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Help with design for new MFT style bench

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I want a replace my current MFT style bench as the top has sagged. The new version should add supports to keep the top flat (the extrusion).

The materials will consist of 45x45 aluminium extrusion (pictured in grey), a Plywood replacement MFT top, and then pine for the base. This will not be moved around, so I want it to be very rigid, with the requirement that you can pull up a chair to sit under it.

This bench is also used as my router table (the red plate), which introduces most of the sagging (Triton TRA001).

I'm quite keen on the 45x45 Extrusion, as I know it will arrive flat/striaght, and I won't have to worry about trying to get straight timber. I am also relying on it to keep the top flat and in shape.

However, I can't decide how I should attach the top section to the wood base?

- should the top section have the extrusion sit on top of the legs/rails? which I think will work best to keep the top flat
- or should the extrusion go around the legs/rails? in which case the middle extrusion would have to be part of the base, or perhaps I notch out a seciton in the base for it.
 

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ScaredyCat

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Doesn't the extrusion have v-slots in it on the bottom? I'd put hole into the top of stretchers and secure the extrusion to the 4 of those in multiple places.


Also where are you getting your extrusion from? I tried KJN but they seem closed down right now.

.
 
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ScaredyCat":11apdx5h said:
Doesn't the extrusion have v-slots in it on the bottom? I'd put hole into the top of stretchers and secure the extrusion to the 4 of those in multiple places.
It's not so much how to affix it, but how attach it in such a way to stop any inaccuracies from the frame being projected into the top. So if I just bolt the extrusion to the frame, and the frame has a little bow or twist, won't the extrusions deflect?

I'll obviously do my best to built the frame flat/true/square and shim any dips from my garage floor, but my I know it won't be perfect, where as the Ply fastened to extrusion should be? ...right? :?

ScaredyCat":11apdx5h said:
Also where are you getting your extrusion from? I tried KJN but they seem closed down right now.
I hadn't gotten that far. Maybe https://www.motedis.co.uk/shop/Slot-pro ... 93455.html ?
 

ScaredyCat

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Does it matter if the extrusion does deflect? If your top is an mft style the holes will all still be correct wont thye?

I'd attach the stretchers to the extrusion first (before the top), then check for any deflections with a straight edge, measuring etc and loosen as required. if you attach in 4 places per side you've got quite a bit of leeway in how you go about tightening up the connectors and how much you tighten them by.

The alternative is to go entirely extrusion based.

.
 

DBT85

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I've used oozenest for extrusion and was happy with the 6020 stuff I ordered so there's that.

I also secured my extrusion to my MFT bench by using the slots in the extrusion. I put holes through the top stretchers and then used the little slot nut things to hold it all in place. Not moving anywhere now. I used mine to help build a pull down track for a tracksaw like the Festool one.

I'll actually finish it one day.

My MFT top is just 50x75mm CLS around all 4 sides and then one in the middle for bracing. It's built into a longer table so there are sections either end and basically all my wood storage is underneath.
 

Sideways

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If I was going to the trouble of alloy extrusion for straightness / stiffness, I'd opt for 8040 size.
Fixing is a piece of pie. Sliding T nuts in the slots on the lower edge of the extrusion and M8 bolts straight through the wood into them.
mark everything up and drill the timber on a drill press before you assemble, that way you'll get your holes properly square. The bolt length will need to be pretty exact as there isn't a lot of depth in the T slot to swallow excess length.
If you want to fettle everything super precisely, you can slacken the bolts and insert shims between the wood and the alloy, overkill, but if you make it flat in the first place and fit height adjustable feet to level everything, the alloy will resist any timber warping.

