Joe's 1000x600x200 steel and aluminium router started

[devmonkey]

Good point..... 'Complexity and Less Overhang' verses 'Simplicity and more Overhang'...

If you have access to a milling machine I would definitely put them top and bottom. If you want to mount them on the face and keep the same rail separation then you need a deeper gantry, if you use 160x80 profile for the gantry then the Y rail centres will *only* be 120mm apart (distance between outermost slots), the same profile with top-bottom mounts gets you ~180mm centres. On my last machine I used 160mm ali box section for the gantry and with face mounting close to the edges got around 140mm centres.

[NordicCnc]

If you have access to a milling machine I would definitely put them top and bottom. If you want to mount them on the face and keep the same rail separation then you need a deeper gantry, if you use 160x80 profile for the gantry then the Y rail centres will *only* be 120mm apart (distance between outermost slots), the same profile with top-bottom mounts gets you ~180mm centres. On my last machine I used 160mm ali box section for the gantry and with face mounting close to the edges got around 140mm centres.

So do you think the difference with 40-50mm extra offset will make any notable difference to stiffness?

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[devmonkey]

So do you think the difference with 40-50mm extra offset will make any notable difference to stiffness?

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Say your overhang to spindle centre from gantry centre (assuming a rectangular gantry profile) reduces from 150mm to 100mm as a result of mounting rails top/bottom then you have decreased the torque applied to twist the gantry to 2/3rds for a given force applied to the spindle, so it really depends on the torsional stiffness of your gantry and the spacing of X bearings. Ideally the front X bearing wants to be at or forwards of the spindle so usually face mounting the Y rails increases the X bearing spacing, this is no bad thing but it does mean the machine needs to be longer to accommodate the same X travel.

For reference the difference in gantry twist for a given spindle force between when the Z is fully up and fully down will be greater than the difference in twist saved by shifting the spindle 50mm backwards.

As everyone says on this forum it depends on what you want to do with the machine as to how stiff it needs to be, having built previous machines that weren't stiff enough I now make every design choice around increasing stiffness. My choice to top/bottom mount the rails was both to increase stiffness and produce as compact a machine as possible as it is going into my garage so I don't have to traipse down to the large workshop in the woods in mid winter.

[NordicCnc]

Say your overhang to spindle centre from gantry centre (assuming a rectangular gantry profile) reduces from 150mm to 100mm as a result of mounting rails top/bottom then you have decreased the torque applied to twist the gantry to 2/3rds for a given force applied to the spindle, so it really depends on the torsional stiffness of your gantry and the spacing of X bearings. Ideally the front X bearing wants to be at or forwards of the spindle so usually face mounting the Y rails increases the X bearing spacing, this is no bad thing but it does mean the machine needs to be longer to accommodate the same X travel.

For reference the difference in gantry twist for a given spindle force between when the Z is fully up and fully down will be greater than the difference in twist saved by shifting the spindle 50mm backwards.

As everyone says on this forum it depends on what you want to do with the machine as to how stiff it needs to be, having built previous machines that weren't stiff enough I now make every design choice around increasing stiffness. My choice to top/bottom mount the rails was both to increase stiffness and produce as compact a machine as possible as it is going into my garage so I don't have to traipse down to the large workshop in the woods in mid winter.

So what I plan to use the machine for is mostly wood and aluminium milling and occasionally very very light stel milling. Based on what you and everyone are saying I am leaning towards top/bottom mounted rails since stiffness is critical.

Have you built any machines with face mounted rails and 160x80 extrusion gantry that was not stiff enough to mill aluminium efficiently?

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[devmonkey]

So what I plan to use the machine for is mostly wood and aluminium milling and occasionally very very light stel milling. Based on what you and everyone are saying I am leaning towards top/bottom mounted rails since stiffness is critical.

Have you built any machines with face mounted rails and 160x80 extrusion gantry that was not stiff enough to mill aluminium efficiently?

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No every machine i've built, including one that was substantially made of MDF with an underslung gantry rail with only a single igus carriage on each X rail on which you could rock the gantry visibly by leaning on the spindle, were capable of machining aluminium. You just need to back off the speed and depth of cut if you lack stiffness, all the accuracy is delivered on the finishing cuts anyway, just as with a normal mill, tool deflection particularly with the thin tools we use in routers is usually as much or more of a problem. That said it is easier to produce accurate work on a stiffer machine, so depends what you are making. If it is machining a few plates out with accurate vertical edges and holes that is one thing, if you are running 3D paths to build molds etc then you need a stiffer machine, and a machine that is significantly stiff over the Z travel you need.

Note though that if you want the rails to sit flat on extrusion it does have to be milled irrespective of which face you bolt them to. I don't think many people do this on this forum but most still get completely acceptable results.

[JAZZCNC]

Have you built any machines with face-mounted rails and 160x80 extrusion gantry that was not stiff enough to mill aluminum efficiently?

Very few on this Forum can answer that question regards 160x80 profile because they won't have needed to change if done one before the other.!

I've built 5 machines that use face mounted and 14+ machines that use top/bot and all cut aluminum perfectly fine. These machine sizes range from 4 x 2 to 10 x 5 and the 6 machines with face-mounted are all 8 x 4 or 10 x 5 machines because saving space wasn't an issue. So in this case, I went for the easiest method of fixing and setup, also fewer parts to machine.

Honestly, it's not something you need to worry about on router that's mostly going to cut wood with occasional aluminum work.

One other method I use often, esp when needing to keep gantry low but with the highest clearance is to mix n match bottom rail on the front, top rail on the top. This gives the most clearance under the gantry and keeps gantry height low.

So like I say use the method that suits your needs because no one is better or worse than the other. You won't see any difference when using the machine.

[NordicCnc]

Very few on this Forum can answer that question regards 160x80 profile because they won't have needed to change if done one before the other.!

I've built 5 machines that use face mounted and 14+ machines that use top/bot and all cut aluminum perfectly fine. These machine sizes range from 4 x 2 to 10 x 5 and the 6 machines with face-mounted are all 8 x 4 or 10 x 5 machines because saving space wasn't an issue. So in this case, I went for the easiest method of fixing and setup, also fewer parts to machine.

Honestly, it's not something you need to worry about on router that's mostly going to cut wood with occasional aluminum work.

One other method I use often, esp when needing to keep gantry low but with the highest clearance is to mix n match bottom rail on the front, top rail on the top. This gives the most clearance under the gantry and keeps gantry height low.

So like I say use the method that suits your needs because no one is better or worse than the other. You won't see any difference when using the machine.

Okay thanks for the information. I might just go with the top/bottom because to me it seems more rigid that way with larger spacing between the linear rails. I think I will be cutting mostly aluminium, some wood and occasionally steel with very light cuts.

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[Kitwn]

As Joe said "if you have a milling machine...".

Perhaps the really clever trick is to design your gantry to take both front and top/bottom mounted rails. Front mounting the rails with a simple Z axis plate would then give you a machine which may be good enough to machine the parts needed to then convert it. Let the machine pull itself up by it's own boot laces.

Kit

[Sterob]

I have been thinking about this. I like the idea of placing the rails on the side of the extrusion( for simplicity), but I would like the wider spacing top and bottom mounting gives.( for ridgidity )

How about using 2 smaller extrusions, spaced apart so the ball screw will fit in between them, and use a say a 10mm Ali backing plate to stiffen the extrusions?
That could bolt to the Gantry arms as well?

I guess aligning 2 extrusions may be a bit of work?

Been thought of already?...lol

[pippin88]

It's been done plenty of times. It's weaker than a large section.