas: wet sanding with CA glue. This might sound crazy but it really works for some cases. You apply a bit of CA to some sandpaper (not the work) and sand the area. This can fill the tiny voids like wet sanding with oil but is a lot quicker since the CA sets up quickly. It will also work better for (relatively) larger voids and cracks. There are also several issues with this method, for example the possibility of gluing your fingers to the sandpaper.

- There is also stabilizing the wood by infusing it with something. Beyond the scope of most people, especially on larger things. This saturates the fibers with something akin to plastic. In general stabilizing not needed with bowls, especially with minor tearout such as yours.

This is all I can think of right now and I need to go move horses to the pasture. But as mentioned often, nothing trumps a clean cut with a sharp gouge. [Warning: soapbox mode] For a beginner turner, the best advice I've heard from the experts is almost never followed by turners whose focus is to turn amazing bowls: Learn turning by first becoming proficient at spindle turning. Experts say this will teach the fine tool control that will let you turn anything. I have references.

(Sorry, I can type faster than I can think and don't have time to proofread at the moment. I hope some of this make sense!)

JKJ

[Reed Gray]

Well, as others have said, those marks are from the tool, and not sanding. If you are already ground off the heel of your tool and still get the marks, you are rubbing too hard. An exercise I recommend for improving here is the one handed push cut, where only the handle hand is used on the gouge. The other hand is completely off the tool, or resting very lightly on the tool, not gripping it at all. The lighter the push, the less burnishing you get... "The bevel should rub the wood, but the wood should not know it."

To remove all tool marks, I prefer a shear scrape cut, and have developed a liking for a burnished burr for the edge rather than the burr from a CBN wheel. On the inside, a ) shaped nose scraper, handle lowered a bit, and very gentle pull cuts. It will take several passes to remove the marks. I hope to shoot a video on that soon, but there are some others up on You Tube. I do things a bit differently....

robo hippy

[Bill Jobe]

"I know of no analogy in metal working except for sub-surface deformation which can cause pitting during polishing". JKJ


There is and I believe it is referred to as "friction welding". (humor)

[John K Jordan]

"I know of no analogy in metal working except for sub-surface deformation which can cause pitting during polishing". JKJ


There is and I believe it is referred to as "friction welding". (humor)

Ha ha.

I used to run a small metallography lab, among other things tasked with checking the grain growth across pressure- and temperature-bonded layers of aluminum that formed cladding for highly enriched uranium reactor element cores by cutting, potting, polishing, etching, measuring, and photographing sections from finished element plates. (That was a mouthful!) I discovered that sub-surface deformation caused by scratches from coarse grits could cause pitting to show up in the polishing step. There was no shortcut - you had to work through the grits and cut down to undisturbed metal below the deformation. Big surprise.

I have a theory that a similar thing can happen with certain woods: a coarse sandpaper grain can not only make a scratch but deform the wood just below the bottom of the furrow. If just the visible scratch is removed a faint line may show up later in the final surface. I've never carefully tested this theory.

JKJ

[Ken Glass]

The ideal solution is not having to sand very much, by using the the best tool control and the right tools.That's not always possible depending on the wood. ..... When I use a gouge or hollowing tool to form the inside of a bowl, I always take the last few passes with an inside scraper which mostly produces very small curly shavings. That will allow you to sand starting with 320-400. Here is an example of the type of Scarper I use.

[Bill Jobe]

That's similar to the tool I used on this bowl as a final cut. I think the problem is in my sharpening skills.
I'll post a pic of that tool shortly.

[Ken Glass]

If sharpened right it is called a negative rake scraper and the only edge touching the wood would be the top edge. Here is a better look at the edge of a Negative Rake Scraper.