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Stiffness upgrade: Replacing V-wheels with linear guide?

Discussion in 'CNC Mills/Routers' started by SecretSpy711, Oct 12, 2017 at 2:45 PM.

  1. SecretSpy711

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    I am upgrading my Shapeoko 2 for more rigidity. X is 1200mm, Y is 500mm. I want to "knock it out of the park," so to speak, without "breaking the bank" so I am thinking of replacing my V-wheels with a linear guide system.

    Firstly, I am thinking of swapping the X axis rail (currently 2 pieces of 20x40 makerslide bolted together) to something like 8040 or maybe even 8080 extrusion. That should take care of the twist around the X-axis which is where most of the flex is coming from.

    However, there is still some stiffness to be had in the way the carriage rides along the rail. The current scheme is delrin V-wheels rolling on makerslide, which is not the stiffest setup. There are some geometry changes I could make to stiffen it, but it will never be as stiff as other systems. I've also found that, unless you use a dust shoe 100% of the time (not practical for me), open wheel systems require a "bellows" (vinyl dryer hose) to keep debris off the rails, otherwise the wheels tend to roll over the debris and cause irregularities in cuts. I would like to get rid of the bellows if I can because it cuts down on usable work area.

    I am thinking of switching to MGN9, MGN12 or something equivalent like AG15 or AG25 (from Automation-overstock.com or robotdigg.com), bolted to the aluminum extrusion. I don't have any experience using that type of linear motion system. Is this a good choice? Advice please?
     
    #1 SecretSpy711, Oct 12, 2017 at 2:45 PM
    Last edited: Oct 12, 2017 at 3:21 PM
  2. Gary Caruso

    Gary Caruso Veteran
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    I would think 9 would be size appropriate, the 15 and 25 would be so huge in comparison to the machine.. You can model the parts easy enough using the library of parts on sketchup.
    Been looking to do something similar for my next machine / Ox upgrade, just can't justify the cost yet :)
     
  3. SecretSpy711

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    That is why I am looking to upgrade instead of build a new machine... but by the time I'm done incorporating all the features I think I want, only some of the bolts would be original parts!
     
  4. SecretSpy711

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  5. Joe Santarsiero

    Joe Santarsiero OB addict
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    I wouldn't recommend them for this application. I've specd them for a couple of jobs in the past. They work great for things like adjustable stops and guides. They don't run as smooth as you would think. Dust likes to embed itself in them. They gouge easily if the track entrance is sharp(which is the case for vwheels). Plus, things might get pretty crummy if lignin and glues rehydrate on them and the track.
    Have you considered making wipers for the track?
     
  6. SecretSpy711

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    Joe, for clarification, you're saying that the polymer-sleeve style bearings are not good for this application? Or the other way around?

    Assuming you are recommending against the polymer style, I think I am leaning towards MGN15 for this upgrade. I put it in my CAD model last night and it looks about the right size to my eye. They come with end-seals by default. Maybe bottom seals also, but if not, I should be able to order bottom-seals from a Hiwin distributor or just buy a block that already includes them. If I mount them on the sides instead of the top of my existing Y extrusion, then I can keep the rail covers I have currently, to help protect from dust.
     
    #6 SecretSpy711, Oct 13, 2017 at 11:43 AM
    Last edited: Oct 13, 2017 at 12:52 PM
  7. Joe Santarsiero

    Joe Santarsiero OB addict
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    I recommend against the sleeve bearings.
    Profile rail is a great choice. The mgn have bottom seals. The wipers should be good enough to push most grime away. These also have grease nipples. You could re add your bellows if you wish. I recommend you add greasing the bearings to your maintanence routine to keep them fresh. Again, if you cut them to length then make sure you file the ends clean just like with vwheels.
     
  8. Florian Bauereisen

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    Hi,
    i would recommend using 20 sized linear rails of hiwin style (tbc/bosh ... get them online there are plenty makes)
    reason is easy: smaller make the rails tend to be not really stiff and easylie get bend or dented while mounting/handling .20 sized offer enough resistence so to speak.
    BUT: when mounting them i would strongly recomend to "surface" your extrusion at the minimum or get a surfaced aluminum plate to mount them on.
    Reason: even using only V0 accuracy wagons the tolerances on the ball/rail are getting so little that any bents/ hills/valleys will easylie cause dozents to hundreds of kilos friction if not carefull. And the rail will follow the shape when srewed down. belive me.

    So as a conclusion: as your gear gets tighter tolereranced the more you have to care about to accuracy.

