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Old 02-09-2018, 02:48 PM
Nutcase Nutcase is offline
 
Join Date: Jan 2010
Location: Vernonia, OR
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Default Floating bridge

Recently, as I was sitting at my desk, it occurred to me that I was on top of a reasonably sturdy and stable platform that pivots easily and has an adjustable height. Used office chairs are cheap and plentiful...

So I took the seat off a chair and attached a 33" long plywood-and-2x4 beam in its place. The chair base is from a stool with a conveniently large range of height adjustment. The seat attachment platform isn't level, so I attached the box with a hinge on the high end and put in a sturdy spacer to prop up the low end. The beam's center of gravity is positioned so that it can pivot from stable horizontal to stable vertical. I attached a bridge to the beam.

So far so good. The top of the beam is itself a minimalistic table. The whole contraption, what I am calling a "floating bridge," is easy to move and easy to store.

I then clamped a 6' 2x10 plank (salvage from an old water bed frame) in Rigid's version of a jawhorse. There are lots of ways to prop up a plank, and I'll want to free up the jawhorse eventually, but this is just about ideal at the proof-of-concept stage. I rolled the floating bridge into place and raised it so that the beam made contact with the bottom of the plank.

The plank supports dimensional lumber very well, and the floating bridge allows for cuts at angles that are too acute for a miter saw.The whole setup also breaks down as easily as it goes up. However, as is, it's more than a bit wiggly, the casters limit small adjustments and the cut line is off to the side from the axis of rotation; in sum, setting up cuts isn't always easy.

So I drilled a 1/4" hole through the top of the beam in line with the chair base's axis of rotation, and through the plank. I then used the 1/4" pin from a door hinge to connect the two. The cut line is still off the axis, but the fact that axis is fixed already makes lining up a cut much easier.

There is still a lot to improve with this setup, but it's already usable.
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Old 02-09-2018, 02:59 PM
kenk kenk is offline
 
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Quote:
Originally Posted by Nutcase View Post
Recently, as I was sitting at my desk, it occurred to me that I was on top of a reasonably sturdy and stable platform that pivots easily and has an adjustable height. Used office chairs are cheap and plentiful...

So I took the seat off a chair and attached a 33" long plywood-and-2x4 beam in its place. ...
I'm so glad you didn't sit on my beloved La-Z-Boy!! Heh
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Old 02-10-2018, 08:15 AM
Nutcase Nutcase is offline
 
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Location: Vernonia, OR
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Quote:
Originally Posted by kenk View Post
I'm so glad you didn't sit on my beloved La-Z-Boy!! Heh
It's not so easy to repurpose a La-Z-Boy. At least, that's what my wife says.
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Old 03-28-2018, 09:16 PM
Nutcase Nutcase is offline
 
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Location: Vernonia, OR
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I've made some interesting progress on the floating bridge, and I finally got someone with a functioning cell phone camera in a position to take a few pictures for me. However, before going further, Iíd like to outline the problem that this project was originally aimed at solving.

I make arched structures out of cattle panels and dimensional lumber. This involves cutting boards to length and cutting the ends at angles down to about 27 degrees. High precision isnít required. Frankly, an experienced framer would just do it all freehand with a circular saw. Me, though, Iíve been using a miter saw for cutting to length and a kludgy single-purpose track saw setup for the angle, swapping these tools in and out of a smallish outdoor working space. I wanted the convenience and superior dust containment of a single track saw setup that I could use for both operations.
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Old 03-28-2018, 10:10 PM
Nutcase Nutcase is offline
 
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The plywood and 2x4 beam in the proof-of-concept version of the floating bridge wasnít very flat or square. I decided to replace it with a 4í long extruded aluminum beam scavenged from a rowing machine some years ago. Itís straight and sturdy, and one side has a track for 3/8Ē carriage bolts. However, it offers no particularly convenient way to mount the B-300 bridge armature on the ends.

