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  1. #1
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    4' x 4' x 8" Router Build

    Hi all, thought it was about time to start a build thread on the Zone. I've been design ing my CNC router for a number of years, my progress so far has been documented over on MYCNCUK.com

    Thi sis the design I have come up with:




  2. #2
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    Re: 4' x 4' x 8" Router Build

    Hi Jonny.

    I like it. Looks pretty rigid.

    Frame is awesomely triangulated, carriages on your long axis are well mounted and gantry ends are well re-enforced. I like the raised X rail design (I am building similar).

    Personally, I don't like t slot extrusion for a gantry, it is comparatively squishy when compared to rectangular tube of suitable wall thickness, especially if you include diagonal bracing internally. Rails mounted over T slots also make space for flex, as the t slot limbs are unsupported cantilevers, and it still requires machining to be suitably flat for good linear profile mounting.

    I cant see how your ACB's are mounted, but consider gussets on their mounting flange to account for axial thrust, the cantilevered mounting plate can flex.

    Are you machining your X rails for the linear rails? Hot rolled tube is not nearly flat enough for linear profile rail, you also need to be able to get the rails level to each other very accurately (though having a flexible gantry can account for some misalignment).

    Z axis plate will let the machine down in terms of stiffness, consider flanges on the sides to give increase its stiffness in its weak plane.

    any reason you selected bearings on top and bottom of your gantry instead of in front? The increased bearing spacing will improve joint stiffness, and overhang, but you sacrifice Z clearance. This may not be an issue if you are only planning on doing flat sheet work...

    An additional spindle bracket, mounted on its upper half, will also improve that joint stiffness, a lot.

    My post reads very critically, I don't intend it that way, your machine looks awesome, and will likely cut wood awesomely and Aluminium OK as well. Lots of people gave me great input when I shared my design intention, and many of these topics have been discussed in this forum in great detail. I will be following on with interest

    Luke

  3. #3
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    Re: 4' x 4' x 8" Router Build

    Thanks Luke, good points.

    My original design was made up from two 80 x 160 extrusions in an L shape:



    I reduced this to a single piece for a couple of reasons:


    1. Cost - each piece is over £500!
    2. Being pragmatic, the single 80 x 160 extrusion has a second moment of area 22x that of C-beam so should be more than stiff enough for the job.


    I'll revisit this decision when I get to placing the order, I'd like to go back to using two pieces as this adds another factor of 7 to the stiffness but it is a lot of money and I'm not convinced it's necessary. I've seen plenty of successful machines built using extrusion - it's the same section that Avid use on their Pro machines.

    I don't know what ACB stands for, do you mean the ballscrew mounts? These are going to be made from 3/8" aluminium angle. Given that the bearings will be mounted in a fixed-fixed arrangement I don't see the forces involved being sufficient to cause much deflection - happy to be wrong though.

    I will be levelling the long axis with epoxy, it seems to work pretty well and easier than machining the frame.

    All the plates are 20mm thick aluminium tooling plate. I get your point about adding side flanges to the Z axis, although I think the bearings are mounted so close to the Z plate that they won't make a lot of difference.

    I put the bearings on the top and bottom to increase the stiffness and reduce the distance from the spindle to gantry. Yes it reduces Z clearance but I've allowed for that in the height of the long rails.

    Hope that all makes sense. I appreciate your input!

    Cheers :cheers:

  4. #4
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    Re: 4' x 4' x 8" Router Build

    T slot is incredibly expensive, it makes perfect sense not to use two - I was suggesting steel or ali square tube, large section, to replace it (should be cheaper than the single extrusion, and will likely be stiffer).

    ACB's are angular contact bearings, so yes, the ball screw mounts on the x axis (the long axis, nomenclature is often inconsistent between people)it seems like you have left a section of the angle unsupported. The gantry axis is more rigidly mounted. As you say, it will likely be fine depending on your cut medium. Are you fully constraining both sides of your screw? You mentioned fixed fixed, but your CAD seems to show fixed-floating (which is better in my opinion as thermal growth wont close your angular contact clearances up).

    Epoxy leveling will work.

    The z flanges are for when the spindle is fully extended (which will likely be most of your cutting, assuming its for sheets of wood etc.), it becomes a long unsupported cantilever, and cutting in the x direction will flex it in its weak direction. Not an issue for wood though, may contribute to chatter in Ali.

    Had a browse through your UK thread, you have obviously put lots of thought into the machine and had lots of input from many people, its going to be a cool build.

