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Thread: CNC Router for Hardwoods: Evaluation and Questions

  1. #133
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    Default Re: CNC Router for Hardwoods: Evaluation and Questions

    Case 51 showed the deflection was down to 0.0022", close to the 0.0015" goal for the structure of these 3 components. The pic for Case 51 shows a 'kink' in the Z plate, where the lower Z blocks attach. So let's try reducing the bearing block force by raising the upper Z block (so the 2 blocks have greater leverage, for less force on each).

    Case 52: raise upper Z block by 3", for a total of 6" Z block vertical spacing (on centers). Also make the Y and Z cars taller to match.
    Deflection = 0.00200". stiffness = 50k lb/in. The pic shows that it reduced the local 'kink' in the Z plate:
    CNC Router for Hardwoods: Evaluation and Questions-52-jpg
    While the structure only saw 0.0002" improvement, the deflections of the bearings themselves will go way down with 3" more separation (a different analysis).

    Case 53: Add a 3rd spindle mount mid-way between the other mounts. It helped a tiny bit:
    Deflection = 0.00196", stiffness = 51k lb/in. Pic is about the same. There is still a bit of bending in the Z legs.

    Case 54: Thicken Z legs to 3/4".
    Deflection = 0.00188", stiffness = 53k lb/in. Pics show the Spindle is still rotating more than the Z car.
    CNC Router for Hardwoods: Evaluation and Questions-54-jpg
    We can see lateral bending in the Z plate, a partial cause:
    CNC Router for Hardwoods: Evaluation and Questions-54b-jpg

    Case 55: Thicken Z plate to 3/4".
    Deflection = 0.00183", Stiffness = 55k lb/in. The Z car has improved noticeably compared to Case 51:
    CNC Router for Hardwoods: Evaluation and Questions-55-jpg
    Certainly, we could do more to the Z car (increase legs from 4" to 5", or change the C-section to a box all around the spindle). But the Y car and gantry tube are bigger fish to fry at this point.

    Case 56: Add 1/2" x 4" legs to the Y plate, to make it a C section, too.
    Deflection = 0.00172", stiffness = 58k lb/in. Pics show the Y legs are very rigid in this configuration (but they'll likely show bending if the Z car is raised up high).
    CNC Router for Hardwoods: Evaluation and Questions-56-jpg

    Here are the side and front views, to show X and Y deflections. It's still worst in the X direction; the side view shows the gantry tube is twisting (lower edge moving out):
    CNC Router for Hardwoods: Evaluation and Questions-56b-jpg
    CNC Router for Hardwoods: Evaluation and Questions-56a-jpg

    Case 57: Thicken gantry tube's wall thickness from 0.25" to 0.50".
    Deflection = 0.00131", stiffness = 76k lb/in. Yeah. The light blue patch on the Case 56 gantry is back to dark blue (especially impressive since the auto-scale has lowered the threshold between those colors):
    CNC Router for Hardwoods: Evaluation and Questions-57-jpg


    - It was a pretty stiff gantry when we started, but after stiffening the Y and Z cars, the gantry then needed more.
    - I'd say for cutting aluminum, a 8x8x1/4" steel gantry (with internal diagonal or bulkheads) is also a good option, even though pretty heavy. It would be even stiffer than Case 57.
    - Deflection is now under the 0.00150" target for the structure of these 3 components, so this could be a good stopping point.
    - Later, I'll try some other variations to see how stiff it can go, and I'll move the Z car up and down to check how that affects results.

    Attached Thumbnails Attached Thumbnails CNC Router for Hardwoods: Evaluation and Questions-52-jpg   CNC Router for Hardwoods: Evaluation and Questions-54-jpg   CNC Router for Hardwoods: Evaluation and Questions-54b-jpg   CNC Router for Hardwoods: Evaluation and Questions-55-jpg  

    CNC Router for Hardwoods: Evaluation and Questions-56-jpg   CNC Router for Hardwoods: Evaluation and Questions-56b-jpg   CNC Router for Hardwoods: Evaluation and Questions-56a-jpg   CNC Router for Hardwoods: Evaluation and Questions-57-jpg  

    David Malicky


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    Default Re: CNC Router for Hardwoods: Evaluation and Questions

    David,

    I'm thinking a good Z plate would be steel Z channel, with the spindle mount welded in, spanning between the channels.

    Next question is Z rails on Y carriage, or on Z plate? I went with on Y carriage (cars on Z plate) on my current router as I felt it gave me the best travel / fixation arrangement.

    Attached Thumbnails Attached Thumbnails CNC Router for Hardwoods: Evaluation and Questions-c-channel-z-axis-png  
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    Default Re: CNC Router for Hardwoods: Evaluation and Questions

    Quote Originally Posted by pippin88 View Post
    David,
    I'm thinking a good Z plate would be steel Z channel, with the spindle mount welded in, spanning between the channels.

