Bookcase project advice.

[Sgian Dubh]

Sgian - Even though the MDF is 26mm thick and the Ply is 18mm Thick?? M

I'll be honest and admit I didn't spot the difference in thickness. All the same, 18 mm thick ply I think will still deflect a little less under load than 26 mm thick MDF, assuming the other dimensions (length and width) are the same. In this case I suspect the ply will deflect about 10 or 15% less than the MDF.

Another factor to consider is creep deformation which is time dependent deformation induced through constant loading. MDF is inherently weaker than ply, so I'd expect shelves made of it, over time, to bend more than the plywood. You may find it curious, but I have witnessed horizontal parts made of MDF simply bend under their own weight, e.g., unsupported long carcase tops. Plywood can do the same, but the deformation will generally be much less.

I suppose a further factor to consider is the weight difference between the two materials. MDF weighs approximately 45- 50 lb/ft³ whereas plywood weighs roughly 30- 35 lb/ft³. So a sheet of 8' X 4' X 1" MDF will weigh ±130 lbs, and the 3/4" ply (also 8' X 4') roughly half of that (60- 70 lbs). I know which I'd rather hump about if I'm on my own.

Sorry about missing the thickness difference. Slainte.

 

[Markymark]

Thanks.

Now 18mm oak veneered ply it is then. I have another question. I have a small ripping table saw and a cheap circular saw which I have to admit I use for most things.

Well I need to cut a straight edge on the 8x4 boards outside, to be able to machine a real timber trim to fit and I have to stop it from Damaging the veneer when sawing. I don't have a fancy laminate blade. Can anyone give me any advise??

M

 

[Markymark]

Quick Update.

I have started this project and have built the main carcasses with dadoes in for the shelves to fit in to. My question is the span is 1 m and the shelves are made of 18mm Oak veneered birch ply. I have decided to go ahead and put a divider in to the bookcases to make a 1 m shelf in to 2 500mm shelves. I am wanting to mill dadoes in to the 18mm vertical divider on both sides opposite each other.

The question is can I do this as I am worried it will take a lot of the strength out of the ply? If I can then wheat depth would you advise. The dadoes on the end panels are approximately 8mm deep on a 18mm panel.

Mark

 

[Markymark]

Ok, I did the Grooves (Dadoes) 8mm and 4mm on the center panels. All has gone well except when I tighten the sash clamps up on the top and bottom shelves it sends the carcass out of square by about 10mm. Does anyone have any good advice for keeping large (7') cabinets square when gluing up?

Thanks

Mark