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Thread: CNC Router for Hardwoods: Evaluation and Questions

  1. #37
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    Thanks for the explanation dmalicky. I am going to have to read this a few more times. The bottom line is that I am going to ditch my single 3060 beam for sure. I also am happier with my choice of having the rails mounted to the z carriage. The variable Z means that I can "stiffen" everything up by raising the cutter if I decide to cut some aluminum on the machine.

    Back to the Y beam: The Misuni is my first choice unless there is something more compelling about the double 3060 beam? The price is similar $424 vs the $477 for the Misuni

    CNC Router for Hardwoods: Evaluation and Questions-sixorwindow-jpg

    Two 3060 beams married together with 1/4" x 6" aluminum plates. The bearing spread of 10.4" changes the forces at the cutter to 105/205 for the bearings and 693 for the effective stiffness of bearing at cutter. (the Misuni plugs into the xls as follows: 183/283 and 323 at the cutter)



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    Quote Originally Posted by Sapele View Post
    Thanks for the explanation dmalicky. I am going to have to read this a few more times. The bottom line is that I am going to ditch my single 3060 beam for sure. I also am happier with my choice of having the rails mounted to the z carriage. The variable Z means that I can "stiffen" everything up by raising the cutter if I decide to cut some aluminum on the machine.

    Back to the Y beam: The Misuni is my first choice unless there is something more compelling about the double 3060 beam? The price is similar $424 vs the $477 for the Misuni

    CNC Router for Hardwoods: Evaluation and Questions-sixorwindow-jpg

    Two 3060 beams married together with 1/4" x 6" aluminum plates. The bearing spread of 10.4" changes the forces at the cutter to 105/205 for the bearings and 693 for the effective stiffness of bearing at cutter. (the Misuni plugs into the xls as follows: 183/283 and 323 at the cutter)
    The 8020 stuff doesn't compare, the wall thickness is probably 1/4 that of Misumi's, which in my opinion negates any benefit of the increased spread. Besides you can get three Misumi GFS8-4590 and arrange in a C configuration, or use two of those extrusions along with a C channel bolted between, which would tuck the ballscrew away nicely.

    But really, if you're going to to just stack two 3060 extrusions like that, you may as well just get 1-14" aluminum tooling plate.



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    I realize that there would be better ways to use the 3060. That was my half hearted attempts just to get a sense of the price difference. The Misumi 100200 is still my choice. If anyone can think of a better beam for a similar price I would be very happy to consider it.

    Louieatienza what size C channel comes to mind in the configuration you suggested? two GFS8-4590 and a 6" or 8" makes an interesting beam.

    many thanks, as always



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    Also "thanks a lot" from a lurker!
    Am new to all this and find the level of discussion flabbergasting! *g*

    I hope to skill myself in the next years to such a level that I can build my own furniture -- dabbling in traditional solutions atm but already looking a few years ahead.

    Be assured that I will come back to this very thread in some moons!



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    Quote Originally Posted by Sapele View Post
    I realize that there would be better ways to use the 3060. That was my half hearted attempts just to get a sense of the price difference. The Misumi 100200 is still my choice. If anyone can think of a better beam for a similar price I would be very happy to consider it.

    Louieatienza what size C channel comes to mind in the configuration you suggested? two GFS8-4590 and a 6" or 8" makes an interesting beam.

    many thanks, as always
    I don't think you have to go too crazy with the C channel width. I'm thinking more like 3-4". You could use another 4590 but it would be more difficult to attach and add undue weight.



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    Yes, it's a good strategy to lift the Z to increase stiffness.

    Also for everyone following this thread, I looked into some alternate gantry designs. The search for 'better' largely depends on priorities--cost, labor, weight, simplicity, stiffness, preferred material and processes, etc. Weight is an issue because larger motors and drives are needed to drive a heavy gantry (or, it will be slower). So, here are 20 options; the most promising ones (IMO) are in bold:
    CNC Router for Hardwoods: Evaluation and Questions-screenshot141-jpg

    Column J calculates the ratio of Stiffness divided by Weight--a measure of how efficient the cross-section is at being stiff. T-slot extrusions are not very efficient for a few reasons. Similarly, Column L is Stiffness/Cost -- 'bang for buck'.

    Designs 3 and 4 are the twin 3060 ideas. Here is #4:
    CNC Router for Hardwoods: Evaluation and Questions-twin3060-c-jpg

    Design 5 adds another C-channel to make it a mostly closed section. It's stiff but quite heavy. Here's the cross-section:
    CNC Router for Hardwoods: Evaluation and Questions-twin3060-2c-jpg

    Designs 6 and 7 use two 40x160 extrusions top and bottom for rail mounting, with alum sheet front and back to make a box tube, plus some channel bulkheads to stabilize the cross-section. Among the T-slot designs, it has the best ratios, but I don't know if the assembly would produce flat rail mounting surfaces. Here's the FEA and channel bulkheads:
    CNC Router for Hardwoods: Evaluation and Questions-efs-jpg

    Designs 8 and 9 are just solid barstock.

