MFT Sagging

[tim]

Not the first post I would choose (and I am a keen festool owner and user) but I have a problem with an MFT. 

I have extended a 1080 with the 2 meter rails and I use it as a site workbench or an extra assembly table in the workshop.  I decided to adapt it to take a router today because I have some long rails to profile.  So after I set the router in the new cut out I realise that there is significant sag in the table - not so much from the MDF on its own but because the whole table is sagging a good 4 mm (3/16 in).  The router weighs c 5kg (11lb) which isn't light but I am surprised that the aluminium box profile can or indeed should flex that much.

Has anyone come across this before and if so was there a remedy?  Any ideas what best to do?  I don't want to have to overly adapt the table so that it completely loses its portability.

Cheers

Tim

 

[greg mann]

If I understand correctly you have an MFT with 2 meter rails. That would make it a 2080 or a 2100 size. Four mm sag seems a little much but I certainly would expect significantly more over two meters than one. Reinforcement is probably your only option.

 

[chughes]

    I did the same extension on my MFT and use the table for everything too.  I have noticed a wee-bit of a sag as well. I think the choices are A. replace the long rails (and don't sleep on top anymore), B. build and install a plywood stiffener ( I could rest on top of the table a little), or C. live with this little bit of deflection.  I'm interested in how you mount the router.  I have my routers mounted in my table saws both in the shop and the field.  I would love to move further from my TS dependency. 

 

[tim]

Thanks for the replies.

My thoughts were based on the fact that those 2m rails are designed to span the 2m without sagging.  Surely if they sag then they are under specified?  Seems to me to be a bit of a design problem...

The router table set up is very simple - I just cut an opening in the table and put the router and plate into it.  The fence is either a cramped batten but I've also drilled holes to mount my incra fence too.

I'm going to see if I can come up with a cunning demountable brace.

Cheers

Tim

 

[greg mann]

I'm not sure we can assume the rails were designed with no sag expected. Rigidity decreases exponentially with length so 2meter rails are probably 4 times more vulnerable to deflection as 1 meter rails are. Could it be that Festool offered 2 meter rails with the expectation that the user would expect to brace them? Either way, if you want to get rid of the deflection that is your only option. I can't imagine that it would be that hard. Even a 2 piece strut out of wood; one with a bolt and wingnut that could ride in a slot placed between the middle of the rail and the ground would take the sag out and be adjustable to uneven conditions. Cut a tenon to fit the slot on the bottom of the rail and make it wide enough to be stable to a little movement. It's a start.

 

[Steveo48]

In a previous thread a FOG member built his own jumbo MFT and used  T-slot aluminum framing instead of the 2M rails.  His reason was he liked the heavier t-slot materials over the Festool long rails.  Search jumbo (Eli told me about that one) and you can see what others have done. 

 

[Michael Kellough]

Tim,

All the comments about the greater deflection of longer spans are right on but there could be an additional factor and potential fix.

Are the legs stretched out away from themselves? On a grippy floor I often pull a pair of legs out (to compress the leg joints) and reduce the wiggling of the table. The stiffness of the rails limits how far the legs can be pulled out and in resisted that force they bend down. I only have regular size MFTs but I can imagine that the extra tension from what I do combined with the extra weight over longer spans of your custom MFT could easily account for your 4mm deflection.

Since your 2 meter MFT is no longer portable it wouldn't hurt to add two tension cables with turnbuckles to pull the legs together springing the 2m rails up to counteract the deflection.
Once you get the table adjusted level you could build a storage shelf onto the leg crossbars to lock in the tension adjustment.

 

[John Stevens]

My thoughts were based on the fact that those 2m rails are designed to span the 2m without sagging.

Well, it's reasonable to think so, but my experience has been the same as yours.  I'm about to start my fourth version of an MFT with the 2m rails.  The first used the legs that came with the MFT, but after that I designed my own (stationary) bases to support the middle.  If that's not an option for you, it mightn't be too hard to design a pair of auxiliary legs that fasten to the extrusion.  Well, other peoples' time is so easy to spend. 

Best of luck to you.

Regards,

John

 

[greg mann]

Since there is a slot on the inside of the rails I think it would be feasable to aquire some thin material, steel or aluminum of about 1/8 inch thickness and about 4 to 5 inches wide, and sister it to the inside of the rail with fasteners about every 10 inches or so. This would stiffen the rail significantly and still be pretty much out of the way.

 

[Ned]

Since there is a slot on the inside of the rails I think it would be feasable to aquire some thin material, steel or aluminum of about 1/8 inch thickness and about 4 to 5 inches wide, and sister it to the inside of the rail with fasteners about every 10 inches or so. This would stiffen the rail significantly and still be pretty much out of the way.

Great and simple idea, Greg.  And like all great and simple ideas others come up with, it gives me a headache--a headache from slapping myself in the forehead.  Why didn't I think of that?

How about an L-shaped reinforcement, attached along the bottom of the extrusion and wrapping around the inside?

Ned