forgetting the 2:1 rule

Saturday began as a beacon of hope, the realization of failure, and then relief at redemption.


Okay that sounds melodramatic. Sorry.

Let's recap the z stage movement systmes considered so far:
-first was the four threaded rod design (Makerbot Cupcake, RepRap Darwin), which was abandoned because of high cost, complexity, loss of building space and known issues evidenced by numerous forum posts
-second was the rollercoaster element bearing , which was abandoned in favour of trying out the igus polymer bearing option (would make the machining a little cheaper and easier)

which leads to where I was when I began machining on Saturday.


Here you can see my attempt at the cantilivered z stage design. The hole in the bottom left corner of the image is one of four on the stage that are used to support the actual build platform. They act as guide/alignment posts, guiding and holding the actual aluminum build platform in place. Sticking in spacer nuts between the build platform and the shown angle frame minimizes the amount of heat transferred to the actual frame of the structure, and allows for easy levelling (this concept can also be seen with the recent makerbot blog post on "instant leveller"). The yellow object on the right of the image is the Igus slider shuttle.


Here's a top view of the stage. The big hole in the center of the frame is where the 7/16" threaded rod goes through, and where the travelling nut fits. The two smaller holes near it are for securing a another aluminum angle to prevent the captured nut from having any rotational slack.










Back view, clearly showing the two polymer shuttles.













Of course as fate would have it, I conveniently ignored the lesson I was taught about the 2:1 rule when it comes to linear plane bearings. The center of gravity of the overall assembly was way more than twice the distance from the bearings than the length of the bearings themselves (you can see that in the photos), so naturally they bound and refused to slide down the aluminum track easily.

With only a couple of hours left in the day before the shop closed, I quickly pulled out my old CAD drawing for the rolling ball bearing design and machined it. With the use of slots instead of simple drilled holes, the bearings tightly hugged the aluminum angle, providing smooth and straight motion. You can see in this photo the "master" bearing.







In this photo you can see the "slave bearing" with bearings clamping only one side of the angle to allow for minor variations in the straightness of the aluminum angle frame.

Hopefully by next week I'll be able to some photos of build platform and z nut attached to the new ball bearing sliders.



till next time,

Eric