    Designs 10-16 are regular alum tube, often with a diagonal sheet inside to stabilize the cross-section. Cost is much lower than T-slot extrusion, but these will generally have more warp in them, so the rail mounting surfaces would need to be machined or epoxied flat. (As I understand it, 8020/Misumi extrusion is generally straight enough to mount profile rail w/o leveling, or at least many people do that.) Of these:
    - Design 12 is attractive because the thick walls don't need an internal diagonal sheet, yet it still performs well.
    CNC Router for Hardwoods: Evaluation and Questions-4x8x-5-jpg

    - Designs 15 and 16 give the highest stiffness and best stiffness/weight. As usual, big tubes rock! But these use internal diagonals or bulkheads, which I've done and can say is a fair bit of work. We used Design 15 on our 4x8 router -- our 8x8 tube came with about 1/8" of twist along its length, so leveling epoxy would likely be needed. This approach is probably overkill for a wood router, but would be a good choice for cutting aluminum.
    CNC Router for Hardwoods: Evaluation and Questions-8x8x-25-jpg

    I also tried some Baltic Birch Ply designs, Designs 17-20 . These perform very well in all categories: light, stiff, low $, reasonable labor. The cross-sections are similar to Design 15 with thicker walls. Well-sealed from moisture, they should be stable. A 12" jointer could flatten the front surface, if needed. The rails would need a stiffer surface (metal) to mount to, so something like a 1/4" alum bar or plate on the front would probably also be needed.

    In general, you can see that to get the highest stiffness and the best stiffness/weight, aim for:
    1) A big cross-section
    2) As much material as possible to the far outside of the cross-section
    3) Bulkheads or diagonals to stabilize the cross-section from 'collapse'. In wood, these are easy to do. In metal, they are doable but not easy. But, if the wall thickness is large enough, the cross-section is stable without bulkheads or diagonals. T-slot extrusion tends to have enough internal material to be pretty stable, although all that internal material makes it pretty inefficient for stiffness.

    So, as with most design problems, there's no "right answer", but there are some interesting and good choices depending on priorities, budget, and capabilities.

    Last edited by dmalicky; 01-10-2014 at 12:11 PM.
    David Malicky


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    Dmalicky-

    This post is going to help so many people like myself. Thanks! The 8020 stuff is all over the internet and becomes an easy choice, that is how I ended up with it in the first place. The chart with the 20 options quickly shows that the convenience of the T-slots comes at a major cost. I have seen your 8' router with the 8" x 8" beam in another thread. I could not find any more information on the diagonal sheet bulkhead. Has it been discussed in other threads?

    great post!



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    David, thank you very much for your information. It is quite useful and perfectly timed (for me). The BB examples are interesting, but I am not sure that I could seal them as required for garage work in Ontario.
    Would there be an advantage to dividing the diagonal panel into ~12" long panels and inserting them in alternating directions (up to the left, then up to the right, then ...)? How would BB bulkheads compare to the diagonal panels?
    Cheers!

    Paul Rowntree
    Vectric Gadgets, WarpDriver, StandingWave and Topo available at PaulRowntree.weebly.com


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    Hi David M.

    Great post, thanks for taking the time. I have a few comments and questions if you have the time.

    Comments:
    1. If somebody is looking to produce a precision machine out of an extrusion, the mounting points for the linear rails will most likely need to be machined. In the many different machines I've dealt with over the years, those made with extrusions had the rail mounting surfaces machined. Of course I would imagine that what one considers to be precision has to be factored in here. You also have to consider excess wear on the linear guides if the mounting surface isn't precise.

    I only bring this up because many people discount steel or aluminum plain tubing because they "require" machining to flatten the rail mounting surfaces.

    Questions:
    1. You show various forms of diagonal bracing, what is your preferred method of fastening the panels in place? In a long tube I can't imagine any that are ideal, mostly due to access. That is welding bolting and riveting all look to be difficult to me. You take a six foot beam as an example, assuming you can get your arms into the tube with the diagonal in place, it just looks to be all around difficult to fix the plate in the tube.

    I'm almost thinking a diagonal bent in a Z profile would be the easy path here. The idea being to drill and tap from the outside of the tube with the diagonal in place. You still have the issue of precise enough fit up in stock heavy enough to do the job. Curiosity is killing me here because it is easy to understand the diagonals benefits but far harder to understand how to pull it off.

    2. Have you eve considered heating a tube up and cooling a diagonal plate down to achieve a loose slip fit? We are probably talking more precision here than is normally seen in structural steel work. I'm just interested in ways to beat the issues in question #1 above.

    3. Instead of a long diagonal piece, how much is lost if plates are installed normal to the walls say every 4 - 8 inches? I see a bunch of plates installed in the tube as a far easier approach no matter the chosen fixing option. That is they can be bolted or welded in place with greater ease than a diagonal plate.

    4. Would epoxy granite or even plain concrete filling the tube be an effective replacement fir the diagonals you design in above. The interest here is fabrication simplicity. We already know that Epoxy granite fills add mass and deaden vibration but how effective are they in improving these simple beams structurally?

