CNC Router for Hardwoods: Evaluation and Questions - Page 5


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Thread: CNC Router for Hardwoods: Evaluation and Questions

  1. #49
    Community Moderator ger21's Avatar
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    Quote Originally Posted by PaulRowntree View Post
    The BB examples are interesting, but I am not sure that I could seal them as required for garage work in Ontario.
    Personally, I don't think the sealing is an issue at all. If there were "ventilation" holes to allow humidity to act equally inside and outside, I believe that even an unfinished BB gantry beam would not have any issues in your area, provided it was properly assembled with good adhesive bonds and tight fitting joints.
    I believe I'm only about a 3 hour drive from you, so our conditions here are about the same as yours.

    The gantry on my current machine is a BB torsion box with 1/2" (12mm) BB skins. I built it 10 years ago. It's unfinished on the inside, with the outside painted with 2 coats of Rustoleum Hammered paint. I suspect it will be in the same condition 20 years from now.

    If you want a really good seal on your parts, just roll on a coat of epoxy on the full sheets before cutting them.

    If you plan on assembling with epoxy, I'd recommend a seal coat on all the edges, as the end grain in BB ply will soak up epoxy until it starts to cure, which coiuld possibly result in a starved joint. There are two options here. One, seal the edges and let them cure, then scuff them up with some rough (80 grit or coarser) sandpaper prior to assembly. Or, two, seal the edges just prior to assembly, and don't assemble until the edge sealant coat starts to gel, where it will stop wicking into the endgrain.

    Make sure to use a thickened epoxy for assembly.

    David, are you using a surface prep or etching solution when epoxy bonding aluminum? West System's 860 surface prep increases bonding strength about 30%.

    Gerry

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    I have a few questions. I was ordering metal yesterday for a wall sculpture and I asked my supplier about a 4" x 8" x 1/2" aluminum tube. Yarde metal does not have it and the wished me "luck" finding it elsewhere. Perhaps I was asking for it the wrong way?

    I very much like the idea of using that beam in my build. The 8" x 8" x 1/4" tube is brilliant, but I don't think it would work well for my particular design. The rectangular cross section swaps in effortlessly.

    I was thinking that I might be better off fabricating a beam from scratch. I think this is what David was suggesting. I would be very curious to know more about this approach. and the fastener requirements. I think it would be much easier to keep the beam flat and the bulkheads would install with ease.

    "- Use Mic6 plate as a front surface and construct the rest of the section behind it. "



  3. #51
    Community Moderator ger21's Avatar
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    Quote Originally Posted by dmalicky View Post
    So that raises another difficulty for the DIY CNCer: the stiffer the gantry, the more important that the rail surfaces are flat. We used epoxy to try to make flat and parallel rail mounts, but our epoxy (Epoxy :┬*Kleer Koat Table Top Epoxy) didn't really level. So then we used small surface plates and long flat tubes to mark, hand-plane, scrape, and sand the mounts flat--quite a pain, and we didn't get them that parallel. Further, I could tell when we bolted the rails down that there was a piece of plastic in there--it just didn't feel rock solid, and I suspect the rails will shift under vibration, and we'll have to redo the whole thing.

    For a 3' gantry, I could see using an inexpensive 2'x3' surface plate to build, sand, or scrap it flat (or mill in a big Bridgeport). How would you recommend a DIYer build or make rigid, flat, and parallel rail mounts that are 5' long and 8" apart? Some ideas I've been tossing about...
    - Use Mic6 plate as a front surface and construct the rest of the section behind it.
    - First pour a large epoxy surface plate (that is really level), then lay down (in order): flat rail mounting bars (Mic6 strips?), JBWeld on top of those, then the 8x8 tube.

    I used to think: first get the gantry tube rigid, then find a way to make the mounts level. Now, I'd rather design the whole thing and process so it comes out rigid and with level and stiff mounts by design.
    Not exactly DIY (unless you already have a 5' DIY router, but I think the method I came up with will work very well for anyone building with wood. Ideally, you still need a VERY flat assembly table to keep it flat and straight during assembly.
    http://www.cnczone.com/forums/cnc_wo...ml#post1168468

    Gerry

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    Dave;

    Thanks for the update, this is providing a lot of insight into how to fabricate a stiff beam.

