Puzzled about acrylic thicknesses

I'm making the Rep(st)Rap redux, using the SVG files from sourceforge:
[reprap.svn.sourceforge.net]

I'm going to use my school's laser cutter to cut out the parts, rather than go through Ponoko. But I'm a bit confused about what thickness and size of acrylic sheet each file should be cut on. The files I am cutting are:

P1-8mm-extruder-redux
P2-3mm-redux
P3-5mm-redux
P3-MDF-beds-redux

I've heard variously that the '5mm' files should actually be cut on 4.5mm acrylic, that the MDF should be 8mm thick, that the MDF should be 9mm thick ...

Please help clear up my confusion! Thanks.

In the Uk the standard Thinknes of MDF sheets are 3 4 6 9 12 18 mm I think I have seen 2mm as well.

s for the Acrilic sheet I think that the 4.5mm was all that was avalible from one supplier. You could make 8mm MDF by sticking two sheets of 4mm together.

It would be worth checking the file by printing the layout and measuring any slots that the parts slot into. Just to be sure that the files have not been adjusted to suit 4.5mm instead of 5mm

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Bodge It [reprap.org]
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My rep strap: [repstrapbertha.blogspot.com]

Buy the bits from B&Q pipestrap [diyrepstrap.blogspot.com]
How to Build a Darwin without any Rep Rap Parts [repstrapdarwin.blogspot.com]
Web Site [www.takeaway3dtech.com]

If you have direct access to the laser cutter, then a quick test print to heavy paper or cardboard goes much faster then cutting the acrylic it self.

Really, really helps during a interactive design process. So paper plus tape is fairly acurate model.