A slight crown in the center may be ideal. If supported at the four corners and maybe around the perimeter with an apron, even a torsion box will sag in the center. Especially one made entirely of MDF.
So tell people the crown was intentional.
Guest
A slight crown in the center may be ideal. If supported at the four corners and maybe around the perimeter with an apron, even a torsion box will sag in the center. Especially one made entirely of MDF.
So tell people the crown was intentional.
Contributor
You really should look up "router sled". It's dead simple to make and use one. One example: http://www.highlandwoodworking.com/f...tharouter.aspx It's just a mechanical process, nothing hard to construct or control about it nor does it make a mess, but it will make a lot of dust unless you connect it to a vacuum.
John
Contributor
Well now you tell us. We're about to go to blows here over a sixteenth of an inch.Originally Posted by Paulh Tremblay
Sounds like you have a 1/32" or less hump in the middle.
Time to start makin' stuff.
Contributor
One question just for my knowledge-
Does the bottom perfectly mimic the top? ie, a slight dip?
Member
In order for the OP to use this technique, he would need a sheet of paper that is 7 feet long. remember the alleged discrepancy is over a very long distance. A foot long straight edge is of no use.
One easy way to measure linearity is to tightly stretch a string on the diagonal above the surface that is very accurately positioned on both ends and then use a ruler along the string to measure discrepancies. Repeat the procedure on the opposite diagonal and that will give you a pretty good idea of how far off the surface is from flat.
Contributor
W/o trying to sound like a smart axx, they make paper a lot longer than 7 feet. I always have a roll of paper in my shop that's about 3 feet wide. I use it for all kinds of tasks from covering my workbench to making full size layouts, to covering the floor in my temporary spray booth.
Everyone needs a long straight edge; in fact, several of different length, and there's no need to spend money for them when you don't need micron accuracy. I have one, for example, that fits perfectly inside door jambs. I made it as I described above, and it's off by an amount I can't measure which makes it plenty good enough for many woodworking uses. I'm sure your string technique works and might be the right tool of choice for some applications, but a straightedge is just a lot faster and easier to use most of the time.
John
Member
Is the high spot on the side you put on the floor when gluing or is it the part that was under the books?
if it is the part under the books, was there an equivalent high spot in the "stuffing" before you placed the top?
if the high spot is in the side that was on the floor, is there an equivalent low spot in the floor?
This is a good case for asking the ultimate first question; "what happens if I do nothing?" Once you have decided that you must do something, there are a good number of suggestions here. Speaking as someone who shims things to get them level in his own shop I can relate to having no empirical reference surface on which to build a flat surface to build things on. Nudging a 4x6 piece of MDF around on a shimmed reference surface is an opportunity for something to go slightly wrong.
As for surfacing MDF with a router, yes you can but, I suggest you do so on a scrap first and see what finish you might use to "reseal" the exposed MDF to a degree that you are happy with. MDF does not come "sealed" but the process does leave the outer surface nice and smooth as opposed to say, the inside of a dado cut in the same material. My MDF workbench is just BLO'd and paste waxed. I have beat on that surface for years and it still look pretty decent so, a penetrating finish and some elbow grease can make things very workable.
"A hen is only an egg's way of making another egg".
– Samuel Butler
[OP]
Member
Good question. When I initially assessed the flatness, it seemed dead on. When I flipped the box, I discovered the lack of flatness. The box weighs close to 200 pounds, so it it not easy to flip it to test my initial assessment.
Member
Simple, turn it back over and be done.
Member
Maybe an outer frame that allows it to pivot.......faceframe clamps on one side,other side gets used for assembly or whatever?....or not.
If not wanting the flip table,I'd run it crown up for a few months,then check it again.You can check the plane with wires in an "X" pattern.Use gage blocks on corners.Best of luck.
Member
I just made my own torsion box top (36 x 48) and am having a similar issue.
I thought everything was flat, but when I check with a straightedge, there is a cup in the middle of the table that varies between 0.01 and 0.02.
The table is constructed of 1/2" MDF grid and skins, and then another 3/4" melamine top with two t-tracks. I haven't mounted the melamine yet, but I plan on running a couple of strips of masking tape down the center of the MDF as a shim. I figure a couple of pieces should bring the whole surface to <0.01, which I would think should be sufficient.
Member
Take your straight edge and lay it on any floor in your house and tell me if it's perfectly flat? Trust me, the framers didn't shim the floor joists or plane the subfloor or knock down the high spots on the foundation wall to be within thousands of an inch!Sometimes we get caught up on things that don't matter in the real world.
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