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Thread: Help!

  1. #1
    Join Date
    Aug 2007
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    Help!

    I'm turning a live edge mortar and pestle. I have the outside finished to perfection but I can't get the inside completely smooth. This is sanded to 320 and you can still barely feel the tearout. Should I take another pass with a fresh bowl gouge or do a carbide scraper?
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  2. #2
    Hit it with the 80-grit gouge.

    If you have anything other than de minimis tearout after you think you're done with your lowest grit, you need to keep sanding with that grit, or even switch down to a lower grit. Power sanding with the lathe off works also (especially for natural edge bowls which are hard to sand any other way).

  3. #3
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    For tearout I do different things depending on the wood and the depth and extent of the tearout.

    I think another finish pass with an extremely sharp gouge is the best, unless the wood is very soft in places.

    If the wood is prone to tearout, I will apply at least one coat of sander sealer. If the wood is REAL punky I might stabilize it with thin CA glue. (Wouldn't make a very good morter/pestle though!) After the sander sealer is dry I will try another finish cut or a rounded NRS.

    Tearout can often be removed with a round-end negative rake scraper without the sander sealer. I like to use these with burnished burrs:
    scrapers_neg_rake.jpg

    Before I resort to sand paper I will use hand scrapers to remove tearout (and tool marks). After hand scraping I can usually get a clean surface with 220 or finer sand paper. I have other scrapers now but this older photo shows some I still use (used some tonight):

    scrapers_.jpg

    Wet sanding by hand with oil another finish will often take care of shallow tearout. The problem with sanding deep tearout is it will take forever and often compromise the intended shape of the surface, especially power sanding.

    I learned a trick from John Lucas for small spots of tearout - wet sand with thin CA glue and fairly coarse paper (by hand, of course). I used that today on a small spot on a cherry piece. Took care of it quickly.

    JKJ

  4. #4
    Quote Originally Posted by John K Jordan View Post
    Tearout can often be removed with a round-end negative rake scraper without the sander sealer. I like to use these with burnished burrs:
    scrapers_neg_rake.jpg
    John, I'd never heard of that, and they appear to be curved in the wrong direction for the spin of a lathe (mine, which has no reverse, at least). Is the burr on that bevel, and the cut takes place there?

  5. #5
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    Roy, you are seeing the tops of the "negative rake". Think of a skew layed on its side but with the bottom at a steeper angle so that the included angle is about 80 degrees. The negative rake allows the tool to set horizontal instead of angled down and makes it easier to use.

    James, since the piece is to be used as a mortice and pestal set, if the wood is too soft or punky, I would find another piece. If it is sufficiently hard I would tend to go with first a clean *sharp* bowl gouge, or fresh scraper. Then, when no tool marks and most of the tear out gone, oil sand with walnut oil.
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  6. #6
    I agree with Thom. If you just try to sand it out you will have ridges/waves in the wood from over sanding. The ridges are caused by the hard wood and soft wood of the growth rings.

  7. #7
    Thanks, Thom. Never considered trying that, but will keep an eye out for a reverse grind tool to give that a go.

  8. #8
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    Quote Originally Posted by Roy Petersen View Post
    Thanks, Thom. Never considered trying that, but will keep an eye out for a reverse grind tool to give that a go.
    Roy, Thom explained it well. You use these laid flat and tight against the tool rest with a short overhang if possible. Unless I'm working deep within a bowl and need a longer overhang I don't use these in handles, I just hold the steel. For smoothing the touch must be VERY light otherwise they will take off a lot of wood.

    I would add that there is a burr on the negative rake scrapers to make them work. Most people grind the scraper and use the burr from the grinding wheel to smooth the work. What I do instead of raise a burr with a burnishing carbide rod in a handle:

    burnisher_IMG_6767.jpg

    I believe the burnished burr lasts longer than the rougher grinding wheel burr. The grinding burr may cut better at first, partially since it is toothed and ragged, but it is soon knocked off the the tool must be ground again. I might use the grinding burr for a short while then hone it off with an extra fine diamond hone, then burnish a burr. The burnished burr is smoother and leaves a smoother surface with fewer scratches.

    If using the grinder burr I would have to go back to the grinder after a short use. If using a burnished burr when it wears and quits cutting I can restore it numerous times before I have to grind again. For me, that alone makes it a no-brainer.

    I keep several scrapers shaped like this and although they are almost identical, they are definitely "one sided" tools, depending on which side the burr is on - they can be used with the cutting edge to the right or the left. I generally keep a couple prepared for each direction so I can use them inside or outside as needed. I write a "T" on the top side so I can pick one up and use it without having to feel which side has the burr.

