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Linear bearing alternatives
The most expensive mechanical bit is the linear bearings, these are the alternatives:
Bart Drings MakerSlide
This looks so awesome I'm tempted to wait for him to get ready to ship: http://www.kickstarter.com/projects/93832939/makerslide-open-source-linear-bearing-system
Angle-profile + V-bearings
This looks both cheap and available: http://www.youtube.com/watch?v=nTQd-TIkAeE
The profiles are available via Sanistaal: https://ehandel.sanistaal.dk/prod/webapp?WSYD_EVENT=SA5fnF&WSYD_SID=22739576&WSYD_SIGN=5E6B2BBD4DE584D5DAD7E82A74EED3C8&SAuwA=4101
V-bearings are available at 7 USD apiece from Bart, once he starts taking orders again.
It might not be the nicest looking or the most compact, but it would be a good temporary solution until Bart starts shipping MakerSlide profiles.
Rollon compact rails
These look interesting: http://www.rollco.dk/p11.asp ... But like all other machine parts in .dk they are outrageously expensive: 10097,- for the entire set.
V-rail + V-groove bearings
The V-rail is available from Klee at 1500 + moms + shipping, they only have crazyexpensive bearings (318 kr/each we need 9), but buildlog.net has cheap bearings and mounting plates at 410 kr and a second package containing v-rails for 700.
The buildlog.net parts don't quite fit the larger size of PS, but I bet it would be possible to get a custom order with longer rails.
I'm jealous of the US for having McMaster-Carr...
less V-rail + V-groove bearings + plain bearings
A way to save some V-rails would be to drop the rails for one side of the double axis and simply use a couple of plain bearings riding on the opposite alu profile, that would drop the cost of rails to a more manageable 1026 kr and remove any alignment problems between the two long axis rails.
Igus DryLin W rails with hybrid rails
Igus makes some very neat looking rails with some very simple bearings and they are very nice, compact, and relatively cheap, total cost including both rails and bearings for X/Y: 1700,- kr
Rail WS-10-80: http://www.igus.dk/WPCK/default.aspx?pagenr=2003&CL=DK-da Bearing WJRM-01-10: http://www.igus.dk/wpck/default.aspx?Pagename=N09_5_12_WJRM&CL=US-en Rail data: http://www.igus.dk/iProsvc/Download.aspx?File=P02600500GBen.pdf&Name=DryLin_WQ10.pdf Bearing data: http://www.igus.dk/iProsvc/Download.aspx?File=P02601700USen.pdf&Name=Drylin_WJRM.pdf
The only problem with this system is that I'm a bit nervous about the tolerances of the hybrid bearings as they are not really made for this use.
Perhaps we could make some nylon or POM wheels for riding on 10mm rails and create our own bearing system to get the best of both worlds (compact, light, nice rails and high accuracy).
Igus Drylin W rails with PCB wagons and standard ball-bearings
Use the same rails as above, but build the wagons out of (CNC cut) PCB material 1.5 mm copper clad, that holds 90 degree ball bearings such as these:
The wagons need 2x2 bearings on top and an adjustable part with 2x2 bearings on the bottom.
PCB fibre glass is both very stiff and very light and it's easy to solder together at 90 angles, with proper design it should be possible to cut it out accurately on the small table saw with the diamond blade and get a very strong part.
When we get the metal-CNC (PCB mill) up and running we can cut these parts out of 4mm alu and get even more legit parts, best of all it all packs flat for shipping if we want to sell kits.
The price would be around 70% of the pure igus solution and I'm pretty sure it will have lower weight and friction, higher stiffness and accuracy.