Confirmat screws and mdf

[brooks]

If you take two pieces of 3/4" mdf and glue them together with Elmer's Probond Wood Glue, can you drive a large confirmat screw into the glue line to attach mdf sides to that first piece?

The basic idea: make up a 1-1/2" table top, with 3/4" sides. I'm told mdf is best fastened with confirmat screws. I haven't done anything with glued up mdf panels before. Will that glue line be okay?

[Tequila]

It might be OK. How big of a table are we talking about, and what's resting on it?

If it were me, I'd change the design so that the top rested on the sides, avoiding the problem alltogether.

[LCHIEN]

If you take two pieces of 3/4" mdf and glue them together with Elmer's Probond Wood Glue, can you drive a large confirmat screw into the glue line to attach mdf sides to that first piece?

The basic idea: make up a 1-1/2" table top, with 3/4" sides. I'm told mdf is best fastened with confirmat screws. I haven't done anything with glued up mdf panels before. Will that glue line be okay?

MDF splits very easily when you drive screws into the "end grain" edges. I think even when glued.

[jAngiel]

I think you would be fine. The main thing to remember is to predrill to the correct width of screw you are using. Regardless of whether there is a glue line or not, you should always predrill before using screws in mdf.

Take this for what it's worth, this is a hobby for me not a profession.

[RayintheUK]

Confirmats were developed for use in particleboard. I prefer to use a straight-shanked screw with a deep, wide, sharp thread. Do not use a tapered wood screw, because MDF fibers do not spring back when cut, they distort. This, together with the tapered shape, results in really poor retention.

Pilot the edge first, drilling 1/4" deeper than the screw length. A dull bit or slow drill speed will burnish the hole, which can lead to crumbling. Use a sharp bit at high speed for a clean accurate pilot hole.

For a #6 screw, drill a 3/32" pilot (in a sheet no thinner than 1/2").

For a #8 screw, drill a 7/64" pilot (in a sheet no thinner than 5/8").

For a #10 screw, drill an 1/8" pilot (in a sheet no thinner than 1").

Do not overtighten the screws, or they will break away. 3/8ths of a turn past flush is maximum. HTH

Ray.

[Tom Miller]

When you predrill, your drill bit could easily gum up with the glue. Maybe you could paste-wax the bit, then pull it out every so often to check/clean it.

In addition to pre-drilling, you should pre-drive the screw with a clamp bracing the piece against splitting. That's assuming you can't have the clamp in place for the final driving of the screw.

Regards,
Tom

[gmack5]

Confirmats were developed for use in particleboard. I prefer to use a straight-shanked screw with a deep, wide, sharp thread. Pilot the edge first, drilling 1/4" deeper than the screw length. A dull bit or slow drill speed will burnish the hole, which can lead to crumbling. Use a sharp bit at high speed for a clean accurate pilot hole.

For a #6 screw, drill a 3/32" pilot (in a sheet no thinner than 1/2").

For a #8 screw, drill a 7/64" pilot (in a sheet no thinner than 5/8").

For a #10 screw, drill an 1/8" pilot (in a sheet no thinner than 1").

Do not overtighten the screws, or they will break away. 3/8ths of a turn past flush is maximum. HTH
Ray.

Ray, I have a question for you regarding the hole you're recommending.
Is that Drill diameter recommended by the material manufacturer, or based on your own experience??

FYI, a screw's dimension can be calculated using a formula:
Screw size X .013 + .060.
As an example, a #6 screw is .138" in diameter.
This can be calculated by multiplying the #6 size by .013 (.078") , then adding .060" for a total of .138" diameter.
With the above in mind it's possible to calculate the hole diameter much closer to "Clearance" than the Drill sizes that you've recommended, while holding a minimum interference to maintain good regestration between the two parts.

As an example, the recommended drill size that you've suggested is 3/32" for a #6 Screw, while a 1/8" is still an interference fit, but not quite as agressive an interference. The 3/32" drill gives you an interference of .04425, while a 1/8" drill will provide an interference of only .013".

Wouldn't the smaller interference be less likely to Split the Material in the near part, if it were re-drilled to the larger diameter, leaving the mating part at your recommended predrilled hole size?

[RayintheUK]

Is that Drill diameter recommended by the material manufacturer, or based on your own experience??

For #6 screws, my own trial and error. Research turned up the #8 and #10 recommendations from Jim Hayden (Designing and Building Cabinets ISBN 1-56158-732-X).

As an example, the recommended drill size that you've suggested is 3/32" for a #6 Screw, while a 1/8" is still an interference fit, but not quite as agressive an interference. The 3/32" drill gives you an interference of .04425, while a 1/8" drill will provide an interference of only .013".

Wouldn't the smaller interference be less likely to Split the Material in the near part, if it were re-drilled to the larger diameter, leaving the mating part at your recommended predrilled hole size?

That may well be, but I haven't tried it, so I couldn't recommend it. My gut feeling is that a 1/8" pilot hole for a #6 screw would be too easy to strip and as I've had no problems whatsoever using the sizes as shown, I'll continue with them (although screwing into the edge is not something I'll do if there's another way around it).

I used to clamp the board in an attempt to stop it splitting, but once I had tried the piloting method and discovered that drilling the pilot 1/4" deeper than the screw length was essential, I don't bother to clamp any more.

The screw needs enough material to bite into, but the secret is to tighten them by hand, because they're so easy to overtighten with a cordless driver.

Ray.

[davidtu]

Here's a link to an article on Woodweb regarding some of these issues...
http://www.woodweb.com/knowledge_bas...at_screws.html

[cabinetman]

Here's what I do. I will predrill a hole with a built in counter sink, the width being that of the actual shaft of the screw, not the width of the threads. I use coarse thread screws. I will drill the hole a tad (like a smidgen) less than the overall driven depth of the screw, so the point of the screw has a bite (at least a full thread). As for length, I will use a 2" screw through 5/8" into 5/8" as a butt joint, with glue. For 3/4" onto 3/4" or 5/8", I'll use 2 1/4" or 2 1/2" screws. You learn how to pilot a hole that centers on the receiving panel.



"I'M NEVER WRONG - BUT I'M NOT ALWAYS RIGHT"