Like this? 
6 Likes
Wha wha what? Did you decide against the strut plates? With three I don’t think you need them nearly as much.
1 Like
Speed is the key. You have to just get the tubing going really fast and it will fit the third one in there.
2 Likes
I had something about lube, but it slipped my mind…
4 Likes
I’m still working on mine (if you’re asking me), I fully intend to cut some strut plates soon, I just got my table put together.
1 Like
I like this option. Do you have any idea if it’s as strong as using struts?
I’m halfway through printing my parts when my ribbon cable died and took my extruder board with it. Awaiting the replacement now, then back to printing. I received the last of my needed parts from Ryan yesterday. I’m looking forward to assembly.
1 Like
Here is a thought for the frame of my table. I’m not sure how crazy this is:
The top is entirely open and some removable ribs (not shown) will be added to support the spoilboard. The frame supports those ribs, but the frame is not depending on those ribs for structural integrity.
The frame top is two pieces of parallel 2x6 lumber that run the length of the Y axis with plywood “wings” (shown in green) to support the belt holders.
The rectangular top has legs (shown in red) to a triangular bottom (shown in blue), and three feet. (The feet are just notional and could be wheels or something.)
Some cross braces (grey pieces) provide support against racking and I think they stiffen the top surface in torsion too.
The plan is something like a truss where all the struts are 1/2" conduit or something that is stiff axially but does not need to be stiff against bending.
The problem I am trying to solve is torsion of the table top that occurs when you put a four-legged table on an uneven surface. The surface in the yard will be extremely uneven (grass, dirt, maybe soft dirt), so one concept is that three feet can’t impart a torque to the table top. The table does have some chance of tipping if the weight is badly distributed at the +Y end, so that’s a hazard I will have to watch out for.
A torsion box with three legs is a possible solution but I also want to have an open frame to be able to surface slabs or cut joinery on the ends of boards. I am not 100% sure of my truss design but on paper I convinced myself it should be pretty stiff in torsion. As a side benefit, it doesn’t use much material, but that’s not really a goal.
What am I not thinking of?
2 Likes
Really interesting ideas. Was concerned at first, but am increasingly curious to see how your project turns out.
Are those gray braces EMTs that’d cause torsion/twisting, undermining being able to have a plane supported by just three points? Or, are the gray cross braces adjustable length (like a curtain rod but stronger, e.g. 1/2" EMT within 3/4" EMT, fastened with… grub screws).
Will material and tools on top be applying downward forces/vibrations that result in torsion/movement that still need to be handled, dampened and transferred-to-ground?
Already seen workbench designs with dampening suspension? Reduces but doesn’t eliminate torsion. Won’t tip, and can be shimmed/locked for “rigid” usage scenarios. e.g.
I like EMT. Would be neat to see you end up with relatively light weight, possibly storable solution. Was contemplating building Torsion box with 4 removable legs, 3 rigid, 1 adjustable length. Ball pivot mounts, with limited range, to torsion box. Are you looking to 3D print and/or CNC fittings for your Workbench? Or, you planning to use something like Maker Pipe fittings, they seem strong and reusable/adjustable. Maker Pipe do a good job of gathering and sharing community concepts/builds to inspire others, e.g. DIY Electrical Conduit Project Ideas | Maker Pipe
Already considered flipping the triangle legs vertically? So four legs still touch the uneven ground, triangle brace attaches to pivot point on your open torsion-box. You could limit twisting range of motion with stablizers/dampers between legs (red EMT) and underside of table’s top (yellow EMT). Add removable pins for optional “rigid mode”.
Honestly, my first thought/concern was…
Good luck!
Full disclosure: Am a software engineer at best, and know squat about Mechanical Engineering. Look forward to reading and learning from others' ideas/perspectives...
3 Likes
Looking at just the top:
- The connection between the green and the yellow is what is preventing it from racking in XY. That’s probably fine. Overkill would be adding more green pieces underneath to give twice the resistance to that racking. Or maybe some corner braces.
- The Yellow 2x6 is probably going to resist twisting pretty well. My only concern is wood movement, and I have an easier time being precise with sheet goods than 2x softwoods. So I would probably laminate two 3/4" plywood sheets to make a beam.
- Adding corner braces under the green into the yellow would avoid any racking in XZ.
I like the idea of a removable bed. I think the end result will be just one very awkward part and a bunch of sticks.
Looking at the base:
I’m looking through the different support sections and imagining the tubes being infinitely strong to compression, weak to tension and almost no torque from the joints:
- The square X end seem fine. It can’t come apart without compressing something. Except maybe the bottom tube.
- The triangle end is pretty good too. Gravity is going to keep it in compression. One of the corners would have to lift, or the whole end would have to rotate to get tension. The top would be able to be pulled apart, but not with the top attached.
- The long sides seem like they could fall over if you pushed the top away from the square end and toward the triangle end. The long angled piece is probably going to be in tension in the final part from the weight of the table.
- The bottom would also fall apart in this thought experiment. But there isn’t a lot of force on it.
My fear is that the connectors won’t be terribly strong in tension or for applying torque at the joints. I cant imagine them being terrible at compression. Maybe adding another cross brace on the long sides would be enough. I would also consider making the bottom out of something like 2x lumber that can be screwed together. But the bottom shouldn’t really be doing that much, so it depends on the connector mechanism.
The nice thing is that you could do this iteratively. If you had that in your yard and you noticed a direction where it was soft, you could add in more. Either corner braces or tubes or screws.