    You don't have to look for one of these to buy but you can get one that Glenn Lucas sells http://glennlucaswoodturning.com/pro...-rake-scraper/, or from Craft Supplies. I ground all of mine, one from a spare Thompson skew and some from Thompson scraper blanks. I use the general profile Glenn Lucas uses but modified it a bit to make a slightly curved but mostly flat tip that works better for some uses, for example I use the tip to help smooth the wings of squarish pieces where part of the time the tool is "cutting" air, something I wouldn't try shear scraping with a gouge. I use these a lot, for example on a piece I finished last night and with a student this morning. Pictures later.

    JKJ

  9. #9
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    Since the tear-out is on the vertical part of the interior I would hit it one more time with a gouge. A scarper will help you on the bottom and making the curve (if you're having trouble with a gouge here), but for the vertical part the gouge is still going to work best.

    Sharpen your gouge, turn the speed up (it's a small bowl, so high speed will help) and take a light pass or two. Be sure to ride the bevel.

    Once you have tear-out, using 80g will just continue pulling the grain and sometimes makes the tear out worse. Nothing works better then a properly presented, sharp gouge.
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  10. #10
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    Mortars shouldn't be slick on the inside so don't sweat it. They need coarseness so you can leverage against whatever you're grinding otherwise it will just slide out from under the pestle.

  11. #11
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    Thank you all for the suggestions! When I get back in the shop tonight I'll give them a try. The wood is hard maple so it's hard all the way through. There is some staining or spalting but its not punky at all. I think I'll try to do another pass with a fresh gouge then clean it up using a round negative scraper.
    Those who dream by day are cognizant of many things which escape those who dream only by night
    -Poe

  12. #12
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    Quote Originally Posted by Rick McQuay View Post
    Mortars shouldn't be slick on the inside so don't sweat it. They need coarseness so you can leverage against whatever you're grinding otherwise it will just slide out from under the pestle.
    I know some of them are. However the medical one I use quite a bit here on the farm is quite slick - both the mortar and the pestle are made of clear glass, polished surface, not ground. I suspect how well it works depends on what is being ground. I use this one mostly to crush pills into powder to mix with animal feed (instead of inserting the pills into the back of the throat).

    JKJ

  13. #13
    Quote Originally Posted by John K Jordan View Post
    For smoothing the touch must be VERY light otherwise they will take off a lot of wood.
    Do you find they're more aggressive that way than a normal grind/burr?
    I would add that there is a burr on the negative rake scrapers to make them work. Most people grind the scraper and use the burr from the grinding wheel to smooth the work. What I do instead of raise a burr with a burnishing carbide rod
    I've never used one (burnishing rod), so likely ground away far more metal than I needed to on my tools. I've gone through a good many heavy bowl scrapers (perfect for inside goblets and steins) over the years as a result.

  14. #14
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    Quote Originally Posted by Roy Petersen View Post
    Do you find they're more aggressive that way than a normal grind/burr?

    I've never used one (burnishing rod), so likely ground away far more metal than I needed to on my tools. I've gone through a good many heavy bowl scrapers (perfect for inside goblets and steins) over the years as a result.
    Not more aggressive, probably less aggressive than conventional scrapers. I guess my point was a NRS is great for smoothing with a light touch but it can take off a lot of wood if you use pressure. (I am careful not to use pressure when holding the bare tool with no handle!) The burnished burr works much like the grinding burr but I think it lasts longer and cuts smoother.

    I found that by burnishing I can adjust the burr to be a more aggressive or very delicate by angle and the amount of pressure from the burnisher. For years I used a much larger diameter carbide burnishing rod but the smaller rods I got from Reed Grey don't need as much pressure for the same effect. BTW, you don't need a carbide rod to burnish - the shaft of a turning gouge works as well.

    Maybe try grinding one. I made some from 3/4" wide tool stock and some 1" wide with slightly different curves. I've also ground negative rakes on most of my other scrapers including straight, round nosed, and box scrapers. I ground a variety of very small NRSs from Thompson round and flat rod and even one from a 3/8" Thompson detail spindle gouge. These have become nearly indispensable for detail work especially on smaller things. They leave a glass-like surface on hard woods like many exotics and domestics like persimmon, dogwood, and osage orange, especially nice to use on end grain such as when turning boxes. (I mostly use a different grind on these small scrapers)

    Negative rake scrapers are much more forgiving in use and difficult to make them catch - some say impossible to catch. (You can easily get a epic catch with a big conventional scraper!) An even bigger advantage is the ease of use since instead of holding the handle high and the cutting edge low as you do with a normal scraper these can be held completely horizontal on the rest.

    JKJ

  15. #15
    Great detail, John, thank you!
    So many tips that I've never heard of to be found in this forum. Why it took so long to find my way here, I have no idea but I could have saved a lot of time rather than the trial and mostly error that made up my original learning curve (which isn't over, by a long shot).

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