Tearout on inside of segmented bowl

[Eric Meier]

Hey guys,

I've been doing segmented turning for about 2 years now, as a nice hobby and to make gifts for people.

Recently, I've been trying to hone my skill in a number of areas, and slowly but surely I'm getting better at a lot of things. But one thing that I can't seem to avoid is a peculiar tearout that occurs only on the inside of a segmented vessel: usually right at the point where two segments meet. Hopefully you guys know what I'm referring to.

I'm not sure how to avoid this, and its very hard to get rid of. I think one of the main problems is just that I have an old lathe, (1980s craftsman), and I can't really get any other tool rests, such as an S-curve one for doing the inside of bowls.

I try to use the bowl gouge for most inside work, but I also use some round scrapers too. I don't think I always have the angle right though---possible relating to the straight tool rests and the poor angle of the tool. I'm not 100% sure what is causing the tearout. I try to keep my tools pretty sharp.

Any pointers?

[Duff Bement]

I would try grinding a scraper to fit the curve of the bowl and sharpen it right befor you take last VERY light cut. Also make sure you keep the edge of the scrapper at or slightly above the center line and tipped just a touch so the burr can take a very fine cut.

[Harvey Crouch]

Eric, I have experienced the same thing. I believe it is not tear out but a vibration as you try to turn near the edge of the bowl half. The vibration causes the gouge or scraper to bounce causing irregular scooping looking much like tearout. I solved my problem by placing a bowl steady rest right behind where I am turning. This eliminates the vibration and allows a smooth cut. Hope this helps.

[Richard Madison]

Eric,
On some of the segments you will be cutting "against the grain", or up grain instead of down grain. Need very sharp tool, very light cuts, can try shear scraping, and tool rest as close as possible.

[Steve Schlumpf]

Eric - I have not done any segment work - yet - but this is one of those times where a few photos would really help folks to diagnose the problem.

[Bob Vallaster]

Eric,
I had the same problem with a run of 7 bowls for the Christmas season. The segmented layers were octagons (maple) with detail rings of 24 segments (mixed maple&cherry or maple&walnut). The damage occurred at the vertical junction of segments in the octagons, and did not look like typical end-grain tearout so much as it looked like chunks lost at the edge of a segment.
My first thought was "Nice glue-up, Bob." But with plenty of wall thickness to go, I experimented. Sharpening didn't remedy the problem. Scraping didn't either. Raising the tool rest helped a bit. Pulling the cut (bottom-to-rim) helped a bit.
If you will put a pencil to paper and draw a segment with its grain orientation, then overlay an arc which is the inside of your bowl. As a gouge cuts the arc, it first cuts downhill...then uphill.
Although I expected the chipout at the uphill portion of the cut (segment trailing edge), I observed more damage at the reversal to downhill (segment leading edge). There is an instant reversal of local grain as the cutter (gouge, scraper) crosses the glueline. This reversal is more abrupt on an 8-sided layer than on the 24-sided. I think Richard has the right idea.
In the end, I was able to cut away the damage and get clean walls with a higher tool rest position, contact point around 10 o'clock, and very light pull cut. (The tool handle was on the back side of the lathe and the gouge rolled well over to the left.)

Good luck with your experiment.

BobV

[Richard Madison]

Bob,
Could you show us a picture of your octagons? Would like to see them. Anyone else want see Bob's octagons?

[Bob Vallaster]

Richard,
Sorry. The digital camera is hard down.
As for my octagons: conventional. I was writing of the segmented rings which are the layers of the bowl, not a high Wow!-factor accent.
I mentioned the segment count because it appeared the higher chord-to-chord angle with fewer pieces in a ring seems to correspond to higher chipout possibility.
Given this winter's experience with chipout, I think future bowl stackups will consist of 12 or more segments per ring.

BobV

[Richard Madison]

Bob,
Some of my brain cells were obviously down too. Somehow I made up a story for myself that you were making octagonal segments, not octagonal rings. With eight segments per ring, turning the o.d. there would be good chance of ripping the corners off some segments. I mostly do 16 segment rings, but only 12's for compound staves. Should do more on some things. Apologize for my mental hiccup, and it's only gonna get worse with time.