I think I would find a strip of 1/4" hardwood and use the 1/4" Slot Cutter bit and insert a full-length spline.

 

That’s an excellent idea John. Thank you.

 

Whatever you do, don't use this joint unless you have a CNC and can stand your MDF sheets on end: CNC Cut End-To-End Finger Joint

4D

 

This finger joint is not an option. I don’t have a CNC

 

I actually have the massive finger joint bit and have never used it. I certainly don't recommend it for a novice unless they have a substantial router table to use it in.
(I don't see this as a "free-hand" tool when other options are available).

 

@bryansong The problem with the lock joint bit is you would have to hold each board vertically against the fence -a big ask, given the size of your boards. Dangerous free-hand, the router would be perched on a 3/4 edge, unless you make a jig.
You could use biscuits, but they might cause bumps in the mdf from moisture absoprtion from the glue (as might the spline).
You could rout out grooves for metal straps across the joint on the undersurface, or get fancy and inlay bowties for the practice.

 

I wouldn't use MDF for a bench top . One coffee spill and it will swell and not be flat any more. I'd use a decent quality plywood for the top. And I'd probably put a replaceable layer of thin ply on top. I'd also build a 2 by ? frame and assemble the legs into it's inside corners. Around these parts MDF isn't much cheaper than ply where I live. For the replaceable top, I'd consider using what used to be called Masonite, which you can get in quarter inch thickness. Screw it down lightly to the ply on top and wax the heck out of it. I hate MDF, Made my first cabinet out of it and never used the stuff again except for small squares I sometimes use as sacrificial push blocks on the router. The ply and thick legs will add weight, which is great for stability. The framework will allow you to add a vise.

 

Hello Tom, how are you?
My 2 benches do have 1/4” Masonite on top of 1/2” plywood which is on a layer of 2”x4”s laid flat 3 1/2” sides up and down running perpendicular to the length of the bench.
The Masonite is screwed down with Sheetrock screws.

Here’s what happened to my workbench area.
My workbench was in a basement garage underneath the front porch. The house is over 100 years old with concrete block foundation. The front steps made out of solid concrete were too heavy and tore the wall busting it up so I had to have some people come in and put three new walls in so they’re approximately 7 foot out from the house and run by 17 feet. All three of those walls were replaced, I had to tear all my bench stuff apart. The floor was broken, where the corner had fallen had fell so I needed to put some levelers on my workbench and replace a bunch of the 4 x 4 legs that had got rotten through the years. Now the bench has adjustable legs a little feet there rated at 2500 pounds a piece each one of those little feet they’re heavy benches now I want to bring the benches together to make one long bench. I want it leveled up and I want a nice flat top. That’s why I thought the MDF three-quarter MDF would be good. I want to put dog holes in it so that’s really what I’m preparing for. I can use plywood, but I don’t wanna go back to the Mason, I’d have to hold it down with something. I want something heavy that I can just attach to the outer edges of each bench, but I wanted to hook in with the both bench tops. That’s why I was asking about bringing the two tops together to be as flat as possible.
I hope I made sense. Thank you all for helping.

Bryan

 

Or if you are careful with a jig saw or scroll saw you could use this joint: A Board Stretcher Joint. Just for Fun..
Inspired by a zipper although it won't work like a zipper. My former department head used a variation of this to connect kitchen counter top sections in the house he remodeled.

BTW, if you used MDF then seal both side and edges with a good oil based finish. As DesertTom mentioned left unfinished it'll swell up when wet.
4D

 

Thanks for the good ideas.
First I need to pull the benches out of the way and DryLock seal the block walls.

 

Here is what I’m working with. I do have some steel cabinets that attach to the underside of the benches that will help hole them in place plus I will be building a wall on the top for my peg board. That will also help with stability.

 

You could simply use a bunch of dowels? Sounds like you primarily want to keep the top surfaces of the two boards aligned in the vertical sense. The 3/4 mdf won’t stay flat if it is bridging a large dip in the current surface of the benches when pushed together, and it will be difficult to see where shims might be needed.
If you are planning an array of dogholes MFT. style, my suggestion of metal straps is void -you din’t want to hit a strap while boring a doghole.

Thinking about your initial lock joint idea, you could rout a dado across the top of one board and the bottom of the other, half the thickness of the mdf. and 3/4 in from each edge. That would lock them together horizontally but not necessarily vertically.