    Next question is Z rails on Y carriage, or on Z plate? I went with on Y carriage (cars on Z plate) on my current router as I felt it gave me the best travel / fixation arrangement.
    Yes, that should be a stiff Z car. In the US, our steel C-channel is hot rolled and has fairly short legs that taper towards the end (American Standard shape) -- https://www.metalsdepot.com/products...cc=%20&aident=
    Hot rolled is not very straight or flat (unless cold finished, which ours isn't), and our short tapered legs are not very stiff in bending.

    We can get aluminum C-channel in 2 profiles, and 1 of them is fairly good for bending (Aluminum Association) -- https://www.metalsdepot.com/products...l?page=channel
    But the legs are not very long, and the AA profiles have fairly thin web thickness (0.29").

    Your drawing shows constant thickness legs, of reasonable length for steel. Sounds like metric steel C-channel has a fairly good cross-section for bending. The web (main plate) thickness of your drawing looks a little thin -- even in steel, it will distort more easily -- that can be minimized by mounting the rails or blocks close to the C legs.

    The welded in spindle mounts should be very stiff. The higher the upper mount, the better for stiffness. Or ideally, make it a similar height as the upper Z blocks on the backside -- then the spindle loads go straight thru to the Z blocks.

    I'm assuming you would stress-relieve it in an oven, since it's not too big (else it could distort over time, as you probably know), and post-machine all mounting surfaces.


    Yes, the rail/block question is an interesting one. I think Z rails on the Z car is overall stiffer, but I've not run that config yet. Z rails on the Y car can work fine if the Y car has long enough legs for its C section.

    David Malicky


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    Default Re: CNC Router for Hardwoods: Evaluation and Questions

    David, I'm not sure on the exact specs of our C channel. I quickly googled and one of the results had a section with 12mm flanges (legs), 6mm web. However it did not gave a drawing, and the flanges may well taper.

    Steel would require machining. Welded sections are stronger than bolted. I don't have the equipment / skills for welding aluminium.

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    Default Re: CNC Router for Hardwoods: Evaluation and Questions

    Casting an aluminium z car may be option. Then machining.

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    Default Re: CNC Router for Hardwoods: Evaluation and Questions

    Yeah, 6mm would be pretty thin for the web/backplate.

    I agree welding has the best joint quality. After welding and stress relief, the backplate flatness would probably be pretty far off, maybe requiring a lot of post-machining. Also post-machining the spindle mount hole seems awkward. Those are certainly doable, but the overall work may be comparable to the next option...

    A combination of bolts and pins should work well for this particular joint. The interface between the channel legs and backplate is mainly in shear, which is relatively easy to control with bolts and pins. (I'm assuming the legs would be mounted to the face of the backplate, not the edge.) Roll-pins are easier than dowels, and have good shear stiffness. I'd probably do a hole every ~inch, alternating bolts and pins. A thin layer of high strength epoxy would make the joint nearly as good as welding, for CNC purposes (where we need stiffness, not strength). With this method, the backplate could start as cast alum tooling plate, which is stable and very flat. I'd do the legs out of 6061-T6, so the tapped holes are stronger.

    Casting would be cool, but for me it would be much more work than fabrication.

    David Malicky


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    Default Re: CNC Router for Hardwoods: Evaluation and Questions

    This set is to try more stiffness improvements, and check different Z car positions. Recall Case 57 numbers:
    Deflection = 0.00131", Stiffness = 76k lb/in.

    Case 58: Thicken Y plate to 3/4"
    Deflection = 0.00130", Stiffness = 77k lb/in. At this Z car position, ~no advantage.

    Case 59: Thicken diagonal sheet (inside gantry) from 1/8" to 1/4".
    Deflection = 0.00127", Stiffness = 79k lb/in. The 1/2" gantry tube wall thickness is already getting thick enough that the cross-section has more inherent stability. A bigger diagonal isn't really needed.

