Friday, February 13, 2009

 

Another off-the-shelf linear bearing experiment

This linear bearing was made completely from items purchased at my local Menards. The square perforated tube (also known as Gridbeam) is 1 inch square with holes spaced 1 inch apart. About $22 US for a 6 foot section. It's easily cut with an angle grinder, chop saw, or other abrasive disc - probably a hacksaw as well. The carriage bolts are 4 inch long 1/4 inch diameter 20 threads / inch. A couple dollars for a pack of 20 or so. Same for the nuts. The 'bearings' are $2.36 each 'sliding door rollers', also 1 inch diameter.

Unfortunately, the sliding door rollers have quite a bit of lateral play, so 8 were needed to give the bearing a solid feel. 4 - with two each on opposing sides - allowed far too much play for any productive use. So this linear bearing cost approximately $25, only allows movement in one axis (as well as rotationally), and functions better than any of my other off-the-shelf linear bearing experiments so far.

I don't think the concave surface of the sliding door rollers provides much, if any advantage with this design. I intend to experiment with regular skate bearings as soon as I have some of the apropriate size. As pictured, the linear bearing would benefit greatly from the addition of some lockwashers. I intend to flesh out a full repstrap using this style linear bearing, and more perferated square tube. My goal is a complete repstrap buildable with off-the-shelf parts, and as little tooling as possible, from a local "big box" or hardware store - with the obvious exception of motors and electronics. Suggestions welcome.

Comments:
Hi Timothy,

I've been playing around with off the shelf materials, but for a CNC build http://flightsofideas.com/?p=377. The funny thing is that I tried a similar approach to yours but couldn't get it to work (using MDF and bad capentry skills). I tried to use square pipe to get rid of the play - have tried your setup with square pipe?
 
Designing a repstrap by yourself seems like a lot of work. Why not buy a laser cut kit?
 
@Andy D.- I don't know about Timothy, but I would say designing your own is fun ;-) For some people the appeal of RepRap is the fabrication process; for others it is about the design and build of the RepRap.
 
Yeah, I see what you're saying, FOI. It can be a lot of fun to design and build something yourself. I think I'm a little jealous that so many people are spending so much time and effort plodding their own path to a RP machine when I'd like to see everyone focusing on perfecting one design branch. Maybe I'm thinking about this wrong, but the whole RepRap project seems segmented and not at all focused right now. Of course Timothy has the right to work on whatever he likes, and these linear bearings may someday be in the main RepRap design or may not. It scares me a bit. I'm not trying to be critical. It's just something I've been thinking of recently, and Tim's linear bearing got me thinking about it. Perhaps it's the nature of open source development?
 
Personally I like seeing the diversity of ideas - but I agree that it can be frustrating if you want a cohesive approach.

I like Timothy's idea because he is addressing the needs of those (all over the world) who can't afford a kit. Making a repstrap from local materials and without expensive machinery could be important to grow the community.

On the other hand, I suppose that if everyone who could afford a kit bought one and started donating reprap parts then repstraps wouldn't be needed.

Perhaps both approaches are valid?
 
This comment has been removed by the author.
 
Well said, FOI. Hear hear!
 
Indeed. Much to agree with on both sides. I'm building my own repstrap for selfish reasons of course, but I think my design principles have some merit for others as well:

- I'm working toward an absolutely minimal BOM - tens of parts, not hundreds.

- The entire machine (sans electronics) will be wrench-buildable. I stipulate that the cost of a machine should include the cost of all the tools required to build it, not just materials, and I'm striving for a very low cost.

- The machine should effectively scale from Darwinish sizes to medium-large sizes (4 feet x 8 feet) and be able to handle much more physical stress than a Darwin - suitable for a CNC router, plasma cutter, etc.

- Ideally, all parts (again, sans electronics) should be obtainable from local hardware or "big box" home improvement stores. At this stage in the design (fairly late - most functional issues have been worked out), this does not look at all unreasonable.

All that being said, if I could find parts appropriate to scale the design down further, and make it cheaper, that would be great! I hope some interested RepRap hacker can make a suggestion or two in that direction... The fastest way to scale RepRap dissemination is to leverage the existing mass-production infrastructure - enabling anyone to bootstrap themselves into RepRap development. I'm just trying to lower the custom-fabricated-parts barrier one more notch -- for myself, and others.
 
@Timothy Schmidt
Nice idea. Can you post some pictures in the blog? I think you're on the right track. A heavier machine than the Darwin with cnc abilities as well as rdm is what I'm trying for as well.
Arvin
 
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