So I made a substitute armature out of two pairs of 3 ĹĒ blunt tee hinges. These hinges are stable, flat, square, and they come with predrilled holes in convenient locations. I mated each pair into a double action hinge about 7Ē tall. Each hinge pair attaches to the track with two track connectors, just like the B-300 armature. On the other end, they each attach to the top of the beam with a single carriage bolt. With this arrangement, the only modification needed was some drilling out of existing holes for variously sized bolts.

Adjustment is simple and easy: square the track end of each hinge pair to the track. Then, bolt down the beam ends so that the track lays flat on some random chunk of dimensional lumber.

Iíve set the bridge up so that the track falls back to lower it onto the material. Consequently, the end of the track serves as the de facto handle for raising and lowering. Iíve also added a stop to keep the bridge from going forward much past vertical, so itís stable in a raised position even with a saw on board.

I have been giving a bit of thought to the potential of this armature in a more conventional setup. With a few minor changes, the hinges could be attached to the sides of a power bench, more or less like the B-300 armature. Alternatively, they could just be directly screwed or bolted to any table top. Something like the carriage bolt track on the beam is very convenient, but it isnít absolutely necessary.

Am I missing something? It sure seems like one can make a sturdy, easily aligned bridge armature with remarkably little cost or effort.óassuming one can find hinges of comparable quality. The hinges Iím using (purchased recently from a local Home Depot) have no translational or torsional free play other than rotation on the hinge axis, and the holes are consistently placed and symmetrical. This isnít always the case.
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  #6  
Old 03-29-2018, 07:52 AM
Absinthe Absinthe is offline
 
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That looks awesome! And you are able to square it and keep it square?
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Old 03-29-2018, 10:29 AM
Nutcase Nutcase is offline
 
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Once the hinges are squared to the track, they seem to stay put pretty well. I just used a speed square as I tightened down the track connector bolts.

If I have one hinge pair only attached to the 52" track near one end, the other end of the track has somewhere between plus or minus 1/4" to 1/2" play before the connection to the beam starts to rotate. It's easy to rotate back into good alignment. It's hard to get significant twisting forces on the track attachments because the single beam attachment slips first. With both hinge pairs squared and tight to the track, one can loosen one or both of the beam attachments and shift them around on the beam without affecting the track attachments at all.

Squaring the track to the plank...See below.
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Old 03-29-2018, 10:53 AM
Absinthe Absinthe is offline
 
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So, how is this intended to be used? You just move it between a couple of supports and cut away?
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Old 03-29-2018, 10:59 AM
Nutcase Nutcase is offline
 
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The next issue was stabilizing the relationship of the bridge to the ďtable,Ē the plank I am using to carry the material to be cut. On the axis of rotation, I substituted a carriage bolt in the beam track for the hinge pin I had been using before. Next, I centered an 18Ē plastic disk (3/16Ē HDPE) on the bolt. Finally, I improvised a clamp to pin the edge of the disk to the beam and another to pin it to the plank. This setup is easy to adjust, and yet quite stable when locked down.

I have put a protractor on the disk. Itís just a protractor image printed on a piece of paper and attached with clear tape. Not durable, but fast and cheap and easy to replace. Because of the double clamping setup, I can zero the protractor easily, using the edge of the beam as a pointer.

To zero, begin by loosening both clamps, then rotate the disk to 90 degrees. Tighten the clamp hidden under the plank, then square the track to the plank. Finally, tighten the clamp on the beam. It is now squared and zeroed. To change the angle, just loosen the plank clamp, rotate as needed, then retighten.

As everything is currently set up, I can get to an angle of about 20 degrees before the bridge runs into the plank.
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  #10  
Old 03-29-2018, 12:01 PM
tofu tofu is offline
 
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Interesting concept. Always fun to read other users' innovations.

Has anyone ever considered a side mounted bridge? Like the old rip sizer, but with arm brackets like the new UEG concept? Since there would be no bridge mounts on the ends of the track, you could have unlimited rip capacity by just sliding and repositioning your wood against the fence.

The arms wouldn't even need to be very stable. Flex doesn't matter once the track is resting on the wood
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