Someone had a brass Kumata SD40-2 that needed some help. This loco has a serious design flaw in that it does not have strong enough insulated screws- and not enough screws total. In order to fix the loco, you must drill and tap the frame for new screws....but you need custom length screws too!
Since this is a procedure I actually do a lot, I thought I would share how I do it.
No doubt most of you do this, but it may help someone.
So I've drilled the SD40-2 frame, tapped it, and screwed in some 1-72 screws. But they are way too long.
Now we are skipping a step. I experimented to find out just the right size screw I needed. Many times when I cut screw, the precise length does not have to be down to the millimeter. But this time it does.
So by experimenting, I determined that a nut and three Micro-Trains washers (you have been saving all those sprues over the years haven't you) would give me what I need.
Now the trick here is the nut. When you cut a screw you obviously tear up the threads. So I use a nut to straighten things out after the cut. So you have to remember to place the nut on FIRST.
So this is ready for the motor tool. I have simply used wire cutters and files at other times, but a motor tool is so much better.
Motor tool cut the screw off even with the nut. Nice and smooth.
This is the screw exactly after I took the nut and washers off. It is NOT useable. The problem is that the last thread is razor thin and will bend if I try to use it. If I were to insert it and try to start screwing, the last thread would fold over and foul up the process. The goal it to get rid of that razor fine material.
So, using an optivisor and a #11 Xacto knife, I stick the blade just under the lip of the last thread and lift up. It comes free VERY easily.
But I need to remove more. It is still too thin. So I do it again.
This is the second lift.
Then I do it a third time. This one is tougher because the metal is thicker. Good. That's what I want.
So after the third lift of the blade, this is what's left. This screw is ready. Notice that I have removed about 1/3 of the last thread- maybe even a little more.
Now, when this screw is inserted into its hole, the thread will properly engage without fouling.
And here is why these screws had to be a precise length. Much longer and they might obstruct the trucks.
And here is the end result of all this work. A better stronger (functional) gearbox.
Now you might ask--- why not cut off the screws when they were screwed into the frame?
Good question. It would be possible.
But the risk is that first turn when removing the screws. Remember the threads are damaged. So they MIGHT do some damage to the threads in the frame. I'd rather use a nut since I have plenty on them.
Now obviously, if i have a LOT of these screws to make, this process would not be all that easy.
And in my EP-2 project, I needed LOTS of smaller screws. So I cut brass sheets, soldered them to the thickness I wanted, drilled and tapped for a tiny metric 1.4 mm diameter screw and created a cutting jig.
You can see this little jig can make screws of two different sizes.
These little screws are made of steel and thus a little tougher to work with than brass (or plastic) but the principle is all the same.
Happy modelling!
