MDF screw holding study

[jppierson]

OK, I found another study that evaluates wood screws in various species, edge, face and end grain. The bottom line looks as if MDF holding capability is similar to Cypress and just under Poplar. The experiments were not identical, in that the wood screw study shows holding power when the overall length is only embedded 3/4 of the nominal screw length, so I compared the results of the actual fully embedded length of machine screw to a longer wood screw result. This may not be correct, but it does reflect the challenges/complaints people report when using MDF. To get the equivalent screw holding strength you need to use more screws or longer screws in MDF. The link below is the wood screw study, its 30 pages and will definitely make your eyes glaze over.

https://nvlpubs.nist.gov/nistpubs/nbstec...ert319.pdf

[rwe2156]

All said and done, why not just buy screws specifically made for MDF?

[Stwood_]

I am sure you are right. I should have stuck to what I know. It has a definite grain and that grain is very prone to splitting.

Yes, I've seen the grain/layer effect. Maybe what we are seeing is the heated rollers (roller after roller) press marks. Each roller that is rolling leaves a flat impression? Maybe...

[Cooler]

All said and done, why not just buy screws specifically made for MDF?

Or dowels which perform better than even the confirmat screws designed for particle board and MDF.

[jppierson]

Or dowels which perform better than even the confirmat screws designed for particle board and MDF.

I started down this rabbit hole by just wondering how the materials differ. Rob Cosman is using a triple layer of 1" MDF as a workbench top and lego style of plywood assembled base for his classroom student benches. I really like the idea of the face vise he uses on his wood top workbench. I also like the idea of not having to re-flatten a top several times while the material finally goes stable. I started wondering if there would be a way to screw a frame structure that would support that style of face vise when opting for a MDF benchtop. So basically one would need to figure out how much holding power is required thus how many screws, dowels, threaded rods or what-not you would need to meet the mission. Now I understand that in this particular situation the force is more shearing than pulling, but ya gotta start somewhere. One other interesting, to me anyway, aspect was that in the pulling force tests the screws would often fail first do to the slots being too deep. The lesson there being to examine your hardware before long term installation to forestall failure during installation. Anyway, I thought that sharing these studies might be helpful to others as well as hearing what others have done.

How would using dowels be much different in a tension stress requirement? I'm not disparaging dowels but just trying to understand all the advantages or disadvantages of various fastening strategies.

[Cooler]

I started down this rabbit hole by just wondering how the materials differ.  Rob Cosman is using a triple layer of 1" MDF as a workbench top and lego style of plywood assembled base for his classroom student benches.  I really like the idea of the face vise he uses on his wood top workbench.  I also like the idea of not having to re-flatten a top several times while the material finally goes stable.  I started wondering if there would be a way to screw a frame structure that would support that style of face vise when opting for a MDF benchtop.  So basically one would need to figure out how much holding power is required thus how many screws, dowels, threaded rods or what-not you would need to meet the mission.  Now I understand that in this particular situation the force is more shearing than pulling, but ya gotta start somewhere.  One other interesting, to me anyway, aspect was that in the pulling force tests the screws would often fail first do to the slots being too deep.  The lesson there being to examine your hardware before long term installation to forestall failure during installation.  Anyway, I thought that sharing these studies might be helpful to others as well as hearing what others have done.  

How would using dowels be much different in a tension stress requirement?  I'm not disparaging dowels but just trying to understand all the advantages or disadvantages of various fastening strategies.

Most often in woodworking the tension of a screw is used to deal with either shear forces or racking forces.

So is you build a bookcase using screws to hold the shelves, the screws are functioning in shear (to support the weight on the shelves) and for racking (prevent the bookcase from tilting over.



While screws do a decent job on shear, they do a very poor job on racking.  Dowels do a very good job on shear, and probably better than screws because of the vastly larger diameter of the fastener.  Dowels far exceed screws in racking.

I read the data and tested myself.  I put four screws through a 3/4" x 12" x 12" board into the edge of another 3/4 x 12" x 12" board.  I repeated that build using 3/8" x 2" through dowels.  

I used glue on the dowels only (none on the board to board junction).

I don't have a tensile tester, but I tried racking the boards and the dowelled board was much more rigid.    

I used construction screws for this test.  Drywall screw would have performed worse.  Worse yet, drywall screws are designed to snap off the heads if overtightened, and are more brittle than construction screws.

There are instances where tensile strength of a wood joint is important, but it is far less frequently encountered than racking strength.  

Also (in regard to racking strength) both of the studies I read said that the fasteners should not be within 2" of the ends of the boards.  The fasteners need support on both sides of the fastener or the boards may split from the racking forces.

[hbmcc]

After breaking some coffee tables several times in my youth, I can assure you that Elmer's glue is the "duct tape" of fiber wood. It has sufficient bonding strength to clear final inspections for rental deposit refunds. 

[Stwood_]

After breaking some coffee tables several times in my youth, I can assure you that Elmer's glue is the "duct tape" of fiber wood. It has sufficient bonding strength to clear final inspections for rental deposit refunds.