Gantry router rails bending.

[DomB]

Hi there,
I am doing some experiments to test various parts of a gantry router desing. The router is a fixed gantry moving table type.

The table (y axis) is fairly heavy and the rails for the table are only supported at the ends. The rails are 16-20mm in diameter, and they span a gap of 450 - 490mm.

When I put a dial gauge on the rain and load the table onto the rail it bows the rods almost 0.5mm for the 16mm rods and 0.2mm for the 20mm rods.

How much bow is not a problem with the sort of work people are doing?

Are people facing items the size of the whole table, therefore a bow in the sheet of this much would be noticable? Or is the small area they are working in at any one time sufficiently close together that the height in the z axis is not a problem there.

Or does the sheet metal bow up from stress relieving more than this anyway.

Are they any ways around the problem without resorting to supported rails along their whole length.

Could wheels be used to take some of the table weight off the rails.

Can a parabolic correction be made in the mach 3 software to compensate for the table (making everything a 3 axis move).

Can the table be milled by the machine itself to automaticially correct for the bow.

Any ideas.
Need more details just ask. No pictures yet though, sorry.

the travel is approx 12" x 12" (XY)

Dom

[CNCkitsandBITS]

Hi,

Will the weight of the parts you intend to cut course the 0.5mm deflexion?

Obviously without supported rail you are going to have problems. Are you using open linear bearings so that you could use a support in the centre of the rails?

Skimming the table is always a good option to increase accuracy even with supported rails.

[DomB]

Its really the weight of the table and vice that will cause the deflection.

The bushes are currently closed type.

The only thing about facing when on the machine is that small router type machines are not too good at leaving a good finish, unlike a conventional mill. (dremel type spindle, trying to face a 350mm square plate).

I was thinking about some sort of bearing pushing against the table at midpoint, like a wheel taking some of the load. I also thought if the movement of the table and spacing of the bushing blocks could be made not to ever cross the midpoint of the rail, a small support could be put there with the original closed bushes. But this restricts the machine movement too much.

Dom

[CNCkitsandBITS]

One possibility is to use cantilever supports at the ends or your rails. Do a search on the zone should be lots of information. I did a quick calculation on Beam boy; simple supports = 0.669mm deflection ,cantilever supports = 0.167mm deflection for 25Kg at centre of 20mm x 600mm round rails.