[JerryFlyGuy]

I'm looking for the same general feed back on how to best orientate the linear rails on my Y axis on my Gantry. I've attached a pdf to show some of the possible style's I've seen or thought of.
Style one is pretty simple.. one rail on the top, one on the bottom, centered side to side on the gantry. This would probably work just fine, however is it possible to improve on this?

Style two is similar yet the rails are spaced side to side on the gantry to give the max 'arm' between the rails. Each rail would be placed on a welded truss member [ see the iso- in the bottom right for the gantry construction style]
this deliver's the max load directly to a truss member and would possibly be more efficent because of the distance between the two rails and their offset.

Style 3 is another style I've seen before. My current structure isn't optomised for this style of rail installation as there would be fairly large side loads imposed on the top and bottom planes of the gantry. I could, however, turn my trussed gantry 90deg onto its side and have a fairly high strength level. [If this was to be concidered the optimum setup]The down side is that the closer the rails are mounted to each other, the higher the stresses induced from a load condition [ assuming the same loads imposed in all three styles]

I also just thought of another style which would have the rails orientated top and bottom again, very similar to style #1, yet the rails would be located at one side of the gantry over/under a trussed member.

As always, opinion's welcome.

Jerry

[WhiteTiger]

Just personal preference but I'd go with style 2, preferably with the rail locations rotated 90 deg ccw about the y axis.

Just strikes me as a sounder strategy for a carriage that is going to have the z w/spindle cantilevered off of it. Puts the near side rail as close as possible to the cutting tool to handle deflection forces and references the z across the longest section of the y box.


Tiger

[High Seas]

None of the above!?
I prefer a dual y axis with the rails on top. The Z axis is betweeen the two Y:
http://www.cnczone.com/gallery/data/...em2_Z_axis.jpg
maybe a picture is a better explanation.
Given you've built the truss - you could make a second just like it! The challenge with this set up is its a bit awkward to change the router/bits etc. But, it does elminate most of the "racking" or torque generated by the router on the Y axis. (and it "eats up" a bit more of the X axis)
:cheers: Jim

[JerryFlyGuy]

Tiger, I like your thinking, however I just wanted to clarify what your telling me. I've attached another PDF showing what I think your meaning. [Style 4] If this is infact the case the only concern I'd have would be to put the rail thats on the top/back side of the gantry on top instead of on the side. Otherwise I don't see any issue's with it. There is going to be a bit more direct twisting force applied to the gantry but that can be taken care of w/ addition truss members if need be.

HighSea's I'm not sure that, that style would work on my Gantry. The reason being that the truss is only strong in torsion when they [ there are two of them joined top and bottom w/ flat plate] are joined together. If I was to split them appart and them place the Z axis in between them I don't think I'd have the stability that I want/need. I could be wrong though... another part of this is that my z axis is around 5' long so.. that gives you an idea of how much force is being applied to the gantry frame. I did some FEA on the basic truss work using style 1, I found that the bottom of the Z axis will deflect up to 0.006" when the full force of the mill is applied to the Z axis post at the bottom. For it's size and weight this is ok in my book's as I don't ever intend to put that much force into the machine while cutting.. it may happen by accident but it shouldn't happen as a rule... Under normal cutting the Z axis post should only deflect not more than 0.001" that is if you have absolute faith in FEA

Anyone else got idea's or thought's to offer??

I'd be glad to hear from ya!
Jerry

[Geof]

The big commercial machines I have seen use the orientation shown in style 3 of your original pdf. In terms of load carrying capacity and stiffness I doubt whether there is much to choose between the different orientations. I suspect the commercial machines use style 3 because it is easier to machine and tap mounting holes on a style 3 and much more accurate because all the machining operations for both rails can be finished in a single setup.

Granted, on a DIY machine you may not be machining the rail mounting surface and may be doing something such as your epoxy embedment. In this case style 3 would be preferable because it is possible to get a direct measurement of alignment between the two rails but with the other styles a direct measurement is not at all possible.

[dpuch]

I'm with Geof on the original style 3 as the best compromise.

Dale

[Splint]

I also like style 3 best, the what I dislike about 1 and 2 is the need for aditional member(s) (which is a source of flex and adds weight) to reach the linear rails.

[WhiteTiger]

Yup, #4 is exactly what I was trying to describe

I suppose if you were going to mount the backside rail on top it would make sense to put the frontside on the underneath. Come to think about it, that would put all your x motion force loading on the mounting screws as shear forces rather than loading the weaker thread elements axially.

Ok, my turn to do a revision. Not a rotation of #2, just a mirror image Still gets you one support right close to the cutter, but with better all round loading on the rail mount.


Tiger

[jimbo]

style 3 is probably easiest to implement as mentioned. but I am thinking of using a rail on top of the gantry away from z, then the second rail on the front of the gantry near the bottom. This puts the two rails at right angles and also puts the rails furthest apart. don't know if there are any real advantages, but in my mind, this allows for each of the forces to partly distributed with a horizontal rail and a vertical rail, instead of two verts or two horizs.

[JerryFlyGuy]

Thanks Guy's, I think Geof's convinced me that style #3 is probably the easiest to impliment. It will require me, turning my truss's on their sides and then adding some members to stabilize the two truss's vertically [it may end up looking like a trussed tower on its side when its done]. Hopefully that shouldn't take to much. I am planning to do the epoxy bed's for these rails also [ for those who don't know what I'm talking about see my other postHERE ]

This will also allow me to put my Rack on the bottom of the gantry so that it shouldn't get as much dust falling on it.

Thanks again guy's!!

Jerry