Need help trying to figure out hinge point - Woodworking Talk - Woodworkers Forum
 
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post #1 of 19 Old 01-22-2015, 10:48 AM Thread Starter
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Need help trying to figure out hinge point

I'm looking to building an outfeed table for my Ridgid TS3650 table saw. I have some gas charged struts left over from a job a few years ago and was tossing the idea around to use them to help flip up the table as well as hold it in place.

I haven't tested it out yet with a full mock up but it seems its not as easy as I originally thought. It appears that it really depends on where the hinge and strut are mounted to be able to make this happen. I was hoping that once you push the table down, it would reach a point that the strut would force it to swing down and hold itself in the closed/down position. The problem I came across is that the strut always wants to push the table back up.

I'm sure there a bunch of guys here that have taken engineering courses and can figure this out quickly. I haven't had the time to mess around with all different scenarios.

It's a little hard to tell in the attached diagram but I was thinking of using 3 pieces of steel/aluminum. 1 piece attached to the table saw, 1 attached to the outfeed table and 1 piece going perpendicular to hold one end of the strut. I'm also thinking the hinge (pivot point) would need to be offset from the perpendicular bar so it would help with keep the outfeed table closed.

Like I said, I haven't mocked it up yet so I know this is just in design phase. I understand some people might not like the idea of just having the struts hold the outfeed in position. The struts I have are rated for 70 lbs and if I use 2 of them, they may be strong enough for minor work. If not, then I can also put some legs that can flip down once the table is in the up position.

Thanks for any help.
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post #2 of 19 Old 01-22-2015, 10:59 AM
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I could be wrong but I don't think what you are trying to do will work. You could put a catch to hold the top down but you would have to have some way to lock it solid in the up position. With enough weight put on the table the struts would let the table down causing a potential safety hazard. There is hardware made for this application that would lock solid for you I would be inclined to use.
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post #3 of 19 Old 01-22-2015, 01:39 PM
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pneumatic struts, to my knowledge, are always seeking to extend.

you can use gravity and leverage to help overcome their resistance past a certain point, but only if your application works that way... Given a blank slate and a few engineers you can design it where leverage produces a desirable balance through the range of motion.

In your case, I think you'll be latching it down, and you'll need to lock it up. the strut will just be a lift assist (and hopefully not one that tries to launch you across the shop when you try to raise it, or requires you to sit on it to push it back down)
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post #4 of 19 Old 01-22-2015, 02:58 PM
where's my table saw?
 
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a mockup is best here

I'm not an engineer, but I would first try mounting the bottom of the strut to the other horizontal brace, so that when it's extended it lays horizontally rather than at a 45 degree angle.

I can't explain why I think that would work... I am not an engineer. I would use a dowel inside a length of PVC that matches the travel and stops of the strut to see where it or IF it will work best.

As suggested you may end up with a folding brace as is the case on my outfeed table on a Craftsman 22124 table saw. If you search Sears Parts Direct they may still make one, I donno?

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 #5 of 19 Old 01-22-2015, 03:24 PM
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I am an engineer, but not that kind. However, it seems to me that you would need to take the point on the vertical leg where it's mounted, and put a bracket there so that the strut now mounts a few inches to the left, away from the vertical. What this would do is when the table is down, it would take the strut past vertical and the effect would be that the strut would now attempt to push the table inward.

This same thing could be accomplished by putting a bracket on the horizontal table so the strut would mount a few inches below the horizontal.

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Last edited by TS3660; 01-22-2015 at 03:40 PM.
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post #6 of 19 Old 01-22-2015, 04:11 PM
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Do you really want an outfeed table that collapses at a certain weight applied? I would at a minimum have a lock brace that keeps that from happening, but would just avoid it if possible.

The tools don't make the craftsman....
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post #7 of 19 Old 01-22-2015, 04:38 PM
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The struts are generally used to either make lifting easier or to slow down the decent of an object, they are not meant to hold it in place when extra weight is applied, you will need some kind of locking device for that.

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post #8 of 19 Old 01-22-2015, 04:40 PM
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IF the table isnt locked, not only would you need a ridiculous amount of tension on the slide when folded but the simple fact is that the weight it will support is equal to the weight required to fold it down... Which I think for safe function of supporting sheets of ply the answer should be "more than you weigh".

Again, this could only be a lift assist, with a lock out at the top. It would be a hazard any other way.
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post #9 of 19 Old 01-22-2015, 05:04 PM
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I attached an axillary piece to the side of my work bench simply to give more table top area to support my projects when they are assembled. And to support sheet goods when they are ripped wider than 24 inches (up to 37 inches).

I used a pair of folding supports that lock in place when extended.
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post #10 of 19 Old 01-22-2015, 05:10 PM Thread Starter
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Thanks for all of the input. Like I said this is a work in progress and plan to full test it before I go ahead and go with it. Right now I'm just trying to figure out where the placement should be of the strut and the hinge should be. Just so everyone doesn't worry about my safety, I'll say it now, "THE TABLE WILL HAVE SOME SORT OF LOCKING FEATURE TO KEEP IT RAISED." Please don't take this the wrong way. I know sarcasm doesn't come across very well on the internet. I really do understand that safety should always be the main goal. It always makes things harder when you start losing body parts.

Quote:
Originally Posted by TS3660 View Post
it seems to me that you would need to take the point on the vertical leg where it's mounted, and put a bracket there so that the strut now mounts a few inches to the left, away from the vertical. What this would do is when the table is down, it would take the strut past vertical and the effect would be that the strut would now attempt to push the table inward.
TS3660 - I think you and I are on the same track. I know its probably hard to see from the drawing but I put the hinge further to the right of the vertical because I was hoping that when the table lowered beyond a certain point, that when the strut expands it would actually be pushing the outfeed table towards the saw. It's just a matter of figuring out where those points are.

