Trivial screw clamping question.

[ayuce]

I'll face join three sheets of 18mm mdf using white PVA glue. Initially i'd use clamps, have no vacuum bag, but i thing using screws as clamp would be more reliable.

I'll drill pilot holes to two bottom sheets together and countersink on both (4 total sides). Make a dey run, sand the bumps of screw holes, with the help of a friend i'll spread the glue, after the last sheet screw three layers together.

It seems simple but cant be sure how many screws will be enough for clamping ? Sheet sizes 155cm x 60 cm. Screws on a 25cm grid, starting 5cm from inside of edge would be an overkill ? Just screws with 30cm spacing around the edges plus few in the middle would be enough ?

 

[Phil Pascoe]

I think I'd drill a couple of holes to insert pins to stop its slipping, but I'd get half a dozen 4' x 4" x 2"s, put a slight curve on them (you may be (un)lucky and buy them with a curve already on them!) and clamp across the boards so the centre tightens first. Thin your PVA slightly or it will be going off before you are ready. You can put it on with a roller.

 

[ayuce]

Thanks Phil. I'm planning to drill two 10mm holes on opposite corners for pins. I could put a curve on 2x4 by hand plane, if i had enough of them Have either 3mt longs for doing smth else or too short leftovers.

Never glued such a wide area on MDF, this PVA setting issue concerns me a litte. Will need time and two more hands to spread glue and aligning three boards. This is why i want to use pre drilled holes + screws. I hope dilluting it by %10 percent would be enough

 

[Phil Pascoe]

I made a large jeweller's bench that way, and I don't think you'll get much pressure using screws - to do much good you'd need scores of them, by which time the glue would be drying. Whatever way you do it, you could join two sheets first, then add the third when the first two are dry.