how to get the flattest work surface

[Alan Berezin]

Hey all,

Ive been mostly just lurking in the backround but thought Id poke my head out to ask a question I'm a bit stumped on.

Im building (mostly built actually) the rolling workcart project from ShopNotes vol15 issue 90. Not a bad design (ill try to snap some pics). Anyway, it is about 21x49. The base is 2 sheets of 3/4 ply (I used Borg ply on this project: never again! That stuff is awefull). The top is one sheet of ply. I was going to put a sheet of 3/4 MDF on top. Here's the question.

The top as it sits now is not very flat. Im not sure if i miscut something or the top ply is just warped and/or not even thickness or some combo of issues. In any case, I dont want to glue/screw the MDF to a not-so-flat surface. I was thinking of the following idea and wanted some opinion.

I can mix a big batch of epoxy (or perhaps polyester resin or something else). Then pour this on the existing ply and spread it arround. Then "float" the MDF on this. I know this isnt a cheap solution (a liter or so of epoxy isnt cheap). Perhaps this is a terrible idea but it seemed like it would work.

[Keith Weber]

How about building a router sled and router the top flat with a straight bit?

Joint two boards so one edge is straight. Attach these boards to each side of your cabinet, jointed side up, with a couple of brads. Make sure that they're parallel and level to one another. Then build an MDF router sled to allow you to run your router back and forth across the top. I'm sure if you did a search on "router sled" or "flattening a table", you'd come up with a detailed explanation. Make sure that any nails in the top are sufficiently countersunk.

If it's really warped and you don't want to rout the plywood. Glue a 1/2" or thinner piece of MDF to the top and route that. Then glue another 1/2" sheet on that and you've got your 3/4" MDF top.

I'd much rather do that than try to monkey around trying to get epoxy level. That would just be a PITA in my opinion!

Keith

[David Rose]

Alan,

Are the outside corners low or high?

This question is really for the attention of those who might have tried this, to convey the information:

If the corners are high, or can be "made" that way, does the MDF need full support of the plywood? If you smeared it with a heavy coat of yellow glue and set the top on without clamping (maybe just using light weights) would that be enough support? I'm figuring that there would be lots of contact points, just not all over.

David

[Allen Bookout]

I vote for the router sled. I flattened my router table that way and was very impressed with the results. The only problem with that solution is that you are going to be routing for a while only taking less than 1/2" wide each time across but the results are worth it.

David's suggestion might work also but I have never tried it.

I think that the epoxy thing would work also but like you say, a little bit expensive. I have worked with epoxy quite a bit and if you put on edges (covered with wax paper) just a LITTLE higher than the table top and flow coated it the top would be very level. Probably a quart would do it. You could add some of West System's thickners if you were going to glue the final top on during the same process.

[Gary McKown]

I don't know how the piece is constructed, but if it is just a simple ply top then I would consider adding jointed supports underneath. If it is already put together, add substantial hardwood "cauls" and draw the top flat with screws. If it is badly warped then you may need to add shims under some of the screws to get enough tension. I have made tops from sagged conference tables using this method.

[Tom Andersen]

The MDF that I get from the lumber yard is almost never dead plane. In fact, I have noted that the slight bending of MDF changes with storage conditions. When I get an MDF board into my workshop, it is important that I store it with air access from both sides to preserve symmetry.

With that in mind, I am not supporting the thought of floating the MDF board on epoxy...

[glenn bradley]

If you haven't already glued the plywood top, I would remove it, correct the fastening points and use MDF. If it's already permenant you would probably be best losing some thickness by router-sledding it and building back up with new material.

I was looking at saving a buck or two on a current project by using some "secondary" plywood along with the baltic birch that will show. After a long ponder and some reflection on other such attempts, I passed. There's too much put into almost any project to sacrifice it over a relativly small amount of cash. Listen to me . . . I'm talking to me, not you. ;-)

[David Rose]

Nope! Glenn, you're talking to me too. :rolleyes: For some strange reason I will waste a ton of time using ever available scrap in some cases. Then of course, I carefully select show parts. Then I damage one at the last minute and can't match it. :( ...best laid plans of mice and men... or ...Mr. Murphy showed up...

