Joe's 1000x600x200 steel and aluminium router started

[Kitwn]

Something else to consider is the complexity of the Z-axis back plate. Rails on the face can allow this to be a simple flat plate whereas rails on top and bottom will require two accurate and rigid square joints.

[Sterob]

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

[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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[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.

[devmonkey]

Progress update!

I got back to the project after a break of a few months to work on other things. I ended up pouring epoxy to bring the X rails into plane, then set the master rail straight using a taut wire, microscope and some software as described here:
http://www.mycncuk.com/threads/12966...r-level/page27

I now have both rails mounted and the gantry moves very smoothly.


Next step is to fabricate the motor brackets and mount the ballscrews and steppers. I've ordered belts and pulleys and am waiting on these so in the meantime I started to fabricate a stand for the machine. Since the machine is itself a rigid structure this stand only serves to anchor it to the floor at a decent working height, hence the lack of bracing. All 4 legs will be bolted to the concrete floor then the machine will sit ontop on leveling bolts, finally the machine frame will be clamped hard to the stand so that it doesn't throw itself across the room (voice of experience...).

Cut and welded yesterday, painting in silver hammerite today. The more I use it the more I love this little MMA stick welder. welded the 3mm box section mitres at only 45amps! Almost as good as a MIG without any complications.