MFT and torsion boxes

[brucew]

I've been using a slab of 18mm hardwood plywood as a worktop with a friction fit down onto a pair sawhorses. There are a few 20mm holes for track saw use and when I clamp a piece down to it, I use F-clamps on the edge more often than not. But of course this single slab is no longer absolutley flat with a *very* slight camber to it (1mm or so) but still rather functional, so was thinking about torsion boxes to maintain the flatness.

The classic torsion box seemed to be made with several cross braces to make a number of "boxes", but an MFT makes use of lot's of 20mm holes for work handling and so those tend to be a single box the same dimensions as the top slab (no overhanging lip). What I'm wondering is why not do a box that is a bit smaller than the top slab and leave a lip for easy edge clamping? The ones I see require clamps at least 6" longer than otherwise.

I didn't post it in the other convo on torsion boxes as it's become mired in arguments mostly unrelated to my question.

 

[Doug B]

I don’t see why you couldn’t leave a lip, good idea depending on how you intend to use the MFT.
This is an unassembled picture of mine, no lip as I intend putting a vice on it but then again even with a lip you could incorporate a vice

A friend cut it out for me & unfortunately I’ve not had time to assemble it yet so don’t have personal experience of it’s longevity but he has had a couple for many years & they are still in remarkably good condition after heavy use.

 

[Jacob]

....... What I'm wondering is why not do a box that is a bit smaller than the top slab and leave a lip for easy edge clamping? The ones I see require clamps at least 6" longer than otherwise.

yes why not? Could be just a frame or with another sheet to make it into a box. Might this interfere with the MFT function though?

I didn't post it in the other convo on torsion boxes as it's become mired in arguments mostly unrelated to my question.

Exactly. Torsion has nothing to do with it.

 

[brucew]

yes why not? Could be just a frame or with another sheet to make it into a box. Might this interfere with the MFT function though?

In the classic MFT, 20mm holes are in a 96mm grid pattern across the whole surface and often rail clamps are used with the rail put through the hole. so, the slab needs to be no more than 19mm thick and there should not be a lot of slat braces across the the bottom as those would annoyingly get in the way of rail clamp use. Also if the "box" has a bottom panel, the height of the sides must be enough for the rail clamps to clear when inserting and removing.

So yes, an inset box would interfere with at least the outmost rows of dog holes. One would need to consider those as sacrifical holes, but then I believe in having as few holes as necessary on these tops and am only adding a few as needed. The full grid of holes make assembly with screws and other small bits rather annoying when they roll around and fall into a hole.

 

[Distinterior]

I use an MFT top quite extensively and have been doing so for many years now.....The 96mm on centres is not really that important in a practical respect. It could just as easily be every 100mm or every 200mm or anywhere inbetween.

If you wanted to make up a "Torsion Box MFT", you could just space the 20mm hole centres at double the space apart (192mm).This would give you plenty of room under the top to allow for the use of MFT or rail clamps in a 360degree arc and still be able to have a grid of 18mm material acting as the torsion bracing.

Obviously, only having half the amount of holes may restrict your clamping / bench dog options to a degree, but then everything in life is a compromise.....

 

[Molynoox]

I've been using a slab of 18mm hardwood plywood as a worktop with a friction fit down onto a pair sawhorses. There are a few 20mm holes for track saw use and when I clamp a piece down to it, I use F-clamps on the edge more often than not. But of course this single slab is no longer absolutley flat with a *very* slight camber to it (1mm or so) but still rather functional, so was thinking about torsion boxes to maintain the flatness.

The classic torsion box seemed to be made with several cross braces to make a number of "boxes", but an MFT makes use of lot's of 20mm holes for work handling and so those tend to be a single box the same dimensions as the top slab (no overhanging lip). What I'm wondering is why not do a box that is a bit smaller than the top slab and leave a lip for easy edge clamping? The ones I see require clamps at least 6" longer than otherwise.

I didn't post it in the other convo on torsion boxes as it's become mired in arguments mostly unrelated to my question.

