Captured Rails Necessary?


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    Default Captured Rails Necessary?

    I am in the doodling stages for my inevitable "next" CNC machine. I am by no means an engineer, but I'm wondering if "capturing" the rails (i.e. carriages/bearings on both the inside and outside of the X-axis rails) is truly necessary?

    For machine v2.0, I'm wondering if an arrangement with a single pair of V-wheels on each X-axis would work. I'm thinking mount the V-track at the inside edge of the rail, and the gear rack at the outside edge, and do away with the outside-edge bearing assemblies altogether.

    I attached a quick sketch below. The view in the attachment would be from the outside of the rail looking toward the inside. You might recognize Ahren's rack & pinion mounting plate on the outside. Just looking for some second opinions on the idea.

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    Attached Thumbnails Attached Thumbnails Captured Rails Necessary?-x-axis_rail_v2-jpg  
    Last edited by adt2; 01-09-2012 at 02:37 PM.


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    In the configuration shown, you will also need V-bearings and rails on the bottom. And a way to adjust them. The weight of the machine is not enough to plat the upper bearings firmly in place.



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    Quote Originally Posted by adt2 View Post
    I am in the doodling stages for my inevitable "next" CNC machine. I am by no means an engineer, but I'm wondering if "capturing" the rails (i.e. carriages/bearings on both the inside and outside of the X-axis rails) is truly necessary?

    For machine v2.0, I'm wondering if an arrangement with a single pair of V-wheels on each X-axis would work. I'm thinking mount the V-track at the inside edge of the rail, and the gear rack at the outside edge, and do away with the outside-edge bearing assemblies altogether.

    I attached a quick sketch below. The view in the attachment would be from the outside of the rail looking toward the inside. You might recognize Ahren's rack & pinion mounting plate on the outside. Just looking for some second opinions on the idea.
    So you're using the pinion assembly to hold the v-bearings on the track? I'm not an engineer as well, but my gut feeling is that any upward or rocking "thrust" that the spindle exerts on your gantry wpuld be better contained and absorbed by a second set of v-bearings/rails, not by your powertrain. Also you'd have decreased side-to-side load.



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    I don't have an answer, but I believe the question is a good one because it is much easier for the DIYer to make one level track than two at a fixed separation (top+bottom). I think Gerry suggested that the cutting forces should <50 lbs, so if the gantry weighed much more than this, it shouldn't be able to jump the track.
    Sideways forces that are not fighting gravity are a different matter.



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    Paul - That's actually what started me down this path. Trying to get two rails on each axis straight, parallel, level, and coincident has caused me all kinds of trouble. My thought is that the weight of the gantry assembly, including the router, is going to be pretty significant (somewhere north of 70 lbs I would guess). All of this weight should be bearing straight down on the V-wheels on either side of the machine. The rack and pinion gear really shouldn't be "holding the V-bearings on the track." They're keeping them from being lifted off, but there really shouldn't be much (if any) lifting force on the gantry.

    After looking at the sketch, though, I think it might be necessary to re-jigger the gantry assembly so the weight of the router doesn't want to tilt it forward. Maybe instead of the short lengths of 1530 bolted to the gantry base, you could use a longer length, say 12", and then mount the gantry riser to the top of the 12" length. Then you could slide the whole gantry assembly back and forth until you found the balance sweet-spot and bolt it down. That way it would be adjustable for use with different routers.

    Attached Thumbnails Attached Thumbnails Captured Rails Necessary?-adt2_v2_perspective-jpg   Captured Rails Necessary?-adt2_v2_closeup-jpg  


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    That is a good point. 50 lbs may not lift the gantry off the rails, but it may tilt it somewhat, much more easily.
    Which brings me to another related idea : it is relatively easy to get rare earth magnets that can have 5 lbs of hold. If the rail or support is steel (like Ahren's CRS), could 3-4 such mags be embedded onto the underside of the carriages to have a friction-free gravity assist? Would eddy-currents limit the speed?
    Cheers!



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    My thought is that the weight of the gantry assembly, including the router, is going to be pretty significant (somewhere north of 70 lbs I would guess). All of this weight should be bearing straight down on the V-wheels on either side of the machine.
    A 70lb gantry is actually pretty light, imo.

    Imagine your 70lb gantry siting on a table, with no bearings. If you push on the center of the gantry with your hand, how much force would it take to tip it over. My guess is 10 lbs or less, as it's very top heavy.

    If you're cutting a circle at high speeds, it's going to be rocking back and forth.

    I believe the Mechmate does use this method, however. But, the gantry of the mechmate is lower, and about 18" wide, so to the force required to tip that gantry over would be much greater than the weight of the gantry.

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

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

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


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    There is MUCH more force to be considered here. Like the forces exerted on the entire gantry when the router plunges into the material being cut. It's not about just supporting the weight of the gantry, sitting on top of some rails, but also HOLDING onto those rails when the Z axis is plunging into the material- it will push the gantry UP! This was an important lesson I learned from my first build, and the reason I redesigned the gantry rail system for a complete wrap-around system. (I used 1.5"x1.5" 1/4" thick square tube as the rails, and mounted the gantry onto 4"x4" 1/8" thick square tubes 12" long. Bearings are mounted onto all thread axles on the top and bottom sides, with the bottom axles inserted through swing bolts (or latch bolts as I have seen them called) which are arranged on both sides of the rails, with the opposite ends (the threaded ends) inserted through bearings and into slots milled into the top side of the 4" truck tubes. This arrangement provides bearings which ride against all four sides of the rails at each end of the 12" length trucks). This design took a bit of adjusting, to properly load the bearings against the rails and to properly align the gantry to eliminate "racking" but has served me well for the 2 years since installing this design. I carve solid oak, with this system as much as 1" deep into the material. The only drawback, is the hot roll steel I used for the 1.5"x1.5" rails is not hardened. And after 2 years of use, the bearings have begun to cut grooves into them- which is not that much of a problem, since I can adjust the load of the bearings to take up for the wear. However, Hardened steel tubes would be an improvement to the design. I am not a metalurgist, so I have no idea how to go about hardening hot roll steel (A36). Perhaps you do!



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Captured Rails Necessary?

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