To secure the top, measure and drill carefully straight through the top, then counterbore. You can put capscrews just below flush and screw straight down into (again) sliding T nuts in the T slots along the top edge of the extrusion. You're not making a portable bench for a van so 25mm MRMDF makes a heavy, solid top and even counterbored, the capscrews have lots of thickness left to clamp down on.
You can choose positions for the bolts anywhere along the T slots so it's easy to keep them out of wherever you may want to cut into the top.
If you regularly tracksaw crosscut in one or two places, rout a groove and cut a bunch of replaceable sacrificial strips to fit it. I've seen UHMW plastic strip used for this but you could use 6mm MDF and just rip strips off a sheet.
 
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Sideways":rs03hdx1 said:
If I was going to the trouble of alloy extrusion for straightness / stiffness, I'd opt for 8040 size.
Fixing is a piece of pie. Sliding T nuts in the slots on the lower edge of the extrusion and M8 bolts straight through the wood into them.
mark everything up and drill the timber on a drill press before you assemble, that way you'll get your holes properly square. The bolt length will need to be pretty exact as there isn't a lot of depth in the T slot to swallow excess length.
If you want to fettle everything super precisely, you can slacken the bolts and insert shims between the wood and the alloy, overkill, but if you make it flat in the first place and fit height adjustable feet to level everything, the alloy will resist any timber warping.

To secure the top, measure and drill carefully straight through the top, then counterbore. You can put capscrews just below flush and screw straight down into (again) sliding T nuts in the T slots along the top edge of the extrusion. You're not making a portable bench for a van so 25mm MRMDF makes a heavy, solid top and even counterbored, the capscrews have lots of thickness left to clamp down on.
You can choose positions for the bolts anywhere along the T slots so it's easy to keep them out of wherever you may want to cut into the top.
If you regularly tracksaw crosscut in one or two places, rout a groove and cut a bunch of replaceable sacrificial strips to fit it. I've seen UHMW plastic strip used for this but you could use 6mm MDF and just rip strips off a sheet.
So would you put the 8040 on top of the wooden frame? Or around it?

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Im wondering now if I should just replace the top wooden rails in my original plan for 8040, and not have the 4040 extrusion on the top piece.

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Sideways

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Ally on top of the wood or around the wood :
It depends what matters to you.
One of the features of the real MFT is the way that fixings can be attached to the vertical surface of the aluminium using the T slots on the face of the extrusion. That works best when the table top is flush with or inside of the outer edges of the ally.
But if you like to hold stuff down to the top using quick cramps gripping the edge of your bench top, then having the alloy in the way is a bit incovenient. Your cramps need to have an extra 80mm capacity to grip the alloy as well as the thickness of the top.
If you put the wood below the alloy (strongest), that makes an even more massive edge to clamp if you like to use your clamps that way. Put the wood inside the alloy and you lose a (insignificant ?) bit of loadbearing strength, a little stability from the reduced footprint, but the benchtop plus alloy is only 98 to 105mm thick.
Of course if you have the clever, skinny clamps, you can feed them through a dog hole and clamp anywhere but you do have the bulk of the clamp sticking up into the working space
If you let your table top overhang the alloy a couple of inches, it's easier to clamp around the edge but if you want to fix stuff vertically it's a bit more complicated.
If using those t slots on the outside faces of your extrusion is important, you need to be able to slide things into the slot. That means nothing overlapping the open end of the extrusion. Your wooden frame will be more important for rigidity and the alloy is i) a set of T slots and ii) a light, straight beam to stop the table sagging.
I've never owned a bench with traditional hold downs or an apron. That got me into the habit of using clamps on the front edge of my benches, none of them aret much over 3" thick. I do this all the time, it's so fast and easy. I wouldn't give up that convenience.

I guess this simply means wood INSIDE the extrusion for me.
 
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Thanks for the info Sideways.

Here is the latest plan.

The top will now use 4080 extrusion which will sit on top of 4 leveling bolts (the MDF resting on the bolts). Once I have it where I want it, I will then screw the extrusion to the base legs using 4 screws per corner. This should now mean the tops flatness is governed by the extrusion and not the frame.

Added some ply to the back and sides to increase rigidity.

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