One sided drive?

[Jeremy]

I'm in the final stages of finalising my "deep router" design and am pondering on the wisdom of using one-sided drive for the X axis. The machine working area is ~800mm x ~600mm, with the Y axis having two widely (~200mm) spaced beams that carry supported rails. The X axis rails are also fully supported and sit on top of tall side members, rather like the Mechmate designs.

I've spotted this design: http://www.durhamrobotics.com/easypage_2.html and have been giving some thought to the idea of only driving one side. I believe that I can make the "gantry" stiff enough, by having the Y rail support beams widely spaced and very well braced, for the light work I envisage this machine will be doing (mainly foam carving and a bit of woodwork, perhaps, but most probably only softwoods/MDF).

I used to have an A3 plotter that was only driven on one side and it seemed top work fine, but obviously the loads were very small. For this machine I'm using fully supported 16mm rails and linear bearings, so should be able to keep friction, and stiction, reasonably low.

Has anyone got any experience of one-sided drive machines please?

Jeremy

[irving2008]

Jeremy

If the gantry can be stiff enough to resist torsion on the XY-plane around the driven side in response to forces on the cutter then it can be done. What were you planning to make the gantry from?

ideally the drive should be outside the X-axis linear rails so that the turning moment due to the cutting forces moment acts primarily on the linear bearing and not on the leadscrew. With a cutting force of 50N (MDF or very light cuts in ali) at 300mm radius (assuming 600mm Y-rails) the turning moment is 15Nm.

By way of example, assuming 16mm fully supported rail on 40 x 30 x 3mm wall box sections 200mm spaced the deflection in the X-direction at the non-driven side will be ~.06mm (so .03mm at the cutter) and in the Y is .001mm. This shouldn't cause any material binding on the rails.

[Jeremy]

Thanks very much for that, Irving.

The gantry members will be two lengths of 2" square x 10g wall 6082-T6 alloy box, with SBR16 fully supported rails screwed to them, so it should be pretty rigid.

I'm thinking of something similar to the layout shown on that Durham Robotics link, with the leadscrew outside the rail, so it should be OK, with luck. The main reason for using this configuration is the reduction in machine height and a possible increase in overall stiffness, as the gantry doesn't need an under-table drive. It also means that I don't need to add the complexity of a double leadscrew arrangement.

Jeremy

[irving2008]

Well given that 50mm square 10g (3mm wall) box section is stiffer than the 40 x 30 x 3 I postulated this should be plenty stiff enough for your needs...

That calculation assumes rigid corner joints and a cross member at each end between the Y-rails of the same stiffness.

The other racking issue is where the cutting forces cause the non-driven upright to rotate in the vertical plane relative to the driven one. Obviously gets worse as the cutter rises above the table, so you need to ensure torsional stiffness of the gantry in the Y-Z plane.

[Jeremy]

Thanks again. I've just done some rough beam bending calcs and, just as you say, it looks like the deflections will be OK if I ensure that the ends are adequately well constrained.

I've assumed, as a worst-case, that the beam length will be 800mm, with the cutting load applied at the 700mm point (this assumes that the head travels over the central 600mm part of the beams). I've also assumed that the load will be shared equally by both beams and that they can be considered to be simple point loaded cantilevers (not quite right due to the constraint on the "free" end, but a pessimistic simplification).

With a 50N force applied at 700mm from the constrained (driven) end, I get a total deflection of 0.01745mm, which is small enough not to worry about!

Torsional stiffness may be an issue, as I'm using two beams that are only constrained at their ends. I want to run the spindle between the beams, so can't box them in to increase the area M of I around the common long axis. I've done a rough calc of the amount of torsional deflection at the centre of a single beam, assuming that the cutter was offset by 250mm from the beam centre, with a 50N side load. This seems to give an angular deflection of 0.000056 degrees. This would equate to about 0.000243mm deflection at the tip of the cutter.

Assuming that the torsional deflection would be at least half this due to being shared between the two rails, and add in the fact that the true rail torsional and bending stiffness would be significantly greater due to the effect of the SBR16 rails bolted to them, and I don't think I need to worry too much at all!

Having also done some rough calculations for unsupported round rails, I am very glad that I opted to spend a bit more on the fully supported type. It would seem that total machine rigidity would be dominated by the rail deflection if using unsupported round rails, which would make building a hefty structure to mount them fairly pointless.

Thanks for pointing me in the right direction with the deflection calcs - I hand't thought to just apply simple beam theory to this problem!

Jeremy

[irving2008]

... Thanks for pointing me in the right direction with the deflection calcs - I hand't thought to just apply simple beam theory to this problem!

JeremyYou're welcome. I used the Framework program (mechanic.zip) at http://members.ziggo.nl/wolsink/ to do a rough 3D analysis. Although it doesnt allow you to model the supported rail joined to the box section, I simulated it by creating an arbitrary shape comprising a 30mm high, 15mm wide steel block on top of a 40 mm x 30mm box.

One way to counteract the vertical torsion is to make the gantry sides solid and put a diagonal brace from the side to the cross beam. Obviously too big a brace will compromise the vertical work envelope.