how flat MDF holey top??

[Lucky Dave]

Planning a custom MFT cabinet.  Thinking about top flatness.  Assuming the supports for the MDF, when cut to about 60 in x 40 in and CNCed for 20 mm holes, are perfectly flat, how flat will the top be? Does anybody know how flat a new MDF sheet is?  Does it depend upon the grade or maker of the MDF? 

Thanks for your input.

 

[Holmz]

Flat when on a table?
Or flat when held by the side and gravity is working on it ?

The boss has a shelf from MDF which was flat and smooth on a table.
It has a 2" bow (50-mm) when resting on the shelf holders.

 

[QuailRider43]

All the MDF panels I've eyeballed look very flat when lying on a flat surface.  I think most MDF is flat from the factory, and then tends to conform to the surface upon which it's laid.  The panels more likely to be bowed are the ones that have been stored vertically at an acute angle, or placed on a non-flat or partially unsupported surface for a long time.  Choose a panel from a supplier that stores them stacked horizontally, and don't pick the ones near the bottom of a forklift skid without checking them carefully.

Since MDF will more or less conform to the surface upon which it rests, the main problem would be sags when unsupported.  If supported on all 4 edges like the MFT/3, and the panel is not overly wide, then I would presume it would stay flat.  The larger you make your custom table, the more likely it might dip in the middle over time.  You could always put a support piece in the middle to prevent center sagging if this is a concern.  To ensure you start with flat stock, bring a long straight edge with you when selecting your piece (or borrow one from the aisle where they sell bubble levels).

 

[Lucky Dave]

According to one web site, thickness tolerance on MDF is + .4 mm which equals .016 in.  That number could be interpreted many ways. If it means that thickness can vary up to .016 within a single 4x8 sheet, that seems very large ( for a worktable top ).

I am using two cross table supports, in addition to edge supports, so the largest unsupported panel is about 20 by 40 in, quite a bit smaller that Festool MFT tops. 

Is there a spec on flatness for Festool MFT tops?

Further concern is swelling due to moisture. My shop is enclosed and attached to house, but does not have conditioned air.  I live in a very dry high desert area. On website mentioned above, there is a note that MDF can have residual swelling after moisture has departed. Does anybody have any knowledge about this residual swelling?

 

[Peter_C]

As others have mentioned the MDF will conform to the surface it rests on. With the size of panel you are talking about I would put 2-4 2x2's vertically for support throughout the field. They wouldn't need to be attached to the top, but would need a solid attachment at the bottom. Rarely if ever would they bother the clamps and a could be easily worked around.

 

[Holmz]

What are you cutting on it?
How with the flatness affect that?

 

[#Tee]

Ya i assume youre doing wood working type work? i mean how flat does it need to be? woodworking is forgiving. Your completed project might bend or warp down the line with wood movement moreso than the mdf top sitting in the shop.

 

[Lucky Dave]

Thanks for your many inputs, I know that you all are trying to be helpful, but my question is about the properties of MDF, specifically about the expected variation in flatness ( or thickness if you prefer) of an MDF sheet, not about how I use it or why.

Yes, of course, it makes no sense to discuss the variation in flatness if the MDF sheet is not properly supported. Also, of course, any variations in humidity have to be excluded for the question to make sense.

Sure would appreciate any numerical responses to the original question.

 

[GarryMartin]

MDF isn't measured according to its "flatness" as far as I know so I doubt you'll get an answer there. Anecdotally though, relative flatness depends very much on how it is stored and the humidity conditions. You mentioned a variance of +0.4mm in an earlier post - that's most likely relative to a standard. For example, I've attached the technical specifications for Medite Premier MDF along with the standard details associated with measuring thickness.

You'll probably find it's "flat enough" for your purposes, but if not, perhaps you could use a router sled to achieve a uniform flatness once it is installed in the same way that CNC machinists often run a flattening of MDF sacrificial boards once installed so they have a uniform reference height for their work.

