Shapeoko 3 - Expansion Packs

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cvoinescu
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Re: Shapeoko 3 - Expansion Packs

Post by cvoinescu » Thu Sep 03, 2015 12:42 pm

The beauty of that table is that you can get it flat without the sheet material being particularly flat to begin with, and without a flat reference surface to work on. It relies on the plywood frame members being cut straight and parallel and all to the same width. As long as you do that, and install them square on the plywood skins, the resulting table will be flat. This is why the frame is plywood: timber would have been easier, but it's not straight enough. (Of course, it does not hurt to have a flat table already.)
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twforeman
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Re: Shapeoko 3 - Expansion Packs

Post by twforeman » Thu Sep 03, 2015 12:52 pm

Here is a good video about building a torsion box table.

No flat reference surface required to start - creating one is the first step.

http://www.thewoodwhisperer.com/videos/ ... rsion-box/
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veng1
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Re: Shapeoko 3 - Expansion Packs

Post by veng1 » Thu Sep 03, 2015 1:59 pm

Most of what I cut is about an inch or two into a box that is 12 inches tall (~300mm). My solution with the SO2 was to build a cart out of relatively large extrusion and then bolt the SO2 to the top. I then built a semi-permanent fixture to hold the part that I want to cut below the normal work surface. This works fine for what I primarily do but not so well when I want to cut "normal" height stuff. For that I slide in a piece of plywood that is also sacrificial that is just discarded when it gets too cut up. Admittedly less than ideal but it makes clamping the work easy as for that I just drive screws into the plywood. Homing switches insure repeatability independent of where the plywood ends up.

I'd suggest if you want a bigger table, make it more rigid than you think you need. And if you are going to do hundreds (I'm close to a thousand) of parts that are the same or at least similar, make your work surface, table, cart, ect. out of metal with a real clamping method. I use Carr-Lane clamps but there are lots of good solutions.

I've learned that poor fixturing can cause one to scrap a lot of expensive materials.

chamnit
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Re: Shapeoko 3 - Expansion Packs

Post by chamnit » Thu Sep 03, 2015 2:40 pm

TomDChi wrote:Watching that video (like a torsion box, but members only running one way, and using 1/4" ply as the top and bottom skin) I was a little skeptical that you'd get good stiffness. But if I understand correctly, you do structural aerospace engineering, so I'm thinking you know what you're talking about!
Yes, I do research in aerospace structures. Norm's table is perfectly adequate for the purpose and the dimension (4'x6'). If you want a bit more rigid surface, you can increase the depth of the table from 4" thick to something like 6" (which I'll be doing). That will greatly increase the rigidity. Much better than adding a lengthwise spar down the middle. This is because plate rigidity (bending stiffness) is a 3rd power to thickness. Simply bumping up the table thickness to 6" will increase the table bending stiffness by over 3x. (8" thick will bump to to 8x stiffer).

FWIW, it doesn't hurt to add one or two more cross ribs to Norm's table either. It doesn't do much in terms of stiffness but it does help with giving better attachment of the stress skin top and bottom, which will help with long term durability. It also helps with sound vibration with the large cavities, which will act like drum heads. I've seen people add loose batt insulation inside that helps tremendously with this.

The problem with the typical MDF torsion box, like the one in the Woodwhisper video, is that they are made up of dozens and dozens of parts. All glued or nailed together. When you build something that is supposed to be precision, you want to make the minimum number of parts as accurate as possible. In this way, the accumulation of error won't effect your final product as much. Norm's simpler table has only a handful of large, easy to cut accurate parts, so the total accumulation of error will be much less. This is the main reason I like it so much.

While the Woodwhisper's torsion box will be stiffer and more durable, it's also a lot more material and weight, probably several hundred pounds total. That weight can also eventually cause the MDF to sag over time. You have to remember that weight is also a big consideration. Plus it's more expensive (more material).

That said, if you are trying to build a 8'x4' table, you may need to add a middle lengthwise spar to help with sag over time since the table will be longer than Norm's 6'x4' one. But, in this case, I would probably avoid an all wood table, but reinforce it with some aluminum extrusions or build a steel base. Or split the table top length-wise into two 8'x2' frames(a la Ron Paulk's torsion box) and screw them together, so the center spar is double the thickness. But, I would make sure to apply the stress-skin top and bottom over both frames as single pieces of material.

