Sunday, August 24, 2008


RepStrap Build Using Laser Cut Parts Kit - Update

It's been a couple of weeks since that last update. I have made a good deal of progress on the build. Things went very well for the most part. I also did as much work as I could on the X and Y axis assemblies. They are almost complete and I will be getting more parts this week.

Here's a picture of the main bot so far:

Things haven't been tightened because I don't have the slide bearings yet and 4 of the side diagonals are missing. I just had to see what it was going to look like.

What went wrong:

1. I broke 2 more of the plastic parts even though I have been extremely careful when tightening bolts.
2. I have not been able to locate a US vendor for slide bearings with the exact specifications as the Skiffy ones from the UK.
3. The side diagonals were short on 2 sides of the main assembly. All of the rods are specified at a length of 600mm and I found that 610mm long rods worked for me for 4 of the rods that make up the 2 side diagonal assemblies. I'd be curious to know if anyone else encounters this. I measured all of the dimensions between rods and I come out with a different diagonal spacing for 2 of the four sides.
4. I've seen the thread on the mechanical forum about splicing the Z belt. It looks like any of the suggestions will work but I'm curious to know what Ian had intended. Hopefully he will add his input to the thread or maybe a procedure on the Bits from Bytes web site. I'm guessing that it's probably the same as for the original Darwin as described by Adrian.


I wasn't happy with trying to adjust the corner brackets that clamp onto the 8mm smooth rods so I used 2 70mm smooth rods and inserted them in parallel with the rod being clamped onto. Here's a picture:

I then took up the slack on the M3 nuts so that the plastic bracket pieces were parallel. Not too tight because you won't be able to remove the 70mm rods. When all of the nuts are snug, remove the 70mm rods and tighten the nuts about another 1/2 to 1 1/2 turns. Once again, not too tight or the bracket parts may crack.

I hope this explanation makes sense.

Love the clear plastic - totally reminds me of Doozers. :)
Which bits broke? Maybe we should do them in 8mm acrylic or provide spares in the kit?

Vik :v)
Well for my laser cut parts I jsut went to a skate shop and asked for ABEC 7 bearings. The closest thing I could get and for the most part seem to work well. ABEC 7 is more a rating so the people at your local skate shop my be confused. They might refer to them as "sevens" as well. Just take a corner piece and and M8 bolt with you just in case you are not sure. The thickness of the bearings is not really much of an issue. so if it is off by a mm or so it will not effect much.
Also I rembered that a fair number of parts for me broke as well. It was worth it to get some "Plastic Weld" basically glue for acrylics. Yes depending on what brand/type the stuff can be toxic. But that is they same with about every thing these days. If you get stuff meant for acrylics you will not have to worry much about the joints being weak. They are less like glues and more like chemical fusing/welding.
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For comparison I have (so far ;-}) broken:

1,40,38 corner pieces by over tightening and them snapping in line with the bar. Oddly I've not broken 2,13,15,39,41 though.

38 One by over tightening the grub screw and another when moving Darwin before I'd added the cross braces.

44 by over tightening the bolt carrying the bearings.

61 trying to seat one of the nuts next to the corner I broke off the corner.

71 I've broken quite a few of these; fortunately there are already 2 spares. When they snap in the wide part they can be glued easily, but I've had some where the plastic above the small hole has broken and I'm not sure how I'd fix them.

88, I broke off one of the small side tabs. I've reattached it, but I don't think it actually does anything.

The parts aren’t particularly weak, but acrylic seems to give very little indication that it is under too much strain before it cracks.
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