Some screw questions

[fablog]

Hi guys,

1) I'm wondering why screws have the first part of their design without any thread. More or less, the first 1/2" of the screw doesn't have any thread but it's what inside the MDF (my cab will use 5/8 MDF and I will countersink the screws) and the thread part will be inside the 2''x2'' pine stud. So, with my noob understanding, it's seems more important to choose a screw for the pine stud than for the mdf if we screw from the outside or am I wrong?

2) Is it better to screw from the inside?

NB: I precise I want to use drywall coarse thread screw and glue.

[Drnick]

Hi guys,

1) I'm wondering why screws have the first part of their design without any thread. More or less, the first 1/2" of the screw doesn't have any thread but it's what inside the MDF (my cab will use 5/8 MDF and I will countersink the screws) and the thread part will be inside the 2''x2'' pine stud. So, with my noob understanding, it's seems more important to choose a screw for the pine stud than for the mdf if we screw from the outside or am I wrong?

2) Is it better to screw from the inside?

NB: I precise I want to use drywall coarse thread screw and glue.

In Short.

1) The unthreaded portion of the shank is designed to slide through the top board (closest to the screw head) so that it can be pulled tight to the board it is being attached to.

2) Yes because then you don't have to cover any unsightly marks before finishing.  (Obviously this is not really an issue if you are laminating.

If you want to use Drywall screws and glue, go ahead.  The Glue should do most of the hard work anyway.  Remember a Drywall screw will try to pull through the wood if you overtighten. They are designed in such a way that the head can easily get below surface level. This makes sense if you are going to plaster over the top of them.

[Vigo]

+1 to the above.

I'm notorious for using less-than-ideal screw for the job, and besides creating a bit more work for myself, I have never had problems. You should be fine as long as the screw is the right length and you drill a good, full pilot hole. I guess the other school of thought is that screws are cheap enough, so it is not a big deal to buy some new ones for the job.

[EvilNuff]

Drnick is spot on.  To elaborate, if your screw has threads covering the entire shaft then any gap between your boards will be maintained by the screw and not reduced.

[fablog]

Thanks guys. So it seems that any regular wood screw would do the job with glue...good news!

I bought a countersing drill bit with a drill head included (I hope you understand what I mean), but this drill part is only 1/2'' long. Is it really important to do a full lenght pilot hole?

[PL1]

I bought a countersing drill bit with a drill head included (I hope you understand what I mean), but this drill part is only 1/2'' long. Is it really important to do a full lenght pilot hole?

With no pilot hole, you're more likely to split regular wood and get "mushrooming" (?) on MDF like the left screw below.

If the pilot hole doesn't go through the first piece and part-way into the second piece, you're likely to get blowout on the back of the first piece and mushrooming on the second.

The right screw below shows the result of using a pilot hole.

If the end of the screw gets too close to the back surface without a pilot hole, it can push the MDF ahead of it and cause it to blow out like in the second picture -- a wood-print shelf used in my pinball controller.


Scott

[JDFan]

Thanks guys. So it seems that any regular wood screw would do the job with glue...good news!

I bought a countersing drill bit with a drill head included (I hope you understand what I mean), but this drill part is only 1/2'' long. Is it really important to do a full lenght pilot hole?

Could always use a regular drill bit to make the full length pilot hole then use the countersink bit to make the countersink before putting in the screw or vice versa (countersink first then pilot hole)

[fablog]

Do I need to do a pilot hole in the MDF and the baton or only in the MDF? If it' only in the MDF then I will keep my countersink drill bit. I will countersink both sides of the MDF.

[PL1]

Do I need to do a pilot hole in the MDF and the baton or only in the MDF? If it' only in the MDF then I will keep my countersink drill bit. I will countersink both sides of the MDF.

Batons are usually narrow enough to make splitting in regular wood likely without a pilot hole.

Mushrooming/blowout in MDF is guaranteed without a pilot hole.

The pilot hole and countersink (red) go through the baton (top tan layer) and into the side. (bottom tan layer)

A very slight countersink where the screw exits the baton and another where it enters the side (wider red area in the middle) should prevent mushrooming between the layers completely.

Could always use a regular drill bit to make the full length pilot hole then use the countersink bit to make the countersink before putting in the screw or vice versa (countersink first then pilot hole)

Well said -- exactly what I had in mind.

Just be sure to clamp before you drill the holes so the baton doesn't shift.


Scott