Getting Rid of Mendel's Nuts

I’ve been working on something for a bit, and the results are starting to surface. I made a rash promise in the #reprap IRC on freenode.net to put up a teaser on Sunday, so here it is. It’s a RepRap variant with no fasteners in the frame, using sockets to accept the (in my case) 9.4mm aluminium tubing struts. You can assemble it with my favourite tool - the mallet! So far the only screws are those needed to hold the motors, belts and extruder guts in place (though as the extruder isn’t built yet so that ain’t saying much). Holes are provided for adding glue to the structure, and for the particularly fastidious they can be drilled and tapped as well. I’ve even made the glue cavity the shape of an M3 nut so you can in theory make strategic joints releasable to aid portability if you really do like your nuts..


Um, ignore the basket of fresh clothes. Suz thinks the workshop is some kind of laundry. Even keeps the washing machine there.

The shedding of M8 rod has other advantages besides removing the need for many fasteners and making the whole thing lighter. A good one is that the frame struts can now be smooth enough to run the axis skids on - running a skid on M8 thread is a teensy bit like sliding down a grater. This removes the need for some lengths, lightening the structure still more. Make it from hunting arrow tubing and you’d be going featherweight. Use bamboo and go eco. Many options.

The design of the new Z axis hinges on the use of a Sarrus Linkage, ahha, ahha. I’m hoping something cunning can be achieved with a DC servo and string, but until then a NEMA17 and a piece of M3 threaded rod will do. No there is not a lot of Z travel in my quick bodge (unless you count the slack when it wobbles up and down at present), but there is enough to get on with. The Sarrus is actually a great fit mechanically for the Mendel-style frame, because the Sarrus gets narrower as it gets taller - just like the Mendel’s triangular profile.

The X axis is right at the top, and a one-piece extruder/carriage slides back and forth across the top two runners. I’m still coding the brackets and idler for that. Yes, coding. The whole thing is parametric. The Y axis is more firmly in reality. This is a cunning sled that pulls itself together with a couple of rubber bands. This causes the sloping faces of the runners to move up the side of the rod as they travel, until they hit the top. This kills wobble automagically and is remarkably tolerant of mis-aligned Y bars. It’ll even work upside-down! The proper Y motor bracket is still being designed, so for mechanical testing I used fancy stuff called string...

The vertexes are going to change a bit from the picture. They’ll have to be a bit wider to accommodate the X carriage, higher so the Z axis does not scrape it’s butt on the ground, and braced for extra stability. None of it needs support for printing.

That’s enough for now. I’ll get back to it. More news & photos as I sort releasable stuff out. Just remember that I’m going to be releasing this as soon as it is vaguely usable, and as yet it isn’t!