mdf vs. ply vs. HDPE

Gents,

first of all, if this is in the wrong section, please feel free to move the thread where it should be.

I know this questions must have been asked numerous times, although I couldn't find to many threads about this on this board. I'm in the middle of prototyping a CNC related machine and the initial idea was to use 18mm MDF as it's flat and cuts rather easily. Recently someone suggested HDPE, and later on ply was suggested, as a natural alternative to MDF. I've looked at HDPE and it looks solid and stiff, although it's not great when it comes to thickness, suppliers can guarantee 18mm +/-2mm. Ply seems to be more stiff and rigid than MDF (which is basically glue and dust), but warping is a problem apparently, although MDF might suffer from it as well.

So out of the three, can anyone suggest one that's better than the others? I'm currently planning to use 18mm MDF, using cross dowels to hold the thing together. The forces acting on the machine are minimal, it's mostly about precise alignment of all the parts, there's two bearings which will be pushed into the MDF (holes 0.1mm smaller than bearing dia) and a z-axis like gantry on linear bearings. Final model will be made out of ali, so this is mostly for dimensions checks and overall idea testing, although it needs to be stiff and precise.

Cheers,
dsc.

It your just looking to build a prototype to check dimensions, work out design faults etc etc then personally I would just use MDF, it's the cheapest of the 3 options & although it does absorb moisture if not sealed properly this happens over time so shouldn't be a problem for you. Same with ply, yes it can warp but generally over time.
What are you using to make the parts?? If your machining them I doubt there is much difference between the 3 but if you are using basic hand tools then again MDF would score over ply as it's easier to cut & drill accurately.

if your pressing bearings into it I would go with HDPE out of the three

I cant compare it with MDF or ply for machining.... it can be a bitch to get a good machine finish compared to acetal (needs a fast surface speed ) and getting rid of burrs is pain, however, it will hold onto bearings better than the other two options but id run a couple of tests first to get a good fit

i'm pretty sure wood would compress and release the bearings if they are under any reasonable labour

Cheers Gents. It is true that this won't be used for a long time before the final model is built, it is mostly to check a few ideas out, but it must also perform it's main function. This covers very light loads which is one of the reasons I picked wood(-ish materials). We are talking hand cranking load levels (manual operation), so nothing above what my elbow and wrist cannot move shouldn't happen. Parts will be CNCed and joined using cross dowels, which means edge drilling is needed. Would that be harder / worse to do in ply considering it's layered character?

Good idea about doing tests for press fit, I assumed (maybe foolishly) that 0.1mm less clearance will work in MDF.

Another thing I'm worried about with HDPE is electrostatic charge. This machine will grind stuff and the output will be rather fine, which makes me worried that it will stick to HDPE (plastic in the end) like crazy with no easy way to discharge (can't ground the chassis with plastic).

Regards,
dsc.

Quote:

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Originally Posted by dsc

Parts will be CNCed and joined using cross dowels, which means edge drilling is needed. Would that be harder / worse to do in ply considering it's layered character?

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Personally I would stay well clear of ply... trying to edge drill that accurately would be a real PITA!

MDF is easier as I said before, easy enough to knock yourself up a little jig to ensure the holes are accurate as well if your using cross dowels.

Cheers guys, I'm sticking to MDF for now.

Regards,
dsc.