  5. #5
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    Re: 4' x 4' x 8" Router Build

    Fair point about extrusion vs steel gantry. My original plan for making all the machined parts was to hand route them - I've seen people having good success doing this, I was trying to avoid having to have a piece of box section machined flat and the expense that went with it. I haven't revisited this decision since getting the price for the extrusion but I like how I can mount everything to the extrusion simply and not sure there would be much of a cost difference after paying for machining etc. I haven't done the calcs on the steel but as an example the 120 x 60 x 5 box section for the frame was over £200 for a length, by the time It's welded up, machined flat top, bottom and both ends plus drilled and tapped I think it'd be around the £500 mark.

    You're right, the bearings are fixed-floating, my mistake.

    You're talking about putting stiffeners on the spindle mounting plate - I thought you meant on the sides of the x-axis (referring to the gantry axis, think we've got our X & Y crossed!). I agree this would make things stiffer, what I keep coming back to are that the forces involved are relatively low (I think). I've done a fair amouunt of hand routing so have a feel for the force required; and calculations I've seen are based around the force to move the cutter being about 5kgf. I appreciate everything needs to be as stiff as possible but I perceive the bearings being the weakest link in the chain. I'm not worried about this amount of force deflecting a 20mm plate backed up with 20mm steel rails - again, happy to be wrong.

    I hadn't thought about doubling up on the spindle mounts, nice idea. I have put several mounting positions so the spindle can be raised or lowered a bit (I'm hoping to have a 4th axis one day so thought best to have a bit of flexibility). these positions could be used to stiffen up the spindle mounting.

  6. #6
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    Re: 4' x 4' x 8" Router Build

    I am building my machine out of reinforced scrap channel, I got all my main axis material for the equivalent of about 150 pounds, but I spend lots of time making it usable... so there is a tradeoff. I did iterative FEA starting with a pair of stacked 80x80 t slot beams, added a 6mm bent c channel front plate with flanges, replaced the ali with steel square tubes, replaced the pair of tubes with 8mm boxed lasercut plates (which I was happy with in terms of stiffness) and then settled on boxed C channel because it was available and cheap... torsion on the gantry with the spindle extended is the biggest contributor to deflection, aside from when I modeled an unflanged spindle plate (which as described before was squishy).

    I did need to spend an additional 250 or so pounds on lasercut interface plates and stiffening bulkheads etc.

    Linear profile bearings (mine are 20mm ZA preload), in a set of 4, are the stiffest component in my (reasonably) stiff assembly... 1840000N/mm of deflection (my 90kg boxed steel gantry, which is only 1.3m long comes in at about 28000N/mm).

    I saw the extra mount holes in your spindle plate, they do give some Z clearance flexibility, you wont regret them

    If its your intention to cut mostly wood, you can pretty much ignore everything I have mentioned, as the build is more than stiff enough for that - the approach I am taking is that it is unlikely I will look at my machine in future and say, "I wish it was a little less stiff"

  7. #7
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    Re: 4' x 4' x 8" Router Build

    Making good progress on the fabrication work


  8. #8

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    Re: 4' x 4' x 8" Router Build

    Quote Originally Posted by JonnyFive View Post
    Hi all, thought it was about time to start a build thread on the Zone. I've been design ing my CNC router for a number of years, my progress so far has been documented over on [URL="http://www.mycncuk.com/threads/11994-4-x-4-x-8-Router-Build?p=128683#post128683"]MYCNCUK.com
    Johnny,

    I enjoy your design. I am curious about your linear guide bearings. I assume they are the recirculating ball type? I use these (9mm size) on my 3D printer with satisfying success. Until recently I never thought about using them for a CNC router table. This is the first design I have seen that uses these bearings in an axial load (shear load between the bearing and the surfaces) instead of a conventional compression load (surface to bearing to surface). In my quick research, I have pulled up some specs that show that these specs are the same. https://www.thomsonlinear.com/downlo...uides_cten.pdf

    BTW- I would give your ACME screw a bit of clearance in case you need to use a large pulley for lower ratio.


    Rick

  9. #9
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    Re: 4' x 4' x 8" Router Build

    Welding very nearly complete!! Very happy with how it’s turned out.








  10. #10
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    Re: 4' x 4' x 8" Router Build

    that look incredibly robust. I have triangulation envy

  11. #11

    Re: 4' x 4' x 8" Router Build

    Jonny,

    Looking good. How do you plan to keep the top of the Y axis rail mounts coplanar?

    Also, how do you insert large photos into your posts?

  12. #12
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    Re: 4' x 4' x 8" Router Build

    Quote Originally Posted by ChipsBFlyin View Post
    How do you plan to keep the top of the Y axis rail mounts coplanar?
    I’m going to construct a bridge between both sides and pour epoxy to level out any undulations in the box section.

    Quote Originally Posted by ChipsBFlyin View Post
    Also, how do you insert large photos into your posts?
    I didn’t do anything special, just used the insert photo button on my phone

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