Does MDF flex?

[j_garcia]

Was going to mention that you might want to pre-drill a bi that guy since splitting is a definite possibility with MDF when screwing together on ends. It is just a stiffener not structural, so it will hopefully be OK.

 

[ParadigmDawg]

Is that going to work?

 

[Rickster71]

The good news, is it now has the spine. The bad news, the finishing nails and drywall screws were too short. I had some 2" cabinet screws and used them. They split the spine in a couple of places but I think it is fine, the glue has most of the holding power...correct?

I kind of forgot that the screws were just to hold everything in place as the glue dries and I went overboard with the screws. I used 8 screws...

That's why I mentioned pre-drilling in an earlier post.
Not overboard at all; 8 screws sounds about right

 

[ParadigmDawg]

That's why I mentioned pre-drilling in an earlier post.
Not overboard at all; 8 screws sounds about right

I pre-drilled the main shelf but forgot to pre-drill the spine. Plus I was a little off of center...

So this just acts like a rigid spine and should stop some flex or are you going to tell be I have to bolt the spine to the back of the credenza...which doesn't have a back.

 

[Rickster71]

I pre-drilled the main shelf but forgot to pre-drill the spine. Plus I was a little off of center...

So this just acts like a rigid spine and should stop some flex or are you going to tell be I have to bolt the spine to the back of the credenza...which doesn't have a back.

No, that should do it, and don't forget to move the foam to the ends closest to the shelf support pins. The shelf is strongest at the sides and along the spline.
So you want to distribute the weight there, by moving the foam.

 

[sawzalot]

Nice job PD. I think you have more screws in the spine then I have in my panel mount
I think you should be fine with that support rail and it is hardly visible with that dark color scheme.

 

[sawzalot]

Oh...and here is one more pic for Sawz....

I wish it was more gross but I have perfect feet.

I guess that could be considered "perfect" as far as a Toe goes, I'm just not sure. Those subs well now thats a different story= perfection

 

[GO-NAD!]

Glad you got it worked out, PD. Took a lot of posts to get you there though...

As for the toes, they look so perfect, I would suggest nail polish.

 

[ParadigmDawg]

Took a lot of posts to get you there though...

I am a tard when it comes to working with my hands....

 

[highfigh]

Was going to mention that you might want to pre-drill a bi that guy since splitting is a definite possibility with MDF when screwing together on ends. It is just a stiffener not structural, so it will hopefully be OK.

How is a stiffener NOT structural? A stiffener adds resistance to bending and it definitely is a structural component.

I agree with pre-drilling, especially if the screw is going into the edge.

 

[sawzalot]

How is a stiffener NOT structural?When it's a non-structural stiffener A stiffener adds resistance to bending and it definitely is a structural component.

I agree with pre-drilling, especially if the screw is going into the edge.

J/K ^^^ I agree any time something considered a stiffener or a support is added in a build it would certainly become part of the srtucture, but in this case the structure would probably be considered the credenza and since the shelf is only resting on the four pins, I would think the definition of the shelf being structural is questionable to some extent. Building/Carpentry 101

 

[Highlander]

ParadigmDawg,

Others have noted surprise that 3/4" MDF has bowed under only 45lbs. I agree. I calculate a maximum vertical displacement of 1/12" (approximately) assuming conservative load and support conditions. The shelf would not normally be expected to displace significantly unless the issue is creep related, i.e. time dependant. If the shelf remains displaced when the speaker is removed then the issue is creep.

...what is a good wood to use for this?

According to the Young's modulus values from here, Oak is only marginally more stiff than Pine, but approximately 2.5 times as stiff as MDF. In other words, all else being equal an Oak shelf will displace 2.5 times less than an identical MDF shelf.

Any wood that's 3/4" will flex and bow over time.

Any material under load will creep over time. The question is whether it is significant/acceptable.

The lip resists flexing in the vertical direction because of its orientation and because there is more structure to it in that direction (on end).

The lip better resists flexing in the vertical direction because stiffness is a function of depth cubed.

MDF is garbage. It works well for speakers because it's density is consistent and lacks the knots that some of the inner layers of Plywood has. Oak is at the other end of the wood spectrum; it's very hard and strong. It can bow if subjected to the right conditions.