    Quote Originally Posted by dmalicky View Post
    Paul, yes, I can see how a Canadian garage would be a tough environment to keep a wood gantry true. I ran some variations on the diagonals, below (#15 vs 21, 22, 23) -- they all perform quite similarly.

    Good question on BB bulkheads in an alum tube. They are tempting since they would be easy to install, esp in 2 pieces. The FEA will say they will work fine as long as they are thicker (to compensate for the lower modulus). It doesn't take a very thick alum bulkhead to stabilize (0.1" is usually enough, although thicker is better to install), so 3/4" thick BBply would be about right. But, I'm not sure how they'll perform in reality, for 2 reasons. Looking at the deformation plots of a stiff gantry, we see the gantry itself is not moving more than 0.0005". So the joints between the BBply and the alum would need to be *really* good, which is hard to achieve over time with dissimilar materials experiencing big temp changes. The coef of thermal expansion of alum is much greater than that of wood, so epoxy bonds will likely break, and the bulkheads will try to punch through the walls when it's cold (distoring the rails). So, I try to stick with the same material.

    I'm not sure this is exactly how I'd do it again, but here's how we constructed our diagonal:
    Interesting though I see all those screws inside the tube as a huge assembly problem. Normally you install a rail precisely as a master rail and then install the second one parallel to the master. Generally that involves some fiddling with the screws on the slave rail. This is why I prefer having all fastening hardware accessible from the outside.

    The big problem with access from the outside is all the drilling and tapping required. That and you need a tube thick enough to support the threads for the linear rail bolts.

    - This CNC was a senior project for mech engr students, and they did almost all the work (I think it's really good for student work, but likely not up to many of your standards).
    Looks pretty good to me!
    - Our rails are the "T" style, so we had to bolt from inside, which means we had to use a diagonal rather than bulkheads. The 3/4" square alum bar is there to give the rails and the diagonal something to attach to, without them interfering with each other.
    That is one way to do it though you could just bolt through the tab on the diagonal.
    - The diagonal is made from 6 pieces of 1/16" alum (20"x~11" each). The students practiced with sheetmetal strips until they found the bend locations to get the diagonal to fit precisely. Each piece was bent individually on a brake, then pairs were riveted and glued together to form a 1/8" thick section Then that was riveted to the 3/4" square bar. After some trimming, the assembly slides in from 1 end (more precisely than I had hoped... they worked quite a while to get it close).
    - Once in place, we used wood blocks and hammered-in wedges to force the sheetmetal and square bar firmly up against the tube walls (no gaps), working in ~2' long sections at a time. That part wasn't easy. Then working from the outside of the tube, we drilled through both tube and the diagonal and riveted the diagonal to the tube -- that part was easy. A good pneumatic riveter makes 3/16" rivets a joy. The square bar was riveted from the bottom surface and bolted from the front surface.
    This is where is see bulkheads as the easier path. You would need to fabricate square plates with tabs folded up on each side. Like the diagonals achieving a tight fit would require a bit of work. Installation wise though you just stiff the bulkhead plate into the tube, find the depth and the drill for either rivets or screws.
    - If doing again, I would put the rivets closer to the sheetmetal bends. That little 1/8" distance between rivet and bend is an opportunity for the diagonal to bend open, reducing its stiffness. So we may add epoxy and more rivets to fix that, if our stiffness test shows it's not performing as it should.
    - The students spent quite a while getting it to fit. It might be faster, and it would probably fit the corners even better, if made like I described earlier: independent pieces joined to each corner, then middle panels join those as a sandwich with epoxy and bolts. But both and many other methods would work.
    I'm not sure that is any better assembly wise.
    - The diagonal doesn't need to be very thick to do its job. The FEA says 1/8" is fine, and thicker mostly adds weight. Basically, once the diagonal is stiff enough to keep the cross-section a square, there's little benefit to more thickness (compare #14, 15, and 23, below). The same can be said of bulkheads (number and thickness).

    Wizard, that's good info that the commercial machines using extrusions machine the surfaces for rails. I'm guessing that many CNCs can bolt them right on because T-slot extrusion is rather flexible and relatively straight. That combo means that if the surface is a bit out of flat, the linear bearing can distort it a little as it passes by.
    That is very possible. The linear systems I was talking about had extremely heavy extrusions so maybe they didn't have an option. Similar machines with steel beams are also machined but that is kind of expected.