I've come across deflection figures for shelving based on different materials and thicknesses before, so perhaps searching for deflection rather than thickness or flatness might help.

 

[Cheese]

Sure would appreciate any numerical responses to the original question.

Here's an excerpt from a specification paper on MDF that I tried to attach but couldn't. While it's not a flatness spec, it does address
the thickness tolerance which can give you a rough idea of what you're up against.

Thickness tolerance (EN324-1)
Whilst board manufacturers have complete control of thickness calibration at the mill and can easily achieve close tolerances, any significant changes in moisture content during transportation or storage can adversely affect the thickness tolerance of boards reaching the user. Consequently attention to correct storage conditions at all stages of the supply chain is recommended. A close thickness tolerance is required for MDF used for wood veneering and foil laminating, finishing through roller coaters and membrane pressing.

The European Standard EN 622-1 specifies a thickness tolerance of ±0.2mm for boards up to 19mm thickness and ±0.3mm for thicker boards.

 

[Lucky Dave]

Thanks for the numbers. Helps a lot.

Made some measurements in my shop. Yes, sample of one, Yes, MDF not in controlled environment, Yes, measurements by inexperienced amateur.

Used a digital Toyo digital caliper calibrated to .001. 

Took measurements of thickness every 4 to 5 in around the edges of a 57 in by 35 in (about) piece. 

Total measurements 29.  Nominal  .759  Average .755    Max .788  Min .748 

Separately examined flatness with a 48 in flat surface with less than .002 error and standard feeler gauge.  The 57 by 35 piece showed a convex bow along both long edges of about .015 and a convex bow diagonal about .025.

Thinking I might be seeing effects of sagging, I flipped piece over. Suprise. Reverse side also showed convex dips. So. Thickness variation in interior of this sheet dominated any error due to sag. Note, this piece of MDF has been lying flat for several months supported by edge and cross mounts that were no better then .010 of flat. Dry, high desert environment.

I have no way of even ball park estimating how much the measurements were affected by environmental effects vs manufacturing variations vs handling (support) effects.

At this time, my personal working conclusion, based on all the info I have at this time,  is that MDF with worktable size dimensions should not be expected to be flat to better than about .020 - .030. or about 1/32.  Another conclusion is that somehow milling the top flat somehow may not be worthwhile in the long term due to humidity variations plus sag effects.

Does anybody have other opinions?

It might be interesting if a couple other guys with reasonably accurate measuring devices tool some similar measurements on their MFT or custom MFT worktops.

 

[QuailRider43]

I wouldn't worry about fractions of mm.  It's wood.  It's going to vary a bit no matter what you do.  Tighter tolerances and perfect flatness would require CNC'd solid synthetic materials, if you can even source them at less than extortion prices.

 

[Vondawg]

Could not agree more.

 

[Holmz]

Flatness I suppose can be related to thickness for the MDF.
Then you talk about smoothness.
If held by the ends, then stiffness also comes into play with respect to flatness.

 

[mo siopa]

There can be a huge difference in tolerances between manufacturers and their "grades."  I just did a quick scan over all of the posts and didn't see if you mentioned where you purchased or who the manufacturer is.  If you have access to Medite or Medite 3d from Roseburg FP (formerly made by Sierra Pine), I think you will have better results.  Also, Plum Creek's Glacier Green but Weyerhauser just bought them and I'm not sure what they are calling it now.

Also, no matter how long you've had material in your shop, the way it was treated before you got it may be an issue.  If your material is thicker around edges, it was likely exposed to either high humidity or condensation before you purchased it- or maybe just not a very good quality material to start with.  When mdf is exposed to excessive moisture, it will expand and may not contract to its original thickness.  If the material was from a "big box" store, it may just be a lower quality.  Some mills will run material for the big box stores through presses that are out of true.

I have people using mdf spoil boards on their CNCs that need to maintain thickness tolerances of .005."  The thickness of the spoil boards will vary with humidity but the variation is fairly uniform across the sheet day to day.