I also agree with your logic that things like uneven ground won’t affect the alignment much on the top. Putting it on a hill might. And just reassembling things may make things slightly different (enough to notice if you had flattened the spoil board). But in general, it would be good.
2 Likes
I agree with Jeff the connections are probably going to be the hard part. The rest looks good to me.
The other route is put more effort into a ultra rigid torsion box table with the removable section you want and set it on a triangular base instead of supporting all four corners. Now that I write that there is not much difference in the structure but at first glance it seems to me like it could introduce less twist into your table.
Using bessel / airy points, could make a for a table that needs slightly less heft to keep it flat and a smaller base. There is so little actual force on the table, and it is almost always evenly distributed. Put the effort into the table box and any base will work fine.
Man…brain twister. I might print out a 3d model to see how it feels. You might be on to something here. I feel like the grey bars could mess with the back end. Decoupling the front and the back feels right to me, but does it actually matter since it is triangular? Might be too early for my brain.
1 Like
Hrmmm… Gut reaction is that triangles in as many places as possible is best, to avoid over-constraining the planes (I’m reminded of a TikTok of three workers carrying a large sheet of glass, and the second a fourth worker touches the glass, it shatters from the torque), since, as you mentioned, you’re dealing with uneven terrain; the key is to arrange them so that you don’t reduce the support too much in any one location. Perhaps multiple columns, rather than trying to force one large one?
1 Like
Thanks guys for the input. In no particular order:
- I think some training wheels (legs) is not a bad idea. I’ll try without it first and see how tippy it is.
- Plywood instead of 2x6 is a good thought, probably better. But I might go 2x6 anyway.
- Gussets in the corners could help the top stay square (or consistent anyway) in the XY plane, and end boards can keep the 2x6 rails upright (also reduce twist).
- I’m hoping the diagonal bars are stiff in tension as well as compression, but not in bending. Potentially stronger in tension because compression can have Euler buckling but maybe stiffness is the same under low loads.
- The connectors havent been fully thought out. I was considering printed joints with all the proper angles and they could clamp onto tubes or drill/screw into the tube to hold it (or both). I wouldn’t be able to use PLA because this part will stay outside all year and even under the patio cover it will get hot.
Other parts are coming along nicely. I’ll post an update on those soon.
I keep forgetting that I need 6 drivers. I’m thinking of wiring Z in series to start with, and staying square in XZ by parking on some blocks.
I am also thinking that the second tool will home toward -X and touch off the first tool, instead of homing toward +X. My thought is that this will produce a more consistent offset between the tools, which is critical. For example if belt tension changes over time, homing on opposite ends will see a larger relative error compared to homing both toward -X.
5 Likes
A bit over-eager in assembly and probably going to need partial disassembly to add endstop wires and belts, but I couldn’t wait to get this together.
I didn’t initially add the three bolts to attach the XZ to the gantry beam (instructions said I could add them later). I found I had to partially disassemble to add them. I’m assuming the access holes in the YZ plates have been enlarged but the DXF I used still had the small holes. No biggie, just a couple minutes.
All in all, it’s been really smooth. I maybe should have used a finishing pass on the YX plates and truss plates because they are slightly oversize (undersize holes) which has made it a bit of a tight fit, but that only slows down assembly slightly.
Soon I will have no choice but to tackle the table.
8 Likes
So my take on this is both are going to do the same thing, correct?
1 Like
Yes in the sense that they will both have routers and cut stuff, but the point is for the two routers to have different router bits and tool paths. Generally they will take turns, not cut out two copies of the same thing.
Which reminds me, I will need a couple more solid state relays if I’m going to turn them off when they’re not being used.
3 Likes
OHHHHH, cannot wait for the youtube on this one, that sounds COOL!!!
3 Likes
Update: I bought some 2x6 lumber and I am changing my mind about the table.
Connecting all those struts at odd angles in the previous design was making me nervous, so I’ve reverted to a simpler, more traditional style, with a short table top at each end made from OSB. The table top is outside the working area, and is for convenience and perhaps more importantly, stiffens the frame from racking in the X-Y plane.
I’ve decided I’d like to use a rail on the left side (negative X) for a hard, straight surface for the bearings to roll on. I don’t have a good way to make a 2x6 perfectly straight, but with shims, I can tweak the rails on both sides to be as straight as I can measure. We’ll see after the humidity changes.
Using a left-side rail means the supporting 2x6 has to be offset downward relative to the belt ends and I’ll need a spacer between the left 2x6 and the table top. On the right side (positive X) I’ll need cutouts so that the top surface remains flush with the top of the 2x6.
Horizontal 2x4’s (green) hold the 2x6’s and support the table top. Vertical 2x4’s (red) serve as legs.
Without the diagonal braces (grey) I’m predicting the table will be very floppy in torsion. I am thinking I’ll shim the feet until the top is perfectly planar (two rails are parallel), and then add the braces. After that, I’m thinking it should be stiff in torsion.
When I go to move the table to a very non-flat floor, I can support it at the middle of the far end (center of blue beam). I can either use it while it’s supported in the middle for a somewhat unstable three-legged stance, or I can shim the feet for a four-legged stance that remains flat.
I am comfortable with this idea, and I have all the lumber and hardware, so now I just need to do the work.
6 Likes
Yes shims will do the job, but maybe the use of mason ruler could help on perfectly straight. I saw 2 different way :
- Use one on top of of your yellow beam and trim it with a router and a copy bit.
- Use 2 mason rulers or steel profils in place of your yellow beams
3 Likes
