Hard Problem: Seeking Guidance - Page 2 - Woodworking Talk - Woodworkers Forum
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post #21 of 27 Old 02-13-2020, 06:48 AM
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If I was facing the problem of producing the skirt I wouldn't hesitate to laminate it from birch plywood and then apply a thick structural veneer for facing.It wouldn't be too hard to apply C shaped segments of the same veneer to the top and bottom edges.It would be very slightly easier to produce two semi-circles and biscuit them together since the laminating former would be simpler and if I did that,I would extend the ends tangentially for maybe three inches to give good seating for some clamped blocks.
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post #22 of 27 Old 02-13-2020, 04:46 PM
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Quote:
Originally Posted by Arbee View Post
The basic principle to expanding the table is to drop the skirt by spining it clockwise, exposing the leaves by spinning the ring clockwise, and then spinning the skirt counter clockwise to raise it and the leaves, (along with the arms and star inside the table) to the base table top height.
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Preface: My focus is the function of the skirt, so I intentionally just skimmed through the rest of the table description.


I don't see them, nor saw a description of them, but I would definitely incorporate cam-follower bearings into the edge of your platform where the "threads" of your skirt will be guided. A "true" cam-follower bearing is too expensive, but you can make your own with a cheap bearing and shoulder screw or even a bolt, since the loading is light. Solely to show you what I mean, here is a "true" cam-follower bearing.

The benefit of this is at least-two-fold: 1) it makes the skirt movement much smoother and easier. 2) it permits you to create a non-linear thread profile for increased lifting force at the closing of the skirt, while reducing the number of revolutions required where no force is required.


Expanding on idea #2, the slope of your threads would be very steep toward the top of your skirt, and very shallow toward the bottom. You could even have 2 of these shallow segments for when the table is in the expanded position and contracted position. (I assume the skirt is higher or lower, depending on whether the skirt is supporting the open leaves, versus covering the edges of the collapsed leaves.)


Dentil Molding
The idea of using dentil molding is in the right direction, except you're just using pre-kerfed wood instead of making it yourself. I think you would be better off making it yourself. Start by making your "cam track" in flat wood, like shown below. You would have an upper and lower cam with a space between them equal to (slightly wider) than your cam-follower bearing. The reason for doing it "in the flat" is because you'll want to make it using a router pattern so you can make (at least) 4 identical copies. (I forgot to mention above that you want at least 4 cam-followers at 90 degrees apart, but even more is better.)



There are 2 different approaches for attaching these to your skirt. Option 1 is to glue them in-the-flat, and then kerf everything at once. This would be preferred, as long as you are able to overcome the single down-side. The down-side is that you need to be more careful when filling your kerfs so as to not "muddy" the walls of the cam with stray resin. (P.S. see "Thick kerfing" below.)


The second option is to kerf these separately, and epoxy-resin them to your finished skirt circle. Hmmmm....I suddenly don't like this option any more, because I just realized you will have the very same "down-side" as mentioned above. So ignore this option!!


Thick Kerf
Because your skirt is now effectively twice as thick as before, you may need wider kerfs....BUT...that doesn't mean the whole kerf needs to be that wide. Make your standard kerfs just like I described the other day (as though your skirt was only 3/4" thick). Only the inside edge of the kerf needs to be wider. So go back with a second-pass of your kerfing, but at half the previous depth, and slightly offset to make them slightly wider (as needed).
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post #23 of 27 Old 02-13-2020, 07:32 PM Thread Starter
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Rick:

Thanks for your wisdom and for taking the time to offer your ideas here. You were right to ignore most of my table's function as it's not skirt related. I provided it though because I thought that giving you the larger context might help, including the downside to the dentil molding: it adds to the skirt's thickness (on the skirt's inside wall) and makes the closed table leaves have less space than the skirt's inner diameter to fit under: even if only very little--and every little bit matters here.

Envision if you will the skirt a reverse threaded nut. In this metaphor, the "X" pattern I have on the base table, which is composed of 4 pieces of 3/4" thick by 5.5" wide wood is the "bolt" to that reverse threaded nut that is the skirt.

You were smart to suggest the use of cam following bearing instead of my existing "X". The cam's diameter is much narrow than this 5.5" thickness and allows for not only a steeper, but varying steepness track on the skirt for which to travel on as you've suggested.

I'm going to see about attaching using cams in place of my exiting "X."
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post #24 of 27 Old 02-16-2020, 11:30 AM Thread Starter
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Rick:

What would you see as the downsides to creating the kerfed skirt in say two or three ring segments that are shorter and easier to work with when kerfed, but un-expoxied and thus still fragile, and then once placed into a form to achieve their correct curve, either before or after expoying them, connected them with biscuit joinery?

I would not kerf near the ends (regardless of 1, 2, or 3 piece biscuiting) so as to preserve enough unkerfed wood at the ends to make a solid biscuit joint possible.

Also, if you're not against this would you biscuit first and then epoxy or vice versa?

Thanks.
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post #25 of 27 Old 02-16-2020, 02:26 PM
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I would do it as a single piece. If you split it up, you're going to have to handle the pieces more than necessary, and that will increase your chances for an accident.



It's not at risk when it's laying flat or standing upright. Only when transitioning from one to the other. So just make sure you support it when making the transition.
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post #26 of 27 Old 02-16-2020, 02:56 PM
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Tape it to a backer .....

Quote:
Originally Posted by Rick Christopherson View Post
I would do it as a single piece. If you split it up, you're going to have to handle the pieces more than necessary, and that will increase your chances for an accident.



It's not at risk when it's laying flat or standing upright. Only when transitioning from one to the other. So just make sure you support it when making the transition.

If you tape to a long 1/4" ply a bit wider than the workpiece, it will not flex that much.... just a thought.

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #27 of 27 Old 02-16-2020, 03:56 PM
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go and look underneath some similar vintage furniture and see how they did it. No need to reinvent the wheel.

Retired engineer-bureaucrat in Oakland, CA
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