There is an engineering rule of thumb that the aspect ratio of a beam or platform must be at least 20:1 (length to depth) to have a reasonable stiffness. For wood, this is probably more like something around 12:1.
Last edited by chamnit on Thu Sep 03, 2015 2:59 pm, edited 3 times in total.

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Re: Shapeoko 3 - Expansion Packs

Post by chamnit » Thu Sep 03, 2015 2:47 pm

twforeman wrote:Here is a good video about building a torsion box table.

No flat reference surface required to start - creating one is the first step.

http://www.thewoodwhisperer.com/videos/ ... rsion-box/
A while back, I've watched that video and almost made his torsion box for my shop. I then watched another video he posted a couple of years later about what he thought about it after using it for a while. The main thing was: He thought it was overkill over what he needed it for. I think this is the video (https://youtu.be/Aq9JWPgXwtc), where he talks about it.

The WoodWhisperer ended up making an out feed table (https://youtu.be/U3Wt3R4S6dw), based on Norm's work table. (Except he made the mistake of not adding the top/bottom stress-skin layers underneath the table top. Not good structurally. It will cause the top to twist like a canvas painting.) He joined the 90deg legs with dominos, instead of biscuits like Norm did, but I think you could do it with butt joints and screws or pocket holes just as well. From what I remember, he's been recommending to build this type of work table instead. It's easier to build, easier to move around, and adequately stiff and flat for a woodworking shop.
Last edited by chamnit on Thu Sep 03, 2015 3:14 pm, edited 1 time in total.

TomO
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Re: Shapeoko 3 - Expansion Packs

Post by TomO » Thu Sep 03, 2015 3:12 pm

chamnit wrote:
TomDChi wrote:Watching that video (like a torsion box, but members only running one way, and using 1/4" ply as the top and bottom skin) I was a little skeptical that you'd get good stiffness. But if I understand correctly, you do structural aerospace engineering, so I'm thinking you know what you're talking about!
Yes, I do research in aerospace structures. Norm's table is perfectly adequate for the purpose and the dimension (4'x6'). If you want a bit more rigid surface, you can increase the depth of the table from 4" thick to something like 6" (which I'll be doing). That will greatly increase the rigidity. Much better than adding a lengthwise spar down the middle. This is because plate rigidity (bending stiffness) is a 3rd power to thickness. Simply bumping up the table thickness to 6" will increase the table bending stiffness by over 3x. (8" thick will bump to to 8x stiffer).

FWIW, it doesn't hurt to add one or two more cross ribs to Norm's table either. It doesn't do much in terms of stiffness but it does help with giving better attachment of the stress skin top and bottom, which will help with long term durability. It also helps with sound vibration with the large cavities, which will act like drum heads. I've seen people add loose batt insulation inside that helps tremendously with this.

The problem with the typical MDF torsion box, like the one in the Woodwhisper video, is that they are made up of dozens and dozens of parts. All glued or nailed together. When you build something that is supposed to be precision, you want to make the minimum number of parts as accurate as possible. In this way, the accumulation of error won't effect your final product as much. Norm's simpler table has only a handful of large, easy to cut accurate parts, so the total accumulation of error will be much less. This is the main reason I like it so much.

While the Woodwhisper's torsion box will be stiffer and more durable, it's also a lot more material and weight, probably several hundred pounds total. That weight can also eventually cause the MDF to sag over time. You have to remember that weight is also a big consideration. Plus it's more expensive (more material).

That said, if you are trying to build a 8'x4' table, you may need to add a middle lengthwise spar to help with sag over time since the table will be longer than Norm's 6'x4' one. But, in this case, I would probably avoid an all wood table, but reinforce it with some aluminum extrusions or build a steel base. Or split the table top length-wise into two 8'x2' frames(a la Ron Paulk's torsion box) and screw them together, so the center spar is double the thickness. But, I would make sure to apply the stress-skin top and bottom over both frames as single pieces of material.

There is an engineering rule of thumb that the aspect ratio of a beam or platform must be at least 20:1 (length to depth) to have a reasonable stiffness. For wood, this is probably more like something around 12:1.
If the overall size of the so3 with the expansion kit comes out to be 5'x5' or larger - do you have any suggestions for making a 2 piece stress skin? Without looking into it - I assume sheet goods larger than 4'x8' are not readily available (I have seen 4'x9' but that wouldn't be any help in this situation).



