Finishing Bowl Bottoms

Discussion in 'Tutorials and Tips' started by John Chianelli, Jan 28, 2009.

  1. John Chianelli

    John Chianelli

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    I use Oneway Jumbo Jaws to finish turn and sand the bottom of my bowls but invariably find that once a bowl is reversed chucked into the Jumbo Jaws it warps the bowl out of round and any tool cuts from the bottom up are out of round with the rest of the already finished bowl. Does anyone have any suggestions on how to keep the bottom of the bowl true to form when reverse chucking? Thanks much for any input......
     
  2. Malcolm Smith

    Malcolm Smith

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    Are you turning seasoned wood? If you are not, it is simply changing as it dries on the lathe. Even if it is carefully seasoned wood, there can be some movement as the wood dries a little more from being turned and sanded. Also, some woods have internal tension which is released allowing the wood to move as one turns it.

    When one reverses the bowl to re-turn the bottom, it's helpfull if there is a center mark or indent that the tailstock can return to. This assumes you started on what is to be the base with the blank between centers before chuck or faceplate mounting to turn the bowl interior. With the bowl mounted this way to turn the base, it can be finished with the least amount of irregularity.

    It's fact that wood moves. One can keep this to a minimum. If you're not doing it, twice turning is a very good procedure.

    Malcolm Smith.
     
  3. MichaelMouse

    MichaelMouse

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    Assuming you're speaking of dry material, though you can do similar with wet material and settle for an oval. Just use a smooth dovetail grip for reversing the piece for hollowing. That way you can sand up the bottom, put a ruffle or flourish in it for the other turners (customers don't care) and reverse to turn the inside. Lazy man's way.

    http://s35.photobucket.com/albums/d160/GoodOnesGone/?action=view&current=BottomHoldProcess.flv

    http://i35.photobucket.com/albums/d160/GoodOnesGone/Birch-Back-Oil.jpg

    I do my TDT stuff the same way, but I don't embellish. http://i35.photobucket.com/albums/d160/GoodOnesGone/ContactSheet-1-1.jpg

    http://i35.photobucket.com/albums/d160/GoodOnesGone/While-The-Glue-Dries.jpg This will (did) ovel slightly. I used some stickyback sandpaper on a board to get it to sit flat before finishing.
     
    Last edited: Jan 28, 2009
  4. John Chianelli

    John Chianelli

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    Thanks for the input Malcom. I am turning seasoned wood that was previously rough turned and has achieved less than 10% moisture content. I typically do not use the tail stock when turning bowls. I suspect it is tension release you say. I was wondering if there is maybe a better way to reverse chuck it? Would attaching wood to the Jumbo Jaws and turning it to hold most of the rim make any difference? This seems to be a lot of extra work to make wooden jaw sets for different size bowls then attach them to an expanding faceplate such as Jumbo Jaws. Not sure if squeezing more rim is worth the trouble.

    BTW, looks like we are neighbors. I am in Tres Pinos, CA (San Benito Co.). Guess I should put this in my profile.....
     
  5. Malcolm Smith

    Malcolm Smith

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    There are two holding methods I use for nearly all the bowls I turn. My first choice is vacuum mounting. I have a Oneway lathe and use their vacuum system. If the regular holders don't work, I have round wood blocks on faceplates that are drilled in the center to allow a vacuum to be created. I turn a ledge that the bowl sits against. This seems to work very well. Having a vacuum system changes everything. Less often, I'll mount the bowl on the jumbo jaws. Either way, all the turning that can be done with the tailstock in place is done that way.

    I don't know what type of lathe you have, but if it can be used with a vacuum system, I would highly recommend getting one. I do realize they represent an investment.

    How are you measuring the moisture content? I have an electronic scale designed for weighing refridgerant bottles. It is extremely accurate. I periodically weigh the blanks and first turned bowls and write down the weight and the date. When they stop losing weight, they are ready.