My original thought was to replace the OP's top too. I just suspected that it wouldn't be easily done. My router table top was found to be "non flat" AFTER the plastic laminate went on! Not a good time to discover that. :eek:

David


If you haven't already glued the plywood top, I would remove it, correct the fastening points and use MDF. If it's already permenant you would probably be best losing some thickness by router-sledding it and building back up with new material.

I was looking at saving a buck or two on a current project by using some "secondary" plywood along with the baltic birch that will show. After a long ponder and some reflection on other such attempts, I passed. There's too much put into almost any project to sacrifice it over a relativly small amount of cash. Listen to me . . . I'm talking to me, not you. ;-)

[Allen Bookout]

Alan,

I am on line reading about building a workbench and found this link to flattening the top with a router sled and thought about your thread. You probably have already done some of this or looked it up but here is the article link.

http://www.woodmagazine.com/wood/story.jhtml?storyid=/templatedata/wood/story/data/408.xml&categoryid=/templatedata/wood/category/data/Woodworking_Basics_Develop_Skills.xml&page=1

[Alan Berezin]

Thanks all. I found another reference to a router sled for leveling a surface, http://www.highlandwoodworking.com/index.asp?PageAction=Custom&ID=58

Im still trying to convince myself that the string methodology (see the link) is correct. Im not sure it is but perhaps the errors introduced are small.

I think I am going to glue my MDF to the existing ply (that is already glued and screwed down) and then remove any unevenness with the sled. Then glue down one more layer of MDF or high quality ply.

Wasnt there some fairy tail about a princess that slept on so many stacked matresses and still couldnt sleep because there was a pea in the middle of the bottom one. Maybe it was one of those Cracked Fairy Tale cartoons :D

[John Stevens]

If you want a really flat top on something like this--like flat within 1/32" or better--you need do two things. First, you need a top that is flat and will stay that way. I'd suggest using stringers with straight edges to support the top sheet, or using a torsion box for the top. For the stringers, I like furniture-grade ply at least 2" wide, because it's strong enough for the job but more stable than solid wood.

Second, always assume that the base will not be as flat as the top, no matter how carefully you cut and assemble it. Try to attach the top directly to the three highest spots of the base, and find some way to fill the little gap(s) when attaching the top to the other
spot(s). Plenty of ways to skin this cat, just use your imagination. Wedges or shims are an easy way to fill the gap or gaps. (For my 39" x 78" workbench, I use a grid of bolts that are individually adjustable in height, but that's a time-consuming thing to make, and probably unnecessary for a top that's only 21 x 49.)

I only have a little experience with epoxy, but I think it would be a little too brittle for an application like this. Whatever you try, good luck.

[Alan Berezin]

This maybe would be cheating a bit (or a lot:) ) but does anyone make a cast aluminum table that is machined flat? Or, some kinda of frame that is machined flat?

[John Stevens]

This maybe would be cheating a bit (or a lot:) ) but

Cheating? It's in the eye of the beholder. I don't consider it "cheating" to use any tool that will help you get the job done...but then again, I don't consider it "cheating" to buy table legs instead of making them. Even the best woodworkers hire employees to divide the labor (and buy shop fixtures instead of making them), so draw the line where it makes sense to you, and don't worry about whether other people think you're cheating, or whether they consider you a "real" woodworker.


does anyone make a cast aluminum table that is machined flat? Or, some kinda of frame that is machined flat?

Sure, how much do you want to spend? ;)

This thread brings into focus some of the ideas that came up in the recent thread about whether to buy or build a workbench. It's hard to build a really flat surface if you don't already have one! But not impossible--it just takes a lot of time and effort that you could be spending on other pursuits.

Are you looking for suggestions? I'll be away from the internet until Tues 11/28...it'll be interesting to see how this thread develops between now and then.

[Bart Leetch]

What are you going to use it for?

Assembly?

Transport items around the shop from machine to machine?

To be used as a general purpose table?

This will dictate how flat it needs to be.

Also to check of how high you need the top to be put your hands down at your sides with the palms parallel to the floor for an approximate height of a bench top. If your top is higher than this & you want a real flat top I would shorten the cabinet & make a torsion box top thick enough to make up the difference. If your going to use this for assembly the top should be a bit longer & wider. Especially if you plan to use it for more than just small projects.