    Case 60: Thicken gantry tube's wall thickness from 0.50" to 0.75". This is roughly similar to a 8x8x1/4" steel tube (Esteel = 3 x Ealum) -- but the steel tube will be stiffer since its metal is closer to the 8" outside dimension (the outside surface does the most work).
    Deflection = 0.00114", Stiffness = 88k lb/in. A substantial improvement, but not as dramatic as the change from 1/4" to 1/2". There are 2 likely reasons:
    1) Adding more material to the inside of a tube is progressively less effective
    2) We're now in the territory of diminishing returns for the gantry -- other parts are more flexy than it, at this point.
    CNC Router for Hardwoods: Evaluation and Questions-60-jpg

    Case 61: Drop the Z car 1.5" to near the bottom of its travel (cutter load is 10" below gantry)
    Deflection = 0.00114", Stiffness = 88k lb/in. It's surprising that it isn't worse! Apparently, while the gantry experiences more torque and must be twisting more, this Z car position puts the Z and Y blocks close to overlapping -- and that is a stiffer config. So the 2 effects happen to exactly cancel.
    CNC Router for Hardwoods: Evaluation and Questions-61-jpg

    Case 62: Raise the Z car 4.0" (cutter load is 6" below gantry)
    Deflection = 0.00123", Stiffness = 81k lb/in. Now the Z blocks are relatively far from the Y blocks, so there is more bending in the Y car.
    CNC Router for Hardwoods: Evaluation and Questions-62-jpg

    Case 63: Raise the Z car 5.0" (cutter load is 1" below gantry)
    Deflection = 0.00141", Stiffness = 71k lb/in. The Y car is the main weak link now. The Z blocks are too far from Y blocks, creating a lot of bending in the Y car.
    CNC Router for Hardwoods: Evaluation and Questions-63-jpg
    (See the pic for case 65 below, to see where the Z blocks are)

    Case 64: Increase Y leg depth from 4" to 6".
    Deflection = 0.00122", Stiffness = 82k lb/in. A good use of added material for this config.
    CNC Router for Hardwoods: Evaluation and Questions-64-jpg

    Case 65: Eliminate Y legs, just to see how bad it is.
    Deflection = 0.00446", Stiffness = 22k lb/in. Clearly, if the Z rails are on the Y car, and the Z car is raised up, the Y car needs a lot of bending stiffness.
    CNC Router for Hardwoods: Evaluation and Questions-65-jpg

    Case 66: Thicken Y plate from 3/4" to 1", to show what thicker can do (not enough). A 1" thick aluminum plate would have similar bending stiffness as a 0.69" thick steel plate. (Based on the same E*I stiffness: SteelThickness = 3^(-1/3) * AlumThickness ).
    Deflection = 0.00316", Stiffness = 32k lb/in.

    Case 67: Increase Y leg depth from 0" back to 6"
    Deflection = 0.00116", Stiffness = 86k lb/in. Compared to Case 64, the 1" thick Y plate shows benefit when the Z car is raised up. Also compare to Case 58, where even a 1/2" thick plate was adequate.
    CNC Router for Hardwoods: Evaluation and Questions-67-jpg

    Case 68: Drop Z car 9" (cutter load is 10" below gantry)
    Deflection = 0.00112", Stiffness = 89k lb/in. Not much different than Case 67. Compare that small difference to the big difference between Cases 61 and 63 (same Z car positions). This indicates that Z rails on the Y car can give consistent stiffness IF the Y car is very very stiff. These pics show the front and side view. X deflection is still dominant.
    CNC Router for Hardwoods: Evaluation and Questions-68front-jpg

    CNC Router for Hardwoods: Evaluation and Questions-68side-jpg

    So, with Z rails on the Y plate:
    Assuming the gantry that is quite stiff in torsion, the highest stiffness at the cutter occurs with the Z car in the low ranges of its travel. The least stiffness is generally with the Z car at the top of its travel. If the Y car is very very stiff, the difference is probably not significant. But if the Y car is just a plain plate, there will be a major stiffness loss in the up position.

    Next I'll look at Z rails on the Z plate.

    David Malicky


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    Default Re: CNC Router for Hardwoods: Evaluation and Questions

    David,

    Nice work. At the end of this, I believe you will be able to put forward compelling design elements for the 'best' machine. A lot of the threads in this forum are repetition of basics (usually with quite poor gantry designs), but that is partly due to a lack of good guides, partly due to a lot of opinion without people crunching the numbers.

    A lot of your results are not surprising, given the basic principle that the larger a section the stiffer, but that only applies in the direction / vector the section is large in. The great example being the classic Z plate (a flat plate) being a weak area, with the addition of legs / flanges vastly improving it..

    With the gantry beam, I think steel wins. I reckon 1/4" 8x8" steel is much more available than 3/4" alu tube (does it exist?)

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    Default Re: CNC Router for Hardwoods: Evaluation and Questions

    Wow, I just found this thread and was blown away by the analysis. Good work David! Do you have any general conclusions from your tests? Did you ever try any cases with the z-rails on the z car? Cheers!



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    Default Re: CNC Router for Hardwoods: Evaluation and Questions

    would love to see you do some numbers on a composite carbon sandwich.
    I have built one with a 1200mm beam but have not done any bending tests on it yet
    beam is 300 square and Y and Z plates have the side plates added with a 12 inch clearance so very similar to the config your playing with

    Mike



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