    That's in contrast to our 8"x8" gantry tube, which we found was quite unforgiving. So that raises another difficulty for the DIY CNCer: the stiffer the gantry, the more important that the rail surfaces are flat. We used epoxy to try to make flat and parallel rail mounts, but our epoxy (Epoxy :┬*Kleer Koat Table Top Epoxy) didn't really level. So then we used small surface plates and long flat tubes to mark, hand-plane, scrape, and sand the mounts flat--quite a pain, and we didn't get them that parallel. Further, I could tell when we bolted the rails down that there was a piece of plastic in there--it just didn't feel rock solid, and I suspect the rails will shift under vibration, and we'll have to redo the whole thing.
    The unfortunate reality is that epoxy stiff enough to support the rails is usually to stiff to flow.

    In the end if people are really in need of high precision I don't think there is anyway to get around the need to machine the rail mounting positions. At times you can get lucky with an extrusion or even a steel beam and have the flatness required but that certainly won't happen with every beam purchased.

    Admittedly this requires finding a well equipped machine shop but this shouldn't add a huge expense to a beam especially considering the overall cost of the machine.
    For a 3' gantry, I could see using an inexpensive 2'x3' surface plate to build, sand, or scrap it flat (or mill in a big Bridgeport). How would you recommend a DIYer build or make rigid, flat, and parallel rail mounts that are 5' long and 8" apart? Some ideas I've been tossing about...
    Sometimes the smart move is to get outside help. I can see hand scrapping a 3' gantry but just adding two feet to that gantry makes the project far more involved. Probably far more expensive too to establish a surface plate to scrap a 5 foot beam against.
    - Use Mic6 plate as a front surface and construct the rest of the section behind it.
    One would conform against the other so I don't see a win here.
    - First pour a large epoxy surface plate (that is really level), then lay down (in order): flat rail mounting bars (Mic6 strips?), JBWeld on top of those, then the 8x8 tube.
    The JB weld might deal with the conforming issue but that would be a tricky glue up. Take from somebody that does a bit of woodworking it will take a bit of planning and some fancy foot work to pull it off.
    I used to think: first get the gantry tube rigid, then find a way to make the mounts level. Now, I'd rather design the whole thing and process so it comes out rigid and with level and stiff mounts by design.
    If you can pull it off that would be great for a manufacture but I really thinking that over all cost wise it would be cheaper to simply have the tube machined for a one off build. It is easy for a manufacture to justify the expense of a surface plate and fixturing to do what you describe. For a guy building a machine for his shop or even the guy that is building one for a factory he is involved in, getting the aid of a machine shop seems like a wiser move.
    1. Yes, the tube length is an issue. Our 8"x8" is 60" long, which coincidentally and fortunately is the longest tube that a ~6' person could reach inside halfway. I'm not sure how to handle something longer. Yes, working from the outside of the tube is very helpful.
    That tube length problem and the related access problem would rule out the design as you have posted here.
    2. I never thought of that. Sounds like a job for a big oven.
    Yeah the more I think about the more I want the idea to go away.
    3. Yes, plates or bulkheads are ~equally effective for stiffness. See runs #24-29 vs #15.
    This is good news! I see bulkheads as the easy do, especially if they can be riveted in place from the outside.
    4. Yes, that would work great for stiffness, although the mass would require expensive motors and drives.
    Or the design could be a moving table design. The other option would Ben a partial fill with a hollow center.
    5. I like that idea a lot.
    The question in my mind is does it offer any advantages over a plain fill of epoxy granite.
    Here are the corrected numbers on the prior runs, and some variations on the diagonals/bulkheads, to get a sense of 'how much is enough' and 'is more much better':
    Would you be willing to post the actual spreadsheet? I ask mostly do to the hope that the data is more legible.



