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Thread: Jaws: Dovetail vs. Straight sides

  1. #1
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    Jaws: Dovetail vs. Straight sides

    i have been turning for 5 years now, and followed the typical Vortex Path. I purchased from PSI some turning tools and a scroll chuck. Of course, over time I have added better tools from Crown, Sorby, Thompson, and others. I also have added chucks from Nova, Vicmark and others. My curiosity has been piqued by the different performance of dovetail jaws and the straight sided PSI jaws. It seems that often times when grabbing the outside of a tenon on a bowl bottom in order to turn the inside of a bowl, the dovetail jaw chucks don't seem to run as true as the straight PSI chuck does. I clearly dislike the bevel gear type method for tightening the PSI chuck, but I will reach for the PSI usually when I am turning anything but a very hard wood. I think that the dovetail jaws can compress unevenly on the softer woods, cut into the tenon sides unequally, and get itself out of balance and run with a slight wobble. I seem to get this result often, and the difference is quite noticeable. Of course it is not so great that I can't turn the item back to round, but there is a difference. Now, I would surely not recommend the PSI chuck over other chucks as the dovetail jaws seem safer, and the better chucks are easier to operate. Hopefully I have explained myself clearly, and if so does anyone have this experience or ideas on this? By the way, all of the chucks (I have 5) run true when alone on the spindle. And none of this has prevented me from turning (and selling) 150-200+ items each of the past 3 years.

  2. #2
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    Apr 2007
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    I have used about every chuck design available, and my preference for jaw configurations is the dovetail. It is not always necessary to create a matching dovetail when cutting a tenon, but for hard woods it does enhance "gripability." It is the best combination of all the other jaw shapes and textures, and I have adapted dovetails to all 5 brands of chucks that I use.

  3. #3
    The dove tail joint provides a mechanical 'locking wedge'. It has been used forever in woodworking as a very strong, long lasting joint. When made properly, it can't be beat for strength. The 'properly' part is the key. The angles must match pretty much perfectly, and the size/diameter must be very close as well. There are specialized tools that make this pretty easy. A pointed set of dividers/compass, to mark the size. I prefer the scraper tool with a dove tail on it, you just line up the tool with the lathe bed and plunge straight in. I cut just inside (I use a recess in my bowls) the line first, then just barely take the line second. I let the scraper rest very lightly on the wood to take out some of the bounce you always get from cutting into side grain. If I was doing long heavy spindles or larger hollow forms, then long straight sided jaws could have some advantages (note here, NEVER use expansion on end grain as you will split it).

    As far as getting the bowl to reverse and run perfectly true, that is impossible. You can get it pretty close. If there is 1/16 inch of run out, that is +/- 1/32 which is very good. You can get uneven grip in very wet wood, but not so much in dry wood, mostly because wet wood will compress more than dry wood. If you grip with 2 jaws on end grain, and 2 on side grain, you can get an uneven grip. Rotate the wood about 45 degrees, and this will eliminate most of this problem. You may need to tighten up again once or twice as you turn green wood as the wood will compress as you turn. Again, not nearly as much of a problem with dry wood. I have a couple of clips up on You Tube showing how I make a recess, if you type in robo hippy.

    robo hippy

  4. #4
    Fred,

    In my experience you observations are correct. I have come to the conclusion that my Oneway stronghold profile jaws re-chuck more concentric then my dovetail jaws on a consistent basis. With the pieces I make this is a very important attribute of the profile jaws. I often have to take a piece out of the chuck more than once before it is complete. I need it to re-chuck as accurately as possible. I do think the dovetail is probably stronger but I have never lost a piece with my stronghold profile jaws. I think the tenon or recess profile is much more critical to getting an accurate repeatable hold with the dovetail type jaw. If you do not need to remove the piece from the chuck or if is does not make a big difference if you have to true up the piece it really does not matter what style jaw you use. I use both but when I am dependent on an accurate re-chuck I use my profile jaws.

    Alan

  5. #5
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    I don't have much experience with a dovetailed tenon, much less remounting a piece.

    I have mostly Nova chucks and until you get up to the 100mm jaws there is not a dovetail on the interior. The interiors are either straight, serrated, or have a lip.

    I do have a Hurricane chuck and the standard 60mm jaws have a dovetail on the interior as well as exterior.
    I will have to watch for a difference when I do use a tenon.
    "I became insane, with long intervals of horrible sanity." - Edgar Allan Poe

  6. #6
    Join Date
    Mar 2005
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    Spokane, Washington
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    I prefer my Oneway chucks with straight (serrated) jaws. I find them at least as secure as the dovetail varieties, and I like being able to cut a straight tenon vs a dovetail.

    Dan
    Eternity is an awfully long time, especially toward the end.

    -Woody Allen-

    Critiques on works posted are always welcome

  7. #7
    The only way I can get things to line up perfectly if I have to take them off the lathe and remount a couple of times is when I am doing threaded boxes. Each piece is glued onto a waste block that is epoxied onto a threaded nut. Even when doing a spindle mounted in a chuck on one end and the tailstock on the other, there will be run out if I have to take it off and remount it.

    robo hippy

  8. #8
    Join Date
    Aug 2011
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    I've found that the runout can be minimized - not always completely eliminated by marking the piece right at the space between two jaws that lines up with the key hole on one the side of the chuck. On OneWay chucks there are two key holes, so I just mark one of them so the piece can be put back into the chuck as closely as possible to the same orientation. Seems to work pretty well.

