Sometimes it really is as simple as two strips of MDF to provide support for soldering subassemblies together.

(Above) I designed this soldering jig so that gravity held the four pieces of each hopper bay in place while heating with a torch. If needed, I could also use a few track spikes to stabilize the pieces. The idea was solid but the slope angle of one or two sides and opening at the bottom was off just enough to make precisely aligning the four sides a constant battle. I got a good fit in one or two corners but the others wouldn’t line up. Instead of fighting the inevitable, I remade the jig, or tried too.

Version Two didn’t live up to the promise of the first one. You can see my compound angles at the four corners didn’t meet at all even though the 45 degree angles of the sides were okay. Again, the dimensions of the bottom opening were still off. So, we move on to…

Version Three, although actually, this is the fourth jig I made. The third one employed a completely different design and failed miserably.

To eliminate the problems with compound angles, V.3 was simplified and worked better. This is the one I used to make the three hopper bays. The slope angles are better and the open design made for less scorching from the torch. While things were still slightly off, I made it work with shims of folded aluminum foil to take up any discrepancies.

(Below) Milling jigs make fabricating parts easier and predictable. I rough cut the 0.032” blanks on the miniature table saw or a cut off disk in a rotary tool. The key to success is to hold the blank absolutely rigid. No. 8 wood screws and washers do the job.

After set up, I mill one edge then flip the piece over and do the opposing edge. A light touch up with a file usually completes the job.

I can nerd out over stuff like this. Jigs like these are fun to design and, usually to build. They aren’t perfect, far from it. Like the project itself, they’re a work in progress.

I’m certainly not a designer or engineer and there must be a dozen ways to make a fixture like this. I simply think about the design for days until an idea comes up. I just do what makes sense to me but I’m not afraid to go back to square one when needed, which, is more often than not. MDF is my material of choice. It’s stable, durable and inexpensive.

Some things to consider are what do you want the jig to do? For soldering multiple pieces at the same time, you want them aligned and stable until the solder flows. Is the job better done in stages, which suggests a two part fixture. Do step one, then bring in the second piece of the jig to complete the work. Since these items are temporary, I don’t put a lot of time into them but if it’s a repetitive task I consider a more permanent fixture. It goes without saying that the accuracy of the part depends on the accuracy of the jig. Time here is well spent.

These jigs were a learning exercise like so much of this build is. They’ve proven their worth as I’m pleased with how the hopper bays turned out.



  1. Simon

    Your hopper jig reminds me of one I made from 1mm thick scraps of card, in this case to make the shape inside some rather lightweight PRS hopper cars. Nothing as fancy as what you are doing, but they ended up as charred as yours, although in this case, I was simply using up bits of low-melting point alloys (from kit sprues, etc) to pride much-needed increased mass low down in the body.

    Not all jigs have to be high-tech!

  2. mike

    Hi Simon
    None of my jigs are hi-tech. I just made a positioning jig to solder the center sill sections in place. It’s three pieces of wood that aren’t even attached to each other. It couldn’t be more basic. -Mike