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    Gerry, I'd not thought of just leaving a wood gantry uncoated to breathe--as long as the RH changes are gradual, that makes sense. For some reason I feel more comfortable trying to seal it, and I like the pre-applied epoxy idea. Or to be extreme, after it's built, I was thinking of 'veneering' alum foil over the whole thing, with alum tape for edges. Or phenolic faced ply would probably offer a good barrier for the faces?
    Thanks for the tips on epoxy assembly and the 860--I didn't know about that. We do the usual scotch brite + alcohol, which works pretty well as long as the surface area is high, but it sometimes fails in high stress areas.
    Also thanks for the link to your phenolic mounts. I was thinking of building the box beam first, then gluing/riveting on the mounts, then machine the surfaces. That way a flat assembly table isn't needed later. Disads to that you can think of?

    Sapele, yes, the larger tube sizes seem to have regional availability. Or you could try a different metal supplier. IMS has it but they are mainly SoCal. Ryerson and EMJ are bigger chains. Reliance owns a bunch of suppliers: Reliance Steel & Aluminum Co.
    There are also the web-based suppliers, but prices are typically much higher.
    I like the 8x4x1/2 as well, as it would be more likely to be straight, and easy to machine flat without the walls getting too thin. If it helps, you could also look for 6x6x1/2, which has a bit better performance (see run #32 in the xls below).

    pippin88, Yes, diagonal screws are an interesting idea, too. They would need to both push and pull (a tool load in the reverse direction distorts the tube the other way). One way to do that is get a grade 8 threaded rod, bevel the end to make it a 'tap', chamfer the outer corner of an alum tube, then drill and tap all the way through along the diagaonal. Nuts on the inside to secure, against triangle pieces like you describe.
    Steel is an easy allegory. Since its modulus and density are both 3x that of aluminum, the aluminum results scale by 3x. Shorter will be stiffer but I don't know of a simple formula. Keep in mind that steel is not really a cost save for a moving gantry machine: the more expensive motors and drives to drive it will more than offset the material savings.
    Yes, even 1 bulkhead helps quite a bit. Endcaps are easy and definitely help. The keyed design looks like a lot of machining to me. Also think about how you would machine an internal rectangle with sharp corners. I think dowel pinning and bolting the plates together would be far easier--that's the usual way.

    Wizard, Thanks for all your input--very helpful. Sure, attached is the xls.

    Interesting though I see all those screws inside the tube as a huge assembly problem. Normally you install a rail precisely as a master rail and then install the second one parallel to the master.
    We installed our master rail using a long precision straight edge for alignment--not as good as butting on a machined edge, I know. We could dab some JBW in the corners to keep it that way. The bolting from the inside was less convenient, but not a big problem, mostly just time consuming. We bolt the slave rail down progressively, 'driving' the gantry car along while it follows the master (face up to avoid gravity droop). I'd prefer rail with bolts on the outside, for sure, but automationoverstock.com only has the Hiwin T style in long lengths.

    That is one way to do it though you could just bolt through the tab on the diagonal.
    Yes, I forgot the other reason for the square bar, which was to give that lower rail a stiffer foundation to bolt to, since it takes the most load. But it may not be that important.

    This is where is see bulkheads as the easier path. You would need to fabricate square plates with tabs folded up on each side. Like the diagonals achieving a tight fit would require a bit of work. Installation wise though you just stiff the bulkhead plate into the tube, find the depth and the drill for either rivets or screws.
    - I'm sure your sheetmetal skills are better than mine, but that sounds much more difficult to me IF it is a full bulkhead attached to all 4 sides. The diagonal's edge could be trimmed to fit, but once a tab is bent, it's tough to move the bend line. Maybe you have something else in mind, though.
    - OTOH, triangular bulkheads that only touch the front and bottom face would be easy and are reasonably effective. I'd probably make them of 3/8" plate--overkill, but easy to tap 1/4-20 into.
    - One complication to bulkheads is that the faces of the alum tube are not very flat. Ours are all concave by about 0.060". The bulkhead would force it flat locally, which would look odd from the outside and introduces a pre-stress, which could hurt long-term stability. A fix for that is to relieve the bulkhead faces except near the corners, like this (5 of these gives a respectable stiffness of 74k, see run #31):
    CNC Router for Hardwoods: Evaluation and Questions-screenshot147-jpg

    I'm not sure that is any better assembly wise.
    Thinking more about it, I agree, thanks.

    The unfortunate reality is that epoxy stiff enough to support the rails is usually to stiff to flow.
    A-ha! I didn't know that at all. Very helpful info.

