Monday, 22 March 2010

Putting X on the map

With general router dimensions defined by the set of shafts (x 1000mm, y 600mm, z 400mm), I gave some initial consideration to the design of the x-axis.

I don't want to move the target surface as this would effectively give me half the run-length, even though it would be a more precise design as the gantry would be fixed.

For the moving gantry approach, I'm thinking about 4 basic options:

1. Floating target with separate support table

The target cutting surface floats like a bridge along the entire x-axis length. The gantry rests on a separate, lower table. The threaded rod that pushes the gantry is centrally beneath the target surface.

Floating Target with separate support table.

Advantages: A single, centrally located driving threaded rod. No gantry weight on target table. Less dust and dirt goes on the rails and threaded rod. Everything is on a table, edge-to-edge (no damage from bumping against the table; easier to move around).

Disadvantages: Big and heavy. Taller design. Threaded rod harder to reach as it's sandwiched between target and table. Target support and table support must be relatively strong to prevent bending. Possible gantry bending error as threaded rod is not in line with shafts/drill point.

2. Floating target

Here, the the target table also supports the weight of the gantry and no separate support table is required.

Floating Target.

Advantages: Simple, light design. Used by various existing routers. Threaded rod kept clean and easy to reach.

Disadvantages: Dirt can reach rails. Target support must be strong to prevent bending. Somewhat complicated under-the-table cross-over. Possible bend error as threaded rod is not in line with shafts/drill point.

3. Dual Motor Design

Here, the target need not be floating and two threaded rods drive the gantry on each side.

Dual Motor Design.

Advantages: Simple, light design. Less bending as threaded rod is at level of shafts.

Disadvantages: Requires 2 motors (about $180 extra for motor, driver, etc) as I'm not keen at connecting the two threaded rods with chains or belts. Threaded rods are not perfect, so the two threaded rods can lose positional alignment which will require software calibration/inversion.

4. Single Motor Design 

Similar as above, but only one threaded rod and motor.

Single Motor Design.

Advantages: Simple, light design. Less bending as threaded rod is at level of shafts.

Disadvantages: Increased probability of positional errors when routing on the side that is away from the motor.

Natural Selection...

Well, I first considered the two-motor design as a good option, but then withdrew. Having to calibrate the two threaded rods, buy an extra stepper, driver, etc, all adds complexity and risk, which makes it comparable to option 1 and 2.

It also seems to me that option 2 wins over option 1. After all if I already have to make a fairly stable supported target table, then I might as well make it somewhat more stable and put the gantry weight right onto it. And the dust problem might be solvable with thin protective "fences".

So, I'm left with option 2 or option 4. I can't help but think that option 4 might create greater errors than option 2. That may require some analysis.

For now, I'll be going for option 2. Others have chosen this path, and in particular for a mathematician, there is safety in numbers....

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