DIY mill...

[Robin Hewitt]

Can you re-arrange and centre support those rails? Would make it lots stronger.

Have you considered reinforced concrete for the base? A bit of granite worktop would be nice. Sand and liquid epoxy if you have the pioneering spirit.

Looks a tadge flimsy as is.

[irving2008]

Robin, how would you support a round rail with a linear bearing that fully encloses the rail?

these are 16mm carbon steel rails supported at each end - according to my calcs a single rail will deflect no more than 0.04mm under a 35kg central point load and a) I dont plan to load them that much and b) the load is distributed across two pillow blocks on two rails so expect the deflection to be <0.01mm...

Where's the flimsy bit come in?

Irving...

[Robin Hewitt]

Robin, how would you support a round rail with a linear bearing that fully encloses the rail?

Where's the flimsy bit come in?

It's not so much the load, it's sympathetic vibration that may give you headaches.

[irving2008]

It's not so much the load, it's sympathetic vibration that may give you headaches.

Robin,

Its an interesting point. a centre support will reduce the deflection but as we already know thats tiny. But I hadn't considered the resonance aspect. I'll do some calcs to find the resonant freq of the bar and see if theres any issue.

Irving...

[irving2008]

Robin,

Its an interesting point. a centre support will reduce the deflection but as we already know thats tiny. But I hadn't considered the resonance aspect. I'll do some calcs to find the resonant freq of the bar and see if theres any issue.

Irving...

Well the natural, undamped frequency for a single rail is roughly 287Hz so I'm not sure theres any issue here - all the possible drivers forcing oscillation are at at a considerably lower frequency (min cutter speed 2800rpm - 46rps) so the damping coefficients for forced excitation are large. Also with the two pillow blocks attached there is a significant damping coefficient. There may just be issues with a combination of cutter speed at 17000 rpm (single flute), 8500 rpm (2 flute) or (4250rpm (4 flute) and/or stepping rate of 172mm/min. (Its years since I did any of this stuff at college... makes my brain hurt just reading the equations!)

Of course the proof will be in the actual result...

[Robin Hewitt]

Maybe I'm just tied up thinking you're going to make deeper cuts in harder materials than you actually are. A speed control can help prevent chatter, but without big heavy slides I'm thinking you really want to get as much spring out of the system as you possibly can :D

[irving2008]

Robin,

Your points are valid, but I think I'll go without the centre support for now, not least because it severely limits the useful length of the table in the X direction as I will have to make the table longer to allow the maximum travel and the length of the pillow blocks means the overall travel is now reduced, see the pic below and compare to the previous one