    In the end if people are really in need of high precision I don't think there is anyway to get around the need to machine the rail mounting positions... Sometimes the smart move is to get outside help.
    Yes, I avoid farming stuff out, mostly for cost as often the quotes are so high. If the quote is similar to the cost of a tool for me to do it myself (e.g., a suitable surface plate), I'd rather buy the tool to improve the shop. I agree it all depends on the level of precision required. I keep thinking there is a clever DIY solution, and I love to find those, so I'll think about it some more. But your point is well taken I'll get some quotes for our job.

    (Mic6) One would conform against the other so I don't see a win here.
    I was thinking that the sides of alum bar (e.g., 3/8"x8") tend to be very straight and could be made right-on pretty easily with a long precision straight edge. So that edge would pull the Mic6 flat. That would make each rail mount straight, but thinking more, there's a remaining question of whether those straight lines would be in the same plane. Finishing the assembly on a 3' surface plate could get that part. But farming out the machining would be more reliable.

    The JB weld might deal with the conforming issue but that would be a tricky glue up. Take from somebody that does a bit of woodworking it will take a bit of planning and some fancy foot work to pull it off.
    Yep, agreed. A suitable epoxy that is thicker (putty or peanut butter) would help the mess a lot. Not sure if that exists... something from Devcon?

    That tube length problem and the related access problem would rule out the design as you have posted here.
    Not sure which design you mean? A 4' travel gantry tube needn't be longer than 60". I put endcap/risers on all the FEA models only because it works for all models. But they aren't really needed for a diagonalized tube.

    4. Or the design could be a moving table design.
    Ah, yes I like those a lot. With weight a non-issue, then we can go to big and thick steel tubes which are massively stiff even without reinforcements (see runs #33-39). For a successful machine tool, one question I have is how to prioritize stiffness and damping. More of both is good, of course, but could enough stiffness negate the need for damping? My thinking is 'maybe': damping is needed to cut vibration amplitude, but extreme stiffness cuts vibration amplitude as well.

    Attached Files Attached Files
    David Malicky


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    Community Moderator ger21's Avatar
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    Also thanks for the link to your phenolic mounts. I was thinking of building the box beam first, then gluing/riveting on the mounts, then machine the surfaces. That way a flat assembly table isn't needed later. Disads to that you can think of?
    Main disadvantage is that it's a lot more difficult to machine the large tube, rather than the single face.
    I'm using an idea I came up with to make assembly pretty simple. The parts themselves will keep the beam pretty flat and straight. I made the top and bottom plates wider than the finished beam, with dadoes to accept the front and back panels. The edges of the front and back are machined for thickness to closely match the dado width. Since the front and back are machined for width, when they are assembled in the dadoes, the entire assembly should be straight in both directions along it's length. The internal bulkheads keep the beam square.


    A suitable epoxy that is thicker (putty or peanut butter) would help the mess a lot. Not sure if that exists...
    Make your own. I use US Composites resin, because it's 1/3 the cost of West System, but start with any good laminating epoxy. Add West System 404 High Density filler or 406 Colloidal Silica or US Composites Aerosil-Cabosil filler to get the consistency that you need.

    I highly recommend reading through the West System How-To guides, and subscribing to their free magazine, Epoxyworks. Lot's of great info on using epoxy, especially on how to bond fasteners into wood. I'm using their techniques extensively on my mostly wood machine.
    How To Use

    You can do some incredible things with epoxy and wood, without needing any metalworking tools and equipment. The downside is you need to put a bit more effort into it.

    Gerry

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    Can a fabricated beam rival a square or rectangular tube of the same size? I suspect the type of epoxy and fasteners are a very important part of the equation. I have not researched either enough to know what epoxy to use other than the offerings from west system. I would like to use 1/4-20 low profile socket head cap screws for the construction, but have no idea about the spacing.

    A fabricated beam is attractive to me for three reasons:
    1-flexibility in getting the right size beam for the machine
    2-the torsion box like construction should result in the easiest path to well fitting bulkheads
    3-aluminum works so easily in a wood shop that I don't see why the finished beam would not be as flat or flatter than a tube.