I have just built an MFT and I did it with an overhang on the 18mm top so that I could clamp around the edges. I'm glad I did because my rail clamps don't fit through the holes, which I thought they would...

Martin

 

[TheUnicorn]

might be worth reinforcing the overhang for longevity

 

[Spectric]

I don’t see why you couldn’t leave a lip

Adding a " lip " or rim to any object will add rigidity, this rim can be a box or a shape with non linear cross section. Think of how car body panels are strengthened by adding what looks like a crease or fold. A good example of this strengthing is shown in the so called Paulk bench.

 

[Distinterior]

I have just built an MFT and I did it with an overhang on the 18mm top so that I could clamp around the edges. I'm glad I did because my rail clamps don't fit through the holes, which I thought they would...

That's odd!....What diameter are your holes and which rail clamps are you trying to fit through....?
20mm diameter hole is the norm.

Unless you mean the clamp arm fits through but it then hits your bracing...?

 

[Molynoox]

That's odd!....What diameter are your holes and which rail clamps are you trying to fit through....?
20mm diameter hole is the norm.

Unless you mean the clamp arm fits through but it then hits your bracing...?

Makita rail clamps, 20mm holes

Doh!

 

[MikeK]

Makita rail clamps, 20mm holes
View attachment 154954
View attachment 154955
Doh!

I have some Bosch and Bessey rail clamps that won’t work on my MFT-style table. The bend radius is too small.

 

[Distinterior]

That is disappointing!
I only have the Festool style clamps, both the screw type and auto one handed clamps and they both fit through the 20mm hole in 18mm thick material.

Could you not just counter sink the under side of your holes slightly, thus increasing the clearance for the clamp arm...?
Would that work...?

 

[frankendoodle65]

Makita rail clamps, 20mm holes
View attachment 154954
View attachment 154955
Doh!

It's a bit of a hassle but if you cut away some of the metal at the outside of the bend with an angle grinder you can make it work.

 

[TheUnicorn]

It's a bit of a hassle but if you cut away some of the metal at the outside of the bend with an angle grinder you can make it work.

surely that would weaken the clamp?

 

[Molynoox]

That is disappointing!
I only have the Festool style clamps, both the screw type and auto one handed clamps and they both fit through the 20mm hole in 18mm thick material.

Could you not just counter sink the under side of your holes slightly, thus increasing the clearance for the clamp arm...?
Would that work...?

That's a clever idea.
Not sure I can bothered though.... £34 for two banggood ratchet clamps, might just buy those.

 

[Jacob]

....... A good example of this strengthing is shown in the so called Paulk bench.

... or any box made since the beginning of time.
Or any ribbed structure such as every timber floor/roof with both boards AND joists/rafters
Or any table with an apron. Is mine a "torque" table I wonder? Must consult the experts!
and so ad infinitum!

 

[mudman]

... or any box made since the beginning of time.
Or any ribbed structure such as every timber floor/roof with both boards AND joists/rafters
Or any table with an apron. Is mine a "torque" table I wonder? Must consult the experts!
and so ad infinitum!

Myself and probably the others involved in the argument in the other thread just gave up and let you have your little say. But here you come again, you just can't let a thing lie can you? You have to come along and ruin any thread for no identifiable reason apart from trying to flog some conviction you have that you are right and everyone else is wrong. It's like talking to a flat earther, maybe you are one. But what ever the reason, I'm sure I'm not alone in wishing that you would just give it a rest and shut the f up.

 

[frankendoodle65]

surely that would weaken the clamp?

Probably, but how much pressure are you expecting to exert on one of these clamps?
Worked for me anyway, though admittedly I have other clamps that I prefer so they're not exactly being abused on a daily basis.

 

[TheUnicorn]

I like the paulk bench (on paper, never seen one), gives a nice all round shelf / working space, there does seem to be quite a lot of space between supports compared to some torsion box style benches

 

[brucew]

Makita rail clamps, 20mm holes

...
Doh!

Odd, I have Makita clamps and 20mm holes on an 18mm top. They fit on my worktop but only when the screw and slide are both loosened all the way.