Interlocking Cross Lap Joint Waffle Grids

[cteno4]

so @kami_illy was doing some beautiful interlocking cross lap joints grids to support his 1/700 airport modules and I realized making waffle grids like this has come up here and there so I thought we could start a topic on it for those interested in discussing it specifically

 

Interlocking cross lap joints grids are created from sets of slats that have notched cut halfway into them at set points. You then assemble the slats into a grid with the notches lining up and nesting into the one perpendicular to it. Think wine box cardboard insert that keeps the bottles apart.
 

Using an interlocking cross lap joint grid like this is the lightest, strongest, and most rigid thing you can do for something like that. Amazing the support a waffle grid gives. Even made fromVery easy to do in wood if you have a table saw to cut your lap slots. Just takes some careful setup and testing of your cuts, but once you have that process dialed in you can make a big grid very quickly. Using a dado blade set to your stock thickness is the best way to cut the slots, but I’m lazy all the time when I just need to cut a few slots for a little grid and use the regular cross cut blade and do multiple cuts to make the slots. It’s a bit more work than cutting with the dado blade and doesn’t make super clean bottoms of the notches (I don’t care usually as these are rarely seen in support things I’m doing) and I don’t have to pull out the dado blade system and set it up and tinker with it (and a decent dado blade set ain’t cheap) Then the grid just needs a small frame around the outside.
 

You can also set up a jig on your miter slide on the table saw to make exact cuts to make very uniform grids. I also cut slots in a bunch of pieces at once so they are all identical. After your first slot cut on an aligned stack of pieces you just put a little piece of your stock onto the first notch and it holds the whole stack locked together for precise alignment when cutting all the rest of the slots. I’ve also cut out grid strips to a bit bigger than the space needed, the assembled the grid, and then cut the grid to size. This can be a bit easier if things are not totally symmetrical in your grid layout to do than trying to cut all the pieces perfect and then slots from the get go.

 

You can do them by hand in wood but that is a lot of work and tricker to get nice, that’s the only downside to cross lap joints with woodworking. With cardboard they are easy to cut by hand by making a slat notch template first and using it to mark your all your cuts. Even if things are jot perfect some glue fillets and even little thin shim pieces of cardboard or wood can make up for any notch imperfections and once glued be quite sturdy.

 

For our club we also made a temporary grid that can be assembled and disassembled as needed to support the modules of one of our club show models. These were cut out of 2” strips of 12mm Baltic birch and the longer beams could be bolted together to make different sized grids as needed.

 

 

Cheers,

 

jeff

 

 

[RS18U]

Excellent!  Thank you for posting this great info Jeff, just what I was looking for.  Interesting in your photo that you are using an L girder for the bottom lengthwise element, do you think this is a necessity, or a nice to have, or did it have a specific reason for being built like that?

 

Will

[cteno4]

That was our temp setup support system for our layout 3.0 sectional layout. It was all modules about 18”x 24-32” and they just sit on top of that big grid made of the L girders and cross slats connected with the cross lap joints. The L on the long girders was to bot stiffen the longer girders and give a larger area to rest on to the centipede support systems top pads that we have leveling bolts built into to pick up the bottom of the Lgurder to level the grid.

 

if you are just doing a couple of sections for a show layout you would not need all this, especially if not big. you can use a cross lap grid as your internal structure supporting your layout top and just trim the outside as desired. For a module of like 3”x3” you can just do like 8”-10” grid of thin ply for an internal structure with a thin (like 5mm) ply top. For your outside frame you want something a little beefier like 10-12mm as that’s the part you will be grabbing and put all your stress on. But really you can get away with a larger grid with 5mm ply on top and just use like 12mm for the internal grid. Take a look at the modules I built for Renato’s home layout. He wanted to be able to potentially reconfigure the shape of the layout so we made it sections so it can be a U, L or I. These were very light and sturdy up to a size of 3’x4’. These could sit on event tables or on a centipede support for a show layout. Renato wanted a support that could disassemble with removable legs so I just made him 2 frames with 6 ikea screw on legs each the modules can rest on. All very light weight and sturdy and fit into his suv for transport home. Sorry wish I had documented these builds more, I get going and forget to take pictures along the way.
 

Layout 3.0 is like 6’x11-17k in size so we needed a big framework to rest 15-20 larger sections on as we did not want to rely on using event tables. The system is slick as 1 or 2 people can have the frame work all set up in under 10 minutes. Then just plop down modules on top of the grid. All the modules have pegs that inter lock with the adjoining modules so they all lock together and joined modules are clamped together then with simple binder clips on the underside. The grid and centered all pack up into one large 4’ long duffle or smaller bundles if needed.