    Sorry about my english i am not a native engligh speaker

    flo
     
  9. JustinTime

    JustinTime Master
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    Florian, do 15 rails really get bent easily if screwed down every 10cm, or what ever the distance is between the mounting holes?
     
  10. SecretSpy711

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    Just for reference, hole spacing is 40mm for MGN15, and 60mm for MGN20 rail.
     
  11. Gary Caruso

    Gary Caruso Veteran
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    Can you share a screenshot of the drawing?
    I think you might be right with 15's, i was looking at the v-slot the other day and realized that 9mm wide rail would not cover the V-slot well, would have to make bridge parts to get the rail to sit flat and stable.
    Been looking at ordering some cheap 250mm or 300mm rails, for a Z upgrade.. cost would be about $80, quality i figure would be good enough that it will be better than polycarb wheels. 12mm would be the smallest you can use with V-rail really. MGN12C on Ebay
     
  12. Joe Santarsiero

    Joe Santarsiero OB addict
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    I just Google image searched MGN 15 and there was a drawing of the components.
    You could take Flo's suggestion and use a stiff backer. Maybe some 1/16" steel flat stock and longer screws for the tnuts would solve some issues.
     
  13. Florian Bauereisen

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    Depending on the make 60mm would be a quite common spacing.
    But all rails will follow the surface they are screwed down to. rails are not structural members... This is why all comerical availible mills have them mounted on a surfaced area, most will even have a ledge incoperated to one side in order to press rail into for keepig it straight ... second side / rail will be adjusted to follow the first rail...


    flo
     
  14. SecretSpy711

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    Flo,
    In order to use base-40 8080 extrusion on the gantry with 20mm rail, the carriages (or "trucks") would be touching each other. (See image) It might work, but I'm not sure this is a good design practice? I can't find any base-20 8080 extrusion, so I would either have to use 2 pieces of base-20 8040 (more expensive, possibly less rigid?), or use special-base 100100 extrusion (also more expensive, and seems excessively large and takes up quite a bit of space on the motor plates.)

    If I order the extrusion with the "milled surfaces" option, do you think 15mm rail is still too small? Misumi does not not appear to have the "drop lock" feature that 8020 Inc. uses, and therefore seems like it would be less prone to causing the MGN rail to have hills and valleys.

    -Brian
     
  15. Florian Bauereisen

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    Why would you use 2 rails? if using 20mm rails one rail is more than enough for your twisting forces (my, mn, mr) Use two trucks lenghwise .
    As a rule of thumb: have them spaced 2(parts) wide apart, while z being 3 parts high max (2:3)

    80X80 will provide you with a tremendous update to your recent setup and quite sufficient for your shape e oko type machine.
    A 15 size rail should be sufficient for us hobbyists but will work only on a surfaced extrusion or a surfaced mounting plate...

    I have gone to 100x100 as well as 20 and 25mm size for one reason: buy cheap buy twice - and have never looked back.
    But it is a different design alltoghether..
     
  16. SecretSpy711

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    All the builds I've seen using this machine layout use 2 rails on the gantry... 1 doesn't seem like enough by looking at it... but here are some numbers...

    According to Robotdigg:
    MGN20, MR (long truck) = 13.4 kgf m = 97 ft lbs
    MGN20, MR (standard truck) = 9.4 kgf m = 68 ft lbs
    MGN15, MR (long truck) = 7.5 kgf m = 54 ft lbs
    MGN15, MR (standard truck) = 4.6 kgf m = 33 ft lbs

    There is roughly 6 inches (0.5 ft) from the bottom rail to the wasteboard.
    For MGN20 long truck, max tool force is 97 ft lbs / 0.5 ft = 194 lbs.
    For MGN15 short truck, max tool force is 33 ft lbs / 0.5 ft = 66 lbs.

    That's a lot of tool force! So, in terms of load capacity, you are correct.... but what about rigidity? Is 1 rail stiff enough to take full advantage of the stiffness of the 80x80 extrusion? By using 2 rails I am spreading the moment out over the whole face of the extrusion.
     

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  17. Florian Bauereisen

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    Sorry,
    mixed up the axis. (for me your y is x) sorry.
    Three choices: i use 120 mm on x (y with me) extrusion
    Use a long and 2 short trucks spaced acordingly
    Mount them on a plate slightly taller than extrusion
    One truck on front while a (smaller one) on top

    Btw a single 25 sized long truck is more than sufficient- been done before.
    I use a single 25 on my z and the mill is stiff as stiff can be.
     

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