I was thinking about my wife's truck before and how there are struts to hold the rear glass open. If I unlock it, it doesn't shoot up in the air so it must be at the point of either not having any strength or if is at the point of starting to push the other direction. I'll have to take a look at it tonight and see it I can apply the design to my table.
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post #11 of 19 Old 01-22-2015, 05:24 PM
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What you need it to do when it swings down is move past the 90 degree point in the swing so that it begins to expand and pushes the table toward the saw rather than up and away from it....see pink line representing the gas strut.

Need help trying to figure out hinge point-imageuploadedbywood-working-talk1421961830.331161.jpg

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post #12 of 19 Old 01-22-2015, 05:26 PM
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And extended Need help trying to figure out hinge point-imageuploadedbywood-working-talk1421961987.871477.jpg

The tools don't make the craftsman....
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post #13 of 19 Old 01-22-2015, 05:39 PM
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What you're looking for is an "over center" mechanism. Here's an example:
https://www.youtube.com/watch?v=1bvAawttIWg

Alexis de Tocqueville was a very smart man.
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post #14 of 19 Old 01-22-2015, 06:02 PM
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The trunk lid doesn't pop open when unlocked because the weight of the trunk is more than the pressure the strut develops. When the trunk lid is up there is practically no weight on it so the strut is able to keep it open. Really for that application that is all the strut is suppose to do is keep the trunk lid open. I just don't understand what the struts are needed for. It would seem to me the outfeed table would be easy enough to lift and all that would be needed is something to lock it in place.
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post #15 of 19 Old 01-22-2015, 06:21 PM
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Yup to the above!

Over engineered, as they say in the industry....

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 #16 of 19 Old 01-22-2015, 06:30 PM
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Has no one ever wanted to do something just cause you could?

I once built an entertainment center out of inch and a half steel tubing and quarter inch plate steel. Conservatively I think it would have supported 4000 lbs or more....but it looked kick ass with its auto quality paint job and industrial design.

Now does the outfeed table need a gas strut, probably not, but it's cool to see if it works out.

The tools don't make the craftsman....
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post #17 of 19 Old 01-22-2015, 08:18 PM
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its very simple then, put piston, put it past center at bottom, hope it doesnt launch the outfeed table into the ceiling when opening.
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post #18 of 19 Old 01-23-2015, 02:08 PM
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Bandit: Okay, I came up with some ideas in the middle of the night- a combination of reading this thread just before bedtime, and that darn prostate

First, I agree with both wwodnthings and ryan50hrl: your idea is overengineered, but sounds like fun, anyway.

Second, I'm not an engineer, and my analytical math and physics peak was sometime during the Nixon administration, so more than a bit rusty.

Anyway, it might make the location of your strut easier if you rule out areas where it won't work. What's left is where you can locate it. Bear with my example, which I think can be adjusted to suit your situation.

Some assumptions: The strongest support for the extension is if the strut is oriented vertically, with the hinge at the far end of the extension. If the hinge is moved closer to the table, there is more leverage from the weights of the table and outfed wood to push the extension downward. If the struct is angled (not vertical), some of the force is exerted horizontally, which won't provide support- think vectors.

You need to know the distance between the edge of the table and the hinge. You also need to know the lengths between the mounting holes of your strut 1) fully compressed and 2) fully extended. Think of the strut as a stick of variable length, but with short and long limits.

Let's say your extension goes out 18", and the strut compresses to 12" and extends to 24". One end of the strut is the hinge at the end of the extension, and you want to mount the other end (the "pivot") somewhere near or along the side of the saw. The pivot location has to be such that the distance between it and the hinge is at least 12" for every point the hinge travels in the arc as you move the table from horizontal (up) to vertical (down).

In the first drawing, the outer arc is the path the hinge travels, and the red hatched area is all within 12" of the hinge. If the pivot is located in the red ("too short") region, at some point the extension will jam, because the strut can't be compressed any more than to 12".

The area where it's okay to mount the pivot is anywhere in the clear area near where the table and extension meet, since all points there are at least 12" from the hinge at all times. The lowest point is the best, since it makes the strut closest to vertical.

Now, let's say you want to locate the pivot away from the side of the saw, say by some sort of outrigger, as in the second drawing. In this situation, you need to make sure that the pivot is at least 12" from the hinge; if it's in the "red zone" the extension will again jam at some point in lowering.

You also need to make sure that at no point is the distance between hinge and pivot longer than the full length of the extended strut (24" in the example). The arcs show the limits from the extremes of the hinge- fully up and fully down. Any point in the green hatched area ("too long") is farther than 24" from where the hinge point can be.

So, the okay region to locate the pivot here is the clear triangle between red (too short) and green (too long). I drew the strut where it's most nearly vertical. By the way, with this orientation, you'd need to make a slot in the extension to allow it to pass by the strut as you raise and lower it.

Sorry about the long explanation- I hope it makes some sense, and that it avoids some hit or miss construction. It's easier redo mistakes on paper than in the shop.

Good luck!
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post #19 of 19 Old 01-24-2015, 11:29 AM Thread Starter
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Thanks guys for the input and suggestions. I agree that this doesn't need to be as extravagant as this might turn out but like Ryan said, at this point it might be more of a challenge to see if I can do it.

I have some angle iron at work that is full of holes so its more like an erector set and I wouldn't have to do any fabrication until I have a design nailed down. I practiced a few layouts yesterday and I did find a layout that worked but I just need to tweak the mounting locations to get the most strength/support out of it. I'll also try the design you guys sent me and see if they would work better than what I came up with. I'll get some pics next week after I get to test out the designs again and show you guys what I come up with.
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