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chamnit
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Re: Shapeoko 3 - Expansion Packs

Post by chamnit » Thu Sep 03, 2015 4:20 pm

TomO wrote: If the overall size of the so3 with the expansion kit comes out to be 5'x5' or larger - do you have any suggestions for making a 2 piece stress skin? Without looking into it - I assume sheet goods larger than 4'x8' are not readily available (I have seen 4'x9' but that wouldn't be any help in this situation).
Hmm, you're right finding sheet goods for a 5'x5' table may be a problem for both a traditional torsion box or Norm's simpler design. I've been thinking in a vein of 4'x4' or for a smaller 1m x 1m SO2 upgrade. Structurally, the spars and ribs of the frame is what provides the majority of the stiffness, while the stress-skin top and bottom keeps the frame from twisting. I suppose that's where the name torsion box comes from (even though it still irks me as an engineer, as it's really a hollow-core plate or stress-skin panel).

If push comes to shove, it's ok to glue the skin from pieces if it's well balanced/uniform and relatively flat. For example, place two 5'x2.5' thin sheets next to each other and then glue two more 5'x2.5' sheets at a 90deg angle from the others. Kind of like making thicker plywood out of thinner plywood. It's not ideal or the prettiest though. You'll also have a lot of waste trying to make one out of 8'x4' stock. A cut list doesn't work out well.

I think it would probably be easier to build two 5'x2.5' Ron Paulk-style torsion boxes (probably not as deep/thick) with cutouts to access the insides, join them together afterwards through the cutouts, and place them on a sturdy base frame. You could use the shims in the base frame (or some other adjustment device) to align the two torsion box halves together with a simple straight edge. Then lay a 4'x4' MDF sacrificial board on top of that... That should work.

EDIT: Probably the best solution for this size table would be welding or building a metal (steel or aluminum) 5'x5' frame like @veng1 suggested. But, not everyone has the $$$ or the equipment to weld it together.

twforeman
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Re: Shapeoko 3 - Expansion Packs

Post by twforeman » Thu Sep 03, 2015 5:57 pm

Ah. Thanks for the feedback on the Wood Whisperer video. I used to watch his videos all the time, but I've been neglecting them.

I agree that lots of parts leads to error, and frankly was a bit concerned about it.

My S1 table is a 2' x 4' sheet of 3/4 MDF glued and screwed to three surfaced 2x4s. It's adequate for that machine.

One thought I had was using 3/4" MDF for the top, but 1/4" or 3/8" MDF (or even plywood) for the bottom skin and ripped strips of 1/2" MDF for the ribs.

Lots of good information here to cogitate on. Good thing I haven't started building yet.
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krtwood
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Re: Shapeoko 3 - Expansion Packs

Post by krtwood » Thu Sep 03, 2015 11:36 pm

TomO wrote: If the overall size of the so3 with the expansion kit comes out to be 5'x5' or larger - do you have any suggestions for making a 2 piece stress skin? Without looking into it - I assume sheet goods larger than 4'x8' are not readily available (I have seen 4'x9' but that wouldn't be any help in this situation).
Baltic Birch usually comes in 5'x5' sheets (3, 6, 9, 12 or 18mm thickness) and is a lot stronger than typical plywood because it's birch all the way through. You won't find it at a typical home center but a decent lumber yard should have it. Alternatively I would do two layers of thinner material and overlap the seams, being sure there was structure under both seam lines.

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Re: Shapeoko 3 - Expansion Packs

Post by WillAdams » Thu Sep 03, 2015 11:58 pm

Yes, the problem is, we want a machine which will accommodate and be able to cut a full 5×5 sheet as one size option. The table for the machine needs to be larger than that.

Hopefully another upgrade will be able to cut a 4×8 (5x8? ) sheet of plywood and I'm hoping that individual axis upgrades will also available --- a 5' X-axis and stock 16" Y would suit me quite well. (yes, l know it would be stiffer going the other way).

One thing I'm still wondering about --- what is the upper size limit on the current frame size and steel thickness? At what size does something need to change?
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