    Here is what I have come up with based on all of the amazing posts about beam design

    the components around the perimeter are 1/2" while the bulkheads are 3/8"
    CNC Router for Hardwoods: Evaluation and Questions-beam-1-jpg

    1/2" x 1" aluminum strips are placed behind the rails for added structure
    CNC Router for Hardwoods: Evaluation and Questions-beam-2-jpg

    CNC Router for Hardwoods: Evaluation and Questions-beam-3-jpg

    3/8" or 1/2" x 2 phenolic strips are glued to the beam
    CNC Router for Hardwoods: Evaluation and Questions-beam-4-jpg

    Not everyone will have access to a cnc router, as ger21 points out, to flatten the phenolic strips. Everyone will have rail and block on hand that can be used to make a flattening jig for a hand router. I realize that the results will only be as flat an co planar as the accuracy of the temporary router sled. Good enough? I am all for the do it in house approach that dmalicky champions and this is the best that i can come up with.
    CNC Router for Hardwoods: Evaluation and Questions-beam-6-jpg



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    If you plan on building an aluminum torsion box, then I'd use aluminum for the rail mounts instead of phenolic. It's not much more difficult to route than phenolic.
    If going this route, I'd bolt a separate small block on the edge for my alignment ledge. This way, you only need to machine a very minimal amount of aluminum to achieve your flat and straight surfaces.

    As far as fastener spacing, that depends a little on the thickness of the materials your fastening. The thinner it is, the more fasteners you need to get it flat. A small prototype should give you a good idea of the spacing required. Personally, I'd probably use a spacing in the 2"-3" range.
    In this application, the fasteners carry the weight and loads. The adhesive makes the structure more rigid by making the structure act as a single piece. Without the fasteners, there's a good chance that the adhesive bond alone would eventually fail.

    I'd do some research on bonding aluminum to find the best adhesive for the job. Aluminum is tricky to bond as it oxidizes nearly instantly. You want to scuff up all surfaces to be bonded with 50-80 grit sandpaper, and use a prep like the West System mentioned above.
    You probably want an adhesive that doesn't get too brittle, as expansion and contraction of aluminum will weaken the bond of a hard and brittle adhesive. I've found that with the US Composites epoxy, the faster curing hardeners produce a harder, more brittle joint than the slower setting ones. So I'd go with the slower setting for a more pliable bond.
    There are also specialty epoxies and adhesives just for epoxy. A quick Google search turned up this.

    http://www.masterbond.com/tds/supreme-11ht


    3M makes some very strong structural adhesives, but they can be extremely expensive.

    Another consideration is the actual choice of aluminum. Aluminum plates can have a lot of warp in them, and may be tricky to get flat and straight. However, aluminum is also fairly easy to work with with standard woodworking tools, so it shouldn't be too bad. You can probably fabricate something very similar to my wood beam with similar techniques.

    Edit. I originally had a link to a stronger adhesive, but it required high temps to cure, which probably isn't feasible for a DIY application. It was quite a bit stronger, though.

    Last edited by ger21; 01-12-2014 at 12:04 PM.
    Gerry

    UCCNC 2017 Screenset
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    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Thanks for the feedback about fastener spacing, glue and the phenolic blocks. I think this approach is worth pursuing for my build. I am not sure why I scaled the outer faces down to 1/4"? I will price the parts on Monday with 1/4" 3/8" and 1/2" faces. I liked the idea of targeting a beam that scored as well as the 4" x 8" x 1/2" tube.



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    Gerry, thanks for the suggestions! Since you've had good success with them, I'd be fine trying other US Composites products, esp at 1/3 the price. I'm not sure why their Table Top epoxy didn't level for us. We followed the directions and our flood coat was a bit more than 1/8" thick, to help it flow. It made a very nice mirror surface with no bubbles, waves, or rubbery spots, but each 'pad' came out ~parallel to the surface below it, instead of perpendicular to gravity. So the 2 pad surfaces were not in the same plane. Maybe its viscosity is too thick to level to gravity. Or maybe it needs to be thicker, but the directions say 1/8".

    For a thickening epoxy filler to bridge between a long aluminum pad and the big tube, how does their Aerosil-Cabosil compare to the "1/32" Milled Fiber" or "1/4" Chopped Strand"? We would want something as stiff as possible when cured. Maybe some of both?