    5. The other idea is a looser fitting diagonal that is held in places with an epoxy granite fill.


    Lots of question but the desire here is finding simplified fabrication methods that provide the greatest mechanical benefit. Understanding the details of how you would realize one of the beams in a shop might help many builders.



  10. #46
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    Great to hear it's helpful! I've run a lot of gantry models the past year; glad to pass on what I've learned.

    I used to avoid 8020 for the low stiffness and high cost, but for a wood cutting machine, I can appreciate the simplicity of it, especially if the rail surfaces come flat enough. But yes, if stiffness or low price are the goals, there are definitely better options.

    I'll take some pics of our 8x8 diagonal installation today. I haven't posted anything about the machine except that bit in the other thread. Also more at the end of this post.

    I just noticed 2 errors in my prior models. First, except for #1 and #2, most/all of the models had an 11" distance from tool to bottom of gantry, instead of 10" (I'm rerunning them all now at 10" and will post updated results soon). It's looking like about a 10% improvement in stiffness, as might be expected. Second, the BB Ply models included some distortion in the YZ car (made of alum, it was ~rigid, but a BB Ply car did bend). So I'm rerunning those with a beefier YZ car, so the gantry comparisons are apples to apples. Combined with the 10" Z clearance, that gives a 15% improvement for the 8x8, on up to 30% for the 11x11. I'm sorry for the errors -- hope to have the updated results later today.

    Good question on the diagonal. I like it starting at the lower rail, since that rail has the most load on it. But for FEA stiffness, IIRC the direction doesn't have much effect (I'll run some models to check). For installation, it's often helpful to have room to put your arm in (esp if using Hiwin "T" rail, whose bolts go in from the back). Here's what an 8x8x0.25 tube does without stabilization (except caps/uprights at the ends):
    CNC Router for Hardwoods: Evaluation and Questions-screenshot142-jpg
    CNC Router for Hardwoods: Evaluation and Questions-screenshot143-jpg

    The distortion is exaggerated a lot so you can see how it deforms. The key is to put in reinforcement (sheet, bulkheads, rods...) that will prevent that "parallelogram mode", where one corner gets closer to the other. So I suspect that alternate direction diagonals would have the same effect as the other solutions, but I've not tested it yet. The harder question is figuring out a design and installation method that isn't too cumbersome to do, fits perfectly without distorting the xsection (prestress) or being sloppy, doesn't interfere with rail installation, and is as stiff in reality as it needs to be. The best solutions I know of use a 2 piece design: 1 piece is fitted tightly to one corner; the other is fitted tightly to the opposite corner. Then they are joined in the middle.

    Wizard, just saw your post, thanks; will reply later.

    David Malicky


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    One possibility that came to mind for diagonals could be using screw 'jacks'.
    Have a triangle shaped piece at each end with a screw in between. Screw is fixed at one end. Have the length such that it slides in with a small clearance, place the jack at the midpoint and then rotate the screw to drive the ends apart. Would need thread locker obviously.

    I'm quite interested in your analysis. Have you done it on steel beams? I'm looking at steel beams ~8x4" by 40" long.

    Even one plate / brace in the middle of a tube is likely to help?
    A fabricated beam could be the strongest / most precise way to deal with bracing needs.
    Build a square or rectangular beam out of 4 plates of ~1/4" to 1/2" aluminium (or steel). Along the length have plates that fit in and are keyed in to the walls.
    See attached.

    I'm planning to build a router similar to matth
    in about 6 months.
    I think his is a great design for a number of reasons.
    I'm planning on steel RHS ~100mmx50mm tubing for the frame
    Then was looking at gantry beams in the order of 200x100 or 150x100 etc.
    My Y travel will only be ~600 to 800mm, so gantry beam of 800mm to a metre.
    Profile rails (Hiwin or similar).
    I'll be bolting my design together rather than welding, to allow adjustment and avoid warping. The X beams sitting on top of the Y cross beams on this designs makes this very easy to do.

    Attached Thumbnails Attached Thumbnails CNC Router for Hardwoods: Evaluation and Questions-fabricated-beam-jpg  
    Last edited by pippin88; 01-10-2014 at 11:17 PM.
    7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)


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    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:
    CNC Router for Hardwoods: Evaluation and Questions-screenshot142-jpg
    CNC Router for Hardwoods: Evaluation and Questions-screenshot143-jpg

    - 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).
    - 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.
    - 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.
    - 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.
    - 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'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 :Â*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.

    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.
    2. I never thought of that. Sounds like a job for a big oven.
    3. Yes, plates or bulkheads are ~equally effective for stiffness. See runs #24-29 vs #15.
    4. Yes, that would work great for stiffness, although the mass would require expensive motors and drives.
    5. I like that idea a lot.

    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':
    CNC Router for Hardwoods: Evaluation and Questions-screenshot147-jpg

    David Malicky


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CNC Router for Hardwoods: Evaluation and Questions
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