is MDF flat and uniform thickness?

[mickthetree]

Hey all

Few threads been around recently about making MDF workbenches.

I have a sheet of MDF from the big orange store which I'm using for a bench top.

It wasnt sitting flat all over when I screwed it down. I put this down to the frame (quick bodge together job).

I've since added a support to the rail which has a very slight dip in it.

However the MDF sheet still sags in the middle??

The frame is flat with my straight edge and when I turn the sheet round the other edge sits flat.

Which leads me to assume that there must be a reduction in thickness in the centre of the sheet.

I assumed (nothing new) that MDF was pretty much flat with a uniform thickness all over. Is this not the case?

Or is it known that shed bought MDF is like this?

Cheers

Paul

 

[jasonB]

Even slightly humid conditions can make it start to swell from the edges

As its not easy to measure the middle you will only know by cutting it down the middle and measureing the edge and centre thickness with callipers

Jason

 

[mickthetree]

NNNGGGGGGGGGNNNNNGGGGGGGG

WILL I NEVER GET A FLAT BENCH??????

Thanks Jason really appreciate the info.

I'm sure it must be the case.

If I'd gone and bought a descent bench I'd have probably saved money by now and could have finished most of the projects I have half done.

Cheers

Paul

 

[big soft moose]

MR mdf - the green stuff - is much less prone to this as , as the name suggests, it is moisture resistant. You can also varnish normal MDF to reduce the likely hood of it swelling.

btw on the buying benches front G&M tools have a decent range of second hand benches

 

[Paul Chapman]

With something like a bench top (or jigs that I intend to keep), I would always lip the edges of the MDF with solid wood and also wax or varnish it. I've found this the best way to stop it absorbing moisture.

Cheers :wink:

Paul

 

[9fingers]

With something like a bench top (or jigs that I intend to keep), I would always lip the edges of the MDF with solid wood and also wax or varnish it. I've found this the best way to stop it absorbing moisture.

Cheers :wink:

Paul

Make sure you seal the underside too!

Bob

 

[catface]

hello mickthetree,
you ask if MDF is flat and of uniform thickness. Flatness is usually ok but this depends greatly on how it is stored after supply (and on moisture, as others have said above).

REgarding thickness there can be significant variation across a single sheet. This is why professional producers of mdf kitchen cabinets which use housings for the joints, will often use an overhead router to form the housings instead of running a sheet over the router table - otherwise when a run of (say) 3 or 4 cabinets is put together side by side, any variation in sheet thickness can result in the cabinets not being quite square once assembled. But using the overhead router does away with this "error due to differences in sheet thickness" by putting the error into the housing depth to eliminate it from the build.

hope this helps, regards, CAtface

 

[mickthetree]

Catface - Thanks. That is exactly what I needed to know.

I'm going to have a look at the MDF this arvo and see if I can measure a difference. Its along the length of the cut edge that seems to have a noticeable difference.

I didnt notice the MR MDF at the shed but I'll take a look at this.

I wondered why the bench build I saw the other day the guy planned the second sheet of MDF before putting the top sheet on. Didnt make sense at the time.

does now!! :wink:

Cheers

Paul