You are confusing stiffness with strength. The two are not the same. Displacment is a function of stiffness alone. The strength, or otherwise, of a material is irrelevant.

*Deflection Plots*

Both plots show a maximum displacement of 0.0006" (what value did you adopt for material stiffness?) though the displaced shape is clearly different. Your lip appears inneffective.

...don't forget to move the foam to the ends closest to the shelf support pins. The shelf is strongest at the sides and along the spline. So you want to distribute the weight there, by moving the foam.

The shelf is equally strong in bending at all points because its cross section is constant over its length. The reason for moving the load to the sides is to reduce the applied bending moment.

 

[j_garcia]

The lip better resists flexing in the vertical direction because stiffness is a function of depth cubed.

Yes, it will bow less. That is not to say that it won't flex at all.

Both plots show a maximum displacement of 0.0006" (what value did you adopt for material stiffness?) though the displaced shape is clearly different. Your lip appears inneffective.

I didn't have a material for MDF, so I used a generic material simply to illustrate how the vertical rib would modify the stiffness of the shelf. The plots are also exaggerated because the actual deflection is almost not visible (and does not exactly correspond to his actual deflection because I do not have the right material).

How is a stiffener NOT structural? A stiffener adds resistance to bending and it definitely is a structural component.

I agree with pre-drilling, especially if the screw is going into the edge.

I suppose it is a structural component of the shelf, but I was thinking in terms of the whole rack like sawzalot mentioned

 

[Stereodude]

MDF will sag under it's own weight let alone when you put a load on it. It's a terrible wood to make any sort of furniture or shelving out of.

 

[sawzalot]

Does MDF Flex ?

Only when it's showing off,

 

[j_garcia]

MDF will sag under it's own weight let alone when you put a load on it. It's a terrible wood to make any sort of furniture or shelving out of.

Except that there is a lot of veneered fiberboard furniture out there on the low end of the spectrum. Again, the only thing it has going for it is: cheap.

 

[krzywica]

Only when it's showing off,

Yes only when placed in the same room with some Beech Wood.

 

[Highlander]

Yes, it will bow less. That is not to say that it won't flex at all.

I never wrote that it wouldn't flex.

I didn't have a material for MDF, so I used a generic material simply to illustrate how the vertical rib would modify the stiffness of the shelf.

Yet all the rib appears to have accomplished is to move the position at which maximum displacement occurs. It hasn't reduced the displacemment - whatever the value - itself, which was the intention. An incorrect value for the Young's modulus for MDF will change only the value for displacement, but in both cases it will remain the same (try it).

 

[highfigh]

Yes, it will bow less. That is not to say that it won't flex at all.



I didn't have a material for MDF, so I used a generic material simply to illustrate how the vertical rib would modify the stiffness of the shelf. The plots are also exaggerated because the actual deflection is almost not visible (and does not exactly correspond to his actual deflection because I do not have the right material).



I suppose it is a structural component of the shelf, but I was thinking in terms of the whole rack like sawzalot mentioned

In any structural analysis, every component is considered separately, WRT its role in the whole structure. If the shelves were fastened to the sides and they were allowed to sag, the sides would pull in and if more weight is on one side, it will rack because of gravity.

The moment of inertia is resisted more when a rib is added to the edge, perpendicular to the direction of the force. If two ribs are added- one at the rear and one at the front, it wouldn't sag much at all and if it's important for them to not be seen, they can be moved toward the center, but not less than 1/3 of the width apart.

With stiffeners in the right places, MDF works great for a lot of applications. If you were to take a sheet of 1/2" MDF and securely fasten 3" high ribs at 8" intervals (along the length and across the width) and then fasten another sheet to the top of the ribs, it will be incredibly stiff. This technique is used for making workbench tops and it's called a 'torsion box'. If you have seen a hollow core door that came apart or was cut, you'll see that it's usually 1/8" hardboard with corrugated cardboard glued to both faces, on edge. Cheap as hell and light as a feather, but really stiff.

 

[ParadigmDawg]

Crap...now I don't know if my self will flex or not...

I wasn't trying to use cheap material, heck I don't care if the shelf cost 150 bucks, I just wanted to replace the glass with wood...