    Sapele, Yes, a fabricated beam can be competitive if it has good bulkheads or diagonals. I'll assume no epoxy or it has failed (worst case).
    - A bolted 90 deg joint is less stiff in lateral bending/opening than a continuous tube joint or welded joint. A continuous tube w/o bulkheads or diagonals depends on that bending/opening stiffness of the 90 deg joints so it doesn't collapse in the parallelogram mode. But adding bulkheads or diagonals changes the loading on those 90 deg joint from lateral bending to longitudinal shear.
    - Shear is very easy to design for: bolts to clamp and dowel pins to locate are the usual methods. Press-fit dowel pins are great for shear and precise locating, although accurately reamed holes that produce a press-fit are a bit risky (if they come out big, the pin is loose... reaming 0.002" undersize helps). Bolts are great as long as they are tight enough; the goal of a good bolted joint is that the contact pressure is so high that the friction prevents any movement. I.e., the bolt itself neither locates nor takes shear; it only compresses the surfaces tight enough.
    - If confident in reaming undersize, a pattern of 2-4 bolts followed by 1 pin might be good. Or just all bolts with Loctite.
    - Good advice from Gerry on fasteners and bonding.

    Your beam design looks pretty stiff.
    - A little deeper section than 3.75" would be best for stiffness/weight ratio.
    - The bulkheads could be every 9" and be ~equally effective.
    - With 1/2" walls, the 1/2x1 alum strip is probably not needed for structure, although it would give added thread depth in case a shallower bolt strips.
    - If using the strip, I would put a small intentional gap between it and the bulkheads (no contact); else, if tolerances are off, the top plate won't sit flat on both the strip and the bulkheads.

    The flattening jig looks promising--thanks! As you mention its effectiveness depends on the straightness and parallelism of those rail mounting surfaces, and (more easily accomplished) the straightness of the lateral sled travel at its ends.
    - To check the straightness/flatness of each mounting surface, a 4'-5' precision straight edge is about $250: Enco - Guaranteed Lowest Prices on Machinery, Measuring Tools, Cutting Tools and Shop Supplies. Shim stock / feeler gauges or sanding can correct those errors.
    - To check parallelism of the 2 mounting surfaces, place the same straight edge diagonally across the mounts at the ends. Measure the depth from the straight edge to a surface (fixed) between the rails. Then place the straight edge along the other diagonal; measure depth again to same surface and compare. (That's how we found out our 8x8 tube had twist warp, and that the epoxy didn't level.)
    - To correct parallelism, careful sanding the assembled rail mounts on a 2'x3' surface plate ($200) might work. Or probably better, place a master precision level perpendicular on the sled and drive lengthwise. Then adjust leveling screws/clamps (on a rigid table) until the bubble is stationary (this is how machinists level lathes and mills). If each of the rails is straight and stays straight, you now have 2 lines in the same plane.

    David Malicky


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    3.75" is the proposed thickness of the core of the torsion box. Does the thickness of the faces add to that number? with 1/2" faces the beam would be 4.75" wide. should I shoot for adding another 1/2" to the width? Another inch? This also brings up a question about weight (and I suppose price of materials too)- do both faces need to be the same thickness? I like the idea of 1/2" on the rail side and 1/4" on the back side. The resulting beam would be 9" high and 4.5" wide. I will adopt the 9" bulkhead spacing. Thanks!



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    For a thickening epoxy filler to bridge between a long aluminum pad and the big tube, how does their Aerosil-Cabosil compare to the "1/32" Milled Fiber" or "1/4" Chopped Strand"? We would want something as stiff as possible when cured. Maybe some of both?
    I've never used the fiber fillers, so I don't really know. The fiber filler that West System sells is recommended for wood, and gap filling. Their high density filler appears to be a bit stronger.

    I would use either the cabosil or West High Density filler. Surface prep is the most important issue in this application imo.

    Gerry

    UCCNC 2017 Screenset
    http://www.thecncwoodworker.com/2017.html

    Mach3 2010 Screenset
    http://www.thecncwoodworker.com/2010.html

    JointCAM - CNC Dovetails & Box Joints
    http://www.g-forcecnc.com/jointcam.html

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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