 

You could simply use a bunch of dowels? Sounds like you primarily want to keep the top surfaces of the two boards aligned in the vertical sense. The 3/4 mdf won’t stay flat if it is bridging a large dip in the current surface of the benches when pushed together, and it will be difficult to see where shims might be needed.
If you are planning an array of dogholes MFT. style, my suggestion of metal straps is void -you din’t want to hit a strap while boring a doghole.

Thinking about your initial lock joint idea, you could rout a dado across the top of one board and the bottom of the other, half the thickness of the mdf. and 3/4 in from each edge. That would lock them together horizontally but not necessarily vertically.

I do have a doweling jig but it can be hard to get the two sheet tight together.
I think I like the slot cutter idea. I have nice cutter I bought and used on some breadboard ends years ago. I think I’ll use John Smith’s idea, slotting both sheets of sanded plywood and stick an oak or maple floating tenon in it.

 

Stick is the person who told me to get that nice Freud slot cutter set. (It’s been a few years ago).

 

Getting the tops level. Option one: If you put leveling bolts on all 8 legs, you could level the tops up. But that may still give you an error and it could be weak.

Option 2: Turn them upside down on a level surface, connect them together firmly with whatever method is easiest. Add a 1x3 across the top of the legs just below the surface pieces. If there is a gap between the two tables UNDER the top, cut a piece to fit and drill a long hole through the two aprons and through the inserted piece and bolt them together with large fender washers. You can shim this if tightening the bolts makes the top ends raise off the floor.

Once it's solid and flat, then flip them back upright. I think that will give you what you want. You can use whatever top you want since it will be dead flat.

BTW, although my diagnosis of a simmering form of a blood pre-cancer stands, I seem to be dodging the onset of the full out condition. Been more that 3 years with no increase in the blood test signs and a slight improvement in some tests. I eat through a tube and have added supplements, and powdered spinach, celery and reishi mushroom extract daily. A Chinese doctor recommended the mushroom when I was first diagnosed, and it and the spinach seem to be keeping me steady.

 

Getting the tops level. Option one: If you put leveling bolts on all 8 legs, you could level the tops up. But that may still give you an error and it could be weak.

Option 2: Turn them upside down on a level surface, connect them together firmly with whatever method is easiest. Add a 1x3 across the top of the legs just below the surface pieces. If there is a gap between the two tables UNDER the top, cut a piece to fit and drill a long hole through the two aprons and through the inserted piece and bolt them together with large fender washers. You can shim this if tightening the bolts makes the top ends raise off the floor.

Once it's solid and flat, then flip them back upright. I think that will give you what you want. You can use whatever top you want since it will be dead flat.

BTW, although my diagnosis of a simmering form of a blood pre-cancer stands, I seem to be dodging the onset of the full out condition. Been more that 3 years with no increase in the blood test signs and a slight improvement in some tests. I eat through a tube and have added supplements, and powdered spinach, celery and reishi mushroom extract daily. A Chinese doctor recommended the mushroom when I was first diagnosed, and it and the spinach seem to be keeping me steady.

Hello Tom, thank you for your ideas. I like the 1x3 idea.
I’m sorry, I didn’t ever read about your pre cancer but I am grateful you are keeping it at bay. I pray for your success and thank God for you.

 

The router base is what keeps the spline in perfect correlation between the two pieces. If a sheet of hardboard will be installed over the MDF or Particle Board, position it so the seams are not over the seams of the base material.

 

John,

Melamine is an option too, I have it for my out feed table and a long work bench but the top surface isn’t always perfect, I mean there are places where the white is worn through already when it’s new.
I still might use that.

 

"Melamine" is not the same animal that I used many years ago. I recently bought some that I thought had the thick, hard plastic surface. It was MDF material with a very light plastic film laminated to it. I had it in my open carport and had several scratches on it from general use. A thick fog rolled in, and the surface moisture made the scratches blow up like water blisters on cheap MDF. Just use your best judgment on how to use what you have. Keep us in the loop.

 

Bummer

 

Just a differernt take on it. Might consider making 2 tables that lock together. Those long tables can be too long sometimes.

 

When I had my last sign shop, I made two tables that were 4x8', one that was 2x6', and two that were 4x4', all the same height and on steel casters, the same height as the table saw and jointer, for just that reason.