    Malcolm Smith.
     
  6. rsser

    rsser

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    How about avoiding the need for reversing to finish the foot altogether?

    So turn a tenon for the foot that's just a bit bigger than your min jaw clamp size. Clamp on that for hollowing; it shouldn't leave marks. Course it helps to have a range of jaw sizes.

    Or turn a recess in the foot to expand jaws into. Course you need to leave enough width in the foot to hold together, and that can be tight with small bowls and the foot width = 1/3 or bowl width rule of thumb.

    Hope this helps.
     
  7. John Chianelli

    John Chianelli

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    Yea, I have been starting to look at vacuum holders. They make good sense as they do not distort the bowl as clamping may and they certainly offer more flexibility. I have a PM3520B so no fit problem for a vacuum system. How do you like your Oneway system? I am sure it is quality as seems to be the case with their products. Interesting you can use a wood-to-wood interface to hold your work. I thought a neoprene gasket is required to created a solid vacuum.

    A use a quality Wagner MC meter that I have base lined using a similar weighing method you describe. The meter has not steered me wrong yet. If I do not have a specific gravity for a certain species I resort to weighing.
     
  8. Steve Worcester

    Steve Worcester Admin Emeritus

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    Are you taking the dry (now warped) bowl and with no other turning, putting it in the cole jaws rim down?
     
  9. John Chianelli

    John Chianelli

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    No Steve, I am second turning a dried bowl that has been trued, shaped and sanded, all except for the tenon and a little ways up the side of the bowls from the bottom. I have sand finished both inside and out and have reversed chucked it to remove the tenon and clean up a little. Straight across the bottom is not a problem, it is when you start moving up the side from the bottom where I encounter tool skip (indicating out of round from what was once round when held by the tenon). Now that it is held by the cole jaws I have trouble carrying the curve to the bottom with out showing a bumpy uneven kind of interface where the reverse chucked cut meets the original tenon held cuts. Ideally you take your cleanup cut to the rim but with cole jaws this is not possible. This I suppose is why the vacuum chuck has been suggested. Hope this makes sense.....
     
  10. Joe Greiner

    Joe Greiner

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    little way up the sides

    I've found the "little way up the sides" troublesome almost all the time. Plywood disk with a groove for the rim, and lately with Longworths. I don't think it would be much different with Cole jaws, or even with a vacuum chuck.

    My workaround: Turn the final shape all the way to the bottom. Reverse mount to finish the bottom. Shave a step, only about 1mm deep and about 1/8-inch wide (varies with amount of ovalling). Dish the interior of the bottom to create a foot ring for better stability. The foot ring itself is concentric, and the step is practically invisible from the side or above. The step also lifts the bowl just a wee amount for a floating appearance.

    This three-lobed bowl from a crotch blank was concentric at different stages of turning; upper, lower, and bottom; but not now. It would be difficult even with a Rose Engine, AFAICT, if the wood had been really dry.

    Joe
     

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  11. John Chianelli

    John Chianelli

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    I am not familiar with Rose Engine or AFAICT......John
     
  12. Joe Greiner

    Joe Greiner

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    By "difficult," I mean it wouldn't make much sense.

    A Rose Engine is a sub-class of Ornamental Lathe, with a rocking headstock guided by cams, sometimes called Rosettes. Usually employed to apply decoration to a previously turned piece, via a high-speed cutter. This Rosette isn't the same as blocks used on window and door frame architraves - See Grizzly's catalog for cutters to make those.

    Google is your friend for unfamiliar terms. There were plans for a Rose Engine in the AAW Journal not too long ago, but I seem to have misplaced my copy. I think they were posted in the forum, which is Search-able.

    For the bowl pictured, some weird cams would be needed for different parts of the piece, including an oval cam for the (now) oval bottom. Transitions between different parts of the piece wouldn't be much fun, either. Easier to let Mother Nature take care of it all.

