acrylic or mdf

Hey reprappers,

I'm scheming to put together a Darwin style repstrap.

I'm looking at getting parts laser cut by a local supplier [www.lasercut.net.au] ( Perth, Western Australia).

Now the plywood design seems to have been superseded by the bits from bytes acrylic design.

I have reservations in using acrylic as it has a nasty tendency to crack and fatigue. Does anybody see any issues with using the same pattern and making it out of 5mm MDF board?

I agree that acrylic can be a pain, I've snapped enough bits building mine, but I don't think you will find MDF less prone to breaking.

From what I've been able to find it looks like acrylic takes almost 2,000 times more load to break than MDF while still being as stiff. The parts I broke most often were the corner blocks; MDF's higher co-efficient of friction should result in less force being required to hold the rods, but I doubt it's enough less to reduce the danger of over tightening.

Acrylic is also easier to put back together after breaking a part than MDF.

Ply is a much stronger alternative with a closer max load, although it is over twice as stiff. The biggest problem with it is that 5 mm Ply doesn't necessarily mean the board will be 5 mm thick it could be 4.5->5 mm. The only parts, I can remember, where this is significant are the X carriage and the X motor mount plates. So to use ply these would need to be altered to take into account whatever size the ply actually is.

The numbers I found are:

Acrylic
3.2 GPa young's modulus
72.3 MPa modulus of rupture

MDF
3.8 GPa young's modulus
32.8 KPa modulus of rupture

PLY (marine ply of unspecified wood)
7.5 GPa young's modulus
4 Mpa modulus of rupture

The difference between the modulus of Rupture of MDF and Acrylic seems very high so if anyone could check them that would be appreciated.

You make a convincing argument.

I shall go for acrylic.....

Now I need to get a quote for the laser cutting.



Cheers.

Be aware there are a few designs now i.e. Vik's plywood, Viks's modified BfB one and our BfB one i think both of Vik's are a work in progress but i could be wrong. Our design use a laser routing technique and takes some trial and error to get right as it is dependent on your particular lasers parameters (we route a grove/ recess for bearings etc) If you have access to use a laser yourself it is quite easily achieved if not I would look to finalize one of Vik's designs. I know the one he's based on our design has been done to remove the need for laser routing.

---

Ian
[www.bitsfrombytes.com]

Hey Ian,

I had a closer look at the deign and realized that some of the parts have partial cuts rather than full depth.

This morning lo a behold the laser cutting place emails me back with a big WTF is with these designs?

hehe.

you are correct , I think I will get out my (t)rusty protractor and compass and look into working on (http://reprap.org/bin/view/Main/PlywoodRepRaps) the slot-A into tab-B design to work with.

Vik informs me that his work has stopped on the plywood design, however I think I will grab it and start hacking away.

The idea of the reprap has grabbed my brain and won't let go...

On another note, rep-rappers beware PIC micros do NOT like 12V unregulated through them

ERK


Simon