There's a technique that has come to be known as Cross Welding, that can be used to create seemingly impossible welds. There are currently no levels in the campaign that requires this technique, but there are plenty of levels where the use of Cross Welds can be helpful. The technique is, however, acknowledged by the game, as there's currently a hidden challenge for the Antenna level to complete it without using Cross Welds. Furthermore, this technique is required to solve the sole level "Cross" in MagmaMcFry's mod with the same name and the Zoologist Chapter in my own mod.

Speaking of the the level "Cross", it's this level (or the shape in it) that is the origin of the name "Cross Weld". A 5-block cross is the smallest shape that is impossible to make from 1x1 parts without Cross Welds. (There are however, 4-block shapes that require Cross Welds to be made from 2x1 parts, but are trivial to make from 1x1 parts.) The technique of Cross Welding isn't limited to making crosses, and can even be useful to weld things together in a different order than what you would otherwise have to.


A cross and a Z-shape, in their parts on the left and welded together on the right. A Z-shape can be made without Cross Welding, but not from two 2x1 parts.

The core mechanic that is used to perform a Cross Weld is how portal faces behave when they are welded to other things. While a portal is active, nothing can weld or be welded to its front face, but when it's inactive all its faces behave just like a normal block. Here's a small example:


The sensor is loose and its positive side is wired to the portal. The two portals are connected but the red portal is not being used.

In the beginning, the sensor will fall and the red block will move towards the portal. After a tick has passed, the portal will deactivate and therefore be welded to the red block. The 2-block piece will then fall. Once the portal reaches the conveyor beneath it, both blocks move to the right, proving that they are indeed connected.


The portal and the red block has been welded together and are now moving in unison.

After a few ticks, the sensor reach the end of the grey column and the portal reactivates. When this happens the portal stops moving (since active portals can't move). More importantly, however, the red block starts falling on its own. This happens because nothing can be welded to the front face of an active portal. We've made a shape that we can break apart on command.


The two blocks have now disconnected as the portal has reactivated.

I'll start by showing you the more generic way of making a cross, but we'll take a look on another approach later. I will demonstrate the Cross Weld by solving MagmaMcFry's level "Cross".

A solution to "Cross".

The image above displays the full solution, but it might be tricky to see what's going on. There are other things than just the Cross Weld happening, so let's split the factory in two: the Cross Weld and everything else.


The non Cross Weld part of the example factory.

Before the Cross Weld can take place, we will group the red blocks into groups of five and weld four of them together into a T-shape. The T-shape will be sent to the Cross Weld, as will the 5th block. On its way there, the 1x1 block will trigger a few sensors. These will control the logic of the Cross Weld. (It controls the Cross Weld of the cross before it.)


The Cross Weld part of the example factory.

With the preparations being taken care of, it's time for the Cross Weld. The double red portal on the left is where the T-shape will arrive (orientated as a T). The 5th block will arrive from the red portal facing left. The signals from the sensors are wired as shown in the image below.


The wires of the Cross Weld. The sensors are triggered from top to bottom.

As soon as the lone block arrives, it will be welded to the deactivated portal above it. Meanwhile, the T-shape arrives from the left and the two pistons extend to help it on its way. When it reaches the blocks on the right it will get welded to the now inactive portal. This creates a large shape consisting of five red blocks and five grey blocks. The portals will then reactivate and the finished cross will fall down as the pistons retract.

Four states of the Cross Weld. Anything but the shapes being welded together has been omitted and inactive portals are just shown as ordinary blocks.

In the image above, we can see (from left to right) how the Cross Weld works through 4 states.

1. Two separate pieces, one consisting of the T-shape and the other of the single block welded to the grey attachment.
   
2. The two pieces are now properly aligned with each other, but not welded together.
   
3. After a weld we now have single piece.
   
4. After the portals reactivate, the red cross detaches from the grey piece.

There's a more straight-forward way we can perform the Cross Weld. However, this approach has a flaw, it won't work with Nortals (while we let our blocks go through portals in the previous approach, it wasn't used in the actual Cross Weld). It's using the same core concept of welding blocks to front faces of portals, but the clever idea here is to use the portals the blocks are already going through.

A clever approach to Cross Welding. This solution to the Cross level is designed by MagmaMcFry.

We begin by grouping the red blocks into groups of five, welding three of them together. The portals are then placed in such a way that the 3-block piece fall vertically with the two loose 1x1 pieces on its side (forming an unwelded cross). As it then fall through the portal that will take it to the output, that portal is shut off for a single tick (it is connected to the positive side of the lone sensor). During this tick, the loose blocks get welded to a side portal each. The 3-block piece does not get welded to any portal, but since it's currently going through the middle portal, it's considered a part of it. The red blocks and the portals are now welded together, but as the portals reactivate we get the cross, already partially through the portals.

Four states of the Cross Weld. Anything but the shapes being welded together has been omitted and inactive portals are just shown as ordinary blocks.

In the image above, we can see (from left to right) how the Cross Weld works through 4 states.

1. Three red pieces and a grey portal piece (plus the output portal for clarity).
   
2. The red pieces have now fallen into place, making the longer one go through the portal.
   
3. The portal is deactivated and the red blocks are welded to them. The two sets of portals are visualised as a single grey piece.
   
4. With the portals active again, the cross detaches from the grey blocks.