Tuesday, May 24, 2011

Winding up the String Wars

I was finally able to finish making the connection between the z-axis lead screw and the cable turnbuckle for the z-axis positioning system. Given that the 3/8-24 threaded rod moves 0.945 mm per full turn and that the 32:12 gear reduction coupled with the 200 step/full turn for the NEMA 23 we get something like 533.33 steps per turn or 0.00177 mm movement per step.


You can see the general layout of the z-axis lead screw with this pic...










I've circled, from left to right, the thrust collar nut, the cable turnbuckle and the linear bearings that make up the elements to transfer power from the stepper motor to the cable.


Here you can see the three connected...







Here is a detail of one of the four cable-driven lifts for the print table with the bracket in place...









I plan on having one fixed joint between the brackets and the table and two sliding joints constrained in the x and y-axes in the two brackets adjacent to it and a sliding joint in the xy plane opposite.  I hope that will be stable enough.


Next, though, I have to see if I can finish the design and printing of the BfB hot end adapted Wade extruder derivative.  If that takes too long I will fall back on the two full BfB extruders that I have in stock.

11 comments:

josh said...

Q: what do you use to predict the performance of this lash-up?

A: String Theory.

Seriously, what's the backlash on those printed gears?

Forrest Higgs said...

No backlash. The weight of the print table keeps them permanently engaged in the down direction. As well, the gears are herringbone, which intrinsically have very low backlash.

josh said...

Nice. Herringbones reduce cogging too. (With your low tooth count you'd get it above and beyond the stepper!)
I'm enthusiastic about the possibilities of printed gearing -- there are lots of configurations that are just too complex to use machined gears for (helical conical internal gears, anyone?). Maybe someone will eventually try Drexler's toroidal worm gear...

Forrest Higgs said...

The ability to print light duty drive trains is one of 3D printing's big advantages, imo. I'd have had to have that gear pair specially cut at a cost of many hundreds of dollars. As it was, I printed it from a couple of dollars of ABS plastic in about 5-6 hours.

Bogdan Kecman said...

How's the lift connected to the string? Is there a loop or?

Forrest Higgs said...

So far, just a friction gripper. More may be required.

Viktor said...

Hi Forrest,

how strong is the string tension?

Can the plastic parts handle this over long time without splitting apart?

Viktor

Forrest Higgs said...

@Victor I've made the tension in the steel cable about the same as you'd find in a drafting table's cabled parallel bar which uses the same technology.

As to the robustness of the solid ABS parts, it is impossible to say at this point. I have made designed for strength and put the grain of the prints at a 45 degree angle to the force lines in virtually all instances.

I think the best test will simply be to run the printer for several thousand hours and see what breaks. :-D

Viktor said...

Hi Forrest,

... let's see :)

I've built my first laserplotters around 1990 with 0.3mm or 0.5mm thick steel wires, but my connectors were solid metall or milled from POM, so they were more stable than the wires ;-)

Viktor

Forrest Higgs said...

@Viktor LOL! I guess it is not readily apparent just looking at the pictures, but the contact surface between the steel cable and the pulley is a steel 604 bearing. The ABS you see acts only as hubs to keep the cables in alignment. I'm not a complete idiot. :-D

Viktor said...

Hi Forrest,

... the critical area is more the body of the plastic parts - if you tense the wires high enough, the constant force may bend or break the plastic.

With my mechanics with solid POM-parts i could measure a significat loss in tension after some months, so i added some stiffening parts to the plastic holders, what too reduced the mechanical oscillation of the optical head ...

Viktor