  9. #9
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    Quote Originally Posted by Alan Trout View Post
    Fred,

    In my experience you observations are correct. I have come to the conclusion that my Oneway stronghold profile jaws re-chuck more concentric then my dovetail jaws on a consistent basis. With the pieces I make this is a very important attribute of the profile jaws. I often have to take a piece out of the chuck more than once before it is complete. I need it to re-chuck as accurately as possible. I do think the dovetail is probably stronger but I have never lost a piece with my stronghold profile jaws. I think the tenon or recess profile is much more critical to getting an accurate repeatable hold with the dovetail type jaw. If you do not need to remove the piece from the chuck or if is does not make a big difference if you have to true up the piece it really does not matter what style jaw you use. I use both but when I am dependent on an accurate re-chuck I use my profile jaws.

    Alan
    Alan, thanks for your response about my query regarding the benefits of the dovetail and straight jaws. I was wondering if you have any idea where one might find non dovetail jaws for certain scroll chucks. I have a couple Nova SN2, and a couple of Patriot (I think that's what they are) chucks and I can't seem to find a source for replacement jaws that aren't dovetail. Working with cedar, spalted wood and other less dense woods causes the dovetail to compress unevenly on the side grain and end grain of either recess or raised tenons. No matter how careful or accurate one might be, it is nearly impossible to avoid this issue. I try to make the tenon as near to the optimum diameter as I can, but that seldom makes a difference.

    Thank you for your time, and may your future be bright,

    Fred

  10. #10
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    Apr 2011
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    Williamston, MI
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    462
    Quote Originally Posted by Jeffrey J Smith View Post
    I've found that the runout can be minimized - not always completely eliminated by marking the piece right at the space between two jaws that lines up with the key hole on one the side of the chuck. On OneWay chucks there are two key holes, so I just mark one of them so the piece can be put back into the chuck as closely as possible to the same orientation. Seems to work pretty well.
    I use the same technique on my VicMarc except that I mark the location of both sides of the number one jaw on the workpiece. I also make sure I mount the jaws to their corresponding number on the chuck.

  11. #11
    Join Date
    Dec 2009
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    Another point to consider is to keep the tenon over-sized if the wood is still green. The first turning does not need to be extremely accurate, since the bowl is going to shrink to an oval shape anyway. The tenon will also be oval shaped in 6 months and it needs to be big enough to be able to turn it back to a round tenon.

    Steve

  12. #12
    I have both and do not have a prefrence.

  13. I'm a bit confused as it seems to me that uneven compression on end grain and side grain, if a real issue, would be the same with or without a dovetail shape as long as the jaw was a perfect angle match to the tenon or recess.
    Donate blood. The gift of life.

  14. #14
    This topic has been discussed many times, and clearly there are two camps on the preference of jaw types. But, just to clarify a point - the Nova/Teknatool bowl jaws utilize a lip vs. a dovetail in compression mode and the tenon/spigot that is cut should NOT be formed for the lip, but instead have sides that are parallel, or straight. In fact, the manual for the SN2 states the following -

    "Mount wood between centres and turn the spigot area. Make the spigot as parallel as possible to
    maximise the efficiency of the clamping action. Only approximate sizing of the spigot is
    necessary, as the jaws will accommodate a wide range of spigot diameters within the spigot limits
    stated above. The 50mm standard jaw has a thin lip or shoulder at the front face. This is designed
    to bite into the timber as the jaws are tightened. DO NOT CUT A RECESS FOR THE LIP TO FIT
    INTO, AS THIS WILL REDUCE GRIPPING POWER."

  15. #15
    Quote Originally Posted by Fred Perreault View Post
    Alan, thanks for your response about my query regarding the benefits of the dovetail and straight jaws. I was wondering if you have any idea where one might find non dovetail jaws for certain scroll chucks. I have a couple Nova SN2, and a couple of Patriot (I think that's what they are) chucks and I can't seem to find a source for replacement jaws that aren't dovetail. Working with cedar, spalted wood and other less dense woods causes the dovetail to compress unevenly on the side grain and end grain of either recess or raised tenons. No matter how careful or accurate one might be, it is nearly impossible to avoid this issue. I try to make the tenon as near to the optimum diameter as I can, but that seldom makes a difference.

    Thank you for your time, and may your future be bright,

    Fred
    I think the issue is not so much the dovetail versus straight, as it is the profiled jaws of the Oneway chucks. With the profiled jaws, there is always a radius that touches the wood, and with non profiled jaws, the jaws grip with the points at the end of the jaws unless the tennon is matched exactly to the size of the nearly closed jaws. When the tennon is larger, the tips or points at the ends of the jaws bite in in differing amounts that results in an increase of the runout. If the tennon is properly sized for the dovetail or other non profiled jaws, the runout will be minimal.

    For me though, I like to be able to use a tennon that is on the large size and prefer to use the chuck for a variety of tennon sizes within it's grip range. When coring, a larger shorter tennon is much preferred (at least by me), and non profiled jaws whether straight serrated or dovetailed will usually cause the corners to start breaking the tennon off during the tightening process unless the tennon is kept at the small end of the grip range. The profiled jaws have a much reduced tendency for initiating cracking of the tennon. Profiled with serrations, straight with serrations, or dovetailed all will get the job done, it just becomes a matter of learning the limitations of each type.

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