Replies

1. 
   BertJ | Aug 28, 2003 10:19pm | #1

   MDF is not a good choice for wood screws. Options are:

   1. Use plywood.

   2. Use MDF with T-nut connectors.

   3. Drill appropriate holes and glue hardwood dowels--then use wood screws.

   BJ

2. 
   Kevin | Aug 28, 2003 10:56pm | #2

   I guess it depends on what kind of jig you want to build. If you pre-drill a pilot hole first, wood screws shouldn't pose a problem in most cases. If strength is a concern, you could go with Bert's suggestion of T-nuts. One nice advantage with T-nuts is that you can build a complicated jig so that it partially or fully disassembles... or you can design it to be multi-functional. T-nuts really are a great way to go. But with a basic jig that's not complicated, I'd just drill pilot holes, countersink if necessary, and use wood screws... being careful not to sink them in too deeply.

   Regards,

   Kevin

3. 
   DavidP52 | Aug 29, 2003 12:12am | #3

   Biscuit jointing works fairly well. The main thing is is to not fasten too close to edges since they'll break out.

   Jigs don't normally get a lot of stress, so this shouldn't be a problem. Just keep in mind that MDF is weak stuff and use it accordingly. I'll often use combinations of hardwood with the MDF if a part is going to be heavily loaded. For example, don't expect to screw a lever or toggle clamp to it as it will tear right out. For guide type jigs, its fine.

4. 
   desertrat | Aug 29, 2003 03:01am | #4

   I try to make all of my jigs multi-purpose and by doing so can afford to spend a little more time and material on them.  I like using Baltic Birch.  Works great.

   I save all the smaller pieces from projects and try to use them for my jigs etc.

5. 
   JohnWW | Aug 29, 2003 03:42pm | #5

   Fred,

   A screw into the face of MDF presents no problems, it has moderately good strength.  The common wisdom that you can't screw into the edge of isn't accurate, you just have to modify your technique.  The core of a sheet of MDF is softer and less dense than the faces of the sheet, if you drive a screw into the edge it will strip out easily and usually split the sheet causing a bulge in the panel over the screw shank.

   The solution to these two problems is to use a longer screw than usual to increase the holding power in the weak material and to drill a larger than normal pilot hole for the screw to prevent splitting.  I've made hundreds of jigs using just screws into the edge of MDF with no problems. 

   If you are joining two pieces of 3/4 inch MDF to make a butt jointed corner, do the following:  Use either 2 1/2  or 3 inch sheet rock screws to join the pieces.  In the piece where the screw goes through the face, drill and countersink a hole as usual, one that completely clears the screw's threads.  

   In the second piece, where the screw goes into the edge, drill a deep pilot hole that is slightly bigger than the inner diameter of the screw shank.  Do a test piece and start with a drill a 32nd of an inch larger than the inner diameter, if the MDF splits when you drive in a screw, go up another 64th of an inch on the size of the pilot drill.  As a further precaution against splitting you could drill the pilot hole to the clearance diameter for it's first 1/4 inch of depth, but in general I don't do that.  Drive the screws by hand at least for the final tightening, you'll have plenty of holding power with only moderate torque on the screw. 

   John W.   

6. 
   rslatm | Aug 29, 2003 04:36pm | #6

   Use coarse thread screws instead of fine thread screws and you won't have any problems. Pre-drill as others have said.  There also an ultra light MDF available. Same quality but about 1/3 less weight.  Best sources are sheet goods dealer not big box store or local lumberyard.
1. 
   JohnWW | Aug 29, 2003 05:25pm | #7

   Both good points, I should have mentioned using coarse screws in my original post.  I use lightweight MDF in my shop and I love it. 

   John W.

1. 
   fredb | Aug 29, 2003 06:26pm | #8

   Hi All,

   I've been reading the posts and want to thank you all.

   I'll get busy as soon as I get my shop back. It's inside out now with an oil tank replacenment going on (outside is the tank - but inside the pipes go around - yep - a basement shop (for now...)).

   Regarding coarse threaded screws, are these available in the big box stores? Would these be coarse drywall screws or wood screws specifically?

   Thanks again!

   Kind regards - Fred

1. 
   JohnWW | Aug 29, 2003 06:55pm | #9

   Coarse drywall screws, ordinary screws won't work for the technique I described.  Home Depot has both fine and coarse, I rarely use fine threaded screws for anything. 

   Be aware that MDF expands and contracts with changes in humidity.  The change can be enough to cause fitting problems, such as with a jig that's used to guide a router to cut out a mortise.  Between winter and summer here in New England a 12 inch piece of MDF will expand a 16th of an inch.

   John W.

1. 
   Kevin | Aug 29, 2003 08:27pm | #10

   Be aware that MDF expands and contracts with changes in humidity.

   That's a good point. Sealing any exposed ends with glue should help mitigate that since the ends expose the least dense, and therefore most prone to absorbing moisture, part of MDF.

   Regards,

   Kevin

7. 
   jc21 | Aug 29, 2003 11:15pm | #11

   Check out FHB #154 April/May 2003 issue; Jim Blodgett has an excellent piece on working with MDF. Plenty of info on fastening.