 

jeff

[RS18U]

Excellent information Jeff!  My layout will be 2 modules around 55" x 30" each, and I am hoping they can sit on a table supplied by the show although I will probably need to have my own legs as an option.  Have to go and check out those links.

[kami_illy]

Seeing your structure has a wider grid (probably bc your base plate will be / is made of wood or similar) and the L-shaped beams gave me another idea. 

If you build the beams -at least in one direction, better in both- as upside-down T-beams / T-girders, you can fill the space in-between with a honeycomb plate and therefor reduce the thickness of the cover plate quite a lot I think. Should be light AND a lot stiffer. Also you should still be able to route cables pretty well. In this case the honeycomb needs to be glued to the plate and then be inserted in the support grid. These honeycomb (or similar) board are usually sold in hardware stores. And since they are made for use as support in floors, they can take a lot of pressure...

You can also glue some supporting braces on the underside of your plates. This will create the same structure to deal with Pull- and Push-momentums like a traditional I-beam. 

And of course change the direction of every layer by 45° to kill any torsion or warping. 

 

Wow, that got nerdy fast, haha. 

Edited January 24 by kami_illy

[cteno4]

Funny you should mention that, when we built the grid for layout 3.0 modules a club member asked if we could use it or something like it for Ttrak. But Ttrak modules are much smaller and don’t interlock (other than the track connection) so you need a much more dense waffle grid to support Ttrak modules well. Using the 3.0 grid would have required pieces of ply to sit on top of the grid and potentially warp and be bulky for transport. I looked at doing a similar large grid with L girders on the outside and inverted T girders on the inside and then just make smaller grids of very thin slat with cross lap joint to drop into each hole in the big grid and as you mentioned be supported by the lip of the L and inverted T girders. The cross slats on the big grid probably wouldn’t need lipstick to grab the little grids, but that then makes the big cross slats bigger and heavier. If you only have the support on the girder sides you just need to be cognizant of the orientation of the little grid so it’s not upside down. In the end I kind of shelved the idea for the time being as we had few events were tables were not easily provided and it then added another pile of stuff to store and transport. But it’s a very doable system as the mini grids could be very minimal like 4mm Baltic birch or very clear 4mm stable dimensional lumber. There is a risk of some warping of thin slats like that, but the grid could probably handle a bit of warping. I should cut out a little mini grids from 4mm Baltic birch ply and let it sit around in the wood shop (not temp or humidity stable place) and see how it holds up! 

 

On the layout 3.0 L girder system this is just needed as a support structure to set the larger modules (18”x24-32”) on top of. Since the modules are all self supporting for themselves (frame of 1.5” h 12mm Baltic birch ply and a couple of cross pieces and topped with 5mm ply) they don’t need support between the openings. We just engineered the grid dimensions of the support girders and cross slats to pick up the modules to support them well enough. Plus since all the modules lock together with a peg system along their edges (and the  binder clips to keep them locked) the modules end up being one big piece that just rests and floats on the framework. 

 

The main thing on the frame work is it needs to be very transportable and we did not want any large sections to transport other than the modules so we did just the simple frame work of the L girders and the cross slats. I only showed it head as the slats and L girders interlock with cross lap joints and it’s a super simple to set up and take down with the most minimal of support structure for the big mat of modules on top.
 

Basically we started with the table idea and started taking away elements to make it into parts more easily transportable than folding tables that are bulky and heavy and don’t transport well in cars (they tend to take up room in inefficient ways in most vehicles and the love to slide around if put horizontal). I played with mounting legs to the L beams and the system then requires more joint bracing to keep the just slip together grid tight enough. The grid is super sturdy on the centipede bases as those have their own internal lateral support. The centipede bases are actually really nice and super easy to set up, much easier than attaching legs to a framework, the only issue is they take up most all the space under the layout for any box storage. In the end the centipede won out being so stable and simple and when folded up each are one small package. It was about the minimum of material we could find to support the large mat of modules and the whole thing be easily to level. With the grid being nice and flat there is no leveling of individual modules. Just level the grid on the outside 4 corners of grid on the centipedes and then just screw all the intermediate leveling points up to match it. Fastest leveling I’ve ever done for a big spread like this! This works as the grid itself is light enough to not sag on its own much so you can level with the outside 4 corners and tweak any middle ones if you visually see any sag. The L girders bolt together in about 44” lengths using plates that span the joints with 1” dia finger screws on each side. With the holes drilled properly it locks the L girders together very solidly.

 

Interestingly the 12mm 1.5” cross slats (with their cross lap notches cut in them) have been hanging in the shop now for about 6 years and they have not warped at all! Always amazes me how well strips of the Baltic birch are so much more warp stable than most all dimensional lumbers of similar sized. Baltic birch still works out to about the price of mid level quality dimensional lumber and usually you can get away with a svelter piece of Baltic birch strip than dimensional lumber.

 

jeff