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Questions about flattening workbench top/bottom

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Forum topic by Sean_F posted 02-28-2021 06:14 PM 591 views 0 times favorited 7 replies Add to Favorites Watch
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Sean_F

10 posts in 46 days


02-28-2021 06:14 PM

Hi all, have lurked here a bit in the past, first time poster.

I am building my first woodworking workbench. Making a smaller version of a Jay bates design which is a simplified roubo.

I have laminated a top from douglas fir 4×4’s (no southern yellow pine here in CT), the size is 48” by 20”. I only have a jobsite tablesaw, planer, router, and hand planes, etc.

I laminated the top in two separate slabs, tried to hand joint one side and ran through planer. Then glued up trying to make the side I wanted for the top flat when gluing. The top is pretty flat now after the glue up and a small amount of handplanting. But the bottom is a little uneven, being 3 1/4” at the thickest and 3 1/8” at the thinnest.

I’m a little stuck on how I should flatten the bottom somewhat parallel with the top before starting the legs. I was going to follow youtubes and hand plane to flatten but getting a lot of tear out when planning diagonally and hitting some knots. Also having trouble getting things this big flat in general with hand planes.

Thinking about using a router and making a flattening jig. However, I have no real flat surfaces to do this on. The concrete floor is wavy and my current plywood workbench has a hollow. How would I go about it? Level the workbench first, put the slab top side down on the workbench, then clamp the rails to the slab and level those?


7 replies so far

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RClark

111 posts in 3236 days


#1 posted 02-28-2021 07:13 PM

I recently dealt with this issue on a very large table slab (about 40 X 78 X 1 3/4).

To get a work surface that large, I used a 4X8 sheet of OSB. I edge jointed two dry 2X4 to get straight edge, and then attached them to the underside of the OSB sheet. I then laid this table assembly on top of saw horses. Since a table of this size will still flex, I used long straightedges to check for flatness, and then put shims between the work table and the saw horses as needed to flatten it all out.

We smoothed one side using a router sled suspended over the workpiece. Once that side was done, we flipped it over and spent a couple hours shimming all of the various components (workpiece, rails, router sled), but after we finally ran the router sled over the work piece, we still couldn’t get the top and bottom exactly parallel. Seeing that we were beginning to go into the “death spiral” in search of perfection, we decided to attack it slightly differently.

- On the bottom, we marked the areas of the bottom where the leg attachment points were going to be.

- We measured the thickness at those points. The thinnest attachment point was the new target thickness, BUT ONLY where the legs/leg assemblies would contact the table top.

- Using the router, we brought the other three contact points down to that thickness.

With such a large bench top in excess of three inches thick, what is going to make that 1/8th difference stand out? If you do make the whole bottom exactly parallel to the top, I’m thinking that any unevenness in the floor surface is still going to have to be dealt with with leg leveling devices of some sort.

For 1/8th inch over that size top, I’d be very tempted to do what I described above, or even work to shim the contact points of the leg assembly.

-- Ray

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Sean_F

10 posts in 46 days


#2 posted 03-01-2021 01:53 PM

Thanks for the advise Ray, I think I may try that. I built the router sled last night. Checking with a straight edge,the sled seems to have a slight hump in the middle of where the router rides. But is flat on the bottom side. Since I already put the sides of the sled on, I guess I’ll try sanding the hump out as it’s very slight.

I took a ton of time picking plywood for this that was flat and straight. Only thing I could find that was like that at big box stores was 3/4 oak ply. Even the 3/4 mdf was warped. Pretty frustrating.

View Robert's profile

Robert

4519 posts in 2531 days


#3 posted 03-01-2021 02:35 PM

Clamp or screw the straight edges to the bench. Use a spacer to the router doesn’t chew up the rail.

Use strings to get the guide rails parallel and planar. Wood Whisperer has a good video on doing this.

-- Everything is a prototype thats why its one of a kind!!

View Sylvain's profile

Sylvain

1222 posts in 3550 days


#4 posted 03-01-2021 03:27 PM

“getting a lot of tear out when planning diagonally and hitting some knots. Also having trouble getting things this big flat in general with hand planes.”

A workbench is not a piece of fine furniture and a first workbench is an opportunity to acquire skills. What is important is that the underside is out of twist at the areas where it will sit on the base. The rest of the underside can be quite rough.

With a top 3” + thick, you can re flatten it when you will have honed your skills.

And as RClark said:
“With such a large bench top in excess of three inches thick, what is going to make that 1/8th difference stand out? If you do make the whole bottom exactly parallel to the top, I’m thinking that any unevenness in the floor surface is still going to have to be dealt with with leg leveling devices of some sort.

For 1/8th inch over that size top, I’d be very tempted to [...] even work to shim the contact points of the leg assembly.”

I have a different workbench but I had to put a shim somewhere; otherwise it would be rocking.
There is no need for a workbench to be perfectly horizontal.

-- Sylvain, Brussels, Belgium, Europe - The more I learn, the more there is to learn

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Giturdone

13 posts in 1222 days


#5 posted 03-01-2021 03:55 PM

In the past have visited the local hardwood dealer to take advantage of the oversize wide belt sander or the local high school woodshop that has a wide planer. Just a thought,

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Sean_F

10 posts in 46 days


#6 posted 03-01-2021 05:54 PM

Thanks guys. I’ll just flatten the bottom since I already have the jig built. And will shim the leg attachment locations if necessary.

Giturdone – thats a good option. I have a local woodcraft store that rents shop time. Maybe I’ll glue up my leg stock and being it there to dimension because I don’t have a jointer.

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Sean_F

10 posts in 46 days


#7 posted 03-04-2021 04:10 AM

It has begun!

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