    AFAICT = As Far As I Can Tell (Internet acronym).

    Joe
     
  13. MichaelMouse

    MichaelMouse

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    Yes, the 'net provides many resources for those who choose to use 'em. www.acronymfinder.com is one for the alphabet soup. For instance, one which I used recently, DRT. Thankfully I haven't used DRTLW since last September.
     
  14. Malcolm Smith

    Malcolm Smith

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    Like everything Oneway provides, the vacuum system is excellent. I have not tried any others.

    I have not had to use neoprene gaskets but do find I need to take some trouble cutting the ledge the bowl sits against so that there is a good fit. It also occured to me that rubber cement might be applied. One would need to be careful that it is dry and isn't sucked back to the vacuum pump. Rubber cement dries fairly quickly.

    Malcolm Smith.
     
  15. Dick Sowa

    Dick Sowa

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    You already have just about anything I could add. However, I long ago gave up on the "little way up the side" at all. Now I turn it all the way to the bottom while chucked. Then, when I finish the bottom, I am not trying to true up the rest of the bowl a second time. Humidity and temperature changes from day to day, can affect the trueness of your bowl. It's amazing how much it can warp with very little change in humidity.

    I think the smart thing to do, is to completely finish the inside and outside while it's as true as you can get it, then reverse chuck it and ONLY turn the very bottom. That way you can avoid the problem altogether.
     
  16. John Chianelli

    John Chianelli

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    I completely agree with everything you say, Dick. My goals is to only have to deal with the very bottom. But every so often I get that bowl with the awkward angled underside or the chuck overhangs the bottom edges keeping any tool away from the last bit of real estate. I have even set up another Stronghold chuck with the cole jaws, so when I take the bowl out of its tenon grip it immediately goes into the cole jaws. Thought a quick turnaround may help. I really did not see much a a difference, if I did it may be imagined anyway. But, I do believe it is better to complete the reverse chucking actions as soon as possible to keep the moving to a minimum. I do believe the longer it sit on the shelf the more it may move as Dick says about temp and humidity daily changes.

    The one thing from this thread I want to try is Malcoms suggestion of vacuum chucking. I cannot help but think that taking the stress off of the rim applied by cole jaws may tone down the warping to the point where sanding covers any minimal tool work up pretty well.
     
  17. Joe Greiner

    Joe Greiner

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    Vacuum chucking may offer benefits other than the objective. But before investing time and money, I suggest identifying the real source of the problem. If your only dissatisfaction with the Cole jaws is the bottom alignment, try wrapping some of the buttons with electrical tape to micro-adjust the bottom orientation, with tailstock assistance.

    Finish turning the bottom as soon as possible might not be the best approach, either. Warping afterwards will produce a rocking bottom. BTDT (= Been There Done That).

    Joe
     
  18. John Chianelli

    John Chianelli

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    Good point.....nobody wants a rocky bottom.....
     
  19. john lucas

    john lucas

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    John I use a tool from bestwoodtools that screws into the chuck and then inserts into the tailstock. This and a vacuum chuck usually gets my bowls centered. If there is warping of course then the bottom may not align perfectly but I can usually sand any discrepancies.
    You will find these tools along with the Oneway adaptor that does the same thing, on our club website under the tips section. check out methods and jigs for reverse turning bowls.
    http://www.cumberlandwoodturners.com/tips.htm
     
  20. Wyatt Holm

    Wyatt Holm

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    One of the simplest and most versatile reverse chucking device I have seen is a tension drive. It is simply a piece of scrap wood turned to a cylinder, with the end of it hollowed out like a shallow bowl. The reason for the concave base is that it allows your bowl to seat flatter. I use soft leather or neoprene to pad the bowl. Tailstock pressure holds it on and you can finish the base. Then you have to sand of the nubbin at the end. I use this mainly for natural edge pieces that either have holes or are too thin to stay in the vacuum chuck.
    Hope that helps,
    Wyatt
     

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